Tuesday, February 25, 2014

March 12-17 Potentially Significant Storm System

I'm seeing the potential for a significant storm system in the March 12-17 period.

Shown above is an ECMWF forecast for the evening of March 5th, depicting 500mb height anomalies over the North Pacific. If we look on the left half of the image, we see a deep trough over Japan, one with strength we haven't seen in some time.If we use the 6-10 day correlation, explained by Joe Renken, to tell us when these two systems may 're-appear' in the US, we find ourselves at a March 12-16 potentially significant storm. If this solution were to verify, we could see the strength of the trough translate over to that March 12-16 storm as well. This could mean a significant storm system, potentially accompanied by significant cold weather. It's too far out to tell if this could be a big snow maker, but judging on how the climate is shaping up for mid-March, I would certainly keep an eye out for some snowy impacts.

The GFS model shifts the timeframe a little further down the road to the afternoon of March 7th, but agrees on the idea of a strong storm system over Japan. Again, it'll take a while before we get into details, but it's clear that there is a chance for a significant storm system over the March 12-17 period. I'll have more details on potential tracks in coming days.


March 1-4 Potentially Significant Snowstorm

There is a rising possibility of potentially significant snowfall over the March 1-4 period.

The first wave of this snow event will come along on March 1st, roughly around this Saturday. Latest model projections take this system as a rather disorganized system, with lobes of vorticity scattered about the general storm. The jet stream around this system has a jet streak just to the southwest, meaning the storm would be expected to strengthen and dig a bit further south in the process. However, seeing as this system never really develops itself, and with a lobe of the polar vortex to the north in Canada, the best bet would be to maintain the storm where it is. Barring any significant model changes in the future, this first wave looks close to being correct.

The second wave is a bit trickier. We have an elongated band of vorticity, which is able to trace its roots back to Baja California. We also see a large system in Montana, which will have quite an influence on the system. The jet stream looks to have this system in its grips, as the band of air is depressed in the Southwest and rises northward as a result. This is also reflected here on the 500mb chart, as you can see the northeast progression of contour lines from the Southwest to the Northeast. On one hand, we could argue that the jet stream may eventually push this storm further north than projected. However, on the other hand, the lobe of the polar vortex to the north could end up pushing the storm back to the south. If there is to be a shift, I would be more inclined to side with a northward shift, but everything remains up in the air until we get these two waves sampled.

The general idea here is that we are likely to see a snow event in the March 1-4 period. How can I be so sure? Take a look at the graphic above. If you look closely you'll see a bump over Japan in the upper left hand corner, and if you were to loop through the 500mb anomaly charts for Japan over the past few days, you'll see that there have actually been two systems moving through Japan. If we use the 6-10 day correlation, explained by Joe Renken, to tell us when these two systems may 're-appear' in the US, we end up with a rough March 2-6 timeframe, and that really boosts confidence in this snowstorm idea. In the past, this 6-10 day correlation has verified with a high success rate, and I expect a similar story to unfold over the next several days.

Continuing to look outside the models, the jet stream in the Pacific is absolutely roaring right now. Wind speeds are in excess of 150 knots at maximum intensity, and if this event takes advantage of the jet stream (which does look probable), we could be seeing not only an increase in storm intensity, but an increase in the severe weather threat.

Just for a little eye candy, here's what this morning's 6z GFS showed for the snow events. Don't buy into this, the GFS gave out some strange solutions in this run that I don't really agree with. The thing to take away here is that two storm systems are likely in the first week of March, and we could see some significant snowfall as a result.


Sunday, February 23, 2014

Thoughts on ENSO, East Coast Snowstorms and Spring

This post will address my thoughts on the ENSO situation, the potential for East Coast snow events in the near-future, some additional spring thoughts, and more information on the upcoming severe weather season. This post will be of the more technical variety, so there will be a summary at the bottom for those who may not know as much weather jargon as some others do.

Above, we see the latest data from each monitoring region of the El Nino Southern Oscillation (ENSO) phenomenon. The blues represent below normal sea surface temperature anomalies (SSTAs), while oranges show above normal SSTA values. The El Nino phenomenon is characterized by above normal sea surface temperatures, and the La Nina is shown by below normal sea surface temperatures. Looking over all of the regions, we see that most of them are in a strong cooling period, actually hitting La Nina criteria of -0.5 degrees C anomaly or below. However, Nino region 4 is experiencing sudden warming to El Nino levels of 0.5 degrees C anomaly or above. Nino region 4 is the furthest west of the four regions, as the graphic below shows.

