Thanksgiving Potential Arctic Outbreak

It is looking increasingly likely that Thanksgiving will contain a significant cold weather outbreak for much of the nation.

850mb temperature anomalies for the evening of November 27

The ECMWF ensembles and GFS ensembles have been telling of the entrance of a very cold air mass into the US, to the tune of nearly 20 degrees below normal, in Celsius (that's 36 degrees below normal in Fahrenheit). This would most likely bring about the coldest air seen in many parts of the nation thus far, with many in the Plains, Midwest, Great Lakes and Ohio Valley in line for temperatures that may flirt with single digits, like Chicago, or smash right through the single digit mark, like many areas closer to the border with Canada.

ECMWF temperature forecast for the morning of November 27

It remains to be seen if the models are correct in their estimation of a brutally cold Thanksgiving, but right now, it looks like such an outcome is within the realm of possibility.

There is also the potential for a Nor'easter in the day or two before Thanksgiving...

Andrew

Thanksgiving Potential Major Nor'easter

There is potential (keyword here is potential) for a Nor'easter to form along the East Coast in time for Thanksgiving, and if true, may drop copious amounts of snow on the inland Northeast.

The overnight 0z ECMWF model projects a rather strong piece of energy to slide along the Gulf Coast in the few days leading up to Thanksgiving, drifting almost due east as the jet stream allocates the energy in that direction. At the same time, another piece of energy will be dropping south and east from Canada, and if you know what phasing is, you know what happens next. The model then has the Canadian and Gulf Coast energies combine into a single system that forms just off the East Coast to form a strong coastal storm, as the forecast below (valid November 27) shows.

The ECMWF model has this Nor'easter bring massive amounts of snow and liquid precipitation to the Northeast, and spreads the wealth north and east to cover most of the interior Northeast. The minimum pressure is projected to be at 991 millibars, which pretty much qualifies the statement that this would be a substantial coastal storm. 

Now, the new runs of the ECMWF and GFS do not show this happening. The ECMWF takes the southern energy and pushes it out to sea as the two pieces of energy do not merge in time to form this possible Nor'easter. So why bring it up? Well, there is most likely going to be a storm system somewhere in the US in the days just before Thanksgiving. As I've told on this blog, there has been a storm forecast to hit East Asia just two days ago. It ended up verifying, and the text from a post on that discussion a handful of days ago is below.

This is a forecast from the GFS Ensembles out to the Hour 60 timeframe. While all of the colors and lines may seem a little overwhelming, we're only going to focus our attention on East Asia, which is under the deep blue swath in the top left part of the left panel. Another correlation that has been proved effective is how weather in East Asia can be reciprocated in North America 6-10 days after the weather anomaly in East Asia. This includes the presence of storm systems or high pressure in East Asia. The November 18 forecast from both the GFS Ensembles and ECMWF model indicates that a deep negative height anomaly swath will slide down over Japan. If we extrapolate November 18th six to ten days out, we come up with a potential cold weather and storm system time frame of November 24 to 28. This narrows down the timeframe previously set out by the Bering Sea correlation, and if we match up the two timeframes (November 24-28 for the East Asian connection and November 25-29 for the Bering Sea connection), we come up with a November 24-29 potential timeframe for a storm, which can be isolated into the November 25-28 period, which is shown by both connections. Based on these two connections coming up with a common timeframe for a potential US storm system, confidence is quickly rising that this event will happen. 

The evidence doesn't stop there- here's text from a post on this storm potential I made back on November 13:

Take a look at the Bering sea in this two-panel reanalysis of the weather on November 8th. If you look closely, you can see that the Bering Sea was in the midst of a decent storm system on this date, just three days ago from today. This storm system passed along those waters in the midst of a mammoth ridge of high pressure just to the south. The 500mb height anomaly chart on the left best reflects this storm system, but its presence is confirmed on the right panel, which displays mean sea level pressure contour lines and denotations, as well as cloud cover. This Bering Sea storm has a connection to this potential Thanksgiving winter storm. Based on research done by Joe Renken, weather in the Bering Sea correlates to weather here in the US approximately 2.5 to 3 weeks after the Bering Sea weather anomaly occurs. This storm system in the Bering Sea happened on November 8th, and extrapolating that out 2.5 to 3 weeks ( 17-21 days) leads us to a potential storm system impacting the US around November 25 to 29. Considering November 28th is Thanksgiving, there does appear to be at least decent potential for a winter storm around the Turkey Day timeframe.

There is something else that must be noted with the ECMWF model I discussed at the top of this post. There has been discussion that the model has a bias, in which it holds energy in the Southwest US for longer than it ends up being there. I have a feeling that if this bias ends up working out in this situation, we would see the merge occur in the Northeast, resulting in that coastal storm. That is, if the energy in the South is able to latch on to the Canadian energy- in the most recent ECMWF model run, the southern energy was unable to phase with the Canadian energy until it was too late and the potential coastal storm was already off to the Canadian Maritimes. To give you an idea of what the snow forecast might look like if this Nor'easter occurred, take a look at the overnight ECMWF forecast below (warning, don't take this as a legitimate forecast just yet):

What I'm saying is, there really is potential here that needs to be monitored.

