Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 232324
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
724 PM EDT Mon Oct 23 2017

.SYNOPSIS...
A few showers may develop well ahead of a slowly approaching
cold front tonight into early Tuesday morning. As the front gets
closer, expect rounds of heavy rain late Tuesday into Wednesday
along with a period of strong to damaging wind gusts possible.
A few lingering showers are possible Thursday, but the trend
towards drier, less humid and more seasonable temperatures are
expected Thursday into Friday. Above normal temperatures may
return by this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
7PM update...
Primary changes with this update will be to hold E flow along E
coasta bit later than previously forecast, allowing some of the
low clouds and fog to linger in the BOS metro. Otherwise,
gradually building of low clouds overnight everywhere. Delayed
SHRA timing a bit given overall lack of forcing, but with
increasing moisture a few could squeeze out during the AM hours.
Overnight mins were raised a bit given slow increase in dwpts
and possibly a slight non-dirunal temp increase overnight.
Otherwise, forecast on track.

Previous discussion follows...

A stubborn area of very low clouds and dense fog has lingered
from the mid coast of Maine SW to Cape Ann down to S of Boston
as seen on GOES-16 prelim, non-operational satellite loop.
Noting light E-SE winds from about KOWD-KBED-KLWM east to the
coast as well as into the seacoast of Maine, while the remainder
of the region has generally S winds at 20Z. This is likely due
to the leftover remains of a backdoor cold front that pushed
into E Mass Sunday. As the S pressure gradient increases with
the slow approach of a cold front, expect winds to finally shift
and allow the low clouds and fog to push offshore. Still some
question when this will happen, but feel it should for a time
this evening.

With some onshore wind component in place tonight, will still
see more low clouds and patchy fog, some locally dense, to push
back along E coastal Mass. Also, with dewpoints in the upper 50s
to lower 60s, will see patchy fog develop away from the coast
as well as temps approach to dewpoints, mainly after midnight.

Scattered showers will develop early tonight across western
Mass this evening, which should become more numerous after
midnight. As an H5 short wave wraps around cutoff low pressure
over the western Great Lakes, will see the vorticity line push
slowly E overnight, pushing the front eastward toward western
New England toward daybreak. Not a lot of instability with the
leading edge, so expect only showers at this point. QPF amounts
generally 0.1 inches or less across central and eastern areas,
possibly up to 0.2 inches across the E slopes of the Berkshires
as plume of higher PWAT air approaches.

The mild S winds will keep temps well above seasonal normals
overnight. Temps will bottom out in the mid 50s to lower 60s.
Could see record tying or setting high minimum temps for 10/24
at our four long term climate sites. Details will be in the
climate section.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...
Expect showers to move slowly across the region during the day
Tuesday. Very sultry airmass in place with increasing S winds.
PWAT values up to 1.6 to 1.7 inches will slide up the coast
ahead of the front. Will also see some heavier downpours develop
with this airmass mainly across western areas during the
afternoon. Heaviest rainfall looks to hold off until Tue night.

Good instability also develops across central and western areas
during Tue afternoon. TQ values increase to the upper teens,
with K indices reaching the lower-mid 30s across central and
western areas. Have mentioned some thunderstorms in the forecast
as well. Also, with strong low and mid level jet associated
with the approach surface front and mid level short wave, expect
S winds gusting up to 30-40 mph, highest along the S coast.

High temps will reach to upper 60s to mid 70s.

&&

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

* Period of strong to damaging wind gusts possible Tue into Wed

* Heavy rain will result in an urban/street flooding threat Tue-
  Wed along with a risk of isolated flash flooding

* Less humid & more seasonable weather finally return Thu/Fri

Overview and model preferences...
Blocking across the W Atlantic continually sharpens up toward
Labrador through the long term. This allows for a near constant
trof reloading pattern over the CONUS, the first of which will
be associated with the heavy rain/strong winds Tue into Wed, but
after taking on negative tilt and lifting NNE late week, a
secondary longwave trof develops in the lee of the continental
divide thanks to reinforcement of arctic influence. Will
continue with a baseline of ensembles given the highly amplified
pattern, which will take into account most solutions in the
envelope.

Details...

Tue Night and Wed...

