Computer Model Snowfall Forecasts

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By Quincy Vagell on February 6, 2013, 4:00pm Last modified: February 6, 2013, 7:46pm

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Short-Range Ensemble Forecast total (liquid equivalent) precipitation for Friday and Saturday.

The computer models are unanimously forecasting widespread snowfall amounts of 1 to 2 feet (12 to 24 inches) across inland Connecticut Friday night into Saturday.

This forecast is very delicate, because several factors may result in extremely sharp cutoffs in snowfall, especially across southern Connecticut. What this means is snowfall totals from town to town may vary drastically. (from 6" to 12" to 24", for example)

The factors include intense bands of snowfall that will localize the heaviest amounts, dryslots (dry air) that may limit totals in other areas and the possibility that warmer air might work in and keep coastline areas more of a slushy mixture instead of all snow.

Sorting it are the forecasts from some of the computer model runs today...
I ran 5 forecasts below and their average ranges from 4" to as much as 24". 

The European (Euro) computer model has been the most consistent with this storm and it has one of the best track records in the past. In fact, it was the first computer model to begin predicting heavy snow and it has stood by that forecast for the past few days. The model paints the picture of a classic southern New England blizzard as low pressure is forecast to briefly stall just southeast of Cape Cod. The result is an intense band of snow sitting over the eastern half of Connecticut, but also enough cold air is in place to bring heavy accumulations down right to the shoreline. 

The GFS is often #2 to the Euro when it comes to accuracy. It originally did not show a storm, but it has become fairly consistent over the past few days with heavy snow across Connecticut. The model brings warmer air a bit further north, which results in more mixed precipitation across southern Connecticut. In the northeast hills, similar to the Euro, it bombs the area with well over a foot of snowfall. The speed of the storm is slightly faster, which does result in somewhat lower snowfall amounts.

The Short-Range Ensemble Forecast (SREF) system uses the high resolution NAM and ARM computer models as a base and spits out slightly different forecasts to account for slight errors. In the end, the average of these different ensemble "members" is taken to give an overall forecast. Here, the SREF is essentially a compromise between the GFS and Euro. It still blasts interior Connecticut with the heaviest snow, but does show some mixed precipitation near the coast.

The NAM has been very inconsistent with this storm. It was one of the last models to show the storm in the first place. Then, it showed a historic scenario this morning with up to 40" of snow in northeastern Connecticut, but the latest 18z run has backed off considerably. The other thing to note is that the NAM shows warm air and dryslotting across Fairfield County, which results in barely any snowfall for areas like Greenwich and Stamford.

For the average that I mentioned, I took the Euro, GFS, SREF, 12z NAM and 18z NAM snowfall forecasts and divided by 5, assuming a 10:1 snow to liquid ratio. Even though the NAM has had two extreme solutions, I believe they even each other out and should still be left on the table.

Stay tuned to for all the latest forecasts and discussions.

I will be posting my own snowfall forecast by 6 p.m. after reviewing more data.
Also, I will lay out a specific time-line to help break down the storm.

Nat'l Weather Service Snow Forecasts

Interact with me via Facebook and Twitter for my most up-to-date thoughts and forecasts.   

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Quincy Vagell

Town: Marlborough, CT  

Reporting for WXedge since January 2012.

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