902
FXUS61 KBOX 290612
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
212 AM EDT Wed Jul 29 2026

.WHAT HAS CHANGED...

No significant changes.

&&

.KEY MESSAGES...

- Prolonged, heavy rainfall continues Wednesday leading to
  potential street and flash flooding, especially across western
  New England.

- Cloudy weather into Friday with onshore flow and off-and-on
  light rain showers, to go along with cooler than normal high
  temperatures.

- Generally dry weather with seasonably warm temps, but turning
  unsettled late in the weekend and especially into early next
  week.

&&

.DISCUSSION...

KEY MESSAGE 1...Prolonged, heavy rainfall continues Wednesday
leading to potential street and flash flooding, especially across
western New England.

We`re approaching the period in which we expect the heaviest
rainfall and thus the greatest flood threat in southern New England,
generally Wednesday morning through Wednesday evening. In the mid
levels, an anomalously strong low center is in the process of
closing off over New York. Over the course of the next 12 to 36
hours this neutral to negatively tilted trough will be slow to move
east thanks to a blocking high over the north Atlantic, setting us
up for a long duration rainfall event. It is a very moist and
dynamic setup, a bit unusual for July in the northeast. Strong mid
level divergence and upper diffluence will induce deep layer ascent.
At the surface deepening low pressure moves up from the Mid Atlantic
causing winds to back to the east and inducing a 30-40kt LLJ. This
leads not only to convergence/rising air on the nose of the jet but
also strong orographic/upslope flow into the high terrain of
southern New England; principally the Berkshires but also the
Worcester Hills. This will allow the axis of heavier rain over
eastern NY to pivot over southern New England, while showers
continue to exhibit training potential, especially over the
Berkshires early. Given the very saturated atmosphere (PWATs over
1.7") this is a setup conducive to big rainfall totals. As we`ve
gotten into high-resolution guidance concern has increased for the
potential of extreme amounts on the order of 5 to 7 inches where the
heaviest bands set up. Unfortunately, exactly where they set up is
nearly impossible to nail down short of a nowcast situation.
However, the greatest threat continues to be western and central SNE
where the Flood Watch is in effect. A more widespread 2 to 4 inches
of rain is more likely for any given area. The most concerning
period for western SNE will be this morning into the early afternoon
when rainfall rates of 1 to 2 inches per hour are possible.
Eventually that shifts into eastern MA in the later afternoon and
evening/early overnight hours. So, the biggest concern for dangerous
flash flooding (especially in urban and and poor drainage areas)
will be for the Berkshire slopes/Hartford area during the morning
commute. This then shifts toward the PVD-BOS corridor for the
evening commute. Even given the extreme amounts, we continue to
expect this to mainly be a flash flood/street flooding concern as
opposed to river flooding, though we are keeping an eye on the
rivers and streams.

By Thursday the best moisture and dynamics have moved off but the
vertically stacked low lingers leading to a damp, dreary day with
lingering showers off and off throughout the day. As for
temperatures, persistent easterly flow and thick cloudcover/rainfall
will depress high temperatures into the low to mid 70s both
Wednesday and Thursday.

KEY MESSAGE 2...Cloudy weather into Friday with onshore flow
and off- and-on light rain showers, to go along with cooler than
normal high temperatures.

The stubborn, slow-moving low pressure near southern CT with wound-
up, vertically-stacked upper-level closed low atop it begins to open
up and fill as we move into Friday, eventually pulling offshore and
away by Friday night.

Unfortunately this means a continuation of cloudy, dreary weather
with at least a modest onshore NE to N wind through the rest of the
workweek. Guarded optimism that later in the day Friday could see
some limited breaks in the cloudiness, but it`s overall a cloudier-
than-not period. Off-and-on periods of showers can be expected as
well too, but with the moist layer shown in model forecast RH cross
sections being about 5000 ft or less in depth from the surface,
these showers should prove light and could resemble more of a
drizzle. Lows Thursday night are in the upper 50s to low 60s. Highs
on Friday though are more challenging in that if we do get any late-
day breaks temps could reach into the low 80s. Better chance of that
in western MA/CT, but otherwise have kept temps in the 70s, with
even some upper 60s out over the Cape due to more clouds/onshore
flow.

KEY MESSAGE 3...Generally dry weather with seasonably warm
temps, but turning unsettled late in the weekend and especially
into early next week.

Weak high pressure then begins to build in for the weekend, so we
should see at least briefly drier weather return, though we could
have to watch for lingering low clouds over the Cape and Islands.
Confidence in this part of the forecast is lower-than-average
though, as the issue is that most of the global ensembles show
another elongated upper trough digging into the OH Valley with
southwest flow aloft developing by Saturday. It`s a pattern which
could spell rain chances coming back into view in the SW flow aloft.
While the non-GFS guidance keeps it a generally dry weekend, the GFS
shows scattered showers/storms later Saturday, with more to come as
we move into early next week. Thinking it turns more unsettled again
early next week, but it is not out of the question there could be
showers/storms popping up again by the weekend. Maintained a model-
blended approach for this period.


&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

06z TAF Update:

Through 12Z...Moderate confidence.

VFR east, MVFR/IFR west to start, IFR to expand east to eastern
MA by 17Z. Sct`d heavy downpours across western terminals.
Though, do not feel these will reach terminals east of PVD. S
to SSE winds. Areas of fog possible ahead of the rain for the
Islands predawn.

Wednesday and Wednesday Night...Moderate confidence.

IFR/MVFR in continued periods of heavy rain. SSE to E winds
10-15 kts with gusts to 25 kts possible. Potential for LLWS for
eastern terminals overnights.

Thursday...Moderate confidence.

IFR/MVFR in scattered showers. NE winds 5-10 kts.

KBOS Terminal...Moderate confidence.

KBDL Terminal...Moderate confidence.

Outlook /Thursday Night through Sunday/...

Thursday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Friday: VFR. Slight chance SHRA, isolated TSRA.

Friday Night through Saturday Night: VFR.

Sunday: Mainly VFR, with local IFR possible. Slight chance
SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Thursday night...High confidence.

An anomalous midsummer low pressure system will continue to bring
a prolonged period of unsettled weather to the southern New
England coastal waters through Thursday. Periods of heavy rain,
reduced visibilities, and embedded thunderstorms are expected
through much of the period. A Small Craft Advisory remains in
effect for all waters through Wednesday, and for the outer
coastal waters through Thursday. South to southeast winds
increase tonight, with gusts of 25 to 30 kt expected on
Wednesday. Seas build to 4 to 6 ft, highest across the outer
waters. Wind direction becomes more variable on Thursday as the
surface low tracks across the southern New England coastal
waters. East to northeast winds ahead of the low will shift to
north to northwest in its wake, although the eastern waters may
remain northeasterly longer before the system gradually departs
Thursday night.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Seas up to 5 ft. Chance
of rain showers, isolated thunderstorms.

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.

Saturday Night through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through this evening for CTZ002>004.
MA...Flood Watch through this evening for MAZ002>004-008>012.
RI...Flood Watch through this evening for RIZ001-002.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ230>237-251.
     Small Craft Advisory until 8 AM EDT Thursday for ANZ250-
     254>256.

&&

$$

DISCUSSION...Loconto/BW
AVIATION...Loconto/BW
MARINE...Loconto/BW