Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 272339
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
639 PM EST Sat Feb 27 2021

.SYNOPSIS...
Rain will come to an end this afternoon as low pressure lifts
through, though some drizzle may persist into the evening. Brief
reprieve as high pressure builds in tonight. Rain returns
Sunday into Monday, especially near the South Coast, as low
pressure passes southeast of New England. A strong cold front
brings a period of scattered snow showers Monday night, along
with gusty winds and well below normal temperatures into
Tuesday. Temperatures rebound back to near to above normal for
mid to late in the week before a cooldown into the weekend.
Mainly dry weather for the rest of the workweek, though turning
unsettled by the weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
630 PM Update:

Reduced PoPs toward sub-mentionable levels a bit quicker per
radar and observational trends, which now show steadiest rains
having cleared into the offshore waters.

However trapped moisture underneath a dry punch of air aloft
(this dryslot seen well in low and mid-level water vapor
imagery) has now resulted in a considerable areal extent of fog
and drizzle across much of SNE. Did opt to increase coverage of
fog and drizzle across a larger part of SNE with this update.
Visbys are mainly 1-3 SM, but locally dense fog is possible
especially near areas of leftover snowpack or bodies of water.
Drier air is right on the western foothills of the Berkshires,
so while fog will likely linger for another couple of hours,
more rapid clearing from west to east is expected by mid-evening
in the interior and late- evening/midnight eastern MA/RI as
westerly flow ushers in these drier dewpoints.

Forecast otherwise appears to be holding up well with no
significant changes needed attm.

Previous discussion:

Temperatures have risen well into the mid 40s across much of the
region, with upper 30s in the higher elevations. As such,
precipitation continues to fall as rain. The back edge of the precip
shield has moved a bit faster than previously thought, already into
western Worcester county/exiting Hartford county at 230 PM. The
steady rain will continue until the back edge of precipitation
arrives, around 2-4 PM for western MA/CT and 5-7 PM for eastern
MA/RI. However, while model soundings indicate moisture quickly
exiting above 900 mb (bringing an end to the steady rain) the lowest
2,000 ft remain saturated for several more hours. This will lead to
some drizzle continuing into the evening along with some dense fog
and low clouds. The column dries out more overnight as the
shortwaves lifts northeast and moderate mid level ridging builds in.
It will be short lived, however, as clouds fill back in by Sunday
morning ahead of the next disturbance. Low temps will be several
degrees warmer than the night previous, in the upper 20s (high
elevations) to upper 30s (coastal plain).

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
The main adjustment with this forecast update was to slow down onset
of rain showers on Sunday a bit. A warm front lifts north associated
with the surface low moving out of the Great Lakes into Canada. This
will keep temperatures quite mild during the day and allow it to be
an all rain event. Rain arrives by early to mid afternoon, spreading
northeast by late afternoon. Along with warm air advection we have
dynamic support from a 30-35 kt low level jet which moves overhead
on the time timeline. A secondary low forms along the frontal
boundary near or just south of southern New England late Sunday into
Monday which will focus better forcing along the south coast or
offshore, so expecting the highest rainfall totals along the south
coast with lesser amounts toward northern New England. Cold front
doesn`t come through until later in the day on Monday, so low
temperatures overnight will be limited by very mild dewpoints...we
should remain above freezing everywhere.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Highlights...

* Arctic front brings a period of brief snow bursts/snow squalls Mon
  night, gusty NW winds and well below normal temps Mon night into
  Tues. Gale-force gusts and freezing spray likely on the waters.

* Temperatures rebound back into the 40s Wed into Thurs.

* Turning cooler Fri into the weekend.

Details:

Monday:

Lingering showers along a cold front, mainly for SE MA into RI and
our southern waters, should be clearing into the offshore waters by
the early afternoon. NW winds and drier conditions should ensue for
the afternoon, preceding a strong Arctic front for Monday night (see
below). Highs mainly in the 40s, with a few lower 50s across eastern
RI and SE MA/Cape and Islands.

Monday Night into Tuesday:

**Arctic Frontal Snow Bursts/Squalls Possible Mon Nite**
**Gusty NW Winds brings low wind chills Mon Nite, Cold Tuesday**

Mon night into Tues looks to be the most active in this long-term
forecast period. A lobe of the tropospheric polar vortex traverses
SE through New England Mon night into Tues, with its Arctic surface
cold front moving through mainly during the evening and overnight
hrs. Though it is brief, it ushers in a significant shot of cold air
(about -35 to -38C at 500 mb, -16 to -20C @ 925 mb). This should
bring a period of isolated to scattered snow bursts/snow squalls
Monday night, increasingly gusty winds overnight into Tues, and well-
below normal temperatures for early March Mon night into Tues.

1/ Arctic Front and Isolated Snow Showers/Squalls

Arctic front looks to progress through SNE mainly during the post-PM
commute period into the overnight (00-06z). Most models depict some
very limited QPF (a few hundredths at best) with the frontal
passage. Fairly robust signals for convective snow bursts and
possible snow squalls centered in this timeframe, including NAM-
based snow squall parameter values in excess of 2 units, owing to
nearly dry adiabatic lapse rates given strong CAA. The depiction of
a strong isallobaric rise/fall couplet and CAPE values around 50
J/kg or so supports this risk, as does the strong background
dynamics associated with the TPV. The shallow instability/CAPE in a
saturated dendrite growth region would support brief but locally
intense snow showers. Though brief low-visibility snow
showers/squalls are possible, with up to an inch of new snow falling
in a short period of time, given the timing more into the late
evening hours and less vehicle traffic it`s questionable if we`ll
really see any impact from these. Have carried 20-25 PoPs for
isolated/scattered snow showers.

