Friday, January 23, 2015

Short Range Outlook (Made January 23, 2015)

This is the latest Short Range Outlook post, where the pattern for the next 0-7 days is addressed. This forecast is valid for January 23rd to January 30th.


500mb geopotential height anomalies (right panel) and means (left panel) over the last 7 days show a general ridging pattern across the western portion of North America, extending a bit into the Central US and southern Canada. This pattern resulted in generally mild conditions for much of the nation, particularly in the Plains region. This warmth was not as enthusiastic in the New England region.

Satellite analysis of the Northeast Pacific shows abundant moisture making its way onshore in the Pacific Northwest from as far west as Hawaii. This will lead to rainy weather in that region for the foreseeable future, while drier conditions should persist in the Southwest. Some of this moisture is associated with a system that will be moving into the Midwest and Ohio Valley over the next few days, dropping accumulating snow as it does so.
Instant Weather Maps

Model guidance sees the aforementioned piece of energy sliding southeast-ward as a weak snow-producing system. From the most recent NAM model, the heaviest snow would appear to fall in northern Indiana, central Ohio, and into the mid-Atlantic, with scattered snow also showing up in Wisconsin, Minnesota, Iowa, Illinois, and points east, all falling this Sunday through Tuesday. A different storm system looks to drop plowable snow in the Northeast in the same timeframe.

After this weak system moves eastward, model guidance sees another small storm system dropping into the Midwest, likely laying down some low-accumulation snows. Latest model guidance indicates this would be another 1-3" snow event.

To summarize:

- Heavy snow looks to impact the Northeast in the next four days.
- A weak storm system will drop from Canada and produce accumulating snow in the Midwest and Ohio Valley in the next four days.
- Following that storm, a weak clipper may lay down another 1-3" of snow in similar regions.


Wednesday, January 21, 2015

Long Range Outlook (Made January 21, 2015)

This is the long range outlook post, made January 21st, 2015. This post will address the upcoming pattern over the next 7-31 days.

We'll begin with an analysis of the pattern over the last several days.

Click images to enlarge
Over the last week, we saw ridging present across the Western US, as exemplified by the green and yellow colors, with that ridge making its way east to provide warmth for many in the Central US. Ridging was also dominant over Greenland and south, into the North Atlantic and Canadian Maritimes. These two ridges combined to force an upper level low into northern Canada, rather than further south towards the United States, a likely scenario if the former ridge out west had been more dominant.

On a more synoptic scale, we recognize high pressure has forced the tropospheric upper-latitude vortex to weaken and splinter, with the prevailing lobe located in western Europe, and other splinters scattered across lower latitudes. This tells us that the cold air up in Canada is more free to move around, rather than maintain its position, locked in the Arctic Circle.

Let's now discuss tropical forcing across the globe to recognize how it has affected our pattern the last several days.

This chart shows a lot of things at once, but for now, we'll take it piece by piece. The first thing to recognize is the blue color shadings on this map. The color shadings are indicative of Outgoing Longwave Radiation (OLR) anomalies, where negative/blue depictions show enhanced convection (thunderstorms), and positive/orange depictions show suppressed convection. Arrows on this image will point away from the blue shadings, as thunderstorms force air up and away, while arrows will compress towards orange shadings, since sinking air (due to lack of convection) drags air down towards the surface. Lastly, the green contours show the intensity of divergence, the action of air being pushed up and away by thunderstorms, while reddish/purple contours show convergence, the action of air being pulled down and compressed towards the surface as the air sinks.

Over the past several days, we've seen an area of enhanced thunderstorms make its way eastward from Oceania, and we now see it beginning to weaken off the western coast of the South and Central Americas. You weather enthusiasts may recognize this as a weakening Madden-Julian Oscillation wave. However, look towards southern Africa. We see a new plume of thunderstorms beginning to develop. It is expected that these storms will propagate eastward and form the new MJO wave that will also push east with time, but that's for discussion later on in this post.

Now that we've gone over the pattern developing in the last several days, let's start looking into the future, beyond the 7 day window.

