December 25-27 Potential Significant Snowstorm

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

We are looking at the possibility of a snowstorm along the central and eastern portions of the nation in the days immediately after Christmas, something that could no doubt cause a travel nightmare. This is a forecast from the National Weather Service's GFS model, also known as the American model. It shows 500mb height anomalies, basically where areas of low pressure are (cool colors), and where higher pressure is located (warm colors).  We see our storm system in the southern Ohio Valley as a deep depression of purples on the morning of December 26th.

Before we dive into the concept of where this system will go, let's analyze the atmospheric structure first. The first thing I want to point out is two low pressure systems to the north of New England. This phenomenon is known as a 50-50 Low, as discovered by wxrisk.com. The 50-50 low acts as an attractant for the storm track to head towards the Northeast. In response to the 50-50 Low, high pressure develops close to or in the Great Lakes. This pushes the jet stream south into the US, pressing into the storm track and forcing storm systems to go into the Northeast, because storm systems do not head into high pressure systems.

Some of you more avid weather enthusiasts may recognize a high pressure system to the west of Greenland as a west-based negative NAO. Not so fast. See that system of low pressure right up against Greenland? That's the Achilles heel of the NAO. Typically, when you have high pressure over Greenland, the jet stream wants to push south and encourages cold and snow to hit New England. However, such an encouragement of cold and snow can be enhanced by the high pressure system being WEST of Greenland, a.k.a. a West-Based negative NAO. I find this potential negative NAO to be too far west of Greenland to be completely effective. The presence of a low pressure anomaly right against the east coast of Greenland then disbands any effects the ridge may have on the NAO. I expect this index to be neutral- neither supportive or unsupportive of snow in the Northeast.

Here's another 500mb height anomaly forecast for the morning of December 26th, but this forecast is from the European's ECMWF model. The ECMWF is commonly known as the most accurate model in the world, and forecasters are frequently watching the ECMWF and GFS to see which one is showing which forecast.

The ECMWF has the system stronger than the GFS, that's the first and foremost issue. A common thing to remember for storm systems is that they will turn north the stronger they are, as is observed with tropical systems. Additionally, the ECMWF is lacking a good 50-50 Low. It has half of the phenomenon, but not the whole package. This, combined with that dicey negative NAO situation, is not all that encouraging as far as snow prospects for the Northeast go. However, that big high pressure over Canada suppresses the storm from the Midwest and thus induces a higher likelihood for a Nor'easter track.

This is an observation map from November 4th of this year, just about 52 days from the forecast date of December 26th. You should see a storm system located in the Southeast. This is another weather phenomenon known as the Lezak Recurring Cycle (LRC). It is associated with a repeating pattern developing every fall, with each repetition falling between 40-60 days. This year's LRC repetition period, known as a 'cycle', is 52-53 days. This is why I went back to find this chart.

I went forward a couple more days with this system and found that it went out to sea instead of riding up the coast, a solution that would be very possible if a negative NAO and/or 50-50 Low failed to formulate. However, I have a feeling that if the huge high pressure system is present in Canada where the ECMWF/GFS are both forecasting it to be, the storm system should rightfully move into the Northeast, bringing significant snows to that region, as well as the Ohio Valley.

Early estimate on best snowfall areas are the Ohio Valley and Northeast. No specific amounts as of yet.

Andrew

December 24-25 Clipper System

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

It is becoming increasingly likely that a small clipper system will deliver precipitation and disturbed weather to the Plains and Midwest during Christmas Eve and the day of Christmas.

Above is the NAM model forecast, valid for the afternoon of Christmas Eve. We see a small, negatively tilted area of vorticity in the upper left hand part of the collage of images above. The system looks to be a clipper, pushing to the southeast from Canada. It's lack of sustained moisture and unimpressive isobar gradient tells me this will not be a winter storm event at all. 6 hour precipitation values in the upper right tell of light snow ongoing in the Upper Midwest in response to this system. High pressure stationed in the Plains will provide a base for cold air in the region.

Two forecasts, both based off of the NAM model but known as parallels (slightly changed from the NAM model itself), show 3 hour snow in north Wisconsin and Minnesota during this system as light, nothing extremely impressive. Based on the low prospect for any big accumulating snows, and the NAM's lack of moisture in the top image collage, this does not look like anything to get excited about. If you're into the traditional snowing on Christmas Morning, those in the West Great Lakes may have a shot at such a prospect as the clipper continues pushing to the southeast.

Andrew

CFS Suggests Cold Opening To January 2013

**This post is dedicated to Josephine Gay, age 7, a victim of the Connecticut shootings.**

The CFS v2 weeklies from WeatherBell are suggesting that January will open on a very chilly note, with temperature anomalies lower than 7 degrees below normal likely.

The CFS v2 has been showing a cold open to January for quite some time, so it's not surprising to see the forecast still standing today. The ECMWF is projecting widespread warming across North America in the stratosphere in 10 days, enhancing the probability that cold weather will push down to the surface in the next couple weeks, as exhibited by the ECMWF forecast below:

Andrew

December 25-27 Potential Significant Winter Storm (Updated 12/20)

**This post is dedicated to Josephine Gay, age 7, a victim of the Connecticut shootings.**

We'll start with the GFS model. This forecast is for the morning of December 26th. We can see our storm system in the Southeast, with a pressure at or just below the 1000 millibar mark. There is a strong high pressure system in the Plains keeping the system from wanting to move north and towards the Midwest, so it would seem the only logical solution is to have the system move towards the East Coast.