So what's making this region warm so quickly? Well, the answer lies in the sharp cooling trend we've seen in the regions further east, like Nino regions 3.4 and 3.

There's a Kelvin Wave afoot.

Shown above is a two-panel graphic, depicting SST anomalies on a depth by longitude chart on top, as well as a mean equatorial temperature depth-longitude chart on the bottom. Looking at the SST anomaly chart, it is very clear that there is a strong Kelvin Wave present under the surface. Kelvin waves are defined as 'warm spots' in the ocean that traverse the Equatorial Pacific, beginning near Indonesia and ending around Ecuador and Peru. The Kelvin Wave is actually a massive-but-small wave, meaning that while the water height only rises by a couple inches when the wave moves through, the Kelvin Wave stretches a vast expanse of water, and its effects on the climate are even more significant. During an El Nino, we see warmer than normal waters over the surface of the four Nino regions. Kelvin Waves can help to speed up the El Nino process by bringing an expanse of warm water into the Nino regions, about 100 to 200 meters below the surface. If it's 200 meters underwater, how can Nino region 4 at the surface be warming up? Well, check out the top of the SST anomaly image. I circled an area where a finger of warmth has propagated to the surface, and that is what's causing the sudden warmth. But the Kelvin Wave isn't responsible for just the warming aspect; it's also responsible for the significant cooling we're seeing in Nino regions 3 and 3.4.

This graphic, drawn up by Mike Ventrice, shows the sort of situation we're experiencing with the Kelvin Wave. As the Kelvin Wave pushes east, we see sea level anomalies rise in conjunction with the anomalous warmth. Ahead of the Kelvin Wave, we then see downwelling, which lowers sea levels and cools sea surface temperatures, leading to a 'false Nina', where the SSTAs might tell us there's a La Nina, but it's actually just a byproduct of the Kelvin Wave beginning to push east.

This phenomenon of warming-before-cooling has been observed well with the last few Kelvin Waves, as this chart shows. We saw one wave crossing the Pacific in September 2013, with little to no cooling before it; we saw a second wave present in November to December 2013, which did provoke some noticeable cooling before the wave traversed the Pacific, and the latest wave looks to be the strongest one of the three, with even stronger cooling occurring just before the warming. If you think about it, a case can be made for the strength of downwelling before the Kelvin Wave determining the strength of that Wave, as it seems the stronger downwelling episodes occur before the stronger Kelvin Wave episodes. The other Nino regions are experiencing anomalous cooling due to this downwelling, as the circled portion in the chart above shows.

The Kelvin Wave has shown up noticeably on sea level anomalies...

...As well as upper ocean heat anomalies.

Both of these indicators point to the idea that we're probably about to exit this neutral ENSO/cool-neutral ENSO situation we've been in for the last couple of years, and a transition to an El Nino seems more likely. Model guidance has been simulating this rise to a moderate El Nino by fall very well.

The next question becomes, will this Kelvin Wave actually make it to Ecuador and Peru? In order to find out, we have to turn to the western Pacific.

Shown here is a map of 850mb winds (arrows), outgoing longwave radiation (contours) and outgoing longwave radiation anomalies (colors) from the period of January 22nd, 2014 to January 28th. We're going to focus in on the portion of this map I boxed in. If we look at that section, we see the arrows pointing down and converging on the Equator before then pointing eastward. This phenomenon is the result of two Pacific gyres on opposite ends of the Equator. This means that there were two low pressure systems, one on either side of the Equator, that were spinning air towards the other in such a fashion that then allowed the air to be pushed off to the east. How is this possible? Considering low pressure systems north of the equator rotate counterclockwise, and low pressure systems south of the equator rotate clockwise, the winds from each system can combine and push their combined winds off to the east, which can then help with the movement of the Kelvin Wave.

The graphic above gives a good picture of what these two Pacific gyres end up doing. Considering we saw one of these double-gyres in late January, and another one is about to get going, the Kelvin Wave should continue to push to the east, and ought to be able to affect all Nino regions. This raises the question, could we see an El Nino by next winter? I'm comfortable putting down a 'yes' for that question.