This is the ESRL ensemble projection of 500mb geopotential height anomalies for 7 days out. Here, we see a strong ridge pumping north in the Western US, resulting in a deep trough (and associated cold weather) in the East US. This fits in well with the expected progression of our new MJO wave east, as the placement of tropical convection just southwest of India supports this type of cold regime. Looking across the northern hemisphere, we can also identify an upper level low over Greenland, a signal for the positive phase of the North Atlantic Oscillation (NAO). The +NAO supports a more low-amplitude jet stream, which is technical-speak for weaker ridges and weaker storms, as well as a faster progression of those ridges and storms to the east. However, at least for now, that +NAO is being overruled by the strong ridge in the West, ensuring a cold period to end January (something we've been discussing for over 2 weeks now).

Going ahead to 11 days from today, the 500mb geopotential height anomaly shows a different picture than the one we analyzed above. The ridge in the West US has been swallowed up by a more dominant ridge stretching across the Bering Sea and into the North-Central Pacific. With support from an inferior ridge along the western North America coastline, it appears a cross-polar flow situation would be in the works, where cold air would be transported from Eurasia, across the Arctic, directly down into the Northeast and Canadian Maritimes, based on the placement of that trough. Things start to improve in the Central US, for those wishing for spring. That +NAO signal we discussed earlier is now in play, as we see the aforementioned West US ridge 'bleeding east' into the Plains. This fits in with the continued progression of the MJO wave east (weather enthusiasts know this part as Phase 3, the image above this one resembled a Phase 2 event). However, I do think we see the ridge retract itself a bit west as the time between this forecast and present day decreases.

We've examined the outlook for the next 7-14 days, so let's start digging into the outlook in the next 14-31 days.

Tropical Tidbits
The image above shows 500mb geopotential height anomalies, forecasted over the West Pacific on the evening of January 22nd. Note the presence of a trough in Japan, in the midst of two ridges on either side of the country. This trough appears to be a storm system that may impact us here in the United States down the road. Using the Typhoon Rule, which states weather phenomenon occurring in the West Pacific is reciprocated in the US about 6-10 days later, we may expect a storm threat around the January 28-February 1 timeframe. As of now, this storm wouldn't be particularly strong, and should be followed by a warm-up. However, it could perk the interest of some severe weather enthusiasts.

Tropical Tidbits
If we fast-forward a bit, we find ourselves looking at 500mb height anomalies on the evening of January 25th, with a much different prognosis than what we saw in the image above. A strong ridge now encompasses all of Japan, bringing a 'heat wave' to the region. This ridge persists for more than a couple of days, which could very well validate my outlook for a mild middle of February. Using the Typhoon Rule here, we look to find ourselves in a warm period around a January 31st - February 4th period, possibly for longer, based on the looks of ensemble guidance further out. This warmth is then interrupted by a brief cool shot before that warmth may return.

I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The forecast for the PNA is positive for the next two weeks, and a sustained strong positive signal at that. This tells me we're looking at that ridge sticking around the West US for a prolonged period of time into February, though it may very well bleed east into the Central US as we already discussed. The NAO forecast is sustained at a moderate level for the entire period, meaning the possibility of that +PNA ridge bleeding east is rather high.
The WPO forecast starts negative, goes positive, and then goes into strong negative territory as that intense ridge on the ESRL ensembles takes over. This should continue for a bit as the MJO wave moves through Phase 3. The EPO follows a nearly-identical path, and both should permit the persistence of cold in the East US. I will refrain from including the Central US in that cold forecast due to the risk of that ridge in the West US bleeding east.

Let's now use tropical forcing to see what we may expect later on in February.

This four-panel image shows OLR anomalies, using the same color definitions as the JMA chart we discussed earlier in this post. In the top panel, we see current OLR anomalies, and that dying MJO wave is observed moving eastward in the next 1-5 days. By the 6-10 day period, our new MJO wave evolves in Phase 2, favoring the cold weather we have discussed earlier in this post. By by the Days 11-15 panel, our MJO wave has shifted east, to Phase 3, favoring a warmer nation as we move into mid-February. As this wave moves eastward over time, it is expected that the wave will go into phases even more favorable for warm weather, which is why I'm maintaining my call for a warm period in mid-late February. Beyond that 31 day benchmark, confidence is too low to forecast further.

