Thursday, January 18, 2018

January 23-27 Potential Winter Storm

This is a brief update to the previous post concerning this possible winter storm, which you can find here.

Tropical Tidbits
Shown above is a near-analysis of the meteorological environment over eastern Asia for the morning of January 17th. At this time, we saw a storm system skirting the eastern coast of Japan, intensifying somewhat as it did so. As I highlighted in my previous post, this will pose a threat for a winter storm in the January 23-27 period.

I maintain that the Midwest/Ohio Valley regions continue to have a better chance of seeing wintry impacts as opposed to the East Coast, but long-range model guidance suggests the Plains may be able to get in to the mix, as shown below.

Tropical Tidbits
This 210-hour forecast from the GFS model has a sub-1000 millibar low pressure system moving north through the Plains, laying down accumulating snow in the northern Plains, with showers and thunderstorms likely occurring in portions of the Midwest down to the Gulf Coast. As is the caveat with any model forecast beyond 84 hours, this is bound to change, and likely will do so drastically. That's why long range model guidance is used more for identifying the *signal* of a storm, not yet the exact location of such a storm. As such, it is a good sign for those hoping for some stormy weather that the GFS is sniffing out such a possibility in this January 23-27 timeframe.

Tropical Tidbits
To exacerbate this point, above is the Canadian CMC's model interpretation of this storm. It predicts the storm will crawl near the Gulf Coast, impact the Southeast and eventually ride up the East Coast. That flies directly in the face of what I said earlier here, but I'm posting it to highlight how these model forecasts will drastically change in coming runs, as is typical. Again, the point is that the model is sniffing out a storm at all- the path will be refined in coming days.

Tropical Tidbits
Just to entertain all crowds, the ECMWF model above shows this storm cutting up through the Ohio Valley, though slightly outside of this timeframe. Details will be sorted out, but model guidance is latching on to the idea of a winter storm around the January 23-27 timeframe.

To Summarize:

- Model guidance is sniffing out the potential for a winter storm in the January 23-27 period, as was highlighted in a previous post from January 10th.
- Significant uncertainties exist with regard to the location of any wintry weather, and such impacts will not be known with any degree of accuracy for at least a few days.
- For now, the Midwest/Ohio Valley/Plains regions look most likely to see impacts, but this comes from the synopsis of the atmosphere back on January 10th and these locations could very well not be affected by this system.


Monday, January 15, 2018

Stratospheric Warming Event Projected To End January

Model guidance continues to indicate that the stratospheric polar vortex will come under further stress as we head toward the end of the month, with a wavenumber-1 event potentially unfolding in the stratosphere and opening the chance for February to be colder than normal.

Shown above is the 240-hour forecast of geopotential heights (contours) and temperatures (colored fill) from the ECMWF model, analyzing the 70-millibar slice of the stratosphere. This is around the middle or lower-middle part of the stratosphere, and as a rule of thumb stratospheric warming events tend to be stronger and more able to dislodge the polar vortex when the warming event is seen throughout multiple layers of the stratosphere. While I'm only showing the 70-milliber layer, model guidance has latched on to this warming potential throughout the stratosphere.

In this image, we see the stratospheric polar vortex still pretty much in control, centered almost over the North Pole. The piece to analyze, however, is the swath of warmer air trying to push into the Arctic Circle, with the warmest temperatures located over the Bering Sea. The placement of these warmer anomalies is important, because when stratospheric warming events push into the Arctic Circle from the Bering Sea region, they tend to be more successful at disrupting the polar vortex. I'm sure there's some academic literature on exactly why this happens, but in the absence of reading such literature I take that as a rule of thumb.

We can get a better look at the polar vortex through forecasted potential vorticity charts.

In a nutshell, these charts show the orientation of the polar vortex at different levels of the stratosphere. This map shows potential vorticity values at the 550 K level, which is somewhere around the 20-millibar level. Warmer colors depict areas of higher potential vorticity - in this case, the location of the polar vortex. Colder colors indicate where the polar vortex is not, in essence. At hour 240, we see the polar vortex in a rather elongated shape, dislodged from the Arctic Circle and positioned from Greenland across northern Eurasia. This shape of the polar vortex signals that it will be under duress at this point in time, as the polar vortex is 'typically' seen in a more circular shape. This elongated shape suggests pressure being applied from the Bering Sea area in the form of a ridge, which is confirmed by the deeper blue area stretching from the Gulf of Alaska to the Bering Sea. Similar to how higher levels of potential vorticity indicate the polar vortex, lower values (especially in this case) can be indicative of a ridge of high pressure.

So far, we've gathered that model guidance projects the stratospheric polar vortex to come under substantial pressure in the next several days, likely continuing through the end of January. This pressure looks to be applied across much of the stratosphere, boosting the chances that any disruption in the polar vortex could then show up in the troposphere in the form of colder than normal weather. Let's sum it all up with a few graphs.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels, as we discussed with the rule of thumb earlier in this post. However, it can still be valuable to look at, as a reversal in wind direction at the 1-millibar level could then filter down to lower levels. In this forecast image, the 1-millibar wind speed is forecast to continue declining slightly from a recent peak, before strengthening again and then once more weakening. This would take it back to roughly the same level we're at right now, if this forecast verified exactly as shown. Let's see if any signals can be identified in other panels.

