Encounter With Polar Vortex On The Horizon in Early-Mid December

It now appears much of the United States will undergo its first encounter with the polar vortex in the second week of December.

Tropical Tidbits

The image above shows 500mb height anomalies over the Western Pacific on the evening of December 1st. In this image, we see what is very likely a lobe of the polar vortex passing just north of Japan, occluding and cutting north as it does so. Minimum values here at the 500mb level in this image, off the GFS ensembles, appear to drop close to 490 dam, an indication of either an incredibly strong upper level low, or the more-likely polar vortex lobe.
This situation appears similar to the case we had last winter, where we saw a very strong upper level low crash into Japan to round out the year 2013, which then led to incredibly cold weather in the first week of the new year.

ESRL
December 27, 2013

If we recall the Typhoon Rule, which states weather phenomena occurring over East Asia is reciprocated in the United States about 6-10 days later, we could expect a bout of  substantial cold here at home in the December 7-11 timeframe. Should this verify, which is becoming increasingly likely given model consistency, the Central and parts of the East US may expect a bout of intense Arctic air to hit in the middle of December.

But it doesn't end there...

Tropical Tidbits

The image above once again shows 500mb height anomalies from the GFS ensembles, now valid on December 6th. Notice that we still see below-normal heights over Japan. This means Japan has been under a strong upper level low's influence for nearly a week, and still going. Building off of the first graphic, it's quite possible that the United States may also undergo a harsh beating from cold weather for nearly a week straight. The degree of this cold air is still under examination, but if it's anything like the upper level low expected to trek north of Japan to open December, watch out.

To summarize:

- Japan looks to undergo severe cold weather to start off December, likely leading to a prolonged Arctic air intrusion in the US in the middle of December.

Andrew

Polar Vortex Split Occurring; Arctic Outbreak Possible in Mid-December

The stratospheric polar vortex has experienced a split as a result of its second interruption by warm air this season, and it appears some cold weather may be on the way as a result.

CPC

The chart above shows heat flux values over the 2014 calendar year, from January 1st to present day. In this graph, shifting our attention to November and December, we find that flux values have skyrocketed to near-record levels, only now beginning to drop down a bit. According to the Climate Prediction Center (CPC), strong episodes of eddy heat flux events to the poles may result in a Sudden Stratospheric Warming (SSW) event. You may recall that SSW events occur when wind direction in the stratosphere reverses from low pressure-favoring winds to high pressure-favoring winds. This decimates the polar vortex, and usually unleashes near-record intensity cold weather on whoever is in the line of fire about 2-4 weeks after the SSW event occurs. Considering we are seeing an eddy heat flux episode of this magnitude, it's no surprise the polar vortex is undergoing some serious damage.

FU-Berlin

The ECMWF forecast of height contours and temperatures (shaded) about 96 hours from now, at the 50 millibar level, shows a Wave 2 stratospheric intrusion event. For some clarification, let's remember that the stratosphere exists primarily from the 1 millibar to the 100 millibar level. Therefore, I'm using the 50mb level as a happy medium, per se, so we don't receive any tropospheric influences that may occur at the 100mb level, but also keep away from vortex events in the far upper stratosphere that will ultimately have little to no effect on the troposphere.
As for the Wave 2 event, stratospheric polar vortex split events are classified into a few 'Wave' numbers. Take a look at this image to see the difference between a Wave 1 and Wave 2 event.

NASA

This image here looks complicated to those who may not be as enthusiastic about the weather as others, so we will focus on the middle column. In that middle column, we essentially are seeing areas of low pressure in orange, and areas of higher pressure in blue. On the top row, a normal polar vortex form is displayed.

Wave 1 events indicate a very strong warming event or body of high pressure is forcing the polar vortex off the North Pole, while still keeping it in one piece.
Wave 2 events are seen on the bottom row, where a weaker, but still intense warming/high pressure event occurs, but now splits the vortex into two vortices.

Using that knowledge back on the ECMWF image, we can confirm that a Wave-2 stratospheric split has occurred.

Now, bearing in mind that it usually takes 2-4 weeks for stratospheric intrusions to result in cold air outbreaks on the surface, we flash forward to a potential chilly period in mid-December.

