December 2013 Weekly & Monthly Forecast

This forecast will give depictions of the conditions for each week in December by text, and then graphics for the monthly averages.

Weekly Forecasts

December 1-7: It appears possible that there will be another outbreak of Arctic air from Canada into the United States. Both American and European ensemble guidance systems confirm this idea, thus it is anticipated that this timeframe will be quite cold. It appears that the precipitation anomalies would be most enhanced over the Plains, Midwest and Ohio Valley with a favorable Pacific pattern, though the East Coast will also need to monitor this timeframe as ridging pops up near Greenland to provide a base for a possible negative North Atlantic Oscillation.

Plains: Very Cold, Possible Above Average Precipitation.
Midwest: Very Cold, Possible Above Average Precipitation.
Ohio Valley: Cold, Possible Above Average Precipitation.
Northeast: Very Cold, Average Precipitation.
Mid-Atlantic: Cold, Average Precipitation.
Southeast: Cool, Average Precipitation.
South Plains: Cool, Possible Below Average Precipitation.
Southwest: Warm, Possible Below Average Precipitation.
Northwest: Average Temperatures, Possible Above Average Precipitation.

December 7-14: The cold pattern should continue, but slowly wind down towards the end of this week. Watch out for a winter storm during this timeframe, which may affect the Midwest or East Coast based on the early week teleconnection patterns. Expect warm weather in the West to continue, but possibly edge eastward as the colder weather does begin that winding down trend.

Plains: Cold, Possible Above Average Precipitation.
Midwest: Cold, Possible Above Average Precipitation.
Ohio Valley: Cold, Possible Above Average Precipitation.
Northeast: Cool, Average Precipitation.
Mid-Atlantic: Cool, Average Precipitation.
Southeast: Average Temperatures, Average Precipitation.
South Plains: Average Temperatures, Possible Below Average Precipitation.
Southwest: Warm, Possible Below Average Precipitation.
Northwest: Average Temperatures, Possible Above Average Precipitation.

December 14-21: The cold pattern that kicked off December should have receded by now, and may very well give way to a warmer than normal pattern. If not warm weather, expect an altered precipitation anomaly pattern, which may shift away from the Midwest and Ohio Valley to the Plains once again.

Plains: Warm, Possible Above Average Precipitation.
Midwest: Warm, Possible Below Average Precipitation.
Ohio Valley: Average Temperatures, Average Precipitation.
Northeast: Average Temperatures Average Precipitation.
Mid-Atlantic: Average Temperatures, Possible Below Average Precipitation.
Southeast: Average Temperatures, Possible Below Average Precipitation.
South Plains: Warm, Possible Below Average Precipitation.
Southwest: Cool, Average Precipitation.
Northwest: Average Temperatures, Average Precipitation.

December 21-31: The remaining week and 3 days of December are rather murky, as it is not known just how long this potential warm pattern will last. Depending on upcoming stratospheric developments, this timeframe may or may not see an active weather pattern. There are no significant signals of either active or non-active weather for this period, so the forecast for this timeframe will follow that of the previous week, but is highly uncertain.

Plains: Warm, Possible Above Average Precipitation.
Midwest: Warm, Possible Below Average Precipitation.
Ohio Valley: Average Temperatures, Average Precipitation.
Northeast: Average Temperatures Average Precipitation.
Mid-Atlantic: Average Temperatures, Possible Below Average Precipitation.
Southeast: Average Temperatures, Possible Below Average Precipitation.
South Plains: Warm, Possible Below Average Precipitation.
Southwest: Cool, Average Precipitation.
Northwest: Average Temperatures, Average Precipitation.

Monthly Averages

Temperatures for the month of December will most likely be determined by the first week or two of the month, with significant cold weather anomalies across much of the nation. Temperatures ought to vary from frigid conditions in the Plains, where the Arctic air will originate from, to cold weather in the Midwest and cool conditions in the Northeast. Average conditions may prevail over the Southwest before slightly above average temperatures show themselves along the West Coast.

