November 25-27 Nor'easter Final Update

Mid-level vorticity analysis reveals that the two pieces of energy that will combine to form the Nor'easter are currently moving in their projected directions, and should merge in just a handful of hours.

It is expected that the rather strong piece of energy in Texas will shift eastward, as that system in Canada drops to the southeast. As we go through the next day or two, the two pieces of energy will merge and shift northeast to provoke a heavy rain event and substantial snow event over parts of the Northeast and Mid-Atlantic.

Snowfall forecasts over the next 60 hours have totals either flirting with or above the 12 inch mark along western New York, west Pennsylvania and northeast Ohio. Lighter amounts are anticipated in the higher elevations in the New England area, but will still allow for plowable amounts. Lower elevation areas should receive a couple of inches from this system, while coastal areas will receive some major rain amounts.

Rain amounts along the coast should push above 2 inches, making this system quite a wet one. I find it possible that some areas will experience flooding, though I'm not sold on a huge flooding event. The more likely scenario is rather heavy rain for most of the Eastern Seaboard in the Northeast and Mid-Atlantic, with lighter amounts inland. Bear in mind that while this forecast shows significant precipitation for the inland areas in line for snow, it's likely that the snow will be heavy, as this is only November, thus light, fluffy snow is rather unlikely.

Stay tuned to your local weather service if you are at risk of this winter weather. Winter storm watches (navy blue), advisories (purple) and warnings (pink) have already been hoisted across the Northeast and Mid-Atlantic as this first snow event begins to unfold.

Andrew

December 4-8 Potential Winter Storm

It looks rather possible that the US will experience a winter storm around the December 4-8 time period.

As the image above shows, there will be a rather deep area of negative height anomalies over East Asia for November 28th, as exhibited by the dark blues in the top left of the left-hand panel. There is a knwon correlation that indicates storm systems or ridges of high pressure over East Asia can be reciprocated in the United States 6-10 days after the event's occurrence in East Asia. As we see a stormy period begin in East Asia over November 28th, I would expect December 4-8 to be the timeframe for a winter storm in the US as a result of this East Asian correlation. There are already a couple of guesses put out by the ECMWF control run. The most recent forecast has a storm system targeting the Midwest and Great Lakes for snow, as shown below (note the yellow arrow denoting the storm track)...

ECMWF Ensemble Control MSLP anomaly projection for December 5th

The second-newest ECMWF control run actually develops a stronger storm towards the end of this December 4-8 timeframe, unlike the December 5th projection from the ECMWF control...

ECMWF Ensemble Control MSLP anomaly projection for December 8th

As you can see, there is still a lot of time for this to be figured out, and the level of uncertainty is very high as far as who may be affected. What I am decently certain of, though, is that there is a chance for a winter storm in this timeframe.

Andrew

Arctic Outbreaks Herald Start of December

A very cold pattern will herald the start of December, and unofficially the start of winter 2013-2014.

The ECMWF ensemble system and GFS ensemble system forecasts agree that we will see a pattern develop in the next 10+ days which will include two substantial bodies of high pressure in the northern Atlantic and Pacific. The Bering Sea will possess one half of the ridging system, while Greenland attempts to reel in the second component. These two ridges are then projected to split the polar vortex into two pieces, one of which will slide down into Europe, while the other pushes down into North America. 

It is expected that there will be a prolonged period of lower pressures across North America in the first couple of weeks of December, and this will lead to below normal temperatures for much of the nation. It is anticipated that this cold air will originate from Canada and will be transported from the Northern Plains into the Central Plains on east. Temperature anomalies from both ensemble systems have deep below normal anomalies stretched out across nearly all of the country. The likelihood of this prolonged cold weather is rather high, as the stratosphere will also be undergoing a Wave-2 response to two high pressure systems applying pressure in just about the same areas as the ensemble projection above shows.

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.

The ECMWF Ensemble set confirms the idea of a particularly harsh outbreak of cold weather to kick off December just over a week in...

Andrew

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