CPC

In the long range forecast from the Climate Prediction Center, the Arctic Oscillation (AO) looks to turn negative in about two weeks, right when the timeframe for any stratospheric consequences opens up. Many signals support the warm start to December ending around the middle of the month, and this stratospheric event only strengthens this potential.

To summarize:

- A stratospheric intrusion may result in a cold air outbreak in the middle of December.

Andrew

December 2-6 Potential Winter Storm

Another winter storm threat is on the horizon.

Meteostar

The image above shows forecasted 500mb vorticity values with superimposed 500mb height contours over Japan for November 26th, off the GFS model. In this graphic, we see a depression in the contours over Japan, as well as a swath of red positive vorticity values,  indicating the presence of a storm system. Using the Typhoon Rule, which states weather phenomena in Japan is reciprocated in the United States about 6-10 days later, we might expect a storm system to trek across the US in a December 2-6 timeframe.

PSU

The ECMWF model is already hinting at this potential storm, as the above frame shows a positively-tilted trough entering the Central Midwest on December 4th and 5th. The extent and intensity of snowfall remains in question, but for now, the possibility is certainly present.

Andrew

Thanksgiving Potential Coastal Snowstorm

It looks like the Northeast will see a pretty white Thanksgiving this year, as a coastal storm will lay down plowable snowfall.

Instant Weather Maps

The image above shows accumulated snowfall from the GFS model, valid over the next five days. We see a swath of snowfall across the Northeast region, primarily coastal areas, as the storm will be located relatively offshore, restricting snowfall. This forecast lays down a general 3-5" of snow for many on the coast, with spots of 6" and slightly above for a handful of areas. Given that this is November, warm air immediately near the coast will be acting to put a damper on any intense snow activities, something that will have to wait until later in the season.

Instant Weather Maps

The NAM model, a shorter-range forecast, puts down a similar 3-5" swath of snow, but enhances the coastal areas with 6-8" of snow, as the pink area shows. Far southeastern parts of Maine even look to see a shot at near 10" of snowfall, if not a bit more.

IWM

Peak storm time looks to be on November 26th, intensifying in New England that evening. Regardless of how this storm ends up, millions in the East US will have to contend with a very messy Thanksgiving commute.

Andrew

November 24th Potential Accumulating Snow

This is a regional discussion for accumulating snow potential in the Great Lakes. Please note these types of regional discussions will become more prominent this winter.

Model guidance supports a swath of potentially plowable snowfall in the Great Lakes on November 24th, into the 25th.

Instant Weather Maps

The image above shows total snowfall from this morning (Sunday) to Tuesday morning. In this forecast, from the NAM model, we can define a swath of 3-6" of snow from Iowa to Michigan in the light blue to dark blue colors, with amounts over 6" (and even approaching the 10" mark) depicted in the pink colors.

This snowfall potential comes from a storm system currently placed in the southern Plains, forecasted to move north in the next day or so. As this happens, it is expected that a band of precipitation on the backside of this storm will form, laying down accumulating snow in parts of the country. The question is, where could this occur? Right now, the NAM model shows the brunt of the snowfall hitting the Wisconsin/Illinois border, but as of its most recent 18z run, snowfall has been shunted to eastern Wisconsin and Michigan. Let's see what the GFS model says.

Instant Weather Maps

The GFS model supports a swath of snowfall hitting primarily Northern Illinois, as well as southern Wisconsin and eastern Minnesota. Amounts would be maximized near Chicago, where the legend tells us 4-5" would be predicted to fall. Lesser totals nearing 4" would then be forecasted in the Northwoods region. We are awaiting word from the 18z GFS model run as I type this to see if we can build any bit of consistency. Why do we need consistency? If you haven't noticed yet, we currently have three completely different solutions:

• Solution 1 (12z NAM in top image): Snowfall strikes the Lower Great Lakes. Rockford, IL into south-central Wisconsin sees the heaviest totals (over 6").

• Solution 2 (18z NAM, not shown): Snowfall fails to fall anywhere, save for Michigan and eastern Wisconsin. Amounts totaling 3-5" in most places.

• Solution 3 (12z GFS, image above): Snowfall impacts North Illinois primarily, laying down just under half a foot of snow.

You can track the latest updates on this event on our Facebook Page, which you can find on the 'Social Media & Contact Info' tab on the right sidebar.

Andrew