2/ Gusty Winds

Strong cold advection should begin to steepen lapse rates and
promote good mixing even into the overnight hours. Model soundings
even during the overnight suggest mixing potentially as deep as 800
mb! Momentum transfer suggests winds at the top of the mixed layer
may support wind headlines. Not yet confident to hoist any wind
advisories as yet but may need to be considered later. However think
its main effect would be to produce low wind chills for
overnight/early Tues AM (5 to 10 below in northern/western MA and
the terrain, around 0 for the Hartford-Boston metros, and low single
digits above SE MA/RI).

Expect winds to become increasingly gusty late in the overnight and
continuing into a good part of Tuesday until the pressure gradient
starts to slacken. Showed gusts in the 35 to 45 mph range similar to
the 90th percentile NBM wind gusts and close to Advisory levels,
peaking thru the first half of Tues. Offered a slow decrease in
gusts for the afternoon as the gradient slackens and mixed winds
diminish. Winds and gusts should diminish even further for the
evening.

3/ Temperatures

Sharp cold advection post-frontal should cause temps to plummet into
the single numbers to teens above zero - on the milder end of
guidance given mixing from winds.

Highs Tuesday in the 20s to low 30s, though the winds/gusts will
make it feel like the single digits to the teens, with mid teens
wind chills across Cape Cod.


Wednesday into Thursday:

The shot of below normal temperatures is brief with SW return flow
resuming on Wednesday and continuing into Thursday. Under high
pressure and mostly clear conditions, highs in the 40s should again
be common for much of SNE with lows in the 20s to near freezing.

Friday into the Weekend:

Forecast confidence then diminishes for late week/weekend. There is
some indication of a southern-stream trough interacting or
potentially becoming in phase with digging northern stream energy
over eastern Canada. This could offer coastal storminess into the
weekend, but too much uncertainty limits predictability at this
point.

&&

.AVIATION /23Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

00z TAF Update:

Tonight: High confidence in trends, moderate to high on timing.

Initial mainly LIFR/IFR due to stratus, fog and drizzle to lift
and improve toward VFR flight categories. Timing 00-02z into
the CT Valley and around 01-03z central/eastern MA; sub-VFR may
linger until midnight for FMH/HYA and ACK. Thus by 04-06z most
if not all areas should trend VFR.

Winds to flip to W around 4-8 kt thru 02z for the interior and
coastal plain. SW winds 10-15 kt gusts to 25 kt across Cape Cod
to become W by late-evening/midnight.

Sunday and Sunday night...High confidence

VFR to start with increasing low to mid level clouds during the
late afternoon/evening. Should see conditions deteriorating to
MVFR and localized IFR as light rain spreads in. Light and
variable winds to start shifting to the SE during the afternoon.
Rain continues overnight.

KBOS TAF...High confidence in TAF/trends, moderate/high on
timing improvement. IFR may lift as soon as 00z but more likely
around 02z.

KBDL TAF...High confidence in TAF/trends. Improvement to VFR may
occur as soon as 00z, but more likely around 01-02z.

Outlook /Monday through Thursday/...

Monday: Mainly VFR, with local IFR possible. Breezy.

Monday Night: Mainly VFR, with local IFR possible. Strong winds
with areas of gusts up to 40 kt. Isolated SHSN.

Tuesday: VFR. Strong winds with areas of gusts up to 45 kt.

Tuesday Night: VFR. Windy with local gusts up to 30 kt.

Wednesday through Wednesday Night: VFR. Breezy.

Thursday: VFR.

&&

.MARINE...
Tonight...High confidence

Winds shifting to the SW/W by this evening as the system exits.
Winds shift to the NW overnight. The strong winds diminish
tonight as high pressure builds in. Waves 5-9 feet diminish by
Sunday morning.

Sunday and Sunday night...High confidence

Light and variable winds during the morning shift to the SE
during the afternoon and increasing as the next system moves in.
Waves 3-5 feet across the outer and southern waters, increasing
5-7 feet for a time Sunday night.

Outlook /Monday through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers.

Monday Night: Strong winds with gusts up to 40 kt. Rough seas
up to 12 ft. Freezing spray, isolated snow showers. Local
visibility 1 nm or less.

Tuesday: Strong winds with gusts up to 45 kt. Rough seas up to
14 ft. Freezing spray.

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Freezing spray.

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

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

&&

.TIDES/COASTAL FLOODING...
Tides are astronomically high Monday, with Boston at 11.0 FT
MLLW (12:36 PM), Nantucket 3.6 FT MLLW (12:02 PM), and
Providence at 5.0 FT MLLW (9:23 AM). Current expectation is
for a surge of less than one foot along the south coast, and
perhaps around 1 foot along eastern MA coast, which would keep
the total water level below the minor flood category for both
coasts.

In addition, winds are expected to shift to the W/NW in the
morning as a cold front moves offshore, lessening the potential
along E MA coast. If front ends up slowing down, it is possible
we could see minor splashover (sub-Coastal Flood Advisory level)
along both coasts, but that seems to be a worst case scenario
right now.

Does not appear to be a prolonged period of strong enough S/SE
winds to build up seas offshore more than 4-5 FT, so beach
erosion is not expected to be an issue right now either.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Watch from Monday evening through Tuesday evening for
     ANZ230>237-250-251-254>256.
     Small Craft Advisory until 1 AM EST Sunday for ANZ231-251.
     Small Craft Advisory until 7 PM EST Sunday for ANZ235-237-256.
     Small Craft Advisory until 10 AM EST Sunday for ANZ250-254-255.

&&

$$
SYNOPSIS...Loconto/BW
NEAR TERM...Loconto/BW
SHORT TERM...BW
LONG TERM...Belk/Loconto
AVIATION...Loconto/BW
MARINE...Loconto/BW
TIDES/COASTAL FLOODING...Team BOX


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