Now, the warming of the westernmost Nino region 4 will likely have impacts on our severe weather season. How? Take a look at the chart above. This graphic shows correlations of 200mb winds based on the Nino 4 phase. In simpler terms, for example, when the Nino 4 has below-normal waters, the oranges and yellows (which signify positive correlation areas) would then experience below-average winds. The areas shown in blue (which signify negative correlation areas) would then see stronger than normal winds. In this case, since we have warmer than normal sea surface temperature anomalies over Nino region 4, we ought to watch for at least a slight strengthening in the subtropical jet stream as we enter March, due to the positive correlation depiction over the southern US. Because the current warming is just a little piece of a Kelvin wave and not yet a legitimate El Nino, I'm not expecting the subtropical jet stream to completely go nuts and strengthen out of control. However, it wouldn't surprise me to see the STJ strengthen in the next couple of weeks as a result of this correlation. And, if we have a stronger subtropical jet stream, the risk of severe weather then goes up. Hence, we need to watch out for the severe weather season to ramp up in coming weeks, not only due to the gradual retreat of this harsh winter, but also due to the slightly-strengthened subtropical jet stream.

Let's switch gears and discuss my thoughts for the end of March and into April (forecast for late February and most of March here), as well as why I feel we need to watch out for a snow event along the East Coast in the middle of March.

This image shows 500mb height anomalies over the Northern Hemisphere during the January 22 - January 28 period of this year. In this reanalysis, we see strong ridging over the west coast of North America, producing a northwest flow situation into the East Coast, where a lobe of the polar vortex sat to the north in Canada. Something we can use here is the Lezak Recurring Cycle. The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a regular, unchanging 40-60 day interval for the next ~10 months before it dissipates over the following summer. If we utilize the Lezak Recurring Cycle for this timeframe, we can extrapolate the January 22-28 period out 57 days (which is the current length of the LRC this season), we arrive in mid-late March. This means that it's very possible we see this late January pattern come back around in late March, northwest flow included. The reason I'm discussing this northwest flow so much is because it has the potential to not only bring us an East Coast snow threat in mid-March, but also has the potential for a severe weather event.

As I discussed above, the subtropical jet stream could be strengthened as we enter March thanks to the warming of Nino region 4. If that occurs while we are in a northwest flow situation in mid-late March, it's very possible we come across a favorable environment for a spring coastal storm. I'm not saying we'll see one for sure, but the last time we saw a northwest flow come about and I thought there might be an East Coast snow event as a result, it ended up coming to fruition.

These images show some possible scenarios for northwest flow severe weather outbreaks, where we would have a lobe of the polar vortex displaced in eastern Canada, with strong ridging along the west coast of North America. Due to the warm pool in the northeast Pacific, it's entirely possible we see sustained ridging stick around in the western portion of North America like we saw in the late January image earlier in this post, which would then lead to some northwest flow severe weather opportunities and snow potentials. Both types of inclement weather potentials would be assisted by the stronger subtropical jet stream.

If we take a look at the 500mb anomaly chart a week after the January 22-28 chart earlier in this post, we see that the environment over the January 29-February 4 period has changed. The ridging has shifted westward into the Bering Sea, leaving the lobe of the polar vortex to weaken and move out of the way, which then results in ridging along the East Coast. I would expect this sort of environment to come back in the closing days of March and opening days of April in accordance with the LRC. In that time period, we may be looking at severe weather threats displaced further west and north, with warmer weather for the East and cooler weather in the West. By this time, we will have probably seen the end of big winter storms, with the exception of a rogue snow event in the northern Plains before winter finally leaves the United States.

To sum up:

•Warming waters in the Equatorial Pacific could have long term consequences for the weather pattern in spring, summer, and into next winter.
•Due to those warming waters, severe weather chances may be enhanced later in spring.
•We are watching for a potential snow event in the East Coast in mid March.
•Late March into April may feature a warm East and cool West.


Friday, February 21, 2014

Long Range Outlook: Late February, March 2014 Forecast

This post will detail the expected weather conditions for the end of February into most of the month of March, 2014.

We'll begin with the weather over the North Pacific and East Asia. Shown above is a previous ECMWF model forecast, showing 500mb height anomalies over the aforementioned regions. If we take a look over at Japan, we see some negative height anomalies present, valid on February 20th. If we use the idea that weather anomalies over East Asia can "re-appear" in the United States 6-10 days later, I would expect a stormy and cool weather pattern for the February 26-March 2nd period. But if we look just to the west, we see strong ridging behind that storminess in Japan.

We eventually see that ridging fill into Japan a couple of days later, as this 500mb height anomaly forecast from the most recent ECMWF forecast shows. With ridging expected to hit Japan on February 24th, a warm-up is probable for the March 2nd-6th period. It's worth watching for another storm threat, as we see a depression in the contour lines right over Japan that indicates the presence of a weak storm system. It is possible we see a storm drag warm air north from the Gulf of Mexico to set-up a severe weather threat, but that's all speculation at this point.