To summarize:

- A period of colder than normal weather across the Central and East US is favored next workweek, likely from Tuesday to Friday to round out February. This cold will be maximized in the Northeast.
- A period of warmth may overtake the Central US in the opening days of February, though the Northeast will remain cold.
- Cooler weather should return to the Central and East US for a brief period around February 6th or 7th, before warm weather takes over.
- A warm pattern may persist into the middle portion of February, possibly into the later part of the month, for much of the nation.


Monday, January 19, 2015

Short Range Outlook (Made January 19, 2015)

This is the introductory Short Range Outlook post of our new posting plan, where the pattern for the next 0-7 days is addressed. This forecast is valid for January 19th to January 26th.

500mb geopotential height anomalies (right panel) and means (left panel) over the last 7 days show a general ridging pattern across the western portion of North America, extending a bit into the Central US. A separate ridge was also observed in the East US. The former ridge permitted the movement of an upper level low further to the south, which gave the Great Lakes and Northeast a somewhat-cool pattern, if not average.

Satellite analysis of the Northeast Pacific shows a healthy storm system moving onshore in southwestern Canada. Abundant moisture from this particular system is being spread across southern Canada, before additional moisture is seen across the Rockies. We can pick out a few more storm systems in this satellite shot, but the main concern will be with that system crossing into Canada.

Model guidance sees the aforementioned piece of energy sliding southeast-ward as a weak snow-producing system. From the most recent NAM model, the heaviest snow would appear to fall in southwest Michigan, with scattered snow also showing up in Wisconsin, Minnesota, Iowa, Illinois, and points east. This will then introduce a period of cooler than normal temperatures.

After this weak system moves eastward, model guidance allows the storm to strengthen quickly along the East Coast, which may drop accumulating snow in the Northeast and parts of the Mid-Atlantic. Amounts are still in question, but this high-resolution model indicates amounts in excess of 6" may impact the region.

To summarize:

- The cool pattern we have observed the last seven days is currently being broken by warmth, but should return in the next week.
- A weak system will drop out of Canada to produce snow in the Great Lakes, re-introducing cool weather.
- Scattered storms can be expected across the Gulf Coast.
- Accumulating, possibly significant snow may impact the Northeast.
- Generally wet and warm conditions may be expected for most of the West.


Sunday, January 18, 2015

The Weather Centre Updated Posting Plan

We’re announcing a new plan of action for the way we post here at The Weather Centre. Effective immediately, we’re beginning a fixed posting scheme, the continuation of which will be based on feedback and ‘success' of posting in this format. The plan is detailed below. The goal of this plan is to end the current erratic method of posting I employ, where I’ll discuss my thoughts on the long range one day, a storm system for a few days, then go quiet for a few days, etc. I’m aiming for continuity to enhance the way this blog operates.

The plan is as follows:

Winter Storm Posts: As Needed
Severe Weather Posts: As Needed
Tropical Cyclone Posts: As Needed
Drought Status Posts: Pending Decision

---WINTER POSTS (September 15 to April 1)---

Short Range Post (0-7 days): Every Monday & Friday
Long Range Post (7-31 days): Every Saturday & Wednesday

—SUMMER POSTS (April 1 to September 15)---

Severe Weather Short Range Post (1-8 days): Every Monday & Friday
Tropical Weather Long Range Post (7-31 days): Every Saturday


Thursday, January 15, 2015

Significant Arctic Outbreak Threat Rising for Late January

Model guidance is combining with the long range tools commonly used on this blog (namely the Typhoon Rule) to portray the country's best shot at true wintry weather to end the month of January.

We'll start off with the Typhoon Rule.

Tropical Tidbits
Click on any image to enlarge
The above image shows the GFS forecasted 500mb geopotential height anomaly forecast in the West Pacific region, valid for the morning of January 17th. In this image, we see warm colors, indicative of positive height anomalies / typically warm and quiet weather, but we see a much different picture over Japan. In Japan, a strong upper level low is impacting the country, sliding to the east as it does so. Notice how the center of this upper level low appears to go directly over far northern Japan.

The Typhoon Rule can be used to predict weather phenomenon in the United States. The rule of thumb indicates that weather phenomena impacting Japan is reciprocated in the US about 6-10 days later. So, if we see a strong (and very cold) upper level low crossing northern Japan on January 17th, we can extrapolate this upper level low to "re-appear" in the US on a January 23-27th timeframe.

