Tuesday 12/15/20: PM Update

Good evening!

Quick forecast update to reflect some significant trends throughout the day.

What’s Changed: I have increased snow totals for much of the region as a large northward trend in the system’s track has become much more apparent, as the relevant energy has become much better sampled and other small atmospheric pieces fall into place. Since last night’s model runs, the northward trend just keeps getting more obvious with this one. For the Merrimack Valley, the forecast certainty in a large snowstorm has increased significantly.

Timing: Snow starts late Wednesday evening, just before midnight. The heaviest snow falls overnight Wednesday night and into the Thursday morning commute. Snow begins to gradually taper off by mid- to late-morning Thursday, with all precipitation done by the early- to mid-afternoon.

Amounts: Expect a widespread 10-15″ snow for much of Massachusetts and southern New Hampshire, with a few spots potentially getting higher amounts. The potential for truly major amounts over 15″ will be limited by the progressive nature of the system, but given the very heavy snow banding as well as the cold air creating a dry, fluffy snow, we shouldn’t have an issue putting up around a foot of snow in the Dracut area as well as much of interior Massachusetts and southern New Hampshire. A few places may underperform if an area of subsistence sets up, but even then, given the amount of moisture and the cold air involved, those locales should sneak over the 10″ mark.

Elsewhere, central New Hampshire will see 5-10″ of snow (with locally higher amounts possible), with a quick drop-off further north as the northern edge of the precipitation shield will probably set up in northern New Hampshire. Southeast Massachusetts will also see around 5-10″ of snow, as the northward trend does mean that areas around and south of Boston will be warmer than anticipated. This means that rain will mix in for Cape Cod, the islands, and parts of southeast Massachusetts. For Boston as well as areas close to the immediate coastline and interior portions of southeast MA, the snow will start as a wetter consistency before gradually becoming fluffier Thursday morning. Cape Cod and the Islands likely see less than 5″ of snow.

Uncertainty: For the Dracut/Lowell/Merrimack Valley area, this is now a high-confidence forecast, with the main uncertainty factor being the possibility of a slight over- or under-performance of the forecast. Even if we get 8″ or 18″, the point stands that we’re in for a respectable and impactful, but light and fluffy, Thursday morning snowfall. Uncertainty is higher in central and northern New Hampshire where the northern edge of the precipitation shield sets up, and uncertainty is much higher for points south of Boston where mixing may become an issue.

Have a great night!
-Nathan

Published by Nathan Coram

Hello! I'm Nathan Coram, a 20 year old meteorology student and weather geek, and am in my junior year at UMass Lowell as a meteorology major. I am the current Vice President of the UML American Meteorological Society Local Student Chapter. Prior to at UML, I attended the Dracut school system for my K-12 years, having graduated from Dracut High in 2018. I first got into weather with the December 2008 ice storm, which knocked out my electricity for 4 days. I had no idea how it could be raining and becoming ice immediately, and how rain can knock out power. (Now I do - warm layer aloft, cold air at surface). But I didn't really get into it until the heat of July 2010 and specifically a few severe weather events during that month, followed by the year 2011, which featured several high profile weather events. Since then I have had a growing interest, and am hoping to make it into the meteorology field, preferably with NOAA/NWS. But for now, I'm blogging here on Dracut Weather (also on Twitter and Facebook), helping with the UML Weather Center social media, and tweeting about the weather on my own account as well. Thanks for visiting!

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