The second panel shows the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where both slices of the stratosphere look to see the strength of the polar vortex gradually decline as wind speeds slow down. While the deceleration here appears modest, there are signs that it will continue through the end of January. Both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, which will lead to a slight deceleration across the 1-millibar, 10-millibar and 30-millibar areas. This will then revert back to near-zero, but at the end of the forecast period the ECMWF sees these fluxes strengthening again, leading to further wind speed deceleration in the aforementioned levels of the stratosphere.  Perhaps most significantly, for this second acceleration in the two fluxes, you'll note how the EP-flux on the bottom panel is pointing straight up, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is also forecasted for the end of the forecast period). As such, while the ongoing increase in fluxes may not do much damage to the polar vortex, the re-configuration of the EP-flux in time for the second projected warming event in about ten days could do far more damage, especially if it is a sustained warming event.

As such, I expect a broadly seasonal to warmer-than-normal temperature pattern for the eastern 2/3 of the U.S. through the end of January, but the chances of colder-than-normal weather increasing, particularly after the first few days of February.

To Summarize:

- Model guidance continues to indicate a potentially-disruptive warming event in the stratosphere towards the end of January.
- As this warming event is expected to affect nearly all sections of the stratosphere, effects in the troposphere are likely in the first and second full weeks of February.
- Colder than normal conditions are expected to increase in likelihood after the first few days of February.


Wednesday, January 10, 2018

January 12-13 Significant Ohio Valley Winter Storm

A significant winter storm impacting the Ohio Valley is expected to lay down accumulating snowfall, potentially in excess in 12", as well as potentially crippling ice accretion.

This post will cover current model guidance, which appears to be forming a consensus as of the January 10 00z model suite.


Pivotal Weather - GFS
We begin with the GFS model's outlook for snow accumulation. The 12z GFS appears to anticipate a stripe of accumulating snow falling from western Tennessee and Kentucky into southern Indiana, much of Ohio and portions of West Virginia. Particularly heavy accumulations are forecasted for northwestern Pennsylvania and western New York, especially near the Great Lakes. Per this model forecast, accumulations look to be in the 2-4" range in western Tennessee and Kentucky, increasing to a broad 3-6" range for Indiana, especially in the southeast portion of the state. Ohio may receive anywhere from 6-12" of snow, with the higher end of this range most likely in the northeast part of the state.

Pivotal Weather - NAM
Next is the NAM's snowfall forecast. As is typical for this model, snowfall accumulations are enhanced, likely beyond what will actually occur. Western Tennessee and Kentucky are projected to receive 4-8" of snow in this solution, a notable bump from the GFS' guidance. Southeast Indiana and a good swath of Ohio are outlooked for a 12-18" snowstorm as per the NAM model, again a substantial increase from the GFS solution. I do not expect the NAM's solution to work out, partially because of its inherent bias of over-doing snowfall accumulations but also the possibility (if not the likelihood) that at least a portion of this projected snow is actually sleet for some areas. This sleet-forecasted-as-snow problem is present in all models for the winter season (and can vary depending on the website you use to get snowfall accumulation graphics), but is particularly maximized in the NAM model. As such, I would take the NAM snowfall graphic with a grain of salt at this point and refer to the GFS and CMC models, the latter of which is shown below.

Pivotal Weather - CMC
The CMC model seems to take a middle ground between the GFS and NAM in a broader sense. On one hand, the CMC anticipates a similar snowfall accumulation story for Indiana/Ohio/Pennsylvania/New York as the GFS, but also sides with the NAM in the idea that snowfall accumulations may be heavier in Tennessee and Kentucky. Adding to the variance is how the CMC's accumulations for Kentucky and Tennessee appear to be slightly west of other guidance. I personally would side with the GFS for accumulations in Kentucky and Tennessee - there will be a very warm air mass in place as this storm gains strength, and I'm not confident in the storm being able to cool off the temperature profiles enough to lay down over half a foot of snow in those areas. I do believe a broad 8-14" range of snow for southeast Indiana, much of Ohio, northwest Pennsylvania and western New York is preferable for now, with higher amounts likely in places nearer to the Great Lakes for Pennsylvania and New York.

Freezing Rain

Pivotal Weather - NAM

Pivotal Weather - GFS
I don't want to discuss freezing rain too much in large part because freezing rain is often a "nowcast" situation, or can only be accurately forecasted immediately prior to the event. I will, however, show the NAM and GFS forecasts for freezing rain to give an idea of where accumulating ice is most likely from this storm. I want to emphasize that, with the above graphics, you should *not* focus on ice accumulations, but *should* focus on the forecasted placement of ice accretion.

The NAM and GFS agree that an ice storm is possible for western portions of Tennessee and Kentucky into southeastern Indiana, as well as south and east Ohio. Further ice accretion is also a possibility for central Pennsylvania, as well as parts of central and eastern New York. The remarkable similarity between projected ice accretion and projected snowfall accumulation makes me more suspicious of the snowfall forecasts, particularly for Kentucky, Tennessee and southeast Indiana, as model guidance could be falsely interpreting freezing rain and/or sleet as snow. As I said, this will likely become a "nowcast" situation for that reason.

