Area Forecast Discussion
National Weather Service Boston/Norton MA
924 PM EST Sat Feb 22 2020

.SYNOPSIS...
High pressure will bring an extended period of dry weather
through Monday. A weak cold front moves through Monday night, 
then stalls south of New England. A disturbance moving along the
front may bring light rain or a light winter mix Tuesday. A 
stronger weather system comes out of the Plains Wednesday and 
Thursday bringing rain to Southern New England and possibly a 
rain/snow mix in Northern and Western MA. Colder weather then 
moves in from the west later Thursday through Saturday.

&&

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

925 PM Update...

High pressure centered to our south will continue to result in
mainly clear skies/light winds across the region overnight. Low
temps will mainly be in the 20s, but a few coastal spots will
be more in the upper 20s to lower 30s range.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
By tomorrow morning, the aforementioned surface high 
pressure will gradually shift eastward over the Mid Atlantic states, 
changing the wind direction to more of a southwesterly or southerly 
flow. Because temperatures will not be as cold to start the day as 
on Saturday, expect highs to be at least a few degrees warmer on 
Sunday afternoon than on Saturday afternoon. Also, the 500mb trough 
mentioned in the near term discussion gives way to weak upper level 
ridging. This will start the warm air advection in earnest. Model 
guidance show 850mb temperatures ranging from -1C in the interior to 
+2C near the coast. Using dry adiabatic lapse rate given the dry air 
mass in place, this would yield afternoon highs in the mid to upper 
40s in the interior high terrain to low to mid 50s in Eastern MA and 
RI. Another factor that favor warmer temperatures are downsloping 
winds. Winds from the SW tomorrow afternoon would help support mid 
50s in the coastal plains. Coastal areas, especially Cape Cod and 
the Islands will only reach the mid to upper 40s due to ocean 
temperatures being in the upper 30s to low 40s. Something to watch 
for Boston tomorrow late morning into the afternoon hours is the 
potential for the development of sea breeze, given the contrast 
between the warmer inland temperatures and cold ocean waters. Should 
a sea breeze develop, that would put a cap on how high afternoon 
temperatures could go in the Boston metro. Nonetheless, still 
looking at at least low 50s given the reasons explained above. These 
forecast temperatures are generally 10 to 15 degrees above average 
for late February. It will certainly feel very spring-like with 
plentiful sunshine. So overall, a beautiful weekend for outdoor 
activities. 

Sunday night...

As for tomorrow night, temperatures should drop off rather quickly 
after sunset under mostly clear skies. However, it will be warmer 
than tonight with warm air advection and rising dew point 
temperatures. Overnight lows are expected to range from upper 20s to 
low 30s away from the coast and mid to upper 30s near the coast.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Big Picture...

Shortwave from the Pacific Northwest digs a closed low over the 
Northern Plains Monday night and Tuesday. The closed low then lifts 
through the Great Lakes Wednesday-Thursday and crosses New England 
Friday. While some precipitation may reach Southern New England 
Tuesday, the bulk arrives Wednesday as the upper flow turns from the 
southwest and reaches maximum in the south-southwest upper flow 
Wednesday night and Thursday. 

Climatological average of 500-mb heights is currently in the 540s. 
Forecast heights are above normal Monday through Wednesday, then 
trend below normal by next weekend. This suggests a mild start
to the workweek, trending to a much colder finish.

Mass fields show a few differences but are roughly similar Monday 
through Thursday. Similarity diminishes by late next week.
Confidence is good through midweek, then diminishes late in the
week.

Daily Concerns... 

Monday...

Offshore high pressure maintains dry weather. Light 
southwest winds bring milder temperatures. Mixing reaches between 
900 and 925 mb, where temperatures are equiv to -2C to -4C at 850 
mb and support max sfc temps in the 50s.

Monday night-Tuesday...

A weak cold front moves across New England Monday night, but with 
only limited clouds. This front then stalls south of New England 
Tuesday. Model guidance suggests a weak disturbance moves along the 
front, passing to our south. This could bring some rain to the 
region Tuesday. But the probability is low until Tuesday afternoon 
and even then the amounts that could fall will be light. 
Temperatures will stay above freezing at night, then the increasing 
clouds will keep readings cooler Tuesday, generally in the 40s. 

Wednesday-Thursday...

As noted, the upper flow turns from the southwest and south during 
this period, drawing increasing moisture north into New England. PW 
values climb to around an inch, which is much above the seasonal 
normal around 0.3 to 0.4 inch. The upper jet also moves into 
position near Southern New England with the left exit region 
supporting lifting of the higher PW air and generating precipitation.

While most of Southern New England looks warm enough for rain, the 
model data does show surface ridging from Nrn New England into 
Western MA and low level temperatures are near freezing along the NH 
border. This suggests a potential for mixing with some light snow or 
sleet north of the Mass Pike and especially north of MA route 2. 
This will need to be watched.

On Thursday, a secondary low moves up the coast and brings the 
strongest lift and precipitation to the region. Timing differences 
between the GFS/GGEM/ECMWF with the GFS already finished by Thursday 
morning while the others show the weather system crossing around 
midday/early afternoon. The closed low and negative tilt orientation 
favor the slower models, so the forecast holds on to the rain 
through midday/early afternoon.

Thursday night-Friday-Saturday...

While the surface low moves off through Maine and the Maritimes, 
expect increasing pressure gradient and gusty west flow. Cold 
advection will encourage mixing, with the mixed layer reaching 850 
mb Thursday and deeper Friday. Winds in this layer reach 25 kt, so 
expect gusts of 25-30 mph. Temps at 850 mb fall from above freezing 
Thursday morning to at least -6C per the ECMWF or to -15C per the 
GFS and GGEM Friday and Saturday. A compromise on these values would 
support max temps in the 30s Friday and Saturday.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Tonight through Sunday night...High confidence. VFR with mainly
clear skies. West to southwest winds less than 10 kt tonight. 
Winds increase to 10-14 kt during the day Sunday. Winds turn
from the south-southwest Sunday night.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/... 

Monday: VFR. Breezy. 

Monday Night: Mainly VFR, with local MVFR possible. 

Tuesday: MVFR. Slight chance RA.

Tuesday Night: Mainly MVFR, with areas IFR possible. Breezy.
Chance RA, chance SN.

Wednesday: Mainly IFR, with areas MVFR possible. Breezy. RA
likely.

Wednesday Night: Mainly MVFR, with local IFR possible. Breezy.
RA, FZRA.

Thursday: MVFR/IFR conditions possible. Breezy. RA, FZRA.

&&

.MARINE...
Tonight and Sunday...High confidence. Gradient weakens a bit, 
so expect WSW gusts to generally remain in the 15 to 20 kt 
range tonight and Sunday. 

Small craft headlines continue across the southern outer waters
through midnight, mostly because of 5 ft seas and occasional 
25-30 ft gusts. Headlines continue closer to the coast through
early tonight due to gusts at Buzzards Bay near 29 kt...this
should diminish.  

Outlook /Monday through Thursday/...

Monday: Winds less than 25 kt. 

Monday Night through Tuesday: Winds less than 25 kt. Slight
chance of rain. 

Tuesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain. 

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

Wednesday Night: Moderate risk for Small Craft Advisory winds
with areas of gusts up to 30 kt. Rough seas up to 11 ft. Rain. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 AM EST Sunday for ANZ254>256.

&&