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FXUS61 KBOX 201110
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
710 AM EDT Tue Apr 20 2021

.SYNOPSIS...
Cold front slides through southern New England today bringing
gusty winds and low relative humidities. Cannot rule out an
isolated shower. Better chances of showers and thunderstorms on
Wednesday as a cold front moves through. Windy and much cooler
weather follows behind the front late Wednesday night into
Thursday, then a warming trend Friday and Saturday. A coastal
storm may bring rain and coastal wind Sunday with improving
conditions next Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
710 AM update...

Patch of mid clouds moving across western MA with more clouds
upstream. Expect bkn cu to develop later this morning into the
afternoon and can't rule out an isolated shower assocd with
convergence along a weak front moving into the region. Current
forecast on track. A mild and breezy day.

Previous discussion...

Highlights

* Elevated Fire Weather concerns across much of southern New England
due to gusty winds coupled with low relative humidities.

* Cannot completely rule out an isolated shower as a cold front
slides through.

Closed off low over eastern Ontario/western Quebec will lift into
northern Quebec later today. A cold front will slide through
southern New England as the day progresses.

Main change to the forecast from the previous shift was to increase
wind speeds and gusts across southern New England. Anticipating
strong mixing per the RAP/NAM/NAMNest and GFS Bufkit soundings.
Should see the mixed layer grow to roughly 750-700 hPa depending on
the model of choice. So, could potentially tap into some 40-50+ kt
gusts if we can fully mix. Have leaned on the HRRRE/NAMNest guidance
in the latest update to increase wind speeds and gusts, which brings
25-30 kt gusts in. Cannot completely rule out a few stray gusts
getting into that higher territory, especially if a heavier shower
or thunderstorm moves through.

Given the strong mixing have decreased the dewpoints and increased
temperatures. Went with the 90th percentile of guidance for high
temperatures today and the 10th percentile of guidance for
dewpoints. High temperatures top out in the low to mid 70s across
much of the region. The only exception is along the south coast,
Cape Cod and the Islands where temperatures range from the upper 50s
to mid 60s. Minimum relative humidities fall into the 25 to 40
percent across the interior and 45 to 60 percent along the south
coast.

Given the cold front working its way through have kept an
isolated shower mention in the forecast. Thinking that the upper
forcing is a bit weak given how far away the H5 cutoff is, but
given the CVA and PWATs in the 0.5 to 0.75 inch range with the
FROPA...think an isolated shower cannot be ruled out.

&&

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

* Warm front lifts through southern New England tonight and cold
front moves through majority of the region by late in the day.
This will bring increasing chances of showers. Expecting some
thunderstorm activity on Wednesday. Could have a few isolated
damaging wind gusts.

Weak mid level ridging over the eastern Great Lakes/New England
tonight while a trough digs into the central Great Lakes. The
trough will lift into the eastern Great Lakes/Mid Atlantic on
Wednesday. At the surface a warm front will lift through
southern New England tonight. Low pressure will lift into New
England and the cold front associated with it will slide
through.

Tonight...

Increasing cloud cover as a warm front lifts into and through
much of southern New England. Models still trending to bringing
in precipitation slower, so have slowed down the precipitation
onset. If there is any precipitation that falls it should be
relatively light. Expect a mild night with 925 hPa temperatures
in the +6 to +10 degree range. Low temperatures range form the
mid 30s to the mid 40s.

Wednesday...

Another gusty day on tap for southern New England within the
warm sector and ahead of an approaching cold front. Expecting a
strong S/SSW low level jet with 925 hPa wind speeds of 35-45
kts. Expecting low level lapse rates between 7 to 9 degrees
Celsius per/km and should be able to mix down the 25-30 kt gusts
fairly easily. This may need to be increased in future updates
if the mixed layer grows a bit more.

Within the warm sector am anticipating temperatures to get into
the 60s across much of southern New England and potentially a
few 70 degree readings with the strong mixing. Dew points will
climb into the upper 40s to the low 50s with southerly flow. Do
have a window where there is roughly a few hundred to 1000 J/kg
of MLCAPE with 0-6 km bulk shear in the 35-60 kt range with the
shear vectors being a bit more parallel to the cold front moving
in. This suggests a linear convective mode, which is what
several CAMs suggest. However, the window to tap into the
instability appears to be short lived and the line of
showers/storms could have a weakening trend as it moves in from
the west. Will be something to keep an eye on as there could be
a fine line, and given the environment cannot rule out some
isolated damaging wind gusts. The SPC risk has shifted the
marginal risk northward in the latest update and just on the
periphery of our CWA. This makes sense given the latest CAMs and
trends models show with the line sliding in.