Does this solution have any support from ensemble guidance? Let's find out.

The image above shows 500mb geopotential height values (not anomalies) from the prestigious ECMWF ensembles set, valid on January 25th. These types of charts are generally used to identify the presence of large-scale ridges or troughs. In this image, we see a very pronounced depression in the contour lines, bringing the 520gpdm line all the way down just south of the Canada/USA border. This depression of contours shows the presence of a strong upper level low, which is actually centered all the way up in northern Canada.

The strong upper level low is being forced southward due to that strong ridge of high pressure developing along the West Coast. Other guidance eventually does much bigger things with that ridge even further down the road, but we'll get to that a little later on in this post. For now, the takeaway from this forecast is very cold conditions might be on deck for the late January period. I have discussed this end-of-the-month timeframe for some cold weather in earlier posts, but its significance is quickly becoming more realized.

Next up, we'll analyze the ensembles off of the ESRL agency, a physics-based modeling branch of the NOAA body. In this image, showing 500mb geopotential anomalies for January 26th, we see a similar layout as the ECMWF ensembles showed. Strong negative height anomalies are developing in the Central and East US as the ridge along the West Coast continues to build and push northward. In these sorts of situations, the ridge may force itself so far north that it becomes a 'blocking' mechanism.

What is 'blocking'? My favorite example is to imagine a highway, with traffic moving along at an even pace. That's a good representation of the atmosphere during 'normal' flow. Now, imagine something happens on the highway that forces the cars to stop - a back-up, perhaps. The cause of this 'back-up' is analogous to the blocking ridge of high pressure; the ridge forces itself towards the North Pole and blocks the atmospheric flow from moving things along. This has been known to produce flooding, long-lasting cold, or intense warmth, depending on the season and who is affected.

The upcoming pattern is very similar to that shown by the negative phase of the West Pacific Oscillation (WPO). In the image above, we see typical temperature (bottom panel) and 500mb geopotential height anomaly (top panel) values for a positive WPO phase. Notice how stormy conditions persist just west of the Bering Sea during a +WPO phase, leading to warmth across most of the Central and East US.

To see the typical conditions during a negative WPO phase, just flip the color scale. We then see intense ridging just to the west of the Bering Sea, leading to sustained cold flooding the Central and East US. The eventual alignment of this ridge into the waters near the Bering Sea, as the GFS ensembles are indicating, would lead to this -WPO pattern.

The final (and what I consider the most surprising) part we have to go over is the analog forecast.

This image shows projected 500mb geopotential height anomalies, valid for about 11 days from today, based on the top ten analogs (dates with the atmospheric pattern similar to the one forecasted to occur 11 days from today) produced at the Climate Prediction Center. In this image, we can see that strong ridge along the West Coast of North America into Alaska, resulting in below-normal anomalies over the Central and East US, eastern Canada, and into Greenland. Just for kicks, let's see what the top analog of 20090118 (January 18, 2009) shows, since it has been deemed the most similar to the forecast down the road.

Above, we see temperature anomalies from January 16, 2009. I went back two days as the cold wave had already pushed east by January 18th; we want to get a diagnosis of the cold wave itself. This graphic is in units of Kelvins, but we can easily convert to Fahrenheit.

The core of the cold extends from southeastern Minnesota into far western West Virginia. A spot of values below -17.5 degrees below normal (Kelvin) shows up along the border of Indiana and Ohio. Let's see how -17.5 degrees below normal in Kelvins translates to Fahrenheit. Doing the math, that area saw temperatures 32 degrees (Fahrenheit) below normal for this cold wave, deemed the most similar to what we could see to end January. That's very impressive; for a scale of how impressive it is, consider that the average temperature in Richmond, Indiana (very close, if not inside that -17.5 Kelvin anomaly) to end January is 37 to 38 degrees F. In this cold snap, temperatures likely dipped down to just above zero, with even colder conditions to the north.

By no means does this mean we'll see something as cold as what happened on January 16, 2009, but it gives you an idea of the type of pattern that's coming down the pipe to end January.

To summarize:

- Model guidance and the Typhoon Rule are in agreement concerning a potentially significant cold blast at the end of January.