Following that initial warm-up in the first week of January, model guidance (the ECMWF Ensemble set, shown above here) wants to bring in a strong ridge of high pressure right over Japan. It looks like this ridge has the potential to bring some seriously warm air into the United States when we see the correlation kick in. With the graphic here valid for February 26th, look out for a warm, quiet weather pattern around the March 4th-8th time period. It's quite possible we see a sustained warm period from March 2nd to the 8th, if these ridges hold together.

Looking out to the last day of February we see the ECMWF bringing in a suppressed ridge into Japan. This tells us to look for that warm pattern to continue in the United States through the March 10th period. Remember to keep track of that Rossby Wave in Alaska, denoted by the powerful ridge.

The weather pattern will then do a flip in the second week of March, as model guidance sees the entrance of a strong trough into Japan on the 3rd day of next month, as the image here shows. This would likely see a return of cold and stormy weather somewhere in the March 9th-13th time period, as we use that 6-10 day gap between East Asian weather and United States weather.

Moving out into the long-long range, we start to key in on the synoptic pattern, and not so much individual features like we were previously analyzing. We've got a good handle on the anticipated weather pattern for the first two weeks of March, so now we can begin forecasting the remainder of the month. This graphic shows you the predicted 500mb height anomalies over the Northern Hemisphere on March 8th. Note the ridging over the Arctic Circle indicating the presence of a negative Arctic Oscillation (-AO) phase, which heightens threats for cold weather in the lower latitudes (including the US). Also note the ridging starting to come back over the west coast of North America. This is part of the Rossby wave train, which is created when several rounds of strong high pressure are pumped poleward. In this case, the train has maintained itself over the northeast Pacific, at times shifting west into the waters just south of the Bering Sea. The way the pattern is aligned here, you'd be hard-pressed to not find a cold forecast for the Central and Eastern US. The flow across the Northern Hemisphere strongly favors a cold middle of March, with the Rossby Waves continuing to form and push northward along the West Coast, leading to cold weather for the populated areas out East. Since this is a 360 hour forecast, things will obviously change, but you get the idea: after the early March warm-up, additional cold weather is possible.

Why is all of this happening? The primary reason is the swath of above-normal sea surface temperature anomalies over the northeast Pacific, as this graphic shows. One of the best indicators of long-term weather patterns is sea surface temperatures. Apart from some sea surface temperature anomalies (SSTAs) defining various oscillations that also affect our weather, these swaths of above or below normal SSTAs correlate to positive or negative 500mb height anomalies, respectively. Thus, it's of little surprise that we are seeing strong ridging over the Northeast Pacific, where this big warm water mass resides. With the warm water still firmly entrenched in the waters south of Alaska, look for a cool spring to also be in store.

What we could end up seeing, if this warm pool becomes a long-term feature, is a northwest flow pattern for the spring season. The screenshot above, from a Storm Prediction Center publication, shows 500mb height contours in two different northwest flow situations, with the severe weather outbreak area circled. As you can see, severe weather events would be more prone to occurring further north during northwest flow severe weather outbreaks, and it's possible we see something like one of these situations evolve if that northeast Pacific warm pool continues to work its magic. I'll detail this more in my upcoming 2014 severe weather season outlook.


Wednesday, February 19, 2014

Second Polar Vortex Intrusion Possible

We may have to deal with another intrusion of the polar vortex in just a handful of days.

Shown above is the GFS ensemble mean forecast of 500mb height anomalies over North America. We see a situation eerily similar to the one that brought about the severe cold in early January 2014. We see a Rossby Wave pushing north into eastern Alaska and Canada, as the strong positive 500mb height anomalies shows. As a result of this strong high pressure, we see a lobe of the polar vortex dropping to the south from Canada. This lobe of the vortex is illustrated well by the deep greens and low contour numbers superimposed on those deep negative height anomalies.

The GFS Ensemble 850mb level (5,000 feet level) temperature forecast for that same timeframe shows how cold conditions may get if this lobe of the polar vortex decides to scrape the United States just to our north again.

The GFS Ensemble Control 850mb temperature anomaly forecast is even stronger, with anomalies as low as -28.4 degrees Celsius in the core of the lobe of the polar vortex.

There's good reason to be concerned about a second intrusion of a lobe of the polar vortex: The Lezak Recurring Cycle supports it.

500mb chart from January 6th
The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a regular, somewhat-unchanging 40-60 day interval for the next ~10 months before it dissipates over the following summer. This season, the length of the LRC has been pegged at 57 days. If we take the January 6th date and move ahead ~57 days, we arrive in the timeframe of late February, a couple of days within the very cold weather the GFS Ensembles showed above. Considering the LRC has been an efficient tool of use in not only this winter season, but in prior winter seasons, this is likely a real threat of a lobe of the vortex coming south once again.

