Friday, November 22, 2013

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.