BOX issues Area Forecast Discussion (AFD) at Oct 20, 1:06 AM EDT046
FXUS61 KBOX 200506
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
106 AM EDT Mon Oct 20 2025
.SYNOPSIS...
A strong frontal system arrives early Monday morning
bringing a quick-moving line of heavy rain and strong winds.
The front moves offshore Monday afternoon with drying conditions
and continued breezy winds. Drier weather returns for Tuesday, but
a stronger cold front for Wednesday ushers in a cooler and
unsettled weather pattern for late in the workweek into Saturday.
&&
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Key Points:
* Few showers/drizzle possible early Monday morning ahead of
approaching frontal system.
Warm and moist air advects into the region tonight out ahead of an
approaching shortwave trough. Cloud cover increases with lower
clouds developing in early Monday morning. This will keep overnight
temperatures on the warmer side in the mid to upper 50s. Winds aloft
steadily increase as a southerly LLJ (40-50 kts) shift into southern
New England. Lack of forcing/CAA should keep the higher winds from
mixing to the surface overnight, but gust 20-30 mph are possible
overnight. High resolution model guidance shows a general consensus
that the main batch of rain/embedded thunderstorms holds off until
4/5am in western southern New England. Can't rule out some showers
or a light drizzle ahead of the main line.
&&
.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
Key Points:
* Strong frontal system moves through region Monday morning.
* Line of heavy rain/embedded storms accompanied by a brief period
of strong to potentially damaging wind gusts.
* Rain arrives in western MA/CT 5-6 am and eastern MA & RI around 8-
11am.
* Potential for very minor coastal flooding for south coast,
especially with AM high tide.
Details...
Monday:
The main forcing from the shortwave trough and surface front arrives
early Monday morning, quickly pushing from west to east through the
morning and early afternoon timeframe. The main line of heavier
rain/embedded thunderstorms will accompany the shortwave and front
along with a period of strong gusty winds. As for timing of the
line, high res model guidance is in good agreement with the general
timing of the main line. The main line will likely arrive in western
MA/CT 5-6am and reaching eastern MA & RI 8-11am. The main batch of
rain will likely exit offshore mid-afternoon.
There is potential for impacts as this line of rain/gusty winds
pushes across the region. Although quick moving, the shortwave
will be potent with strong surface forcing/convergence and
therefore pack a "quick punch" as it moves through. A strong
southerly 40-50 kt LLJ positions over the region helping
transport a plume of moisture (180- 220% of normal) into the
region. Model soundings show sufficient warm cloud depths to
support efficient rain processes. Most model guidance also
indicates marginal instability with MUCAPE values 200- 400 J/kg.
Together, this should support a batch/line of widespread rain
with embedded heavier rain and a few thunderstorms. This will
favor a period of heavy rain rates in areas as the line goes
through, potentially exceeding 1-2"/hour. The positive is that
this should move through quickly, but the heavy rates may impact
the morning commute and/or cause minor nuisance flooding in
urban/poor drainage areas. Strong to potentially damaging winds
will also accompany this line. The question will be how much of
the winds aloft from the strong LLJ will translate to the
surface level. High- res guidance has been aggressive with a
period of strong gusts accompanying the line; however, the early
AM timeframe may limit efficient mixing. The strong forcing
should be enough to bring at least gusts 30-40 mph along the
line. Can't rule out localized damaging gusts 50-60 mph within
any stronger cells/storms. It will likely remain breezy behind
the line/front with SW/WSW winds with gusts 20-25 mph.
Temperatures will likely not follow a true diurnal pattern.
Temperatures top out in the upper 50s to mid 60s in the morning, but
are expected to slowly fall through the afternoon/early evening as
cold air continues to advect in. A secondary push of colder air late
afternoon-early evening will switch winds more W. There is a
weak signal for a period of breezy winds along the higher
terrain and downslope of the Berkshires late afternoon-early
evening with gusts 25-35 mph.
Monday Night:
No concerns for impacts Monday night with the frontal system exited
to the east. Winds/gusts gradually decrease Monday night, although
should be slowly to decrease across the waters. Temperatures drop
into the the 40s.
&&
.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Key Messages:
* Drier Tue but a mix of clouds and sun with seasonable temps in the
60s.
* Rainy conditions with southerly breezes for Wed AM as a cold front
moves through.
* Cooler and more unsettled for late in the week into early weekend.
Details:
Shortwave trough at 500 mb to open Tue transitions rather
quickly to a period of brief height rises/ridging aloft.
Increasing SW breezes and warming temps will be met with full
sun and dry weather, with highs in the 60s. Tue seems likely to
be the pick of the workweek weather-wise.
Another rather strong cold front moves in on Wed, and while there is
still some uncertainty on the timing of the front, guidance
continues to favor an earlier-in-the-day passage similar to the
frontal system for Mon (early AM to off the coast by early PM).
Winds don't look as strong as the Mon frontal system, and there
isn't as much moisture to work with either, so impacts could be
lesser than the frontal passage on Mon. Cloudy conditions with a
period of breezy/gusty showers again for Wed.
A cooler and unsettled weather pattern then establishes itself
across the Northeast underneath deep cyclonic flow aloft late this
week into the early part of the weekend. Expect partly cloudy
conditions for most, with partly/mostly cloudy conditions western
New England and temps running a couple degrees cooler than normal in
the 50s. Couldn't rule out a passing shower underneath some of the
stratocu at times but the vast majority of this period is dry.