The upside with the front moving in is we will receive more much
needed rainfall. Right now expecting roughly 0.1 to 0.75 inches
of rain on Wednesday, though these amounts will be higher
especially if there are any thunderstorms that move through.

&&

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

* Showers/isolated t-storms exiting eastern New Eng Wed evening
* Windy and much cooler late Wed night and Thu
* Dry with moderating temps Fri/Sat
* Coastal storm may bring rain and coastal wind Sunday

Wednesday Night...

Surface wave will be lifting north of the region Wed evening with
attendant cold front moving offshore. Lingering showers and isolated
t-storms assocd with deep moisture plume and marginal elevated
instability along the front will be exiting eastern New Eng during
the evening. Behind the front, strong cold advection will result in
gusty W/NW winds developing and much cooler temps as 850 mb temps
drop to near -10C by 12z Thu. Lows will will fall to the low/mid 30s
with upper 20s over higher terrain. Some upslope snow showers may
spill across the eastern slopes of the Berkshires during the late
evening and overnight.

Thursday...

Closed upper low will move east from northern New Eng with cyclonic
flow across the region. Other than a few flurries or snow showers in
the Berkshires and Worcester hills, mainly dry conditions expected
with a mix of clouds and sun as bkn strato-cu develops given shallow
low level moisture and steep low level lapse rates. Gusty winds and
much cooler temps will be the main story for Thu. Soundings show a
deep and well mixed boundary layer with potential for gusts 40+ mph
with a lower risk for gusts to 50 mph. A wind advisory may
eventually be needed. Temps will range through the 40s but the winds
will make it feel in the 30s.

Friday and Saturday...

Height rises through Sat as upper trough exits with shortwave
ridging moving in. High pres builds south of New Eng with dry
weather and moderating temps. Highs should get back to near 60 on
Fri in the coastal plain and interior valleys, warming to 65-70 Sat.
It will still be a bit breezy on Fri.

Sunday into Monday...

While specific details and timing are uncertain at this time range,
deterministic and ensemble guidance indicating a potential coastal
storm may impact SNE on Sunday with rain and coastal wind.
Conditions improving by Mon but there is uncertainty with how
quickly the storm will exit and whether showers may linger into Mon.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels.

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

12z update...

Today...High confidence.

VFR conditions expected with gusty WSW to SW winds. Most areas
will see speeds of 10-15 kts and gusts of 25-30 kts. Not
completely out of the question for a few 35+ kt gusts especially
if any heavier showers move through. Though am thinking the
shower activity will be isolated in nature. Have not included a
mention in the latest set of TAFs as am uncertain on how
widespread the coverage will be. Expecting the mixed layer to
decouple between 22-01Z, which will bring an end to the gusty
winds.

Tonight..High confidence.

VFR conditions expected with increasing clouds. Should see cloud
bases generally between 5-10 kft. Scattered showers possible
after midnight for far western areas after midnight. Light winds
shifting to the S.

Wednesday...High confidence in trends moderate in timing.

Conditions deteriorating from VFR to start to MVFR/IFR and maybe
some localized LIFR as showers and thunderstorms move through. S
winds of 10-15 kts and gusts of 25-30 kts. Winds shifting to the
SW/W across western areas late in the day as a cold front slides
through.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Mainly VFR, with areas MVFR possible. Windy
with gusts up to 35 kt. Chance SHRA.

Thursday: VFR. Strong winds with local gusts up to 40 kt.

Thursday Night: VFR. Windy with gusts up to 30 kt.

Friday through Saturday: VFR. Breezy.

&&

.MARINE...
Today...High confidence

Have a period of time this afternoon into the evening as a cold
front approaches that there will be gusty SW winds at 15-20 kts
and gusts of 25-30 kts at times. Have hoisted a Small Craft
Advisory as a result.

Tonight...High Confidence

Southwest winds diminish and shift to a southerly direction.
Waves 2-4 ft.

Wednesday...High confidence

Increasing southerly winds and gusts as the day progresses as
another cold front moves in. Cannot rule out a few rumbles of
thunder later in the day.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Strong winds with gusts up to 40 kt. Areas of
rough seas. Chance of rain showers.

Thursday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft.

Thursday Night: Strong winds with gusts up to 35 kt. Rough seas
up to 11 ft.

Friday: Strong winds with gusts up to 30 kt. Rough seas up to
10 ft.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Watch from late Wednesday night through Thursday evening
for ANZ231>235-237-250-251.
Small Craft Advisory from 1 PM this afternoon to 8 PM EDT this
evening for ANZ230>237-250-251-254>256.
Gale Watch from Thursday morning through Thursday evening for
ANZ230-236.
Gale Watch from late Wednesday night through Thursday evening
for ANZ254>256.

&&

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