Andrew

Polar Vortex Split May Lead to Cool December

A split in the polar vortex as a result of a changing pattern may lead to a colder than normal December.

A look at the relative atmospheric angular momentum chart gives us a good look at what we will be dealing with in the coming few weeks. While the things that go into this chart are complicated, dissecting it is relatively easy. Basically, in order to see wintry conditions over the US, one would want to see positive AAM anomalies propagating upward past the 60N line, with negative AAM anomalies in pursuit, but only propagating to the area below 60N. In the same sense, -AAM anomalies moving north past the 60N line and +AAM anomalies shifting into the regions below 60N would favor a very anti-winter environment. A glance at the current chart reveals that we see a mass of +AAM anomalies pushing up past the 60N parallel, with -AAM anomalies in hot pursuit below the 60N parallel. This tells me that the atmospheric pattern is due for a change, likely in a favorable direction for winter weather lovers in the US.

The change that is coming appears to be in the stratosphere, and will eventually be reflected on the surface.

The ECMWF model has been forecasting the development of a strong Wave 2 stratospheric event, with the development of a polar vortex-disrupting high pressure system that will aid in the cold weather potential for December. The image above shows the Wave 2 response, with the highest values stretched across the far upper stratosphere to the middle of the stratosphere, with a secondary, minor blip on the radar around the jet stream. As we see the +AAM anomalies propagating to the regions above the 60N parallel, I would dare to theorize that this stratospheric response will be at least partially initiated by the AAM development.  If that is to be true, then this modeled Wave 2 response would become far more likely, as the +AAM anomalies are already happening rather than just being forecasted.

Now, there are two main types of stratospheric events that disturb or split the polar vortex. There is a Wave-1 response, which involves the polar vortex becoming elongated and/or weakened, however a split does not occur. This can result in displacement of the vortex out of the Arctic, but that prospect is not as likely as it is in a Wave-2 scenario. A Wave-2 stratospheric response involves the polar vortex being split into two main vortices, as the image from NASA shows above. While the temperature images show the Wave-1 and Wave-2 (top and bottom rows, respectfully) responses occur over Greenland and Eurasia, especially with the two split vortices where one vortex goes over Greenland and one goes into Eurasia, the split can lead the vortices into any land mass, not just those two regions.

This split is confirmed by the ECMWF potential vorticity forecast by Day 10, with one piece of the vortex going into Canada and the other, larger piece heading into Eurasia. Keep in mind that the polar vortex is one huge storm system that holds the pure Arctic air in its place, so when it splits and heads towards land masses, it's bringing that intense cold air with it. If the model projections verify, those in Canada should prepare for a big cold shock for wherever that polar vortex roams; the same goes for Eurasia.

It now looks like the ridge that interrupts the polar vortex will pop up in the area around Greenland, and this is good news for winter weather folk in the East. With the polar vortex splitting and a piece apparently dropping down into Canada, it only makes sense that a strong ridge will form in Greenland to counteract the deep negative height anomalies of the vortex. Ensembles have started to back off on this negative North Atlantic Oscillation (top), and also the Arctic Oscillation (bottom) in terms of how negative they will get, but I'm pretty sure they will end up more negative than neutral, as the ECMWF ensembles show above.

There are the usual caveats of long range forecasting in play here, along with a few other extra items that have not been mentioned. However, as long as the +AAM anomalies do their thing, and the stratosphere responds as it is expected to, I see a possibility of some cooler weather coming in for December. No word yet on how cold, but with a portion of the polar vortex in Canada just a few hundred miles from the US border, I would expect that colder weather to have an effect on the US as well.

Andrew

PDS Tornado Watch - Illinois, Wisconsin, Indiana

URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 561
   NWS STORM PREDICTION CENTER NORMAN OK
   840 AM CST SUN NOV 17 2013

   THE NWS STORM PREDICTION CENTER HAS ISSUED A

   * TORNADO WATCH FOR PORTIONS OF
     EASTERN IOWA
     ILLINOIS
     NORTHWEST INDIANA
     NORTHEAST MISSOURI
     SOUTHEAST WISCONSIN
     LAKE MICHIGAN

   * EFFECTIVE THIS SUNDAY MORNING AND AFTERNOON FROM 840 AM UNTIL
     400 PM CST.

   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...

   * PRIMARY THREATS INCLUDE...
     SEVERAL INTENSE TORNADOES LIKELY
     SEVERAL LARGE HAIL EVENTS LIKELY WITH A FEW VERY LARGE HAIL
       EVENTS TO 2 INCHES IN DIAMETER POSSIBLE
     SEVERAL DAMAGING WIND GUSTS WITH A FEW SIGNIFICANT GUSTS TO 80
       MPH POSSIBLE

   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE
   MILES EAST AND WEST OF A LINE FROM 25 MILES SOUTH SOUTHEAST OF
   SCOTT AFB ILLINOIS TO 40 MILES NORTHEAST OF MADISON WISCONSIN.
   FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH
   OUTLINE UPDATE (WOUS64 KWNS WOU1).

   PRECAUTIONARY/PREPAREDNESS ACTIONS...

   REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
   TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS.

   &&

   DISCUSSION...ONE OR MORE BROKEN BANDS OF SUPERCELLS ARE EXPECTED TO
   DEVELOP THIS MORNING AND RAPIDLY SPREAD EAST/NORTHEAST ACROSS THE
   WATCH AREA TODAY.  GIVEN THE COMBINATION OF MODERATE INSTABILITY AND
   VERY STRONG VERTICAL SHEAR...CONDITIONS WILL BE FAVORABLE FOR LONG
   TRACKED...SIGNIFICANT TORNADOES.  LARGE HAIL AND CORRIDORS OF WIND
   DAMAGE WILL ALSO BE LIKELY.

   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   500. MEAN STORM MOTION VECTOR 24050.

High Risk Issued for November 17

The Storm Prediction Center has issued a High Risk outlook for today, as the threat for severe weather has been deemed to be very high.

The High Risk has been issued for eastern Illinois and much of Indiana, likely pointing to the Storm Prediction Center's concern over potentially tornadic cells that may develop as they coagulate with one another to form a powerful squall line. Categorical outlooks issued below tell the story; tornadoes and wind are the top two concerns for today.

Hail Outlook

Tornado Outlook

Damaging Wind Outlook

High resolution models have been continually projecting discrete cells in Illinois developing into an extreme squall line over Indiana and Ohio. The cells that develop in northern and central Illinois will have great potential to be tornadic, especially if they become supercells right off the bat. This environment is very dynamic with a 120kt+ jet stream and 60kt+ lower level winds combining with strong winds from the south to produce incredible shear and a very potent place for thunderstorms.

Initiation of these storms is expected around the noon hour Central Time in Illinois, and further intensification will occur explosively until the 3:00 PM hour, before coagulation occurs and the storms race eastward towards the Ohio Valley at over 50 knots, per recent sounding analysis.

This is a dangerous situation. If you are in the Moderate Risk (red) or High Risk (pink) areas on the top image, you are advised to prepare for potentially extreme weather, which may include tornadoes (some strong), potentially extreme damaging winds, and hail. The scope of this now-likely outbreak is displayed on excerpts of the Storm Prediction Center write-up below.

...TORNADO OUTBREAK WITH MULTIPLE SIGNIFICANT TORNADOES AND
   WIDESPREAD DAMAGING WINDS EXPECTED ACROSS THE OH VALLEY AND SRN
   GREAT LAKES REGION TODAY...
...THE DEVELOPMENT OF A SQUALL-LINE APPEARS LIKELY DURING
   THE LATE AFTERNOON ACROSS THE OH VALLEY WITH THE LINE DEVELOPING SWD
   INTO THE TN VALLEY DURING THE EARLY EVENING.
0-3 KM STORM RELATIVE HELICITIES OF 400 TO 450 M2/S2 WILL BE
   FAVORABLE FOR STRONG TORNADOES WITH SEVERAL LONG-TRACK DAMAGING
   TORNADOES EXPECTED TO OCCUR FROM ERN IL ENEWD ACROSS MUCH OF WRN AND
   CNTRL IND FROM LATE MORNING INTO EARLY AFTERNOON. AS CELL COVERAGE
   INCREASES AHEAD OF THE FAST MOVING FRONT...A SQUALL-LINE SHOULD
   ORGANIZE LATE THIS AFTERNOON. WIDESPREAD WIND DAMAGE WILL BE
   ASSOCIATED WITH THE LINE ALONG WITH TORNADOES WITH ROTATING CELLS
   EMBEDDED IN THE LINE. THE SEVERE THREAT SHOULD RAMP UP SOUTH OF THE
   OH RIVER DURING THE LATE AFTERNOON AS CELLS INITIATE SWD ALONG THE
   FRONT. THE POTENTIAL FOR WIND DAMAGE AND TORNADOES SHOULD DEVELOP AS
   A FAR SOUTH AS THE TN VALLEY DURING THE EVENING AS AN EXTENSIVE LINE
   OF SEVERE STORMS MOVES EWD INTO THE UPPER OH VALLEY AND CNTRL
   APPALACHIAN MTNS.
 SEVERAL FACTORS APPEAR TO BE COMING TOGETHER FOR A TORNADO OUTBREAK
   ACROSS THE OH VALLEY. THE FIRST FACTOR IS THAT THE SYSTEM IS
   NEGATIVELY-TITLED AND VERY WELL-ORGANIZED. THE SECOND IS THAT THE
   LOW AND MID-LEVEL JETS WILL BECOME FAVORABLY COUPLED IN THE OH
   VALLEY TODAY COINCIDING WITH AN OUTBREAK OF STORMS ACROSS THE WARM
   SECTOR. THE THIRD IS THAT SUPERCELLS ARE EXPECTED TO REMAIN DISCRETE
   INTO THE MID-AFTERNOON. THESE FACTORS COMBINED WITH EXTREMELY
   FAVORABLE LOW-LEVEL SHEAR PROFILES JUSTIFIES THE ISSUANCE OF A HIGH
   RISK ACROSS ERN IL...IND AND SW LOWER MI.

Andrew