To Summarize:

- A significant winter storm is expected this weekend in Kentucky, Tennessee, Indiana, Ohio, Pennsylvania and New York.
- Snowfall amounts of 2-6" are possible in Kentucky and Tennessee, while a range of 6-12" is possible for southeast Indiana into western and central Ohio. Northeast Ohio and western New York look to receive 8-14" of snow by the time this event is finished.
- An ice storm does look possible from this system. While I am not confident in forecasting accumulations, it looks as if central Tennessee, western Kentucky, southeast Indiana, central Ohio, central Pennsylvania and central/eastern New York will be at risk for non-negligible ice accretion.
- The location and intensity of freezing rain will most likely only be accurately determined immediately prior to the event, if not determined during the event (i.e. "nowcasting").


January 23-27 Potentially Significant Winter Storm

A potentially significant winter storm looks to impact the United States around the January 23-27 period.

Tropical Tidbits
The GFS model forecasts a strong surface low crossing the central and northern parts of Japan on Wednesday, January 17th, producing strong lower-level winds offshore the eastern coast of the country as a result. As I've discussed in the past, the East Asian Rule can be applied here. Recall that the East Asian Rule is a leading indicator of sorts for weather in the United States, by a matter of 6-10 days. In this case, the presence of a storm system over Japan on January 17th suggests a storm threat in the United States sometime between January 23rd and January 27th.

This is a little far out to be anticipating a storm system in Japan, so let's see if any other model guidance agrees.
Tropical Tidbits
The Canadian's global weather model, abbreviated often as the CMC model (Canadian Meteorological Centre), agrees to some extent with this phenomenon. The model anticipates a trough to cross Japan at roughly the same time period as the GFS, albeit at a weaker magnitude. Such variations in strength are to be expected, and it's entirely plausible that the "potentially significant" tag will need to be dropped if/when this storm comes into view, both for Japan and then the U.S. For now, however, the key thing is that both models do show a storm system impacting Japan during this time period, a good signal for a storm in the U.S. around January 23-27.

Tropical Tidbits
The GFS ensembles, valid for the same time as the CMC model image, also agree on the presence of a storm system impacting Japan around January 17th. The above image shows the mean MSLP from the individual ensemble members through the black contour lines, and shows individual ensemble member lows in the red numbers. Higher pressure values are shown in the blue numbers. The color represents the normalized spread among members, and as the graphic indicates there is considerable spread / uncertainty around the idea of a storm hitting Japan. However, with the mean MSLP contours indicating the presence of the storm, it seems to confirm the ideas of the GFS and CMC above, that indicate a storm system is expected to hit Japan on January 17th.

So, we can now say it appears more likely than not that a storm system will hit Japan on January 17th. This would correlate to a storm system in the U.S. around January 23rd-27th. But we can go further. We can pinpoint where the storm may end up tracking, to at least some degree.

We'll begin by using the Madden-Julian Oscillation (MJO).
Dr. Roundy:
The screenshot above shows a Hovmoller forecast (from the bright green line upward) of outgoing long wave radiation (OLR) anomalies through early February. Outgoing long wave radiation is used to identify areas of enhanced or suppressed thunderstorm activity in the tropics - here, it analyzes such activity or lack thereof between the 7.5 degree North line of latitude and the 7.5 degree South line of latitude. Cooler colors indicate stormier weather, while warmer colors indicate the absence of thunderstorms. Given that the Madden-Julian Oscillation is identified by the placement of enhanced thunderstorm activity along the Equator, this helps us identify where the MJO may be.

In the above image we will focus on the solid red line, which outlines where the Madden-Julian Oscillation appears to be tracking in terms of longitude (on the x-axis) as based on OLR anomalies. We see that the MJO is currently shown to be maximized just west of the 90 degrees East line of longitude, as shown by the solid red contours. As we move forward in time to roughly January 21, however, that MJO "pulse" (which just so happens to be progressing with a Kelvin Wave, as the thin pink lines show) weakens and peters out, while another "pulse" fires up around the 135 degrees East line of longitude. Around January 25th, in the middle of this potential storm's expected timeframe, the deepest negative OLR anomalies look to be placed around the 140 degrees East line of longitude.

We've established that the MJO is expected to be active during this potential winter storm, and we know where this enhanced thunderstorm activity is expected to be, but we don't yet know what phase of the MJO this corresponds to. If you'll read on, however, you'll see that we can also figure that out!

The above image is a composite graphic from the Japanese Meteorological Agency (JMA), showing 200-millibar velocity potential anomalies across the globe during the month of January when the MJO is in Phase 5. The bottom panel shows OLR anomalies during the month of January when the MJO is in Phase 5. Put our earlier discussion and this image together, and it looks like the MJO is projected to be in Phase 5 when this winter storm is forecasted to impact the United States. We figured this out by seeing how the minima of the negative OLR anomalies were centered in the region around the longitude lines of 130 degrees East and 140 degrees East. We can now see the typical precipitation effects of when the MJO is in Phase 5 during the month of January.

