textproduct: Boston / Norton

SYNOPSIS

High pressure moves offshore this afternoon then a fast moving low pressure system will bring accumulating snow overnight into Sunday morning, especially south of the Mass Pike. Ocean effect snow showers will linger into Sunday afternoon towards the Cape and Islands. A brief shot of arctic air follows Sunday night into Monday with wind chill dropping to between 5 below and 5 above. It remains cold Tuesday, but a pattern change with a significant warming trend is likely Wednesday into Thursday. The milder will likely be accompanied by a period of gusty winds and showers sometime in the late Thursday into Friday time frame.

NEAR TERM /UNTIL 6 PM THIS EVENING/

Today...

Mainly dry weather today with high pressure in control. Will see increasing mid and high clouds as a disturbance approaches from the Great Lakes. 850mb temperatures will warm to around -5C by the afternoon with a light return flow behind departing high pressure. High temperatures will be warmer than what we've recently seen with highs well into the 30s with even some lower 40. Mainly dry today...but a few spot snow showers are possible across interior or rain showers near the coast after 21z with weak lift ahead of an incoming shortwave.

SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY NIGHT/

Key Messages...

* Accumulating snow very late tonight-Sun morning especially south of I-90...highest accumulations near south coast

* Advisory Level snowfall expected towards the south coast, Cape and Islands.

* Ocean effect snow showers linger on the outer Cape into Sun night with additional minor accum possible

* Brief shot of arctic air results in wind chills dropping to between -5 and 5 above late Sun night into early Mon morning

Details...

Tonight...

Starting to see some better agreement among guidance with respect to the season's first widespread snow event along the I95 corridor slated for tonight into Sunday morning. Notably, the GFS has trended further north with the higher QPF to be more in line with the CMC and ECMWF. As a result, global models are nearly identical in terms of QPF placement and amounts. High res guidance is generally similar with the exception of the HRRR which has the bulk of the precip shield further offshore and thus a drier solution.

Later today shortwave energy rotates around a positively tilted trough north of the Great Lakes. Trough starts to dig overnight bringing a deep moisture plume northward into SNE. PWATs increase from 0.3" to 0.5-0.6" overnight so there will be plenty of moisture to work with. Further aloft, the region will find itself under the right entrance region of a 140-150 kt upper jet overnight into Sunday morning. High confidence that this combination will bring the first widespread accumulating snows for much of RI, and areas S of the Mass Pike. Precipitation will likely start at light snow late tonight for much of the region except for the Cape and Islands where temperatures will take some time to cool. Still expecting much of the accumulating snow to fall over a 8-12 hour period later tonight into Sunday morning. High res guidance has also honed in on the chance for lingering ocean effect snow for the Cape and Islands into Sunday afternoon. While this certainly does not look like a heavy snow event, BUFKIT soundings have become somewhat more impressive for this event. Specifically the 00z NAM has modest amounts of omega on the order of -10 ubar/sec collocated with a 6-8kft DGZ across much of SE Massachusetts. Thus, unsurprisingly, the HREF shows a brief window of hourly snowfall rates between 0.25 and 0.5 inches mainly across S RI and SE MA between 08 and 12z Sunday morning.

In terms of totals, while this won't be a major event it will be the first accumulating snow of the 2025-26 season for many of us. Despite a somewhat deep DGZ and temps in the mid and lower 20s snow ratios will be limited by a lack of strong mid-level frontogenesis and omega. All told, this will be a general 1-3 inch event S of the Mass Pike with highest amounts along the south coast. Expecting around 4-4.5 inches on Cape Cod and the south coast. Meanwhile, just a coating to an inch for areas north of the MA Pike. Think the RRFS shows a reasonable worst case scenario if there is a northern shift there could be up to 6 inches across RI and SE MA and Cape Cod but this is still a low probability (less than 25%) outcome. If the system trends south it would be closer to 1-2 inches near the south coast with decreasing amounts northward.

Sunday and Sunday Night...

Steadiest snow will quickly end from west to east through mid morning Sunday. Expecting ocean effect snow showers to linger the outer Cape Sun afternoon and night as arctic air pours into the region on NNW-NW flow. Ocean induced CAPES expected increase to 500- 600 J/kg with SST differential around 25C. An additional inch or 2 is possible on the outer Cape but boundary layer temperature profiles may be a bit warm for additional accumulations during the afternoon.

A short duration shot of arctic air will pour into the region Sun night and Mon. Lows will drop into the single numbers and teens with wind chills bottoming out between -5F and +5F late Sun night and Mon morning, with -10F possible in the Berkshires.

LONG TERM /MONDAY THROUGH FRIDAY/

Key Messages...

