DAYS 1-5 (DECEMBER 16-20)
A nice cold and tranquil day that starts with sun & high clouds and will end with a slate grey overcast in advance of an approaching low pressure area that will bring us a mid December, or late autumn “winter storm”. Pretty certain now that low pressure will track just southeast of New England Thursday morning and midday, it’s weakening and elongating center passing south of Cape Cod and the Islands, with cold high pressure to the north in eastern Canada. This is a recipe for a decent winter weather event, although the low will actually be on a weakening trend instead of strengthening, like many do. Nevertheless, it will still have plenty of moisture with it and pack enough punch to deliver decent snow to most areas. Coastal flooding will not be a major factor with this event, with just some minor flooding around Thursday’s high tide, favoring east-facing and north-facing shores. Wind will be notable but not ferocious, as you’ll see in the detailed forecast below, but the wind direction will be key for snowfall, which leads me to the most difficult aspects of this forecast – the position and movement of a coastal front and the amount of mixing with and changing to rain takes place. Right now, the best bet is that mixing is a safe bet for the South Coast of RI/MA and up along the South Shore of MA to perhaps the Plymouth area during Thursday morning, and a quicker change to rain is pretty likely for a good portion of Cape Cod and the Islands. The determining factor in how much snow ends up falling in these mix/change areas will be how quickly that coastal frontal boundary moves back to the east as the storm goes by, returning cold air to these locations. These areas along and east of that frontal boundary that see a wetter snow and/or rain will also have a high risk of a flash freeze when that cold air returns. Further northwest, it’s a pretty straightforward situation: lower water content fluffy snow, with the only real complicating factor being potential drying patches or bands where intensity may get much lighter keeping some areas near the low end of the forecast range, or even them under-achieving expectation, so we will have to keep an eye out for that. Snowfall amounts will appear in the detailed forecast following this.. After this system departs, it’s dry and cold for Friday and Saturday with temperatures running below average for mid December. A bit of a moderation follows on Sunday but that’s also when we will be dealing with our next unsettled weather – a weaker system coming in from the west, with some light precipitation, favoring snow but with some mix possible as well.
TODAY: Sun followed by increasing clouds. Highs 27-34 Wind N up to 10 MPH.
TONIGHT: Thickening overcast. Snow arriving late evening and overnight, mixing with or changing to rain Cape Cod & Islands. Lows 18-25 except 25-32 South Coast. Wind N to NE 5-15 MPH.
THURSDAY: Overcast with snow except mix/rain South Coast and Cape Cod for a while before changing to snow during the afternoon west to east. Expected snow accumulation: 1-2 inches lower portion of outer Cape Cod (Chatham) as well as Nantucket and Martha’s Vineyard, 2-4 inches upper portion of outer Cape Cod (Provincetown) as well as Mid Cape, 4-8 inches Cape Cod Canal area as well as South Shore up to about Plymouth and westward to the immediate coast of RI, and 8-14 inches elsewhere. Highs 20-27 except 28-35 South Coast and 36-43 outer Cape Cod and Islands. Wind E to NE 15-25 MPH with gusts 35-45 MPH southeastern MA and southeastern RI, shifting to NW during the afternoon, and NE to NW 10-20 MPH with gusts 25-35 MPH elsewhere.
THURSDAY NIGHT: Clearing. Blowing snow likely. Lows 10-18. Wind N to NW 10-20 MPH, higher gusts. Wind chill often below 10.
FRIDAY: Sun and passing clouds. Highs 27-34. Wind NW 10-20 MPH.
FRIDAY NIGHT: Clear. Lows 10-17. Wind NW under 10 MPH.
SATURDAY: Mostly sunny. Highs 30-37. Wind W 5-15 MPH.
SATURDAY NIGHT: Variably cloudy. Lows 20-27. Wind SW up to 10 MPH.
SUNDAY: Mostly cloudy. Chance of light snow/mix. Highs 33-40. Wind variable 5-15 MPH.
DAYS 6-10 (DECEMBER 21-25)
Potential unsettled weather events December 22 and 24-25. Temperatures variable, averaging near normal. Will re-evaluate these events next update.
DAYS 11-15 (DECEMBER 26-30)
One or two precipitation events possible during this period. Temperatures variable, averaging close to normal.