To sum up:

•The atmosphere looks primed for a strong cold blast in late February that will likely produce anomalously cold weather.


Kelvin Wave Provoking El Nino; Atmosphere in Disagreement

A Kelvin Wave currently moving across the eastern Pacific appears to be trying to provoke an El Nino, but the rest of the atmosphere is in disagreement.

The graphic above shows water temperature anomalies across the Equatorial Pacific, with longitude values on the bottom legend and depth values in meters on the left legend. The Kelvin wave is clearly shown by the strong positive water temperature anomalies from the 100 to 200 meter depth in the central/western ENSO monitoring area in the Equatorial Pacific. With a Kelvin Wave, you have a mass of warm water pushing east under the surface, and pushing east across most (if not all) of the Equatorial Pacific. This wave warms the waters below the surface, and if those subsurface waters are mixed to the surface, we can see an El Nino develop. Due to that process, Kelvin Waves are typically associated with El Nino formations, and that is why we are monitoring this event for possible El Nino provocation down the line in spring.

However, the rest of the atmosphere does not agree that we will see an El Nino anytime soon.

One feature we see over the Eastern Pacific is a swath of anomalously strong upper level winds, as the purples and yellows on the attached image denote. Thanks to the Earth System Research Laboratory (ESRL), we can compare these uppr level winds to see what ENSO state the atmosphere is currently in- a La Nina, El Nino, or neutral-ENSO state.

If we look at the ESRL's 200mb wind anomaly composite for February and March over the Equatorial Pacific, we see that El Nino's are typically recognized in those time periods by strong upper level winds due west of Mexico, with anomalously weak upper level winds over the ENSO monitoring area. Taking a look back at the latest observed upper level winds, it is clear that the atmosphere is in more of a La Nina state, as we usually see strong upper level winds over the eastern Equatorial Pacific during an El Nino instead of weak upper level winds we are currently seeing.

Another item we can use to judge what state the El Nino-Southern Oscillation phenomenon is in is the sea level anomaly. In other words, we can see if the waters over the eastern Equatorial Pacific are higher or lower than normal, and use that to help tell us what phase the ENSO is in. If we check out the composite graphic above, it is clear that the El Nino phenomenon is identified by higher than normal sea level anomalies, with the La Nina producing lower than normal sea level anomalies. The November 1997 sea level composite on the top of this graphic shows the El Nino sea level anomaly, while the February 1999 composite below that depicts sea level anomalies for a La Nina.
Taking a glance at the latest observed sea level anomalies, it is more apparent that while we aren't in an El Nino, which would show up as a swath of yellows and oranges to indicate higher than normal sea level anomalies, we also aren't in a defined La Nina, which would appear as a swath of deep blues on the right hand side of this chart. We do see some blues in the Equatorial Pacific, which could argue for a weak La Nina if you really want to be picky about it, but I'm not really seeing any defined ENSO state going one way or the other.

This chart, showing the anomalous depth of the 20 degree (C) isotherm over time, gives you a hint at what the progression of a Kelvin Wave looks like, with two clear examples of Kelvin Waves observed in November 2013 into January 2014, and from September 2013 into October 2013. The latest observation of this 20 degree isotherm confirms that we are looking at a Kelvin Wave. with this wave looking to be the strongest out of the last three events. It's possible that this sends us into an El Nino pattern when spring and summer come around, but for now, an El Nino does not look to be on the way, at least for the near future.


Tuesday, February 18, 2014

February 20-22 Potentially Significant Severe Weather Event

Model guidance is beginning to converge on the idea of a potentially significant severe weather event over the February 20-22 period.

The Storm Prediction Center has already outlined a large area of severe weather potential for February 20th, centered from northern Louisiana into eastern Arkansas, eastern Missouri, a large portion of Illinois, and eastward into Ohio. States like Indiana, Kentucky, Tennessee, Mississippi and Alabama may be affected the most by this severe weather event.

For February 21st, the Storm Prediction Center then outlines an area of Georgia to southern New Jersey for an enhanced severe weather risk as the frontal boundary associated with this dynamic storm system moves to the east. In this post, because the threat area is a little more specific, we will focus on the February 20th severe weather risk.