* Blustery and bitterly cold Mon AM with Wind Chills between 5 below & 5 above and afternoon highs only recovering into the 20s

* A few brief snow showers possible Monday night, otherwise dry and still chilly Tue with highs mainly in the lower-middle 30s

* Pattern change with significantly milder temperatures Wed-Fri with unsettled/mainly wet weather possible sometime late Thu into Fri

Details...

Monday...

A brief shot of arctic air and gusty NW winds will result in bitterly cold wind chills early Mon morning in the 5 below and 5 above range. Under a mixture of clouds and sunshine will result in Mon afternoon high temperatures only recovering into the 20s.

Monday night and Tuesday...

A fast moving moisture starved northern stream shortwave may bring a few brief snow showers to the region Monday night. Otherwise...the main story will be the continued chilly weather. Lows Mon night will generally be in the teens. Highs on Tue will range from the upper 20s in the highest terrain...to mainly the lower to middle 30s elsewhere.

Wednesday through Friday...

The long range model guidance continues to be in very good agreement in a significant pattern change. A -PNA developing will allow for milder Pacific air to overspread much of the lower 48. This is quite the change from the very cold weather we have seen for the first of of December. High temperatures should reach well into the 40s on Wed and perhaps 50+ Thu and/or Fri. In fact...Thu night may end up quite mild with gusty southerly winds and steady/rising temps if current model timing pans out. This a result of shortwave energy moving into the Great Lakes resulting in strong surface low pressure tracking well to our north across Quebec. This would allow a strong southerly LLJ to develop...2+ standard deviations above normal also bringing showers and the potential for gusty winds sometime in the late Thursday to Friday time frame.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

12z TAF Update...

Today...High confidence.

VFR. SW wind gusts of 15 to 20 knots develop this afternoon with a few gusts near 25 knots possible near the south coast. Low risk of just a brief spot rain/snow shower later today with the greatest risk towards the Cape and Islands.

Tonight and Sunday...Moderate confidence.

VFR conditions dominate during the first half of the evening. MVFR-IFR conditions in light snow overspread the region in the 03z to 07z time frame. Ptype will mainly be snow...but a few hours of rain or mixed rain/snow are possible near the Cape and Islands. Steadiest snow south of I-90 and especially towards the south coast where brief LIFR conditions are possible in snow. Snow winds down between 12z and 16z from northwest to southeast...but lingers into the afternoon towards the southeast New England coast. Otherwise...improvement to VFR conditions expected. Light SW winds tonight shift to the NW at 5 to 15 knots on Sunday.

Sunday night...High Confidence.

VFR outside the the outer-Cape/Nantucket where ocean effect snow showers may persist Sunday evening. This activity should dissipate though after midnight. Otherwise...NW wind gusts of 20 to 25 knots with a few gusts near 30 knots in the high terrain and near the coast develop Sunday night.

KBOS Terminal...Moderate confidence in TAF. Snow likely develops after 06z with the bulk of it ending before lunchtime Sunday. Total snow accumulations of 1-2" possible.

KBDL Terminal...Moderate confidence in TAF. Snow develops after 03z/04z and ends by mid-late Sunday morning. Total snow accumulations of 1-3" possible.

Outlook /Monday through Wednesday/...

Monday: VFR. Windy with gusts up to 35 kt.

Monday Night: VFR. Breezy. Slight chance SHSN.

Tuesday through Tuesday Night: VFR. Breezy.

Wednesday: VFR. Windy with local gusts up to 30 kt.

MARINE

Forecaster Confidence Levels...

Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent.

Today Through Sunday Night ...High confidence.

Small craft advisory conditions redevelop later today into this evening with gusty SW winds/choppy seas developing ahead of an approaching cold front. Gale watch Sunday night into Monday as strong CAA will result in excellent mixing over the waters. Areas of very light freezing spray possible Sunday night.

Outlook /Monday through Wednesday/...

Monday: Low risk for gale force winds with gusts up to 40 kt. Areas of rough seas.

Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Slight chance of snow showers.

Tuesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft.

Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas locally approaching 5 ft.

Wednesday: Strong winds with gusts up to 30 kt. Seas up to 5 ft.

BOX WATCHES/WARNINGS/ADVISORIES

CT...None. MA...Winter Weather Advisory from 10 PM this evening to 7 PM EST Sunday for MAZ020>024. RI...Winter Weather Advisory from 10 PM this evening to 7 PM EST Sunday for RIZ003>008. MARINE...Small Craft Advisory from 10 AM this morning to 1 AM EST Sunday for ANZ231>235-237-250-251-254>256. Gale Watch from Sunday evening through Monday afternoon for ANZ231>235-237-250-251-254>256.