Area Forecast Discussion
Issued by NWS Boston, MA

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595
FXUS61 KBOX 211759
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1259 PM EST Tue Jan 21 2020

.SYNOPSIS...

High pressure will build over southern New England through much
of the week, bringing dry weather and moderating temperatures.
There is the potential for a coastal storm this weekend, but
uncertainty exists in regards to timing and precipitation types.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

1245 AM Update...

Increased sky cover a bit across Cape Cod and the Islands for
the rest of the afternoon based on satellite observations. No
other changes since the previous forecast update.

10 AM Update...

Nudged down high temperatures for today a few degrees based on
the latest obs as we are just a bit too warm. Otherwise
forecast remains on track. Light ocean effect snow showers still
possible across Cape Cod. Best opportunity for this is across
the outer Cape.

Previous Discussion...

High pres in control. Column is rather dry and sunshine will
give way to increasing high clouds later today from the north as
mid level shortwave moves into northern New Eng. The one caveat
for today is potential for ocean effect clouds and
flurries/snow showers for the outer Cape. Steep low level lapse
rates over the ocean and delta T from SST to top of the shallow
boundary layer around 17-18C so enough low level instability for
clouds and perhaps a few flurries or snow showers. N wind
trajectory favors the outer Cape and some of the hi-res sources
are indicating some light QPF so have indicated chc pops here.
Low risk for a coating to one half inch for the outer Cape. Low
level cold air remains in place with 925 mb temps around -10C so
expect highs 25 to 30.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...

Tonight...
Any ocean effect clouds or flurries over the outer Cape will
dissipate as low level moisture decreases and inversion lowers.
Otherwise, some mid/high clouds expected to move through the region,
especially across northern and eastern MA as shortwave rotates SE
through the Gulf of Maine. Potential cloud cover may have some
impact on low temps so went a bit above MOS guidance with temps
mostly in the teens, with some single numbers in western MA.

Wednesday...
Mid level shortwave moves well east of New Eng with ridging building
into the region. High pres will remain in control at the surface.
Expect lots of sunshine with light winds and moderating temps. Highs
will range through the 30s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Highlights...

* Milder, above normal temperatures Thu/Fri with continued dry
  weather

* Coastal storm this weekend brings rain/snow chances Sat/Sun,
  favoring more rain than snow along the I-95 corridor, but
  uncertainty in precip type remains.

Details...

Wednesday night through Friday...

Dry, quiet weather rounds out the very quiet work week. This under a
building ridge of sprawling high pressure. A noticeable change from
the first half of the week will be the rebounding temperatures.
We`ll be back toward normal and then some as low and mid level flow
comes out of the S/SW on the back side of the high and draws in
milder air. A subsidence inversion under the high pressure will keep
mixing very shallow (model soundings indicate to around 950 mb);
should see highs in the low to mid 40s each day. A weak gradient may
allow for sea breezes keeping things cooler along the coast. Lows
dip into the 20s/low 30s Thursday and Friday nights, colder on
Wednesday night (teens) due to light winds, little cloud cover,
and resultant radiational cooling, in addition to the colder
airmass we start with.

Saturday and Sunday...

For Saturday we are becoming increasingly confident that our stretch
of quiet weather will come to an end, though details regarding exact
onset time and precip type are less certain. A digging 500 mb trough
and sfc low over the Ohio Valley move into the mid-Atlantic,
generating a coastal system that will move east in the vicinity of
southern New England from Saturday through Sunday, perhaps even
lingering into Monday. At this point the best chance for snow
continues to be in the interior, especially the higher elevations
given the marginal/warm antecedent airmass and the expected low
track too far north to bring with it the cold air necessary for snow
to the coast. Wintry precip, however, is certainly still a
possibility anywhere. EC ensemble probabilities of >1" snow continue
to exhibit a SE creep over the last several model runs, so plenty is
still left to be seen in regards to any snow potential for the rest
of southern New England. Blocking high pressure in eastern Canada
will play a role, potentially assisting with colder air, and slowing
the low as it ejects. This will be a very wet system (QPF totals up
to over 1" possible), and slow moving, with potential to drop a
significant amount of snow in interior southern New England that
would be disruptive and impactful. Stay tuned.

Monday...

Still some question as to how quickly the weekend system exits but
Monday should be dry or drying out as the low exits to the northeast
and high pressure builds in from the west.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels...

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

18Z discussion...

Expect VFR conditions at all terminals as high pressure builds
in from the Mid Mississippi River Valley. There is potential
for MVFR ceilings at HYA and ACK as winds are conducive for
ocean effect clouds and possibly showers to move in. Expect
these clouds to dissipate between 00Z and 03Z as drier air moves
in per the latest RAP/HRRR guidance.

Winds generally less than 10 knots at terminals out of the
north/northwest. Winds become light overnight and gradually back
to a westerly/southwesterly direction by Wednesday morning.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Friday: VFR.

Friday Night: Mainly VFR, with local MVFR possible.

Saturday: Mainly VFR, with areas IFR possible. Breezy. Chance
RA, chance SN.

&&

.MARINE...

Short Term /through Wednesday/...High confidence.

Diminishing wind/seas becoming light today with light winds/seas
continuing through Wed as high pres in control. Scattered snow
showers possible today and this evening over waters east of Cape
Cod.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: Winds less than 25 kt.

Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.

Friday: Winds less than 25 kt. Local rough seas.

Friday Night: Winds less than 25 kt. Rough seas up to 9 ft.

Saturday: Winds less than 25 kt. Rough seas up to 9 ft. Chance
of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...KJC/BW
NEAR TERM...BL
SHORT TERM...KJC
LONG TERM...BW
AVIATION...BL
MARINE...BL



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