Friday, December 21, 2012

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 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.


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.