Note: All model guidance images in this post will be valid for the evening of February 20th.
Model guidance indicates we will see a strong storm system lift into the upper Plains, tilting negatively as it heads towards the upper Midwest on February 20th. The jet stream projection above for the evening of February 20th shows the highly dynamic situation at hand here. We have a jet streak in the range of roughly 110 knots to 130 knots, centered over the southern Midwest and Central/Southern Plains. Analysis of upper level divergence suggests strong values located over those same areas, and for that reason we are looking for the strongest storms to hit the area of southern Illinois, southern Indiana into western KY/TN, and Missouri into Arkansas.

A forecast image of the 850mb level, located about 5,000 feet above the ground, gives us the full picture for the evening of February 20th. We see a strong moisture fetch from the Gulf of Mexico, shown by not only the northbound wind barbs from the Gulf Coast, but also the swath of high relative humidity values along the frontal boundary. We see a sharp gradient in theta-e values along that frontal boundary, denoted by the dashed pink values, and this also spells out the potential for at least a decent severe weather event. Enhancing this event's potential even further is the 50 to 60 knot lower level wind speeds right over the severe weather threat areas we discussed above, and these lower level winds ought to really help out with storm development. However, the lack of directional wind shear (changing of wind direction with height) means any storms that form are unlikely to contain a vast number of tornadoes. I expect tornado reports from this event, but not that of an outbreak level.

A forecast graphic for the 700mb vertical velocity on the evening of February 20th shows intense lifting located over the same regions we discussed above, and this lifting will lead to the development of strong thunderstorms. Precipitable water values exceeding 1" will ensure that this will be a heavy rain system, and model guidance (as well as the SPC) seems to agree on the idea that this will be a squall line event, rather than multiple tornadic supercells. The squall line lowers the tornado threat, but greatly raises the damaging wind threat. It appears that the damaging wind threat will really maximize this severe weather potential.

Adding fuel to the fire is how the subtropical jet stream will really help this severe weather event become the beast I expect it to be. See that long string of water vapor extending from the Central US all the way down towards Hawaii? That is a classic mark of the subtropical jet stream, close relative of the polar jet stream. During the springtime, the subtropical jet stream acts on storm systems crossing the US, and pumps warm, humid air into the warm sector of storm systems to enhance severe weather prospects. Additionally, we typically see stronger wind speeds as a result of the STJ, and this can enhance both directional wind shear and vertical speed shear (the change of wind speed with height).

All in all, I've got a feeling the Storm Prediction Center will nail its current outlook for the February 20th period. Model guidance seems to agree on a squall line event which will likely produce damaging winds, as well as an isolated tornado report.


Saturday, February 15, 2014

Long Range Discussion: Severe Weather, Snowstorms To Kick Off March

This long range discussion will focus on the expected weather to end February and kick off March.

The image above shows the latest 500mb height anomalies across the Northern Hemisphere on the left, with cloud cover, pressure contours and high/low pressure denotations on the right. If you've been with The Weather Centre for a while, you know how we use a rule created by Joe Renken that states a storm system in East Asia then results in a storm in the United States 6-10 days later. This situation is no different- today, we saw ridging/high pressure emerge in East Asia. If we extrapolate that out 6-10 days, we arrive at a warm-up in the February 20-24 timeframe. I'll discuss this more a little later down this post.

Lately, as the image above shows, the weather pattern has been dominated by west-northwest to northwest flow, thanks to deep troughing in the Gulf of Alaska and suppressed ridging in the Southwest producing a pattern favorable for East Coast snow events. This whole pattern is expected to flip in the medium range.

The changes begin as ensemble members foresee strong ridging shifting east into far eastern Russia, which will then shift the Gulf of Alaska storminess to the east, resulting in the loss of high pressure in the Southwest US. That Southwest ridging then propagates east into the contiguous United States, producing mainly zonal flow, which keeps anomalous warm or cold outbreaks out of the picture. However, with the ridging now gone, the storminess in the Gulf of Alaska can drop south into the Western US. The result is a stormy West US, and a very warm East US.

Model guidance (with the exception of the CMC model on the far right) is in agreement on a pattern change evolving, where we see the troughing drop south into the West US, as high pressure prevails yet again in the Gulf of Alaska. The storminess in the West US isn't as well defined on the ECMWF forecast (left) or GFS forecast (center), primarily because this image shows projected 500mb anomalies over the 8-10 day forecast period. This means the forecasts may seem a little less 'intense' than they end up being, because the anomalies are averaged out over a 2 day period, rather than a 6 hour period like most models print out.

Regardless, the pattern will involve high pressure setting up along the East Coast in response to troughing out West, and this sort of pattern just screams for not only a snow event, but a severe weather event. We will see anomalously warm and humid conditions present across the Central and Eastern US as the jet stream bends north to accommodate for the ridging out East. This ridge also tells me we should keep an eye out for a storm system cutting north through the western Great Lakes, something model guidance is already picking up on. I have a feeling such a storm cutting north would result in not only a northern Plains snowstorm, but also a severe weather event for the Central US, something I discussed in yesterday's post.