Met overview...
A cold front becomes gradually parallel to the low-mid lvl flow
as strong upper lvl trof digs and takes on a negative tilt late
Tue night into Wed. The progression of the front across S New
England is very slow as a result, and is likely to take until
Wed evening to fully shift offshore. The parent trof has a draw
of subtropical air with PWATS reaching very close to 2.00 inches
(3 std deviations above normal). As dwpts respond to this
moisture in the low lvls, soundings become conditionally
unstable, resulting in 300-500j/kg of CAPE available. Finally, a
strong LLJ, which peaks at about 60 kt at H92 and near 70 kt at
H85 shifts N-S through the area late night into early Wed. This
will lead to a very unsettled, prolonged period of wet wx,
potentially exceeding 24 hrs.

Wind/severe threat...
LLJ peaks late Tue night into early Wed, with H92 speed values
around 60 kt even on the typically more conservative ECMWF/EPS
members. Combination of precip drag and some BL mixing thanks to
conditionally unstable lapse rates will likely lead to some of
this momentum reaching the ground. Widespread 35-45 mph wind
gusts are likely, even into interior New England. However, 50-60
mph are also possible, especially across SE MA/RI where the
core of the LLJ peaks. With some instability, thunderstorms,
and/or even a fine line of convection will more efficiently mix
these winds down, potentially leading to localized higher impact
damage. While this is possible anywhere in New England, highest
winds will still be featured across the SE. Wind advisories
will be hoisted, and given fully leaved trees, will highlight
all of S New England for this risk, but still monitor for
localized higher impacts in thunderstorms/fine convective line.
This coincides with the SWODY2 upgrade to a slight risk, given
localized gusts could produce more damage in convection. Peak
will be overnight into Wed morning. One final note, cannot rule
out a brief spinup given the high shear (helicity 300+) and low
LCLs within the moist airmass. This is a very low/localized
risk.

Heavy rain...
High PWATs and the influence of convective potential could
yield pockets of heavy rain with rainfall rates exceeding 1in/hr
at times. The combination of the slow movement of the front,
and strong S LLJ suggests a risk for training heavier cells,
which could lead to prolonged periods of high-rate rainfall. A
slight E component to the low lvl flow could lead to enhanced
upslope in E slopes of the Berkshires and Litchfield hills. Even
though dry, with FF guidance nearly 3-4 inches/hr, the long
timeframe could exceed 3 or 6 hr FF guidance. Flashier streams
will see a risk for flooding. Therefore, will be hoisting a FF
Watch with this update mainly for W portions of the forecast
area, where the enhancement is most apparent. Further E,
training/heavy rain is still an issue, but the risk would be
more in the form of urban/poor drainage flash flooding,
exacerbated if stripped leaves clog drains. We may need to
expand the FF watch for this risk in later updates. Overall
widespread 1.5-2.5 inches are expected with localized 3-4 inches
certainly possible.

Thu...
Dry air moves in through the column aloft first, trapping some
lower lvl moisture into the day on Thu. The combination of this,
some cold advection and strong cyclonic curvature aloft suggest
a risk for lingering clouds and SHRA through much of the day,
but not nearly as widespread as the previous 24-48 hours. H92
temps drop to about +6C on average, so expecting seasonable
temperatures with highs mainly in the upper 50s and low 60s
while weak flow overnight and gradual clearing should allow mins
to drop into the 40s.

Fri and Sat...
Dry period as weak mid lvl ridging forms between the exiting
cutoff/trof to the NE and deepening longwave trof shifting E
into the prairies. H92 temps hover around +9C to +10C each day,
suggesting highs will be seasonably warm, mainly in the mid
60s, although overnight lows will be seasonable thanks to some
radiational cooling especially Fri night. Dwpts in the low-mid
40s during the afternoon suggest these will be near the
overnight lows.

Sun into Mon...
With a very amplified pattern timing/mesoscale features are not
resolved well this far out, but given a similar setup with a
gradually tilting trof with some subtropical moisture associated
with it, will have to watch for another round of low pres with
widespread rainfall. Will favor the slower timing of ensembles
given the meridional nature of the pattern. At the very least,
another unsettled wet period is possible early next week.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/....

