Area Forecast Discussion
National Weather Service Boston/Norton MA
712 PM EST Wed Feb 24 2021

.SYNOPSIS...
A cold front will bring blustery weather and a return to 
chillier temperatures Thursday and especially by Thursday night
and Friday although with diminishing winds. Periods of 
unsettled weather this weekend into early next week with a 
series of fast-moving features passing by southern New England 
every couple of days into early next week. Mild over the 
weekend, then colder on Tuesday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...

7 PM Update...

Cold front moving across central NY led by 1 mb/hr pressure
falls across New England and the Hudson Valley. Satellite shows
clouds across Western MA and Northwest CT moving east. One patch
of showers was crossing Northcentral MA, with others along the
front in Central NY. Also, BOX radar shows a 40-50 kt low level
jet at 2000-3000 feet AGL supporting wind gusts of 25 kt/30 mph 
and potentially 30 kt/35 mph at the surface.

Forecast expectations continue, with a period of clouds moving
across the region tonight, generating widely scattered showers.
Movement of the front extrapolates to the Berkshires around 11PM
or midnight, Central Hills around 1 AM, and MA East coast around 2
or 3 AM. Gusty south winds will turn from the west after the
front moves through. Temperatures will fall from the upper 30s
to mid 40s evening to the upper 20s and 30s by morning. 

As everything is roughly on target, no changes are expected.

Previous Discussion...

After the mildest day in some locations in quite a while, temps
will not drop that much this evening. A modest southwest LLJ of
55-65 knots will move overhead this evening ahead of an 
approaching cold front. The result will be gusty southwest winds
of 20 to 30 mph developing this evening and perhaps up to 35 
mph across the high terrain and portions of the Cape/Islands. 
This will only allow temps to drop into the upper 30s to middle 
40s during the evening hours, although it will feel a bit 
cooler with the gusty winds developing. 

The main impact of the approaching cold front this evening will
be an abundance of mid level cloudiness and the gusty SW winds.
Low level moisture is lacking, so not much more than a brief
sprinkle/spot shower or two expected with the front. Therefore,
just included a low pop in the forecast.

Winds will shift to the west behind the front toward morning.
Temperatures will have dropped into the 30s by daybreak across
most of the region, but still remain above freezing in most 
locations.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Thursday...

It will be quite blustery on Thursday with strong cold advection
aloft resulting in excellent mixing with NW wind gusts of 25 to
35 mph. However, the cold advection aloft will be offset by a 
relatively mild start and plenty of late February sunshine. This
should push highs into the upper 30s/near 40 across the high 
terrain and well into the 40s elsewhere. It will feel a bit 
chillier though given the windy conditions.

Thursday night...

High pressure building in from the west will result in
diminishing northwest winds and a return of seasonably chilly
temps. Low temps by daybreak will be down into the middle to
upper teens across the normally colder outlying locations; to
the lower to middle 20s in the urban centers.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Big Picture...

Duelling jet streams. The southern stream is dominant through 
Sunday, with one shortwave in particular forecast to move through 
the flow and up the St Lawrence Valley. The northern stream then 
takes over across the Northeast USA the first half of next week. A 
trough from Hudsons Bay moves through the flow and across Eastern 
Canada, swinging a weak cold front across New England Monday evening 
and night. 

Model mass fields are similar through the weekend, then diverge the 
first half of next week. As shortwaves are the weathermakers, 
confidence in timing is less than for a mature weather system. 
Thermal fields are similar through Sunday. These fields indicate the 
same trend to colder temps by Tuesday, but show different magnitudes 
with the GFS the coldest. Overall, moderate confidence through the 
weekend, trending to low confidence Tuesday-Wednesday.

Details...

Follow the shortwaves during the long term. 

One shortwave in the southern stream races up the St Lawrence Valley 
Saturday, passing to our north and swinging a cold front through 
during the late day/early night. Temperatures Saturday may initially 
be cold enough for snow, but likely just a coating, before changing 
to rain for the midday and afternoon/evening. IVT continues to show 
a glancing blow by the vapor transport...most passes to our south 
Saturday 18Z-00Z, but GFS brings 700-800 kg/m-sec to Srn New 
England while ECMWF barely reaches 700 on the South Coast.  

A second shortwave shows in the southern stream on Sunday night, but 
with disagreement between the models on how much it spreads into Srn 
New England. GFS says yes, the ECMWF says offshore. Will combine
the effects of this shortwave with the third shortwave, which  
swings a cold front through on Monday. Will feature chance pops
for mainly rain showers Sunday night and Monday morning.

A fourth shortwave, also in the northern stream, passes north of New 
England on Wednesday. Best dynamic lift remains to our north,
but close enough to mention low-end chance pops for rain or snow
showers.

Mixing temps Friday support sfc highs in the mid 30s to low 40s, 
then mid 40s to low 50s Saturday and Sunday. Radiational cooling 
Friday night would suggest upper teens to mid 20s. Dew points in the 
30s Sat night and Sunday night suggest min temps no lower than the 
30s each night.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

Tonight...High Confidence.

Mainly VFR despite an abundance of mid level cloudiness for a
time tonight. Perhaps some brief marginal MVFR ceilings impact 
portions of the interior with the best chance across the high
terrain. A few brief spot showers possible through 06z ahead of
an approaching cold front. Main impact will be for SW wind 
gusts of 20 to 30 knots ahead of the front that will shift to 
the W toward daybreak behind the front. A few hours of LLWS is 
also expected overnight.

Thursday...High Confidence.

VFR with NW wind gusts between 25 and 30 knots on Thursday.

Thursday night...High Confidence.

VFR with gradually diminishing NW winds Thursday night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR. Breezy. 

Saturday: Mainly MVFR, with local IFR possible. Windy with
local gusts up to 30 kt. RA likely.

Saturday Night: Mainly VFR, with local IFR possible. Windy with
gusts up to 30 kt. Chance RA.

Sunday: Mainly VFR, with local MVFR possible. 

Sunday Night: Mainly MVFR, with local IFR possible. Chance
SHRA.

Monday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SHRA, slight chance SHSN.

&&

.MARINE...

Tonight and Thursday...High Confidence.

A modest southwest LLJ developing ahead of an approaching cold 
front will bring gusty SW gusts of 25 to 35 knots. Winds will 
then shift from the WNW with gusts between 30 and 40 knots on 
Thursday...with the strongest winds across our eastern/northeast
outer-waters. the forecast has strong Small craft to Gale 
Headlines for all waters into Thursday for all waters. Seas 
over the open waters will build to between 4 and 8 feet.

Thursday night...High Confidence.

NW winds may still gust between 25 and 35 knots early Thu
evening, but will diminish to between 20 and 25 knots 
overnight. This in response to high pressure building in from 
the west. Seas will also diminish, but may still have some 
marginal 5 foot seas lingering across the eastern outer most 
waters through 12z Friday.
 
Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Seas locally approaching 5 ft. 

Friday Night: Winds less than 25 kt. 

Saturday: Moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Local rough seas. Chance of rain. 

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Rain likely. 

Sunday: Winds less than 25 kt. Local rough seas. Slight chance
of rain showers. 

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers. 

Monday: Winds less than 25 kt. Local rough seas. Chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 PM EST Thursday for ANZ232>235-
     237.
 Small toCraft Advisory until 7 PM EST Thursday for ANZ230-236.
Gale Warning from 10 PM this evening to 4 PM EST Thursday for 
ANZ231-250-251-254. Gale Warning from 10 PM this evening to 10 
AM EST Thursday for ANZ255-256.

&&