The above image shows just that. It shows typical precipitation anomalies over the United States during the months of December-January-February (centered on January) when the MJO is in any of the phases above. Looking at the graphic for Phase 5, it seems that wetter than normal conditions tend to show themselves during the winter months in the Midwest, Ohio Valley, Central Plains and Southern Plains when the MJO enters Phase 5. Additionally, drier than normal conditions appear to present themselves in portions of the Southeast, as well as the northern Plains. Much drier than normal conditions are typical along portions of the West Coast when the MJO reaches Phase 5. We can derive a couple of things from those anomalies.

For one, the dry patch in the Southeast tells me that the (in)famous Southeast Ridge may make an appearance. This is not what winter weather fans along the East Coast like to see, as it discourages storms from running up the coast and forming into "Nor'easters". This is exactly what winter weather fans in the Midwest, Great Lakes and Ohio Valley like to see, however, as that Southeast Ridge can often help storm systems move northeast and bring heavy snows to the aforementioned regions. In this case, while we of course cannot say for certain yet, the presence of a Southeast Ridge would certainly boost the chances for this potential winter storm impacting the Midwest, Great Lakes and Ohio Valley with heavy snow as opposed to the East Coast. 

So far, we have established that a winter storm is quite possible in the January 23-27 timeframe for the United States, as model guidance seems to agree on a storm system impacting Japan on January 17th (albeit with varying magnitudes). We've also used forecasted OLR anomalies to show how the Madden-Julian Oscillation looks to be in Phase 5 around the time this winter storm would impact the United States, and then used precipitation composites to show how this possible winter storm has a better chance of hitting the Central U.S. with heavy snow than the East U.S. 

But we can go further with this.

Shown above are forecasts for four major teleconnections: in clockwise order, the NAO (North Atlantic Oscillation); the EPO (East Pacific Oscillation); the WPO (West Pacific Oscillation); and the PNA (Pacific-North American index). Glancing over these, only two of them look to be strong enough to actually influence the weather pattern by January 22nd, with the EPO not registering a signal and the NAO descending further into barely-positive territory at that time. The WPO is forecasted to be positive on the 22nd, but given how far upstream it is I generally don't account for it in my forecasts. The PNA, however, is forecasted to be moderately negative right before the timeframe for this potential winter storm opens up, and that provides us with another indicator to use. Let's apply it to precipitation anomalies.

Shown above are four panels showing precipitation anomalies' correlation with the PNA index. Brown colors indicate precipitation anomalies that have a negative correlation with the PNA index, while green colors indicate precipitation anomalies that have a positive correlation with the index. For example, in the brown-colored areas, if the PNA were positive, this says that precipitation anomalies would be below normal (negative, in effect). Similarly, if the PNA were negative, those same areas could then expect above normal precipitation. 

The top-left panel shows this correlation with the PNA index for the month of January, and that panel is what we'll focus on. The most important feature on that panel is the swath of brown colors encompassing the Midwest, Ohio Valley, slivers of the Gulf Coast and portions of the East Coast. The darkest brown shades are located in the Ohio Valley, almost into the Mid-Atlantic. With the PNA forecasted to be negative when the storm threat rolls around, this panel tells us that above-normal precipitation is favored in the Midwest, Ohio Valley and Mid-Atlantic in such a pattern. This is slightly different from the MJO Phase 5 composite in that it doesn't have that Southeast Ridge component apparently visible, but what counts is that both the MJO Phase 5 image and this PNA image indicate that the synoptic weather pattern will be ripe for wetter than normal conditions in the Midwest, Ohio Valley, Great Lakes, and perhaps the Mid-Atlantic and sections of the Northeast, as well as parts of the Plains.

To wrap up, let's assuage the discrepancy I highlighted in the last paragraph, how the MJO phase 5 typically brings about a Southeast Ridge but a negative PNA in January does not appear to do so.

The above image, from North Carolina State University, shows a 'textbook' negative PNA weather pattern. This is seen by the below-average heights in the Western U.S., indicative of stormy weather that could then push east in the form of a storm system (perhaps multiple, but that's a case-by-case basis). On the flip side, a 'textbook' negative PNA will tend to pump up a ridge in the Eastern U.S., as a consequence of the deep negative anomalies in the Western U.S.

But wait a second, didn't that PNA precipitation image show that there wasn't really a ridge in the East or the Southeast? Yes, but it must be remembered that the atmosphere is not rigid. A storm system will not turn due north in the above graphic once it hits the oranges and reds of that ridge. Instead, as I learned quickly (through failed forecasts), storm systems mesh with their surrounding features. Storm systems will press into the ridge and ride above it - typically not pushing it off to the side but also not treating it as a brick wall and having to change direction. We apply this here by recognizing that a storm would likely drop south and into the Southern Plains, because geopotential heights are so low in the West and thus are suppressing the jet stream south, but the storm would then move northeast as it starts bumping up against that ridge. This would result in a path through the Midwest and the Ohio Valley, perhaps exiting through the Northeast or the Mid-Atlantic. This would lead to the above-normal precipitation anomalies for those regions as seen in the MJO Phase 5 graphic and the PNA graphic, but also favor the idea that a ridge *would* be present somewhere in the East US that would allow this storm system to take such a track. Keeping with such a theme, I expect this potential winter storm to take a similar path.

