BOX issues Area Forecast Discussion (AFD) at Sep 17, 2:06 PM EDT817
FXUS61 KBOX 171806
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
206 PM EDT Thu Sep 17 2026
.WHAT HAS CHANGED...
No significant changes to the forecast.
&&
.KEY MESSAGES...
- Muggy with seasonably warm temperatures today. Scattered
showers arrive by the evening, continuing through Friday
morning. Isolated moderate to heavy showers possible.
- Next chance for rain Sunday into Monday. There remains
uncertainty in the details such as coverage, timing, amounts.
Showers may linger through early next week.
&&
.DISCUSSION...
KEY MESSAGE 1...Muggy with seasonably warm temperatures today.
Scattered showers arrive by the evening, continuing through
Friday morning. Isolated moderate to heavy showers possible.
Today a shortwave trough approaches from the northwest increasing
rain chances toward the evening. While much of the day stays mostly
dry, a plume of anomalous moisture advects in resulting in plentiful
moisture and humid/muggy day. PWAT values rise to around 2.0" which
is fairly anomalous (180-200% of normal). Across most of the CAMs,
showers begin to arrive after 2 PM in western MA/CT spreading
eastward this evening while becoming higher in coverage. There
continues to be variance in the exact track of this shortwave,
particularly with how far north or south with respect to southern
New England. This will likely influence rain amounts and how
widespread the showers will be. A more north track will shift the
stronger forcing to our north thus resulting in lighter rainfall
amounts overall. The 00Z HRRR, RRFS, and NAM (to some extent) have
lean more south favoring higher rain amounts with the support of
stronger ascent and marginal instability for isolated downpours. The
split continues to bring uncertainty in the expected totals and the
potential for some isolated downpours this evening and tonight.
Leverage the full range of guidance, most areas should see between
0.1" to 0.6". However, pockets of higher amounts of up to 1.0" are
possible within any embedded downpours. Mid-level flow should be
strong enough to keep the showers moving which will reduce the flash
flood threat. Additionally the latest HREF doesn't highlight any
potential areas for potential to exceed 1hr and 3hr FFG.
Cold front moves through early Friday morning with winds shifting to
the NW behind it. Showers ahead of the front push across the
Cape/Islands, exiting by mid-morning. Breezy NW winds in the
afternoon with gusts 20-25 mph, mainly across the interior.
Temperatures are likely to be in seasonable ranges in the 70s. A
cooler than normal airmass settles across the region by Saturday
bringing Fall-like temperatures in the 60s. Surface high pressure
dominates bringing dry conditions.
KEY MESSAGE 2...- Next chance for rain Sunday into Monday. There
remains uncertainty in the details such as coverage, timing,
amounts. Showers may linger through early next week.
The main surface high exits offshore Saturday night into early
Sunday as the next system approaches from the west with our next
chance of precipitation. Shower/rain chances increase Sunday morning
into Sunday afternoon from west to east. Plentiful moisture advects
in within the southerly flow to support showers as the warm sector
pushes northward into southern New England. PWATs increase to 1.0-
1.8" with the higher values across CT and RI. Soundings show deep
moisture profiles further supporting efficient rainfall. Rain likely
continues into early Monday morning. There is some question in if a
frontal boundary stalls to the south which may result in lingering
showers in areas Monday (south coast, Cape/Islands).
The potential rain amounts will depend on the exact track of
the system which is still variable across guidance this far out
in time. This variance translates quite clearly to the rain
totals across both deterministic and ensemble guidance.
Ensembles range generally between 0.70 to upwards of 2.0". The
GFS and Canadian ensembles have leaned on the lower end of that
range(even lower than 0.70" in areas) compared to the ECMWF
ensemble. On the other hand the AI- ECMWF ens and AI-GEFS have
less than 0.60" QPF with similar solutions in the AI
deterministic runs. A decent amount of spread still exists in
how much rain will fall Sunday into Monday, but overall looks
like a good shot for a soaking rain. Depending on if and where a
stationary boundary sets up nearby, showers may linger through
the early part of the week.
&&
.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18Z TAF Update:
Through Tonight...Moderate confidence.
Predominantly VFR with areas of MVFR this afternoon. SSW winds at 5-
10 kt. Exception is BOS, where ENE winds continue through late
afternoon. Showers arrive during the afternoon hours from W to E,
with chances continuing to increase through the evening. Showers are
more likely to develop after 21z for the western terminals and after
00z for the more eastern terminals. Overnight, MVFR with areas of
IFR and patchy fog. Showers continue to move across the region from
west to east, exiting the Cape/Islands around 12z. SW winds 5-10 kt,
shifting to NW 06-12z as the front moves through.
Friday and Friday Night...High confidence.
Areas of MVFR linger across Cape Cod and the Islands through 14/15z,
otherwise VFR. NNW winds 8-12 kt with gusts up to 20 kt, though
gusts diminish after sunset. The exception will be the Cape and
Island terminals, where gusts continue into the overnight hours.
Across the interior, northerly winds less than 5 kt.
KBOS Terminal...Moderate confidence in TAF.
ENE winds become S around 23/00z. SHRA chances increase after 23z;
have opt'd for VCSH during that time since probabilities are lower.
Higher confidence in SHRA after 01z, continuing through 07/08z with
MVFR ceilings, though cannot rule out briefly falling to IFR during
periods of SHRA. VFR Friday with NNW gusts up to 20 kt.
KBDL Terminal...Moderate confidence in TAF.
Outlook /Friday through Tuesday/...
Friday through Friday Night: VFR. Breezy.
Saturday through Saturday Night: VFR.
Sunday: Mainly MVFR, with areas IFR possible. Breezy. SHRA.
Sunday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA.
Monday: VFR. Breezy. Slight chance SHRA.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Through Saturday...High confidence.
A round of showers pushes across the waters tonight into early
tomorrow morning. Seas remain 2-3 ft, though portions of the
southern waters may approach 4 ft heading into tomorrow morning.
Winds shift to the NW tomorrow morning, with some gusts up to 20 kt
possible. High pressure builds in from the WNW Friday afternoon and
remains with us through Saturday. Briefly Friday night into early
Saturday, gusts may approach Small Craft Advisory criteria.
Confidence in gusts exceeding 25 kt is low; however, gusts of 20 to
25 kt seem likely. Future forecast updates may include an advisory
if confidence in higher gusts increases. Otherwise, nice conditions
on Saturday with seas 2-3 ft and ENE winds less than 15 kt.
Outlook /Friday through Tuesday/...
Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.
Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft.
Saturday: Winds less than 25 kt. Seas up to 5 ft.
Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft.
Sunday: Winds less than 25 kt. Chance of rain showers.
Sunday Night: Winds less than 25 kt. Local rough seas. Rain
showers likely, slight chance of thunderstorms.
Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
$$
DISCUSSION...Mensch
AVIATION...BW/Dooley/Mensch
MARINE...Dooley/Mensch
Source:
BOX issues Area Forecast Discussion (AFD) at Sep 17, 2:06 PM EDT----------------
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