Long Range Predictor Suggests Wintry Cold Blast to Start October

One of the long range predictors used in our winter discussions is now hinting at the possibility of an intense, winter-like cold blast to start October.

Plymouth State
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The image above shows the 500mb geopotential height forecast across the Alaska and Bering Sea regions. This forecast was produced by the ECMWF model, and is valid for Friday morning, September 12th. In this image, we can see a very strong upper level low in the Bering Sea, centered just north of the westernmost Aleutian Islands.

By observing trends in the Bering Sea, weather conditions in the US can be predicted 17 to 21 days in advance. For instance, if the Bering Sea experiences a deep storm system, cold weather may be expected 2.5 to 3 weeks later. The same goes for high pressure in the Bering Sea. If we extrapolate the image above, valid on September 12th, we might expect a strong upper level low (and resultant cold blast) in the US around September 29th to October 3rd, and possibly further. This long range predictor has had great success in its use across the meteorological community, and if the ECMWF forecast verifies, its success could be something that we'd be frowning upon.

If the ECMWF outlook above verifies, indications are that a very strong upper level low would push into the United States and remain there, possibly for a number of days. If this occurred, much of the nation could reasonably expect to go under significantly-below normal temperatures, with snowfall not out of the question. The availability of cold air in Canada after this week's upcoming cold blast only magnifies that possibility.

It's still unknown as to where this upper level low would originate from, but in the spirit of including all possibilities, I suppose there's at least a slight chance that this upper level low may originate as a sheared-off piece of the polar vortex. Then again, it's certainly possible the storm forms as a rapidly-strengthening storm system in the Plains, dragging cold air down behind it. The point is, we just don't know yet.

What we do know is that if the ECMWF forecast does come true, millions across the country should begin watching for frosts, freezes, and general anomalously cold weather to end September and begin October.

September 9-10 Potentially Strong Severe Weather Event

A multi-day severe weather event looks to unfold this Tuesday and Wednesday.

SPC
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The image above, from the Storm Prediction Center, shows the severe weather outlook for Tuesday, September 9th. In this image, we see the states of Kansas, Nebraska, Iowa, Minnesota, Wisconsin, Illinois and Missouri under the Slight Risk demarcation. The outlook from the SPC indicates a strong low pressure will traverse the Midwest and Plains, with a large airmass of warm, humid air battling a swath of cold, dry air from the north.

Instant Weather Maps

Although there won't be too much instability to provoke a huge severe weather event, the strengthening of the low pressure system as it pushes northeast, combined with the high precipitable water values across the Plains and Midwest, should lead to the chance for severe weather. Initially, individual cells should be the primary threat, before congealing into a squall line, potentially with damaging winds.

SPC

On Wednesday, the SPC outlooks some of the same areas that were threatened on Tuesday, highlighting Illinois, Wisconsin, Michigan, Indiana, Ohio, Pennsylvania, and New York for potential severe weather. As the low pressure system previously discussed continues to push northeast, it should lead to a more enhanced risk of severe weather, possibly more than on Tuesday.

Instant Weather Maps

The GFS model shows the low pressure system rapidly strengthening by Wednesday afternoon, now displaced into northern Michigan. The minimum pressure has dropped to about 995 millibars, not atypical of autumnal low pressure systems that drive through the Upper Midwest and Plains with tremendous strength.

The set-up on Wednesday will be considerably different than that on Tuesday. The wind field on Wednesday is expected to be more 'charged', due to the rapid cyclogenesis occurring with that low pressure system in northern Michigan. Consequentially, the SPC is indicating a severe wind threat may evolve over the highlighted regions, as a squall line may form and bring potentially widespread damaging winds to the Midwest and Great Lakes.

Additional updates on this event will be posted in the near future.

Andrew

September 10th Potentially Strong Severe Weather Event

September 10th is being monitored for the emergence of a strong severe weather event.

SPC
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The Storm Prediction Center has outlined an area from Chicago to western New York for potential severe weather on Wednesday, September 10th. The outlined region includes states such as Michigan, Indiana, Illinois, Wisconsin, Ohio, Pennsylvania, and New York.

Instant Weather Maps
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On the morning of September 10th, a surface low is expected to track northeast into the Upper Midwest, rapidly strengthening as it does so. This sort of rapid cyclogenesis was discussed by the SPC in their outlook for this day's severe weather, and it does appear to be a valid concern. The wind field around this surface low is expected to tighten and intensify, as the close proximity of isobars on the map above shows. With cooperative surface winds, severe storms become quite a bit more likely (assuming all other components for severe weather are in place, of course).

Instant Weather Maps
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Another item mentioned in the SPC outlook was the presence of a moisture-rich environment in the outlined area. A look at precipitable water values confirms this idea. Precipitable water values are determined by theoretically taking a single column of air and collecting all the water from it. For instance, PWAT values of 1" suggest if you were to compress that column of air and squeeze all the water out of it, you would see 1" of water. Typically, values above 1.5" tend to be humid and uncomfortable for many, while values above 2" can feel 'soupy'. All of the moisture projected to be in the air on September 10th, per the image above, should allow for any strong storms to possess the potential for torrential rainfall, with flash flooding a possibility.

Additional updates will be posted in the near future.