To Summarize:

- A potential winter storm looks to impact the United States in the January 23 - January 27 timeframe.
- Multiple atmospheric oscillations appear to be favoring the central Plains, Midwest, Ohio Valley, Great Lakes, and perhaps portions of the Mid-Atlantic and Northeast for the heaviest precipitation that would be associated with this storm.
- Any snowfall associated in this storm would likely impact those same areas, though depending on the orientation of the ridge in the East this may not be the case for the Mid-Atlantic.
- Substantial uncertainty remains over all parts of this potential winter storm, including if the storm itself will even transpire.
- However, based on the evidence presented above, I do feel confident in the chance for a winter storm in the country sometime between January 23 and January 27, with precipitation chances potentially favoring the mid-section of the country, from the central Plains through the Midwest and Ohio Valley.


Tuesday, January 9, 2018

January 12-14 Potentially Significant Winter Storm

A potentially significant winter storm is expected to impact the Ohio Valley and Northeast in the January 12-14 period, with all modes of wintry precipitation possible.

As of this morning, a strong upper level low was located offshore California, moving into the Southwest. As discussed in a previous post about the January 10-12 Winter Storm, this upper level low will actually be torn into two pieces by January 11th, as a piece of energy currently skirting the U.S. / Canada border near the Pacific Northwest will help a portion of the upper level low to become negatively-tilted and create the winter storm over the 10th through the 12th. This process is shown below.

As this piece of the low is taken northward and creates that winter storm, however, a deep positively-tilted trough is left behind in the southern Plains. This will be the energy that creates the January 12-14 potentially significant winter storm, expected to primarily affect the Ohio Valley and Northeast with wintry precipitation.

By Friday evening on January 12th, the aforementioned energy in the southern Plains is forecasted to move east into the Southeast and strengthen, to the point that it becomes negatively-tilted (shown in the northwest-to-southeast orientation of the vorticity maxima), indicating a mature storm system at peak / near-peak strength.

Tropical Tidbits
A few hours before that image is valid, the surface low will have already formed and is projected to be near eastern Tennessee by Friday afternoon. The swath of warm air that the January 10-12 Winter Storm had to deal with will again be present in the Northeast, allowing this event to begin as rain for the entirety of the Northeast and portions of the Ohio Valley. Northern Ohio and portions of Indiana may be able to retain temperature profiles cool enough to support wintry precipitation for nearly the entire event, but I have a feeling that will be more of a "nowcasting" determination.

Tropical Tidbits
By Saturday morning, as the trough continues to be at its peak intensity, the surface low is projected to be moving northeast and strengthening below the 1000-millibar level. As such, the deformation zone looks to be well-developed and producing heavy wintry precipitation from Kentucky through Ohio, much of New York, and slivers of Pennsylvania, Vermont and New Hampshire. As the graphic shows, a region of significant sleet and/or freezing rain is being outlined in eastern Ohio, northwest Pennsylvania and western New York at this time. I personally see freezing rain and, to a lesser degree, sleet as precipitation types that can only be accurately predicted immediately prior (i.e. 24 hours or less) to the event, simply because more factors are involved in getting freezing rain relative to the factors involved in producing snow or rain. So, while I will refrain from calling for a significant ice event from this storm, I certainly recognize the possibility of such an outcome, and model guidance on ice accretion will be analyzed further shortly.

Tropical Tidbits
By Saturday morning, the surface low is forecast to have strengthened further, with the deformation zone now in place from the northern Ohio Valley along the Great Lakes into extreme southeast Canada. Again, the GFS here anticipates a not-insignificant region of freezing rain and/or sleet, particularly from New York through northern Vermont and New Hampshire, as well as a good section of Maine.

In terms of accumulations, everything but the kitchen sink looks to fall from this storm.
Pivotal Weather
The heaviest accumulating snow is expected to fall from Kentucky into Ohio, with the highest totals projected in southern Canada along the Great Lakes. As is generally the case with dynamic winter storm forecasts more than 48 hours out, snow accumulation forecasts are rife with caveats. While it is wise not to see these numbers as written in stone, it is also wise to believe that this storm has the potential to produce significant snow totals in excess of 12". Exact amounts and the location of relative maxima in amounts will change as we get closer to the event, but it is quite apparent that this storm has the ability to lay down a swath of over 12" of snow across a sizable section of the Ohio Valley.

Pivotal Weather
I show this image of forecasted freezing rain accumulations as a word of caution more than anything. First, notice how jagged and strange the placement of these accumulations are, almost like a contorted snake. This is a symptom of the GFS being unable to resolve the environment to a fine enough degree - whereas the NAM model has far higher resolution (that's why NAM hi-res graphics look so pretty), the GFS and other global models have lower resolution, simply because they forecast for the entire world, as opposed to the NAM, which forecasts for the United States. If the GFS model was made to create global forecasts at the NAM's resolution out to 384 hours, the forecast run could take hours to complete! So, let's take a look now at the NAM's forecast for freezing rain accumulations.
Be warned, the NAM does not yet have the full timeframe of this storm in its sights yet, so ice accumulations will seem far lower than they may actually turn out to be.