Tonight...Moderate confidence.
Mainly VFR outside of BOS which will remain IFR as long as winds
remain E to start the night. Once winds shift around to the S it
will improve at BOS, but CIGS will gradually lower everywhere
else. MVFR CIGS possible after 04Z, widespread by 08Z except
Cape/Islands. Some SHRA, mainly W MA/CT overnight with reduced
vsbys. Winds increase out of the S, with gusts 20-25 kt mainly
after 06Z.

Tuesday...High confidence in trends. Moderate confidence in
timing.
Expect mainly MVFR-IFR conditions in low CIGS and areas
of fog. Expect areas of -SHRA/SHRA to push slowly into the
region during the day. Rain heavy at times late in the day
across W areas. Occasional TSRA at times Tue afternoon. S-SE
winds gusting up to 25-30 kt, possibly to 40 kt in any
thunderstorms. Highest gusts along S coast, Cape Cod and the
islands. Areas of LLWS likely, mainly after 15Z.

KBOS Terminal...Moderate confidence in TAF.
Mainly IFR-LIFR CIGS and IFR VSBYS lingering through at least
04Z, possibly later. E winds up to 10 kt should shift to SE-S
overnight. Remaining IFR during at least early portion of the
morning push.

KBDL Terminal...Moderate confidence in TAF.
VFR through 00Z, then CIGS lowering to MVFR. May see MVFR-IFR
VSBYS in scattered showers by around 10Z or so. S winds gusting
up to 20-25 kt by around midday Tuesday.

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Mainly IFR, with local MVFR possible. Strong
winds with gusts to 40 kt. RA, chance TSRA, patchy BR.

Wednesday: Mainly VFR late, with local IFR possible. Windy with
gusts to 35 kt. RA, patchy BR.

Wednesday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA, chance RA, patchy BR.

Thursday: VFR. Breezy. Chance SHRA.

Thursday Night through Friday: VFR.

Friday Night: VFR. Patchy BR.

Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Have converted Gale Watches to Warnings with this package for
all waters.

Expect increasing S-SE winds tonight, gusting up to 20-25 kt
after midnight. Seas build up to 5-6 ft. Winds and seas highest
on the southern waters. Reduced visibility in areas of fog.
Scattered showers across the southern waters late tonight.

On Tuesday, S winds continue to increase, with gusts up to
30-35 kt. Seas continue to build, up to 6-9 ft. Visibility
restrictions develop in areas of locally heavy rain and patchy
fog. Scattered thunderstorms possible. Gale force wind gusts
likely Tue afternoon across most of the waters.

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Gale force winds with gusts up to 45 kt. Rough
seas up to 11 ft. Rain, chance of thunderstorms, patchy fog.
Local visibility 1 to 3 nm.

Wednesday: Gale force winds with gusts up to 40 kt early. Rough
seas up to 12 ft. Rain, chance of thunderstorms, patchy fog.
Local visibility 1 nm or less.

Wednesday Night: Winds less than 25 kt. Rough seas up to 11 ft.
Rain showers likely, rain, patchy fog.

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers.

Thursday Night: Winds less than 25 kt. Local rough seas.

Friday: Winds less than 25 kt. Seas up to 5 ft.

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for
     CTZ002>004.
     Flash Flood Watch from Tuesday evening through Wednesday
     morning for CTZ002.
MA...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for
     MAZ002>024-026.
     Flash Flood Watch from Tuesday evening through Wednesday
     morning for MAZ002-003-008>011.
RI...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for
     RIZ001>008.
MARINE...Gale Warning from noon Tuesday to 11 AM EDT Wednesday for
     ANZ231>234.
     Gale Warning from 8 PM Tuesday to 9 AM EDT Wednesday for
     ANZ230-251.
     Gale Warning from 2 PM Tuesday to 8 AM EDT Wednesday for
     ANZ236.
     Gale Warning from 11 AM Tuesday to 8 AM EDT Wednesday for
     ANZ235-237.
     Gale Warning from noon Tuesday to 2 PM EDT Wednesday for
     ANZ250-254.
     Gale Warning from 11 AM Tuesday to 11 AM EDT Wednesday for
     ANZ255-256.

&&

$$
SYNOPSIS...Doody/EVT
NEAR TERM...Doody/EVT
SHORT TERM...EVT
LONG TERM...Doody
AVIATION...Doody/EVT
MARINE...Doody/EVT



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