Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 111421
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1021 AM EDT Sun Apr 11 2021

.SYNOPSIS...
A backdoor cool front advances through Southern New England this
morning, spreading cloudy skies cooler onshore flow across the
region today into Monday. Meanwhile, a frontal system passing to
our west will also bring a period of light rain for the
interior later today. Two weather systems pass Southern New
England, one on Tuesday and the other Thursday- Friday. Each
will bring showers, but with light amounts.

&&

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

1015 AM Update...

Let the Dense Fog Advisory expire as vsby has improved. Mesowest
obs shows the backdoor cold front reach the CT River valley
with Day Cloud Phase satellite imagery showing the associated
low stratus well. Forecast remains largely on track. Main change
with this update was to bump up temps along the CT River valley
given the partial clearing (a narrow time frame between mid
level clouds pivoting in from the west and the low stratus
moving in from the east) and the fact that it doesn`t take much
sunshine in mid April for temps to pop. Also lowered the PoPs
given current radar trends, with better forcing for meaningful
precip remaining to our south and west. Overall, it will be a
rather raw and dreary day with the occasional mist and drizzle
after the anomalous warm stretch of weather. However, for
perspective, the normal highs for this time of the year is in
the mid to upper 50s. So we are actually right about average for
this time of the year.

PREVIOUS DISCUSSION...

645 AM Update...
Backdoor cold front at 6 AM had moved through Boston Metro-west
area. May take until midday to get through the remainder of Srn
New England. Observations showed lots of low clouds and areas of
fog. The fog will thin out, but the clouds will linger for a few
days.

No significant changes to the forecast. Continue to expect the
dense fog along the South Coast to thin around 10 AM or a little
after.

We hope you had an opportunity to enjoy the sunny and mild
weather pattern over the last handful of days. Starting today, a
pattern change towards cloudier, cooler and frankly downright raw
weather conditions takes hold of Southern New England at least
into early in the workweek.

Several features of note this morning. Weak high pressure is still
producing SW winds across most of Southern New England. This is
allowing for still quite mild temperatures away from the southern
coastline with temperatures in the mid 50s! SW winds have also
permitted locally dense fog to return back northward across the
South Coast into Cape Cod, the Islands and adjacent waters. Given
visibility observations and after coordination with NWS New York, a
dense fog advisory has been hoisted thru 14z for southern RI into
coastal SE MA and Cape Cod/Islands. The primary feature of interest
is a backdoor cold front which now extends from Laconia-
Concord- Manchester-Nashua line in NH on just south of Lawrence
and Beverly in MA. The front is pretty clearly evident in METARs
with a stratus field apparent in nighttime microphysics RGB
imagery into these same areas. Fairly substantial airmass change
lies behind the front - temperatures at Beverly fell some 10
degrees between 06 to 07z and winds flipped to NE upon its
passage. The other notable feature, more so for later today into
tonight, is a closed upper low over southern MI which has a NW-
SE oriented rain shield from NW PA southeast into the Delmarva
Peninsula.

So for today, we are looking for increasing clouds which will mainly
be in the form of stratus, along with cooling temperatures and
lowering dewpoints. Winds will also shift to NE and increase with
speeds increasing to around 10-15 mph. This will occur as the
backdoor front surges southwestward across Southern New England. The
air mass change will be quite noticeable, and certainly compared to
the last few days where many areas were in the 60s to the 70s. There
may be a few hours of some heating to offset this across western MA
and parts of northern CT. However many areas will see hovering to
falling temperatures through most of if not all of today. Have again
utilized higher-res guidance to best capture the expected fall in
temperatures. This translates to early-day highs across eastern and
central MA into much of RI, and highs occurring around late morning
to midday across western MA into northern CT in the upper 50s to
lower 60s before falling. Upon passage of the backdoor front in
eastern/central MA into RI, most areas will struggle to reach 50
degrees. The onshore winds will also make for a rather raw day.

Late in the day, the rain shield now over the mid-Atlantic into the
eastern Gt Lakes progresses NE into CT and into western MA.
Soundings again show some dry mid-level air that needs to be
overcome first, but have offered PoPs increase into the solid Likely
range between 5-7 PM here, while areas along and northeast of a
Worcester to Providence to Newport line may be tough to see much of
any precip. PoPs decrease to the NE into the Chance to Slight Chance
range.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Tonight into Monday:

In essence, really more of the same general raw and cloudy
conditions. Shallow PBL cooling continues to take place which will
help to lock in areas of stratus and mist, along with slowly cooling
temperatures. Some differences in the models with regard to how the
evolution of the rain shield is handled. Most of the synoptic models
more or less fractures the band as it tries to lift northeast, but
the higher res guidance sends the steadiest rains on northeast into
NH/ME. Stuck with the synoptic models depiction with rains more or
less confined to our western and southwestern areas with PoPs more
in the Chance range. Slow cooling down to rather uniform lows into
the low to mid 40s, with a continued though lighter easterly onshore
wind.

Still cloudier than not for Monday, but there is some optimism early
in the day for some breaks in clouds (towards more broken lower
clouds). However a reinforcing backdoor front and renewed
enhancement to onshore flow should again allow for lower clouds to
fill back in into the afternoon. In addition, as the closed low over
the eastern Gt Lakes moves east, it may induce another period of
steadier rains across western MA into northern CT. Temperatures will
again struggle to climb much if at all - in fact the 3-hourly temps
on the MAV/MET guidance show maybe a degree or two of warming if any
occur at all. Thus, looking for highs in the mid to upper 40s,
perhaps near 50 in the CT Valley. East to northeast winds will pick
up again to speeds around 10-15 mph with strengthening easterly
pressure gradient.