Pivotal Weather
Now that's a lot easier on the eyes, isn't it? The NAM's ice accumulation forecast has a swath of freezing rain that looks far smoother, far less jagged and contorted than the GFS' image. Again, however, I post this partially as a word of caution. The NAM is notorious during the winter season for amplifying wintry precipitation accumulations beyond what they end up being. For example, in previous seasons it hasn't been uncommon for the NAM to project 8" of snow in one area while the GFS and ECMWF support up to 4" at best. This applies to freezing rain as well, and I believe that while the NAM is almost certainly over-doing ice accumulations here, it gives a good idea of where freezing rain *could* accumulate when this storm begins. Areas from northern Mississippi and eastern Arkansas through western Tennessee, western Kentucky, Indiana and Ohio could all be at risk for some ice from this storm. I'm a bit skeptical of the projected freezing rain in Louisiana, but with a storm system this dynamic it certainly bears watching.

To Summarize:

- A potentially significant winter storm is expected to impact the Ohio Valley and Northeast in the January 12-14 timeframe.
- While these regions will initially experience rain, a heavy snow event is expected for portions of Kentucky, Indiana, Ohio, and Canadian areas near the Great Lakes.
- Given the dynamic nature of this storm and the body of warm air affecting a region that only recently escaped from a "deep freeze", accumulating freezing rain is possible, particularly in a zone from western Tennessee/Kentucky and eastern Arkansas into Indiana, Ohio, and northern portions of Pennsylvania, New York, Vermont, New Hampshire and much, of Maine.
- While amounts will be refined in coming model runs, snow accumulations in excess of 12" are possible, particularly in Ohio and southern Canada.
- While amounts will likely be determined only shortly prior to the event, ice accumulations in excess of 0.25" are possible, particularly in Ohio, northern Pennsylvania, New York, Vermont, New Hampshire, and Maine.


January 10-12 Winter Storm

A winter storm is expected to impact the Northern Plains over the January 10-12 period.

As of this morning, a strong upper level low was located offshore California, moving into the Southwest. In addition, a piece of energy was located moving onshore just north of the Pacific Northwest. These two features will combine to create an accumulating snow event for portions of the central U.S. in the next couple of days.

By Thursday morning, this upper level low is forecast to have moved into the Central U.S. and separated into two primary parts - the majority of the energy is projected to take the form of a positively-tilted trough, bottoming out in western Texas, while a smaller part of the low will break off and ride the ridge over the Ohio Valley to become negatively-tilted. At the same time, that small piece of energy we currently see moving along the U.S. / Canada border near the Pacific Northwest will help to pull that smaller piece of energy northward and help give it that negative tilt to create the winter storm for the Plains.

Tropical Tidbits
Also on Thursday morning, the winter storm is projected to be ongoing for the Plains, with a surface low located along the Iowa / Wisconsin border producing a band of accumulating snow from Kansas to Minnesota and the northern portions of Wisconsin. The GFS model portrays a sliver of precipitation to fall as sleet or perhaps freezing rain, but due to how conditions must be "just right" for freezing rain to occur, and to some extent also for sleet, the location and intensity of either freezing rain or sleet will likely only be accurately known just before the event begins. Rain is expected to fall and chip away at any enduring snowpack in Iowa, Illinois, Michigan, southern Wisconsin and parts of Missouri.

Pivotal Weather
Snow accumulations are expected to be maximized along a bad from central Kansas through eastern Nebraska, northwestern Iowa and southern Minnesota, with a stripe of particularly-heavier accumulations forecasted from southern Minnesota through northern Wisconsin and the upper peninsula of Michigan. Light snowfall is possible in the wake of the storm system as colder air filters in behind the low, resulting in the 1-2" accumulations depicted across southern Wisconsin, northern Illinois and parts of Iowa and Missouri.

The snowfall in the primary axis of heavy snow is expected to be of the heavier and wetter kind, as opposed to the light & fluffy snowfall seen from Alberta Clippers. As such, if you reside in an area expecting accumulating snowfall from this storm, be sure to take frequent breaks if you decide to shovel, and to not exert yourself when doing so. Snowblowers are often the best idea for moving substantial accumulations of this kind of heavy, wet snow.

To Summarize:

- A winter storm is expected to affect portions of the Central U.S. between January 10th and January 12th.
- Accumulations of 6-9" are expected in a band extending from central Kansas through eastern Nebraska, western Iowa, southern Minnesota, northern Wisconsin and the upper peninsula of Michigan.
- This snow is expected to be more akin to heavy, wet snowfall than lighter, fluffier snowfall. As such, take precautions when shoveling snow, and use a snowblower or plow if possible.


Long Range Outlook: January 9, 2018

This is the long-range outlook, published on January 9th, 2018.

I'll begin with a topic that I've discussed a few times on this blog since November 2017: that of the stratospheric warming event that unfolded in December.