Moving ahead to the morning of December 27th, we see that the system has bombed out and is moving up the East Coast with a minimum pressure of 988 millibars. The proximity to land suggests much of the Northeast and Mid-Atlantic would be severely affected by this solution, should it happen. We still see our high pressure system in the Plains, keeping the system away from the Central US.

We now move on to the ECMWF model, forecast for the same timeframe of December 26th in the morning. Once again, we see the storm system in the Southeast and a strong high pressure in the Plains in an act known as suppression, where the storm is forced south and thus suppressed from moving north. This is a typical negative NAO pattern, something I will address below.

Moving to the morning of December 27th, the reliable ECMWF model brings the system inland into New York, with a minimum pressure just below 980 millibars. This strength difference does prove key, and will prove key in future forecasts. We still see a high pressure system in the Midwest and Plains, but not as strong as the GFS for this same timeframe.

Here's the issue I see with the forecasts the ECMWF and GFS are putting out. They both suppress the storm to a massive degree and then take it up the East Coast in a strikingly textbook negative NAO pattern. But looking at the teleconnection forecasts for the NAO, we actually see a neutral NAO. Such a neutral NAO would not push the storm up the coast, but rather could push the storm out to sea without going up the coast.

The Pacific North American index (PNA) forecast from the 12z models shows a spread in the forecasts during the time period this forecast is valid for. The GFS/GFS Ensembles take a positive PNA, while the ECMWF goes negative PNA. The ECMWF Ensembles are indifferent to either phase. If such a positive PNA evolves, you are not going to see the storm be suppressed south and then go up the East Coast. A positive PNA induces conditions that prefer a storm track in the Ohio Valley and Midwest, like previous ECMWF runs were showing.

If the PNA were negative like the ECMWF is showing, high pressure would build in over the Southeast and provoke a storm track more tailored to the Plains, like we saw earlier in the month. The ECMWF is not handling its teleconnections correctly. A neutral NAO and negative PNA would not suppress the storm south and bring it up the coast, unless this is an anomaly situation, which it should not be.

My feeling is the models are having troubles. I have found that the timeframe between Hour 120-Hour 204 is the most turbulent time in model forecasts. The system remains out in the Pacific, meaning weather balloon data is unable to sample it, and the model forecasts are thus hindered in their accuracy.

If anything, such a strong storm in the East would try to provoke a ridge in the Rockies, possibly then inciting a slight positive PNA as the GFS is suggesting. I won't believe that big suppression forecast unless I see it.

Andrew

December 25-28 Potential Significant Winter Storm (Updated 12/19)

We continue to see the potential for a significant winter storm somewhere in the East US between December 25-28. Let's see what the models are saying today.

I've decided today will be dedicated to the ensembles, because I just don't trust the operational models this far out to handle a storm like this on their own. As we saw with the Dec. 19-22 blizzard, it can take a while before a reliable solution can be developed, so we'll look to the ensembles for the time being. This forecast is for Christmas morning, and we can see our storm system sitting in the Gulf Coast region. The ensembles seem pretty disorganized as far as exact location goes, seeing as there is no defined low pressure area on the image for this body of precipitation.

Moving ahead to the morning of December 27th, we see the ensembles have solidified in a system more confidently, with a low pressure signature now showing up just offshore of New Jersey. We see the rain/snow line hugging the storm as well, a solution that would bring heavy snow to much of the Northeast.

Considering the general neutrality of the North Atlantic Oscillation through this timeframe, with a slight positive PNA, I wouldn't go as far as to completely trust this solution. It will need to be watched, but I'm not totally confident in it.

Moving on to the Canadian GGEM Ensembles, we find our system stationed in the Southwest, producing copious amounts of precipitation on the evening of December 26th. Keep an eye on these timing issues. The GGEM Ensembles appear to be following a track similar to the GFS Ensembles, although they appear to want to pull the storm slightly further north, possibly in response to a weak positive PNA.

The morning of December 28th brings the storm well offshore per the GGEM Ensembles. Some precipitation is reaching the immediate coastal areas as snow, but with the system so far offshore, I would like to disprove that precipitation forecast.

I don't trust the GGEM Ensembles usually due to their base off the GGEM model, but they do seem to agree with the idea of a slight positive PNA having an effect on this system. It will have to be watched.

Morning of December 26th

Morning of December 27th

The ECMWF Ensembles take a totally different track on this system, preferring to have the system start out in southern Tennessee before moving northeast into the Northeast US, not offshore. A sharp cold outbreak then follows the energy. Such a track would likely bring the best snows to the Ohio Valley and central Great Lakes, but because the ECMWF EPS precipitation graphics are so expensive (if the European weather center even produces them), we have no way of knowing.

For those wanting the ECMWF model itself, here's a couple precipitation graphics:

All precipitation above the dark blue line would be snow- and that would be a LOT of snow for the western Great Lakes and Midwest, possibly in excess of 10 inches.

Forecast preference: ECMWF ENS
Confidence: 20%

Andrew