Moderate Snow Likely, Heavy Snow Possible Saturday

This post is the first official Free forecast for the upcoming storm threat, though we must advise that given the extremely low confidence with this storm a number of possibilities remain.  All of this forecast was made available to Premium members, who just received a more comprehensive version of this forecast summarized and emailed directly to them.  It contains an analysis of all the most recent weather model guidance that drove this forecast.  We have attached some of this analysis as well to demonstrate the thought process behind this first take.  To begin receiving these live forecast updates as the storm develops, subscribe here.  Otherwise, the first free forecast is below:

FORECAST ACCUMULATIONS: The latest snowfall forecast is unchanged from yesterday: Areas north of I-84 see 2-4 inches of snow, with the higher amounts closer to the highway.  Inland portions of Fairfield and New Haven Counties see 3-6 inches of snow, again increasing in amount as we get close to the coast.  Inland Westchester and coastal Fairfield and New Haven Counties see 4-8 inches of snow, while southern Westchester may see 5-10+ inches.

This is a blend of global weather models and short-range high resolution models.  The global models have been weighted slightly higher as they fit my overall expectations for the storm, and I do believe that the short-range models may be over-amplifying the storm leading to more dramatic scenarios than are realistic.  Additionally, the global ensemble members that do show a more dramatic hit across the region keep liquid amounts much more limited than the over-amplified SREFs and NAM, meaning I believe those two models are printing out too much snow even if they are handling energy better aloft.  Accordingly, I see upside for snowfall likely capped in the 12-15 inch range, with a relatively small chance that amounts exceed those levels.  An even slight tick further south could mean that areas north of I-84 see even less than 2 inches of snow, but I do think some moisture makes its way up there.

FORECAST SNOW TIMING: Models generally agree more on timing.  It looks like snowfall will start from southwest to northeast between 7 AM and 11 AM on Saturday.  The slower-moving the precipitation shield is the less snow we get, so if the global models are right and amounts are lower we could see snow start very late Saturday morning.  Meanwhile, if the high resolution models are correct, a start closer to 7 AM is most likely.  The heaviest snow would likely be between 11 AM and 8 PM on Saturday, with the threat for lingering snow showers remaining overnight Saturday night before all snow should end between 10 PM Saturday and 4 AM on Sunday.

WINDS FORECAST: The latest GFS here show sustained winds approaching 25-30 mph at the immediate coast 1 PM on Saturday.  Gusts up to 45+ mph at the immediate coast are possible.  However, once you move inland only a couple of miles sustained winds will be just 15-25 miles through the storm with occasional gusts close to 40 mph, and inland towns likely do not see gusts much above 30 or 35 mph.  Any wind damage would thus be near exposed areas at the immediate coast, where the most drifting snow would occur, though these winds are unlikely to be strong enough to cause anything more than widely isolated power outages and occasional downed limbs.

WHAT WE KNOW: The timing of the snow has slowed down, and the heaviest snow can be expected from late Saturday morning to Saturday evening.  Winds are not looking quite as bad and should be gustiest early Saturday afternoon.  Coastal flooding is a threat but winds out of the northeast should keep it from being quite as bad.  A very tight snowfall gradient will set up somewhere, where the difference between a few inches and over a foot could be decided by less than 50 miles.  Short-range guidance is further north than the longer-range guidance, and we are beginning to approach the time where I typically take short-range guidance more into consideration.  Should I see more significant northerly trends on the global weather models we can expect a more significant storm, and the 18z GFS did trend slightly further north from the 12z (in line with expectations) and now matches the forecast almost perfectly, as seen here.  Finally, we know the worst impacts would be Saturday afternoon and evening, with snow leaving by early Sunday morning.  The closer you are to the coast, the worse the impacts can be expected to be.

WHAT WE DON’T KNOW: The most obvious thing we don’t know is which model camp will dominate and accordingly where the sharp snowfall gradient will set up.  This means we can’t really be confident at all in total snow accumulations.  We don’t know if sleet will mix in, as some models are showing is a possibility depending on the exact track.  This also means we don’t know exactly what snowfall ratios are going to be, which is something that we will need to work out as the storm approaches.  We don’t know exactly what impacts will be without knowing exactly how far north that heavy snow will get.

Multiple Premium updates can be expected tomorrow on the storm as the latest weather model guidance comes out and even more short-range guidance gets in range.  A Free update to this forecast will also come tomorrow evening.  A number of shifts are possible, and again I want to emphasize that there could be dramatic changes in total snowfall accumulations depending on where models finally settle on this gradient setting up.  The accumulation forecast listed above is considered most likely at this time given all available guidance, but this is an extraordinarily low confidence forecast.  Please be sure to stay tuned.

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