Shown above is a line graph showing the 30-millibar level temperature from September 2017 to the present day. I had highlighted how a stratospheric warming event appeared imminent in late November, though as mentioned this was not expected to be a sudden stratospheric warming (SSW). This is what transpired, though this warming event was prolonged and came in a number of waves after the initial failure of the first wave of warmth to penetrate the stratospheric polar vortex in the Arctic Circle. As the above graph shows, however, the warmth did eventually make its way into the stratosphere, weakening the stratospheric polar vortex and delivering the colder than normal temperatures to the eastern 2/3rds of the country to kick off the new year, as pictured below.
Following the stratospheric warming event, the warmth in the stratospheric levels of the Arctic Circle has dissipated, leading to the sharp drop in the 30-millibar temperature that has reversed only recently, though still remains well below normal. Similar to how a warming event in the stratosphere can lead to cooler than normal weather in the eastern 2/3rds of the country roughly 2-3 weeks later, a rapid cooling of the stratospheric Arctic Circle region can encourage warmer than normal temperatures in the United States, as the colder than normal air becomes "locked up" in the upper latitudes. We'll get to the forecasts for the medium-range shortly to see if this does end up being the case.

For the time being, though, the Pacific is showing some very interesting features that I'd like to review.
Above is a four-panel graphic showing the global orientation of 200-millibar wind speeds (color fill) and geopotential heights (contours) in five-day periods, from December 18th to the present day. From the five-day period including Christmas Day to today, the Pacific jet stream has strengthened and become extended to the waters due south of the Aleutian Islands. A rule of thumb when observing the Pacific is that an extended jet stream can portray the threat of a storm system in the following days, and this does appear to be the case for this week, but will require a post of its own. For now, it is worth noting the extended nature of the Pacific jet stream, as this will more than likely lead to an opportunity for unsettled weather in the U.S. in the next two weeks.

Let's now take a look at one of my favorite longer-range graphics to watch, the 8-10 day geopotential height anomaly forecast from the three primary global weather models.
The ECMWF model's forecast for 8-10 day geopotential height anomalies is shown on the left-most panel, with the GFS model's guidance in the middle panel. The CMC (Canada's global weather model) provides its outlook in the right-most panel.

The three models certainly have some key differences in the projected pattern for the eight to ten day period, but there are a few similarities to be gleaned from the guidance. For one, all three models indicate the presence of a Rex Block in western Canada into Alaska and the Gulf of Alaska, albeit to varying magnitudes. The GFS is most pronounced with this pattern, producing a 'textbook' Rex Block in the form of the strong, cut-off ridge over a portion of the Arctic Circle and Alaska, as well as the negative anomalies in the Gulf of Alaska, offshore the Pacific Northwest. One could argue the actual Rex Block is formed with that ridge and the apparent upper level low over north-central Canada in the GFS panel, but such an orientation (slanting to the southeast, from the ridge to the upper level low) is rather unorthodox.
The ECMWF and CMC generally agree with the idea of the Rex Block, though neither model cuts off the ridge as the GFS does, instead setting up a positive-PNA-looking ridge from the American Rockies into a portion of the Arctic Circle, as one continuous ridge. Both the ECMWF and CMC still appear to support this as a Rex Block, however, with the zonal downstream flow in the United States, save for the waters just offshore of New England, where a suppressed ridge holds in place as a consequence of strong negative geopotential height anomalies in Europe and a mixed North Atlantic Oscillation (NAO) pattern. Those two features will combine to discourage colder than normal air buckling south into the eastern two-thirds of the country, instead supporting seasonal temperatures.

It's not enough to simply eyeball weather models, however. We now turn our attention to the Madden-Julian Oscillation.
Above is the ECMWF model forecast for the Madden-Julian Oscillation (MJO), with the ECMWF ensembles displayed individually via the thin yellow lines. Although this forecast period began on January 1st and ends January 15th, more-recently issued forecasts still support the outlook of the MJO transitioning from Phase 2 to 3, and perhaps then to Phase 4, albeit at a weaker grade than the previous phases. As the screenshot below and on the right shows, from the Climate Prediction Center,
temperatures in the United States tend to be cooler than normal for much of the Central U.S. and the majority of the East U.S. during Phase 2 in the winter months. This follows the rule of thumb (portrayed by these same composites) that Phases 8, 1 and 2 of the MJO are most favorable for colder than normal and snowier than normal conditions in the eastern two-thirds of the country during the winter months. Similarly, Phases 4, 5 and 6 are most encouraging for warmer than normal temperatures in the United States for the winter months.

The expected movement into Phases 3 and 4 of the MJO by the middle of the month suggests warmer than normal temperatures for much of the United States around the same time period. This seems to go hand in hand with the three primary models' projections of the 8-10 day period, and indicates that the middle of the month (likely a little bit beyond the middle of the month) should be warmer than normal for much of the country. I expect the warmer than normal tendency to continue through the later part of January in part due to that rapid cooling in the stratosphere that was observed at the end of December, and will translate to the warmer than normal weather for the middle and end of the month.

But what should we be on the lookout for at the end of the month? Will the warmth continue, or will the cold return?

