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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 061508
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1008 AM EST Sat Mar 6 2021

.SYNOPSIS...

Dry weather persists through the middle of next week. Colder than
normal through the weekend, but then a significant warm up will
lead to well above normal temperatures away from the immediate
coast by mid week. A cold front moves through on Friday bringing
our next chance of precipitation.

&&

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

950 AM Update...

Made minor adjustments to cloud cover and temperatures across
southern New England today. Have increased highs toward the 75th
percentile of guidance, which only brings up temperatures a few
degrees. Most notable difference is across the interior where
some spots get into the lower 30s.

Increased sky cover toward the latest HREF/HRRRE guidance, which
brings in some of the cloud cover from the eastern Great Lakes
this afternoon. Rest of the forecast is on track.

710 AM UPDATE...

Forecast remains largely on track so only made minor changes to
bring near-term forecast in line with latest observations.

4 AM Update...

Low pressure lingers over Labrador and Eastern Quebec, although
there are signs of weakening.  Expect continued west-northwest winds
into Srn New England with gusts of 15-20 kt, maintaining a feed of
colder than normal air and drier than normal air (dew points in the
single numbers). Mixing will reach to at least 900-mb, tapping temps
that support max sfc temperatures in the upper 20s and 30s.

&&

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

The upper low in Canada starts drifting east. High pressure from the
Midwest starts building in bringing lighter wind. But the cold dry
air will linger tonight and Sunday with dew points in the single
numbers. Light wind tonight will allow min temps in the single
numbers and teens, perhaps a few 20s along the coast. Mixing on
Sunday will bring max sfc temps generally in the 30s, possibly some
upper 20s in NW Mass.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

Highlights...

* Significant warm up for the new work week with the peak of
  warmth on Wednesday and Thursday with highs in the 60s away
  from the immediate coast.
* Next chance of any meaningful precipitation does not arrive till
  Friday with a cold front.

The new work week looks to feature a transition into a +AO/+NAO/-PNA
regime, which is favorable for a more significant and extended
period of warmth. Not surprisingly, the CPC has 80 percent
probability of above normal temperatures for Southern New England.
Along with the lack of snow cover away from the interior and
downsloping effect from strong southwesterly flow, blended model
guidance will likely be too conservative with the high temperatures
for most afternoons. Lending further confidence to anomalous warmth
during this period is that both the NAEFS and ENS Situational
Ensemble Awareness Table show heights at all levels and 1000mb
temperatures rising to 90th percentile for Tuesday and 97.5th to
climatology maxima for Wednesday and Thursday. 850mb temperatures
spike to between +9 to +11C as 572 Dm heights build into Southern
New England on Thursday. These are all above 90th percentile values
according to SPC sounding climatology for Chatham, MA. A look at the
850mb and 925mb wind fields really show the low level jet cranking
up late Wednesday into Thursday. With the precipitation and the cold
front holding off till Friday morning, Thursday could well end up
being the warmest day of the week and of the year so far. Also, with
such a strong southwesterly flow, onshore flow is not expected to be
an issue for Eastern MA coast, including Boston. The south coast,
Cape and Islands as well as locales around Narragansett Bay would be
cooler due to SSTs still in the mid 30s to low 40s.

Given such a strong signal for temperatures to overperform, have
used the 90th percentile of guidance for forecast highs for Tuesday
and Wednesday while the maximum of guidance for Thursday. Should
there be more sun than clouds on Thursday, would not be surprised to
see upper 60s or even a few 70 degree readings especially in the CT
River valley and Merrimack Valley, which tends to see temperatures
overperform due to downsloping. Forecast wise, we have mid to upper
50s on Tuesday, upper 50s to low 60s for Wednesday and low to mid 60
s for Thursday. As with a stout southwest flow, locations away from
the immediate coast will be warmest with coastal locations around 10
degrees cooler due to cold SSTs. Low temperatures overnight Tuesday
into Wednesday as well as overnight Wednesday into Thursday are also
adjusted using the 75th percentile of guidance as warm air advection
continues in earnest. It is not out of the question that we could
see 60s on Friday as well, especially locations in Eastern MA and
RI, especially if the wind shift does not occur till the afternoon
hours and the sun can break out following morning precipitation.
Then it looks like much cooler but seasonable temperatures for the
weekend.

As for precipitation chances, a passing shortwave to our north could
produce some showers on Tuesday but it does not look to produce any
meaningful precipitation. But the dry cold frontal passage would
lead to gusty conditions at times alongside with a wind shift to the
southwest, which allows the warm air advection to begin in earnest.
Then for later in the week, models have slowed down the frontal
passage to Friday so Thursday is looking to be dry. There are
indications that the wave could be deamplifying as the attendant
surface low lifts north from the Great Lakes into Labrador.
Therefore, we have kept precipitation probability no higher than
chance.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...

Forecaster Confidence Levels.

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

VFR with dry but diminishing west-northwest winds today through
Sunday. Strongest gusts today will be near 20 kt, on Sunday near 15
kt.

KBOS TAF...High confidence in TAF. VFR.

KBDL TAF...High confidence in TAF. VFR.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday: VFR.

Monday Night through Tuesday: VFR. Breezy.

Tuesday Night through Wednesday: VFR.

&&

.MARINE...

Diminishing west-northwest winds through the weekend as low pressure
moves off from Labrador and high pressure approaches from the
Midwest. Wind speeds 20 to 25 kt today, mainly on the outer
waters this morning, and around 15 kt Sunday.

Seas linger around 5 feet on the outer waters today, with 1-3 feet
closer to shore. Seas further subside from late today through Sunday
with heights generally 1 to 3 feet.

Still some areas of light freezing spray expected this morning
and again tonight. But with diminishing wind, this will be
harder to come by compared with past days.

Small Craft Advisories come down on the nearshore waters, but linger
on the outer waters today for seas and a few leftover gusts to 25
kt. Expect the last of the small crafts to expire/cancel by early
tonight.

.Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday Night: Winds less than 25 kt.

Tuesday through Wednesday: Winds less than 25 kt. Seas locally
approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 9 PM EST this evening for
     ANZ254>256.

&&

$$
SYNOPSIS...WTB/BL/Chai
NEAR TERM...WTB/BL/Chai
SHORT TERM...WTB
LONG TERM...Chai
AVIATION...WTB/Chai
MARINE...WTB/Chai