Precipitation anomalies are set up in accordance with the first few weeks of the month, with a wide, rather uncertain swath of above normal precipitation stationed over the Plains, Midwest and Great Lakes. The Great Lakes will be inundated with lake effect snows as a result of the cold outbreak at the beginning of the month, with another one possibly following close behind. Other areas of the nation are too uncertain to delineate, but we will most likely see precipitation anomalies outlined in the first days of the month.

Andrew

Thanksgiving Potentially Significant Nor'easter

I believe that the probability of a Nor'easter around the timeframe of Thanksgiving is now high enough to be labeled as potentially significant.


Model guidance has been somewhat consistent on the development of a coastal storm system out of two pieces of energy, one from the Gulf Coast and one from Canada. The prognosis right now is that the piece of energy along the Gulf Coast will push east along the coast before the Canadian energy drops down far south enough to pick up the coastal energy. When the two pieces of energy begin to interact, it is expected that the Canadian energy will pick up the Gulf Coast system, the two will shift northeast and form one coastal storm system that will then become a full-blown Nor'easter. Now, model guidance is not completely on board with this system. The European ECMWF model has been in favor of a Nor'easter for a while, but the GFS model has been flipping back and forth between a storm or no storm. For reasons of consistency with the ECMWF, and agreement among the prestigious ECMWF ensemble prediction system (EPS), we will go with that model system.

Precipitation projections are tricky right now, as model guidance is still not 100% sold on the Nor'easter even occurring. However, if it is to occur, it is anticipated that coastal regions will receive rain while inland areas get potentially significant amounts of snow. This is due to a variety of factors, including the proximity of the storm to land, the anomalously warm water temperatures along the coast, and the fact that it's only November, and therefore there will still be warm temperatures to interfere with snow prospects for some areas.

This ECMWF image gives you an idea of what kind of snowfall could be seen with this storm system if it occurs as is shown right now by the model system. It is still subject to potentially drastic change, so this is just shown to give you an idea of possible snowfall amounts with this storm.

So, what do we have to go on that supports this potential likely winter storm? We'll start with data I discussed on Wednesday's post.

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. (End post)

The East Asian and Bering Sea correlations alone are enough for me to be decently confident in this event occurring, but there are other items that further enhance the probability of a winter storm along the East Coast.

This is the North Atlantic Oscillation forecast from the ECMWF for the next 10 days. The North Atlantic Oscillation, or NAO, involves pressure anomalies over Greenland that influences synoptic weather over North America, and around the world. The positive phase of the NAO involves stormy weather around Greenland, and this translates to warmer weather across the US, as well as a zonal flow across the nation. The negative NAO comes about from high pressure stationed over Greenland, and this permits cold weather to enter the East US, and enhances the chances of a coastal storm. The area where this storm may occur is circled in red, and as you can see, the model projects the NAO to be switching phases during this timeframe. It has been demonstrated that when the NAO is switching phases, coastal storms become more likely. Thus, it isn't such a far-fetched idea that a Nor'easter could occur in the day or two prior to Thanksgiving, possibly into Thanksgiving itself.

(Image removed for security purposes)

Now, I've discussed how the GFS and ECMWF are at odds with each other over this storm, but there is one atmospheric feature that they do agree on with this storm system. Both model guidance systems indicate that there will be a rather strong storm system around the far eastern portion of Canada. This is called a "50/50 Low", and is called such because of the presence of a low pressure system around 50 North and 50 West on the latitude and longitude scales, respectively. When a 50/50 Low forms, it is not uncommon to see a coastal storm to occur in the 2-3 days after the phenomenon. That said, I believe the ECMWF when it comes to the projection for this Nor'easter, in addition to the Bering Sea and East Asian correspondents.

I would make an outlook map, but I'd like to wait a bit longer before I do so, mainly because I would like to see the GFS come on board, and I want to see where that snow-rain line places itself, as that will make a big difference in my forecast. But right now, I'm feeling pretty good about a potentially significant coastal storm in a day or two before Thanksgiving, maybe into the actual day of Turkey Day.

Andrew

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