Just as this post began with the stratosphere, it will close out with the stratosphere. Above is a multi-panel forecast from the ECMWF model for different variables in the stratosphere. We will focus in on the top three panels to make some prognostications for the end of January and into early February.

The top panel shows observed zonal wind speeds at the 1-millibar level of the stratosphere - in other words, the strength of the far-upper stratospheric polar vortex. The 1-millibar level isn't as important as the 30-millibar or 50-millibar levels. As an example of this, it has not been uncommon for a warming event to occur at the 1-millibar level but not the lower levels of the stratosphere, resulting in the effects of that 1-millibar level not showing up in the troposphere. Regardless, it is useful as an early indicator of a weakening of the stratospheric polar vortex, which can then show up in weakening at other levels of the stratosphere. The black line shows the ECMWF forecast of the 1-millibar wind speed, and as the wind speeds strengthened with the rapid cooling of the stratosphere (indicating a strengthening of the stratospheric polar vortex, as is to be expected), they look to again weaken over the next couple of weeks, albeit still remaining well above levels that would imply a "weak" stratospheric polar vortex. If you're a 'glass half-full' person and enjoy cold weather, then this is a good sign. Personally, the weakening of the wind speeds from a projected 90 m/s to a projected 50 m/s at the end of the forecast period, while a notable change, isn't something to really pay attention to just yet. If the wind speeds continue to weaken, then it could be something to take note of, but for now the upper-stratospheric polar vortex looks set to remain in firm control.

A similar forecast is shown in the second panel for the 10-millibar zonal wind speed (blue line) and the 30-millibar zonal wind speed (red line), where a strengthening of the wind speeds is then followed by a weakening, though still to rather-elevated speeds. To make matters worse for those who are seeking some colder weather, both the geopotential flux and heat flux indicators in the third panel are set to significantly increase during the forecast period, usually a positive sign for those who want the polar vortex to weaken. However, if you've analyzed these graphs before, you'll note how the EP-flux on the bottom panel is pointing up and to the left, when looking at it two-dimensionally. Stratospheric warming events are most probable when the geopotential and heat fluxes are high (which is forecasted) and the EP-flux arrows are pointed directly up (which is not forecasted). As such, the stratospheric polar vortex looks set to keep the colder air locked up in the Arctic Circle through at least the second-to-last week of January, helping to severely hurt any chances of a sustained colder-than-normal pattern for the eastern two-thirds of the country through at least early February.

To Summarize:

- The recently-discussed stratospheric warming event has ended and has been followed by significant cooling.
- Atmospheric oscillations, the stratosphere, and medium-term weather model guidance agree on warmer than normal temperatures more likely than colder than normal temperatures in the eastern two-thirds of the country through at least mid-January.
- The stratosphere supports this likelihood of a warmer than normal weather pattern persisting into at least early February.
- A separate post on the potential for a strong storm system this week is forthcoming.


Friday, January 5, 2018

Tropospheric Polar Vortex Dislodged, But Warmth To Prevail

Medium-range forecasts have the tropospheric polar vortex dislodged from the Arctic Circle, typically an opportunity for a colder than normal period in the United States, but model guidance foresees abundant ridging in the Rockies to disperse any such opportunities.

Both the GFS and ECMWF models, in their 8-10 day 500-millibar geopotential height anomaly graphics, show strong ridging pushing into the Arctic Circle, forcing pockets of cold air to lower latitudes. However, with a strong ridge over the Atlantic in a less-than-optimal position and a broad ridge over the Rockies just screaming to bleed east, any opportunities for cold weather look to be transient.

The ridge in the Atlantic looks to try and push northward, but an area of well below-normal geopotential height anomalies just east of Greenland will not only stop the ridge from moving north, but due to its positioning near Greenland will also promote conditions similar to the positive phase of the North Atlantic Oscillation (NAO). Such a pattern is conducive for warmer than normal conditions over the Central and Eastern U.S.

Also helping prospects for warmer than normal weather in the eastern two-thirds of the country is the ridge over the Rockies, something that may be recognized as the positive phase of the Pacific North American (PNA) index, but this is a 'false positive'. The 'passive' nature of this ridge, as seen in its relative weakness compared to the ridge over the Atlantic and the trough over the Aleutian Islands of Alaska, will let the Pacific jet stream just ride over the ridge and *not* proceed to plummet south into the Plains, as would typically happen in a +PNA set-up. Instead, it looks like the jet stream will be coerced into staying north, possibly even meandering in Canada in this timeframe, storing the colder air up there and in the three lobes of the tropospheric polar vortex, expected to be over the Aleutian Islands, northwest Europe and northern Eurasia.

In the longer-term, prospects for sustained colder than normal weather look similarly-cloudy, with model guidance foreseeing the Madden-Julian Oscillation (MJO) transitioning to Phase 4 by the middle of the month. Phases 4-6 are notorious for encouraging warmer than normal conditions in the eastern two-thirds of the United States during the winter month, and as such, further shots of cold air may prove to be transient through the end of January.

To Summarize:
- Following the recent period of colder than normal weather, warmer than normal (if not seasonal) temperatures for much of the country are expected for much of the remainder of January.