Moderate Accumulating Snow Likely Into Friday Morning

The below Free forecast update outlines a number of model solutions and forecast timing for precipitation with the storm moving in tonight into tomorrow, going in depth with the latest accumulation forecast.  Travel and school impacts are considered possible, and Premium subscribers just received a detailed update on school closure and delay percentages.  To view our impact analysis, be sure to subscribe here.  Otherwise, below is the latest Free forecast for the upcoming storm:

PRECIPITATION TYPE: The storm is going to transition from rain to snow, of that I am quite confident.  As can be seen here, already there is some rain approaching the region.  Obviously, right now surface temperatures and temperatures aloft are too warm to support snow.  Precipitation thus does likely start by 10 PM (if not sooner) across the area but it will start as rain.  On RAP guidance, we see here at 7 PM that the 850mb freezing line (bottom left, dashed black) moves across the region, and that is the first step in a gradually cooling atmosphere behind this cold front that needs to go right before we can finally see snow.  The model shows us relatively dry through this point, and by 10 PM herewe see that the 850mb freezing line is continuing to progress across the region, but on the top left surface temperatures are still in the low 40s, again unlikely to support snow and certainly not supporting any snow sticking o any surfaces.  Precipitation is falling but is really just beginning with the storm.  RAP guidance then gives us a break in precipitation at 1 AM here, where temperatures across the region have fallen into the mid 30s inland and upper 30s at the coast.  This is where precipitation at the coast is likely rain and inland snow if it is falling.  And finally, by 4 AM here we have very steady, moderate/heavy snow moving in with surface temperatures falling into the low/mid 30s region wide and snow falling with such intensity that it will stick.  Generally, most other model guidance agrees with this timing as well.  The latest HD NAM shows scattered showers around midnight here being rain south of I-84 but by 3 AM here snow is clearly taking over.  Thus, we can expect scattered rain showers through midnight or 1 AM.  Around 1 AM far inland areas begin turning over to snow, and this is a process that gradually works down to the coast through around 2 or 3 AM.  By 3/4 AM, the entire region should see moderate snow falling, and from then forward it should begin accumulating.

SNOWFALL TIMING: The timing is generally outlined above, with rain starting late this evening and turning to snow from 1-3/4 AM across the entire region.  However, heaviest snow is generally seen continuing through around 8 AM, but again there is some disagreement.  The HD NAM, which shows the least snow, shows precipitation moving out of the region by 8 AM here.  The latest 18z GFS got much more aggressive with snowfall and has moderate snowfall continuing through around 10 AM, as seen here.  The low-resolution NAM here keeps moderate snow across at least New Haven County through 10 AM.  So once we turn to snow, much of the region can expect steady moderate snow through at least 8 AM, winding down from west to east between 9 AM and 12 PM.  In Westchester County, we could see snow winding down as early as 7 AM depending on the exact track, as that is the region where confidence is lowest right now as it is on the western edge of the main precipitation shield.  Overall, though, that 3-8 AM timeframe looks to hold the steadiest snow.

SNOWFALL ACCUMULATION: I have had to update the latest snowfall accumulation forecast to account for recent trends across model guidance.  The latest forecast is as follows: New Haven County sees 3-6 inches of snow, with Fairfield and Westchester Counties both seeing 2-5 inches. Isolated amounts of 6+ inches of possible in far eastern New Haven County County, while in Bridgeport and eastern portions of Fairfield County we could see accumulations approach 6 inches, though I expect most 6 inch accumulations will be off slightly further to the east where there will be even heavier precipitation.  If any part of the forecast is too low it is for coastal areas, where I expect we see the heaviest precipitation.  Confidence is relatively low with this forecast overall, however.

To view our latest school impact analysis for the upcoming storm, along with more examples of model output and specific accumulation forecasts from this output across the SWCT/NY, be sure to subscribe here and receive detailed email updates every time the forecast shifts and school or travel impacts become more/less likely.  Another Free update will come around 9 or 10 PM this evening with the latest short-range weather model guidance, so be sure to check back in then.

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