Andrew

September 14-18 Potential Wintry Storm System

A storm system looks to affect the United States around the September 14-18 period, and may have some wintry implications.
* Note: This is not expected to be a significant winter storm, and any wintry implications should be minimal. Read below for additional information.

Tropical Tidbits

The image above shows 500mb geopotential height anomalies over the Western Pacific. Oranges and reds indicate the presence of ridging, typically resulting in warm, quiet weather. Blues tell the presence of below normal height anomalies, normally seen by stormy and cold weather.

On the forecast image above, valid on September 8th, slightly below normal height anomalies are observed just to the east of Japan. As has been discussed consistently for the past couple of years, the weather in East Asia can have a significant impact on weather here in the United States. Utilizing the East Asian correlation of ridging over Japan equals ridging in the US 6-10 days later, and the same situation with negative height anomalies, we can foresee long range weather patterns weeks out at a time. This mechanism is referred to as the Typhoon Rule, and states that weather patterns found at the 500mb level can replicate themselves over North America 6-10 days later after they appear over Japan.

If we extrapolate the storm skirting just east of Japan on September 8th, we can estimate a storm affecting the US around a September 14-18 period.

But that's not all... long range models are hinting at some winter weather.

COD
Long range caveats apply

The image above shows the GFS model projected 850mb temperatures over the United States on September 15th, in the middle of our September 14-18 storm system timeframe. The model indicates a swath of cold air will surge south, dropping temperatures below the freezing mark only 5,000 feet above the surface! It is expected that this cold air would drop south on the eastern side of a ridge in the Rockies, intercepting... you guessed it... a storm system.

COD
Long range caveats apply

The image above gives us a view of precipitation accumulations over a 12-hour period, mean sea level pressure (MSLP) values in the blue contours, and the 1000-500mb thickness lines in yellow. This graphic is from the GFS model, valid at the same time as the 850mb temperature one above. On this picture, we see our storm system traversing the East US, likely the storm system in the September 14-18 timeframe we outlined above.

Take a look at the Upper Midwest. Weather enthusiasts analyzing this chart may recognize a familiar reading on the yellow 1000-500mb thickness line- the 5400m demarcation. This value is commonly used around the weather community as a rain-snow line. Areas north of the 5400m line are presumed to see snow, while rain is favored south of the 5400m line. As this model is showing, the 5400m line is forecasted to stretch down to Illinois, encompassing a swath of precipitation in Wisconsin and Minnesota in the process.

Just for kicks, if we use a heavier-than-average snow ratio of 8 inches of snow :1 inch of water (average is 10:1), the GFS lays down up to 6 INCHES of SNOW across portions of northern Wisconsin and northern Minnesota.

However, don't get too excited just yet. The GFS is notorious for forecasting snow in situations when snow is not climatologically likely, and this appears to be one of those situations. While a reservoir of cold air is expected to be dangerously close to the US, I'm not expecting anyone to see any major accumulation from this system, which is still in quite a bit of uncertainty. If any wintry weather is to occur, which is a significant uncertainty in its own right, it would likely be in the form of a sleet-like mixture to some non-accumulating snowflakes.

This is all subject to potentially drastic change, given the long range nature of this event.

Andrew

Model Guidance Foreshadowing Cold, Stormy September Ahead

Latest model guidance has continued hinting at the idea of a cold and stormy period in September, now possibly spanning the remainder of the month.

Tropical Tidbits

The image above shows a five-day average of 500mb geopotential height anomalies over the Western Pacific. In this graphic, warm colors correspond to positive height anomalies, which generally bring about quiet and warm weather. On the other end, blues indicate negative height anomalies, which are acknowledged by the presence of cooler and stormy weather. This five-day average is valid for the period from September 8th to September 13th.

By observing the weather occurring over East Asia, we can actually determine the weather for the United States a handful of days down the road. For example, if there is a storm system over Japan, we can generally expect a storm system affecting the United States around 6-10 days later. The same rule applies for high pressure and warm weather, with the 6-10 day lag period.

If we extrapolate the forecasted cold and stormy weather over Japan around September 8-13, we can expect similarly cool and stormy weather here in the United States in the September 19-23 period. Considering this forecasted cold/stormy period lasts longer than the five-day average in the image above, this sort of pattern may last longer than September 19-23.

Tropical Tidbits

In the image above, we once again see a five-day averaged 500mb geopotential height forecast from the GFS Ensembles, valid for the same September 8th-13th timeframe as the image at the top of this post. Similarly, warm colors correspond to ridging (warm and quiet weather), while cold colors indicate troughing (cooler and stormy weather). This time, instead of looking at the outlook over the Western Pacific, we are looking in on Alaska.

Once again, we can use weather occurring in the North Pacific to predict weather in the United States. By observing trends in the Bering Sea, weather conditions in the US can be predicted 17 to 21 days in advance. For instance, if the Bering Sea experiences a deep storm system, cold weather may be expected 2.5 to 3 weeks later. The same goes for high pressure in the Bering Sea.

Per the graphic above, a steadfast trough looks to position itself in the eastern Bering Sea, with ridging to the west. This tells us that the long range may feature a period of below normal temperatures, possibly with storm systems, before brief ridging returns. Very long range ensembles then have troughing return to the area with force, but that'll have to be examined in coming days. For this image, however, we might expect this cold weather in the final days of September and into October.

To summarize, colder weather looks to make a comeback in the last few weeks of September, possibly into November.

Andrew