Today will feature the arrival of a strong late-season winter storm as one area of low pressure falls apart over the Great Lakes and another explosively intensifies in the Gulf of Maine. This storm will bring heavy snow to the foothills and mountains while a mix of rain and snow falls along the coastal plain, and the coastline itself experiences heavy rain. Meanwhile, wind gusts will pick up especially along the coast east of Brunswick. This post will be a little longer than usual as I walk through the storm’s impacts step by step.
Radar imagery this morning shows most of the storm’s precipitation back off to the west as the primary low over Ontario still has control of the situation. As the day goes on, that cluster of thunderstorms rolling through DC will become the secondary low responsible for producing most of our heavy precipitation.
Precipitation will arrive in the area as light rain in SW NH between 8 and 9 this morning as a weak wave of low pressure develops along the primary low’s cold front over New York. When that first round of precipitation reaches the mountains, expect precipitation to fall as snow.
By 3 PM, heavier precipitation will begin arriving as the secondary low begins developing over MA/CT. This will fall as heavy snow in the mountains and heavy rain elsewhere. During this time, thunder is likely in southern NH as a the northern edge of a strong squall line moves through. Any severe weather associated with that line should remain confined to south of the MA border.
The secondary low will really begin to intensify as we move into the evening hours. As it does so, a combination of cooling due to intense upward motion, cooling due to the latent heat exchange that occurs when snowflakes melt, cold air advection on northeast winds, and cooling due to the sun going down will bring the rain/snow line back towards the coast. I think the ECMWF might be a bit optimistic about heavy snow falling in Freeport/Wiscasset/Rockland, but you won’t have to go very far north/west of those towns to get in on the action. While temperatures will remain above 32F outside the mountains, the intense dynamics associated with the rapidly intensifying storm will allow snowfall rates to push into 2-3″ per hour territory. When it’s snowing that hard, you’ll get accumulation even with surface temps 33-35F.
Heavy snow will continue into the evening as the secondary low rapidly intensifies just southeast of Portland. This map is valid at 8 PM and shows heavy snow continuing for most of Maine (save for the immediate coast east of 295 and south of Route 1 while precipitation comes to an end in New Hampshire. This mid/late evening window is when thundersnow becomes a distinct possibility across western Maine. It’s hard to exaggerate just how energetic this storm is going to be.
Mid/late evening is also the time when wind gusts will pick up along the coast east of Brunswick. Right ahead of the storm’s core, gusts over 50 mph are possible south of Route 1 where warmer temperatures will help strong winds aloft mix down to the surface. We’ll also have to worry about coastal impacts from spashover during high tide later this morning and tonight. No major coastal flooding is expected (thankfully the peak storm surge will occur right at low tide) but the typically vulnerable spots are likely to see some water.
The storm will wind down later tonight as the secondary low moves past Bangor towards Houlton. We’ll still be stuck under its influence tomorrow when upslope snow showers will try to move southeast in the form of squalls (possibly with thunder and hail) as a disturbance rotates around the storm. More on that this time tomorrow morning.
So how much snow are we going to end up with when this is all over?
For many of us, the answer is “a lot”. The base map here (shaded) is the NWS official forecast for snowfall totals today. I’ve penciled in my opinion with the contours. For the mountains, this is mostly a 12-18″ storm with locally higher totals in the typical favorite spots (Sugarloaf and the Whites above 2,000ft). That 12″+ zone will bleed east a bit into the foothills NW of Waterville. Most of the Bethel-Augusta-Belfast corridor should end up with 6-12″ (except for Belfast proper) with totals tapering off sharply south of that. The Fryeburg-Lewiston-Camden corridor is the most uncertain but I think 3-6″ is a decent bet. Should the storm end up pulling in a little more cold air, those areas are quickly headed for 6-12″. If the change to snow takes a little longer than expected, 2-3″ will be a struggle to pull off. Right now, based on current observational trends and high resolution guidance trends, I’m leaning a bit on the colder side but there’s still plenty of room for surprises. Farther south, I think the 1″ line makes it close to Gorham/Brunswick/Bath/Wiscasset/Rockland. Again, small forecast errors make a huge difference here. These towns could easily end up not seeing a single flake, or could wind up with a “surprise” 2-4″. Once you move south of Route 1, confidence in an all-rain event is much higher.