&&

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

A series of closed upper lows stretch across the USA Northern Tier,
but dominated by one closed low over the Great Lakes. A part of this
upper low ejects south of New England Tuesday. The bulk of the
system over the Great Lakes then moves east over the Northeast USA
late in the week. An upper ridge then builds from the Southern
Plains into the Northeast and bringing drier weather for the
weekend.

Upper contours are forecast below long-term normal midweek, then
trend to normal by late week. Surface flow varies between northeast
and north during this time, reinforcing the expected cold pattern
midweek that moderates to around normal late week.

The various model mass fields are similar through midweek, but then
diverge late week. Forecast confidence is low-moderate.

Details...

Monday night through Wednesday...

Surface low associated with the ejecting fragment from the Great
Lakes low moves south of New England. This maintains the low-level
northeast flow...a marine flow...over Eastern MA and RI. The
position of this coastal upper low favors a few showers over Western
MA, Northern CT, and parts of RI. The northeast flow favors
scattered showers over Eastern MA. Put it together, it suggests a
period of scattered showers. Although QPF placement varies model-to-
model, all clearly indicate light precip...generally less than one-
tenth inch.

The coastal system moves off to the southeast on Wednesday.
Lingering showers in the morning should diminish...will indicate
slight chance pops in the afternoon.

Mixing profiles are shallow Tuesday and Wednesday. This suggests
Tuesday highs upper 40s near the east coast and lower 50s
inland...Wednesday highs in the 50s. Low level temps are
consistent with mins in the upper 30s and low 40s.

Thursday through Saturday...

The Great Lakes upper low ejects southeast to the Mid-Atlantic
coast. The 00Z ECMWF shows significant rainfall Thursday and
Friday. The 00Z GFS shows lighter precip over us and heavier
amounts staying offshore to the south. The GFS also shows an
intense surface system spinning up with a tight pressure
gradient over the ocean waters to our south, but a lighter
gradient over Srn New England. GFS shows northeast winds of
40-50 kt between 900 and 950 mb, supporting wind gusts of 25-30
kt. Blending the precip suggests a chance of showers both
Thursday and Friday with the best chance in areas south of the
Mass Pike.

The system pulls away during Friday. Eastern Canada high
pressure then builds in with dry weather for Saturday.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels.

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

Today: Moderate to high confidence.

IFR-LIFR stratus for most in NE onshore flow, with fog for the
southeastern airports. May be a period into the aftn where
ceilings are more IFR than LIFR. Better confidence in categories
across central and eastern airports where NE flow should be more
established. Have lower confidence in the TAFs at BDL and BAF,
where VFR/MVFR stratus may be more probable than IFR. Late in
the day (after 19z), shield of VFR-MVFR vsby rain begins to
overspread BAF, BDL and perhaps into ORH/PVD. Remaining TAFs
should be dry.

NE to E winds increase to 8-12 kt interior sites; around 10-15
kt near the eastern MA coast, with gusts up to 20 kt.

Tonight: Moderate to high confidence.

IFR-MVFR cigs early, with intervals of MVFR mist. MVFR-IFR more
likely western airports. Appears modest improvement toward
MVFR- VFR possible overnight for the eastern airports, but
confidence here is lower given the persistent ENE/NE flow which
tends to keep categories lower. NE/ENE winds around 7-10 kt.

Monday: Moderate confidence.

IFR-MVFR early, deteriorating to IFR-LIFR ceilings as
enhancement to NE flow resumes. Timing of deterioration is still
uncertain but currently anticipate degradation into the aftn
hrs. NE winds 8-14 kt, gusts near 20 kt along the eastern MA
coast.

KBOS TAF...Moderate confidence in TAF. LIFR-IFR ceilings
developing around pre-dawn (08-10z) then may trend IFR by late-
AM.

KBDL TAF...Moderate confidence in TAF. VFR-MVFR early, IFR
possible but less certain.

Outlook /Monday Night through Thursday/...

Monday Night: MVFR/IFR conditions possible. Breezy. Slight
chance SHRA.

Tuesday: Mainly MVFR, with local IFR possible. Chance SHRA.

Tuesday Night: Mainly MVFR, with local IFR possible. Chance
SHRA, patchy BR.

Wednesday: Mainly MVFR, with areas VFR possible. Breezy. Slight
chance SHRA.

Wednesday Night: Mainly VFR, with areas MVFR possible. Slight
chance SHRA.

Thursday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA.

&&

.MARINE...
Small craft advisories remain in effect for later today into Monday
on the southern offshore waters, as seas project to build to 5-6 ft
and easterly flow becomes enhanced. SCAs will also need to be
considered for Monday as building seas over a generally persistent
easterly fetch should bring SCA conditions to the eastern waters as
well.

Initial SW winds will transition to NE and increase to speeds 10-15
kt with gusts in the low-20s kt from N to S today. Seas generally in
the 2-4 ft range for most waters except 5-6 ft on the southern
offshore waters.

For tonight, expect a decrease in NE/E winds on the eastern waters
to around 15 kt gusts, while gusts on the southern offshore waters
should reach into the lower-end SCA levels (25 to near 30 kt). Seas
2-3 ft eastern waters, increasing to 5-7 ft on the southern offshore
waters.

Into Monday, enhancement to NE/E flow should bring gusts to near or
over SCA levels on most waters. Seas will also build further into
the 5-7 ft range offshore and around 2-4 ft nearshore.


Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

Tuesday through Tuesday Night: Winds less than 25 kt. Local
rough seas. Chance of rain showers.

Wednesday through Wednesday Night: Winds less than 25 kt. Areas
of seas approaching 5 ft. Slight chance of rain showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 5 PM this afternoon to 8 PM EDT
     Monday for ANZ254>256.

&&

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


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