&&
.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
06z TAF Update:
Today: High Confidence
CIGS will gradually drop to MVFR ahead of the frontal system
approaching for this morning. Said frontal system will bring a
round of heavy rain, Strong winds in excess of 30-40 knots, and
IFR/LIFR CIGS. Guidance is in good agreement with the timing of
the heavy rain, reaching the CT river valley between 10-12z, and
reaching the I-95 Corridor between 13-17z. There could be a
rumble or two of thunder with the line of heavy rain, however,
confidence in thunder is low, so opted to use shra vs tsra.
CIGS behind the front are a bit more uncertain, but will
generally be a high end MVFR to low end VFR with breezy SSW
winds up to 25 knots.
Tonight: High Confidence
VFR. WSW winds at 10 to 15 knots. Wind shear at 2kft around 30
knots from the west
Tuesday: High Confidence
VFR. Decreasing southerly winds.
KBOS Terminal...High confidence
VFR CIGS will gradually become MVFR this morning ahead of the
front. The heavy rain is anticipated to arrive around 16z with
IFR/LIFR CIGS and strong winds gusting 35-40 knots. After the
heavy rain moves out, CIGS should rebound quickly to around the
VFR/MVFR line. Winds behind the front remain gusty from the S
at 15-20 knots
KBDL Terminal...High confidence
VFR CIGS will gradually become MVFR this morning ahead of the
front. The heavy rain is anticipated to arrive around 11z with
IFR/LIFR CIGS and strong winds gusting 30-45 knots. After the
heavy rain moves out, CIGS should rebound quickly to around the
VFR/MVFR line. Winds behind the front remain gusty from the S
at 15-20 knots
Outlook /Tuesday Night through Friday/...
Tuesday: VFR. Breezy.
Tuesday Night: Mainly VFR, with local MVFR possible. SHRA
likely.
Wednesday: Mainly VFR, with areas MVFR possible. Breezy. SHRA
likely.
Wednesday Night: VFR. Breezy. Slight chance SHRA.
Thursday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.
Thursday Night: VFR. Slight chance SHRA.
Friday: VFR.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Through Monday Night...High confidence.
Key Messages:
* Dangerous Marine conditions expected Monday morning to early
afternoon
* Short period of heavy rain with potential for gale force
winds possible.
Winds continue to increase overnight, gusting 20-30 knots. A
strong line of showers and thunderstorms will approach the
waters from the west Monday morning. This line is expected to
bring a brief period of heavy rain and strong winds near gale
force. Given the short duration, likely under 2 hours, this
line will best be handled with short-fused special marine
warnings rather than longer-term gale warnings. Nonetheless,
dangerous marine conditions can be expected on Monday morning.
Seas increase to 4-8 feet behind the line and winds remain,
turning SW at 20-30 knots for Monday afternoon.
Winds continue to decrease Monday evening, though may be slower
to decrease across the southern outer waters. Seas 6-8 ft
decreasing early Tuesday morning.
Outlook /Tuesday Night through Friday/...
Tuesday: Winds less than 25 kt. Local rough seas.
Tuesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers.
Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft. Rain showers.
Wednesday Night: Winds less than 25 kt. Local rough seas.
Chance of rain showers.
Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Chance of rain showers.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers.
Friday: Winds less than 25 kt. Slight chance of rain showers.
&&
.TIDES/COASTAL FLOODING...
330 PM Update:
South-southeast winds will increase tonight and into Monday morning
along the southern and eastern coasts. We are most concerned for the
southern coast around the Monday morning high tide around 8 AM. At
Fox Point/Providence, the astro tide is around 5 ft MLLW. Storm
surge values early Sunday morning were as high as 1.5 ft at Fox
Point/Providence, and it stands to reason that as southeast winds
increase, storm surge values should be in the 1.5 to 2 ft range
along portions of the southern coastline.
Though splashover with limited overall impact is the most likely
outcome for the RI/MA South Coast, we opted for a coastal flood
advisory in case we do see a storm surge as high as 2 ft, which
would result in some minor inundation on vulnerable shoreline roads.
But overall, we expect only minor impacts.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Coastal Flood Advisory from 5 AM to 11 AM EDT this morning for
MAZ020-021.
RI...Coastal Flood Advisory from 5 AM to 11 AM EDT this morning for
RIZ002-004>007.
MARINE...Small Craft Advisory until 4 PM EDT this afternoon for ANZ230-
236.
Small Craft Advisory until 6 PM EDT this evening for
ANZ231>234.
Small Craft Advisory until 5 AM EDT Tuesday for ANZ235-237-250.
Small Craft Advisory until 8 PM EDT this evening for ANZ251.
Small Craft Advisory until 8 AM EDT Tuesday for ANZ254-255.
Small Craft Advisory until 8 AM EDT Tuesday for ANZ256.
&&
$$
SYNOPSIS...Loconto/Mensch
NEAR TERM...Mensch
SHORT TERM...Mensch
LONG TERM...Loconto
AVIATION...KP
MARINE...Loconto/Dooley/Mensch
TIDES/COASTAL FLOODING...Loconto
Source:
BOX issues Area Forecast Discussion (AFD) at Oct 20, 1:06 AM EDT----------------
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