As we head into the last days of February, ensembles give the Central and East US another shot of cold weather, as shown by the GFS Ensembles' portrayal of the trough at the 500mb level on the left panel. Again, the pattern we discussed above would strongly support this solution. It wouldn't surprise me to see additional chances for storms, both snowy and severe, before the month of February ends, apart from that other storm system we discussed yesterday in the link above.

For the opening days of March, I wouldn't be surprised to see us start on a chilly note before rebounding to rather seasonal temperatures. Depending on how the sudden stratospheric warming event continues to evolve, we could be looking at a cool March, though the chance of that would be maximized in the north-central US (primarily the Plains), and minimized along the East Coast and West Coast, where ridging and warm weather would be expected.


Friday, February 14, 2014

February 15-16 Potential Blizzard

It looks like there is now the potential for a blizzard this Saturday and Sunday.

The GFS model shows extreme wind speeds along the coast of the Northeast as another strong storm system rides up the coast in similar fashion as the storm that just ended for the East Coast. We see 10 meter wind speeds over 44 knots just offshore land, with widespread 25-35 knot wind speed forecasts at the height of this storm.

The GFS model indicates the storm will drop accumulating snow along the coast, as the 984 millibar low comes close to land. Despite the proximity of the low pressure system to the coast, the precipitation here will not be incredibly intense. Accumulation is expected, and the wind will be that of borderline-blizzard conditions, but the accumulation will not be significant.

The real fun begins overnight of Saturday into Sunday, as the GFS projects this storm will rapidly strengthen to drop significant snow accumulation on Maine, and this should continue into the day on Sunday. The system will be moving rather quickly, so while significant amounts are expected, it will not be a prolonged event. This also means good news for those of you who like to watch snow come down at high rates (i.e. 2 inches an hour or higher), as we'll likely see some pretty high snow rates in Maine from this storm.

This 6 hour snowfall forecast shows the intense snowfall overnight Saturday into Sunday, with the GFS depicting amounts as high as 12 to 14 inches of snow falling in that 6 hour period. If you were to average that out, it would be equivalent to 2" of snow falling every hour, but as we all know, snow does not fall in uniform and consistent rates like that. It's more probable that we would see very high snow rates (possibly 4" an hour at best) for a period of time, with lighter rates mixed in (i.e. 1" an hour or lower).

When all is set and done, here is the total snowfall put out by the GFS model. We can see those high amounts in Maine, but also some good totals in coastal portions of Massachusetts and Connecticut. Again, totals aren't looking to be too bad (I'm not so sure we see the GFS' predicted 10" of snow hit those areas in MA/CT), except for in Maine. The wind will certainly make it feel like a big snowstorm for those further south, though.

The NAM model has a different take on this event, bringing heavy snow to only the extreme eastern portion of Massachusetts, with that jackpot in Maine also moved to the east. Considering we saw the snowstorm along the East Coast a few days ago also further east than what model guidance suggested, it's possible we see a solution like this work out.

I'm wrestling with which solution I like better in this case. On one hand, we have the GFS solution, which seems somewhat-reasonable (compared to the NAM) with the slightly-lower inland snow amounts, but on the other hand, the NAM model being to the east may verify, considering the most recent East Coast storm appears to have been further east than modeled. Needless to say, this forecast will see some fine-tuning in the hours ahead.


Sudden Stratospheric Warming Commences in Northern Hemisphere

The winter's most sizable sudden stratospheric warming event to date has commenced.

The animation above shows us temperature anomalies in the 10 millibar level of the atmosphere, which is located way up in the upper stratosphere. We see how the stratosphere has been rumbling since mid-January with attempts at a stratospheric warming event, but it is only as of the last few days that the stratosphere saw a massive warming event, titled a sudden stratospheric warming (SSW) event. To see how big this event is, let's head down to the 30 millibar level, located further down in the atmosphere compared to the 10mb level.

The 30mb temperature anomaly animation above shows how the stratospheric polar vortex has been under attack for a longer period of time compared to the upper stratosphere. We see a large swath of warmth had been developing and manifesting itself in Eurasia in January into February before finally bursting into the upper latitudes. It is now clear that the sudden stratospheric warming event has extended itself into a decent chunk of the stratosphere. Let's check up on the 50 millibar level next.

