Nov 23, 2010: Wintry Mix Potential On Thanksgiving

Today was a mostly cloudy day across the area, with high temperatures in the mid to upper 50s inland, lower to mid 60s in the immediate NYC area, and in the upper 50s to lower 60s in Long Island/S CT, which was near the expectation.

A colder air mass is going to move into the area tonight, bringing much colder temperatures for tomorrow, with high temperatures returning into the 40s. Even though a storm moving through the Great Lakes will affect the area on Thursday and Friday, it appears that the cold may stay long enough for precipitation to begin as a wintry mix west of New York City.


Tomorrow's Outlook:

Tomorrow will be a mostly sunny and colder day, with a NW wind expected. High temperatures will be in the lower to mid 40s inland with a few upper 40s possible and in the mid to upper 40s for the rest of the area, with a few lower 50s possible south and SW of NYC.

Looking at the rest of the region, tomorrow will be a cold day in the interior Northeast, where highs will only be in the 20s to the lower 30s. Virginia will see highs in the lower to mid 50s.


Thanksgiving Storm: Wintry Mix To Rain

While at first, the models were not consistent with the storm, in fact the GFS model was showing a very weak and suppressed storm solution at one point, the models came into better agreement on Sunday, which is still present, though there is still some uncertainty with the timing of the storm. Below, I will discuss the expected set up for the storm and the forecast for this time frame.


Storm Set Up: A broad area of low pressure tomorrow expected to be near Kansas will split by tomorrow night into Minnesota with another low pressure further south, which the Minnesota low pressure becomes the dominant one by Thursday and intensifies while moving ENE. Unlike a typical storm in that area, however, the low pressure will expand southeast while becoming negatively tilted, and redevelopment is expected near the New England coast on Thursday night moving NNE towards the main storm. This redevelopment potential was already shown on the models for the last several days. The storm will bring its cold front through on Friday for most places, with a much colder air mass including scattered flurries behind it.


Thursday: As the storm moves into the area in the late morning hours, surface temperatures will be in the lower 40s inland and lower to mid 40s in the immediate NYC area. Dew points are expected to be in the 20s, with a high pressure over the area moving east. A CAD (Cold Air Damming) scenario is expected to take place, where despite the storm moving into the Great Lakes, instead of bringing a warm air mass into the region, it traps some cold air in Pennsylvania, with a small area of 850 mb temperatures below 0c.


With the set up mentioned above, a band of precipitation will move into Pennsylvania tomorrow morning, with heavy precipitation affecting western PA by the afternoon and late afternoon. The rain will run into the type of airmass mentioned above, which should lead to a mix of snow, sleet and freezing rain falling in central Pennsylvania. While the set up will not be as favorable in the area, the timing is the main question, as if the storm can arrive fast enough, by at least 11 AM-noon on Thursday, it could produce frozen precipitation as far east as the immediate north/west suburbs of NYC. The timing and the temperatures are likely to be favorable enough for at least a little snow/sleet to fall in the interior parts of the area.


Thursday Night - Friday: As the first rain band associated with the warm front lifts out of the area, warm air advection will take place, changing any leftover frozen precipitation in the Mid Atlantic to rain, with an occasional shower overnight and rising temperatures, which could reach the lower 50s in NYC. This band will move on to produce a wintry mix in other parts of the Northeast.

By Friday, the cold front should move through the area, producing a round of moderate rain in the morning into the early afternoon hours. Due to a strong cold air mass behind this cold front, it will be able to produce back end snow into Pennsylvania, and may lead to a few flakes in the interior parts of the area as temperatures start to drop in the afternoon. Meanwhile, a secondary low pressure will develop in New England, which will enhance the rainfall amounts in these areas.


Friday Night - Sunday: Much Colder
The coldest air mass of the fall so far will move into the area behind this storm, which is the leftovers of the current extreme cold spell in the northwestern and north central US, where low temperatures will drop below -20 degrees in the mountains. The GFS model, before trending to the currently expected scenario with the Thanksgiving storm, had a large source of cold air spilling into the Northeast, leading to very cold, if not near record temperatures. The storm, however, will block this source of cold air, leading to a cold air mass but not as extremely cold as modeled to be at first. High temperatures are likely to be in the upper 30s to mid 40s for the area on Saturday, with overnight lows ranging from the lower-mid 20s inland to the lower to potentially mid 30s in NYC.

By Sunday, the cold air mass will start to weaken, with slightly warmer temperatures, however another storm will start to organize over the Rockies, which may later on move into the Great Lakes, producing another round of brief warmth and rain for the area for early next week followed by another round of colder temperatures. Stay tuned for more information on this time frame.