The 50mb level is also seeing significant warming in the upper latitudes, which only confirms the idea that this is indeed a sudden stratospheric warming event, quite possibly the first one we've seen this whole season.

Why is this a big deal? The sudden stratospheric warming phenomenon allows the polar vortex to weaken, and this enhances the possibility of Arctic cold outbreaks not unlike those we saw earlier in January. I'm not saying we will see January 2014-level cold again, the prospects of cold outbreaks rise roughly 2-4 weeks after the sudden stratospheric warming occurs. This would place cold weather prospects in the February 18-March 4th period. It's already looking like we'll see a cold blast to kick off March, and this could be related.

The ECMWF model sees this intrusion on the stratosphere continuing, forecasting continued elevated geopotential-Wave 1 activity at the 10 day forecast period. We see the elevated values maximized around the 3mb to 30mb levels, with lesser influences downwelling as low as the 500mb level. It is likely that this will be the biggest stratospheric intrusion this season, as we are just about to wrap up the meteorological winter season, and the "final warming" (where polar winds reverse to signal the end of the SSW season) isn't too far away.


Thursday, February 13, 2014

February 15-16 Potential Coastal Snowstorm

Yet another snow event is possible for the East Coast, with this next threat coming in the February 15-16 period.

The GFS model indicates we will see another trough develop in the Southeast, with an uncanny resemblance to the storm we are currently fighting through along the East Coast. The GFS sees this storm coming about as a result of the strong-yet-suppressed ridging in the West US, which leads to a west-northwest or northwest flow across the US, a favorable pattern for East US snow events.

The GFS model brings a 984 millibar low pressure system along the waters just off the East Coast, dropping snowfall in the eastern portions of North Carolina, Virginia, Massachusetts, Maine, and all places in between. This snapshot, valid on the evening of February 15, shows the heaviest snow staying offshore, but moderate snow still being able to impact the easternmost parts of the Northeast. As of now, accumulations for regions up to Maine look to be in the 2-5" range.

The storm system really gets going when it approaches Maine, with the MSLP for this storm clocking in at a minimum of 973 millibars. We see 6 hourly snowfall amounts in eastern Maine approaching 12", which would average out to 2 inches per hour for those 6 hours overnight from February 15 to 16. Now, heavy snowfall doesn't fall for 2 inches an hour at that steady rate- my guess is that we would see a period of intense snowfall rates (i.e. 3" an hour not unreasonable) with some lighter snow rates mixed in.

Only a slight shift west or east could flip the forecast- stay tuned for more details about this potential snowstorm.


February 22-24 Potential Severe Weather Event

Model guidance is latching on to the idea of a severe weather event in the February 22 to 24 timeframe.

Tropical Tidbits
The GFS model brings unseasonably warm temperatures north on February 22nd, as the graphic above shows. Projected temperatures range mainly in the 60s and 70s, where the warm sector is exerting the most influence. We see the cold front clearly defined, stretching through the Plains and Upper Midwest, with the warm front pulling that warm, moist air far to the north in what would likely be a set-up for the first severe weather event of the season.

Tropical Tidbits
Jet stream analysis from this same GFS model shows the storm system positioned in Wisconsin as a strong 988 millibar low pressure system. We see a jet streak in the 130 knot range positioned to the east of the trough, indicating the storm has become negatively tilted and is expected to continue lifting north and east. These strong jet stream winds, combined with the warm and moist air (containing up to 1.6" of precipitable water across the warm sector), would likely produce a severe weather event. The negative tilt of the trough, shown below, would further enhance severe weather prospects.

Tropical Tidbits
The negative tilt of the trough, shown best when maximum vorticity values are positioned on the eastern side of the trough and contour lines seem to be pressing in a southeast direction, indicates that the storm system has reached maturity, and is therefore more able to produce significant weather. In this particular case, with temperatures only projected to be in the 60s, PWAT values to only be around 1.6", and the jet stream maxing out at 130 knots, I wouldn't be holding my breath for a big severe weather outbreak. However, this solution looks good for at least some severe weather.

Tropical Tidbits
The 700mb winds and vorticity chart for February 12th shows us that lower level winds will be reaching into the 40 knot and 50 knot range, which, while not all that impressive, solidifies my concerns that this may still be a severe weather event, albeit not as intense as a full-out "outbreak".

Because this event is still in the long range, model guidance will inevitably change with this storm system. It is possible we don't even see a severe weather event at all, and it's also possible the severity of this event increases as we get closer to the event. I would bet my money more on the weakening prospects, as the atmosphere still looks a little 'wintry' to me, if you will, and doesn't seem all that conducive for a big severe weather event.