483
FXUS61 KBOX 272329
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
729 PM EDT Mon Jul 27 2026

.WHAT HAS CHANGED...
Flood Watches have been issued for western portions of Southern New
England for Tuesday afternoon into part of Wednesday evening, with
an increase in rainfall totals in interior Southern New England.

&&

.KEY MESSAGES...

- Flood Watch issued for interior western New England, with
  heavy rainfall leading to potential street and flash flooding
  Tuesday afternoon through Wednesday evening. Heaviest rain in
  eastern New England is expected on Wednesday, with street
  flooding possible but less favored than further inland.

- One final push of rain Thursday with isolated flash flooding
  possible. Slow-moving system may keep periodic showers around
  into Friday.

- Potentially unsettled conditions linger into the weekend,
  although confidence remains lower.

&&

.DISCUSSION...
KEY MESSAGE 1...Flood Watch issued for interior western New England,
with heavy rainfall leading to potential street and flash flooding
Tuesday afternoon through Wednesday evening. Heaviest rain in
eastern New England is expected on Wednesday, with street flooding
possible but less favored than further inland.

An anomalously strong upper-level cyclone for late July is expected
to close off into a slow-moving upper low over central/south-central
New York Tuesday into early Wed, then is forecast to slowly meander
ESE through the lower Hudson Valley Wed into Wed night. This system
will draw in rich subtropical moisture (e.g. PWAT values around 1.5-
1.7") along/ahead of a frontal boundary, a boundary will tend
to slowly lift or pivot E/ENE Tue night/Wed. Additional
enhancements to rainfall in an easterly upslope flow in interior
Southern New England are also anticipated. Put all that together
and we are looking at a soggy, dreary midweek with rounds of
rainfall, some of which could turn heavy at times. Because of
the slow eastward progression and timing, we`ll break this down
regionally below.

For interior portions of Southern New England: moderate to heavy
rain with some embedded rumbles of thunder into expected to develop
Tuesday afternoon and continue into Wednesday across western New
England, especially into the CT Valley and northern CT. Heaviest
rain is expected to fall late Tuesday night/overnight and continue
into Wednesday. With a longer duration of this type of rain,
the risk for street flooding as well as rises on creeks and
streams looks to be the greatest in this general area. We`ve
issued a Flood Watch to best message this potential for the CT
Valley in MA into Hartford and Tolland Counties in CT, and this
runs from Tuesday afternoon through early Wednesday evening.
Although it is still uncertain what the maximum rainfall will
be, this general area seems more primed to be at least in the
vicinity of seeing that maximum; we`ll also need to monitor
where a pivot point ("deformation zone", in the weather
vernacular) develops as the frontal boundary lifts northeastward
as an area potentially favorable for heavier rain
accumulations. This is also reflected in the coarser-resolution
global ensemble QPF probs from the ECMWF, Canadian GEM and the
GEFS, which show high probs of 2+ inches of rainfall, with low
(<20%) probs of 4 or more inches of rain. We`re also not yet
really into the higher- resolution guidance yet, so we still
have to digest that data as we move through the next 24-36
hours, but the synoptic-scale picture paints the potential for
locally significant rainfall accumulations in western New
England. This is also reflected in the ECMWF Extreme Forecast
Index (EFI) and Shift of Tails guidance and justifies the Watch.
Although a slight risk for excessive rainfall was collaborated
with WPC, a zone of moderate risk has now been introduced for
points just to our south and west into the lower Hudson Valley
in New York, which speaks to the risk for flooding. Additional,
but lighter rain is expected even into Thursday. I`ve shown rain
totals through 7 AM Thursday which range from 2 to 3 inches,
with locally higher totals nearing 4 inches along the east
slopes of the Berkshires.

For eastern portions of Southern New England, roughly Worcester and
Windham Counties eastward into RI and eastern MA: outside of
isolated to widely scattered rain showers associated with a
northward-moving warm front Tuesday morning, much of this region
ends up being mostly dry on Tuesday with a relative minimum in PoPs
(chance or less). The frontal boundary will be gradually moving in
either late Tuesday night (after midnight), but more likely by early
Wednesday. It looks like Wednesday is the period of moderate to
heavier rainfall for this portion of Southern New England, only
slowly decreasing in intensity as we move into Wednesday
afternoon to early evening. There could be some risk for street
flooding here too, especially in the typically more vulnerable
urban areas (not limited to Boston, Providence, Worcester, Fall
River and New Bedford), but felt the risk was less than into
western portions of Southern New England who are poised to see a
longer duration of moderate to heavy rain. Later shift(s) could
consider expanding the Flood Watch into some of these locations
if rain totals continue to increase, but it seems more
conditional/marginal. Rain totals through 7 AM Thursday range
from 1.5 to 2.5 inches, with the highest rainfall totals in
hillier Worcester County as well as into the Merrimack Valley.

In terms of river response...obviously one of the bigger challenges
when it comes to projecting impacts is that we`re still dealing with
a stubborn drought where streamflows in several locations are still
below-normal. Right now there are no rivers forecast to rise into
minor flood stage per NERFC forecasts, although rises can be
expected over the next few days.

The persistent rain and cloud cover with easterly onshore flow will
keep diurnal temperature ranges narrow, with milder than normal low
temperatures but cooler than normal high temperatures.

KEY MESSAGE 2...One final push of rain Thursday with isolated flash
flooding possible. Slow-moving system may keep periodic showers
around into Friday.

Rain continues into Thursday, with the potential for one final surge
of heavier rainfall during the second half of the day. The unsettled
weather is driven by a slow-moving area of low pressure that becomes
vertically stacked through the mid levels, with little eastward
progression due to blocking over the northwest Atlantic. While
Thursday lacks the robust upper-level jet dynamics of the previous
day, PWATs increase to around 1.7 inches (roughly one standard
deviation above climatology), and strong mid-level vorticity will
support periods of widespread rainfall. Rainfall rates should
generally be lower than Wednesday`s, with precipitation becoming
more stratiform than convective. Even so, antecedent wet soil
conditions warrant continued monitoring for isolated flash flooding,
particularly in urban and poor-drainage locations where heavier rain
bands develop. The system is slow to depart, resulting in periodic
showers lingering into Friday. Confidence in the exact timing
remains limited, but Friday does not currently appear to be a
washout.

Both days will feature below-normal temperatures due to abundant
cloud cover and persistent east to northeast winds, with little
diurnal temperature range. Highs on Thursday are expected to reach
the lower to middle 70s, warming modestly into the middle and upper
70s on Friday.

KEY MESSAGE 3...Potentially unsettled conditions linger into the
weekend, although confidence remains lower.

Forecast confidence decreases heading into the weekend, with
conditions largely dependent on how quickly the downstream ridge
breaks down and allows the slow-moving low pressure system to
finally exit the region. Model consensus remains poor at this lead
time, resulting in lower confidence in the forecast. Given the 5 to
6 day forecast horizon, additional changes are likely.

That said, periodic showers remain possible through the weekend.
With a continued trend toward greater cloud cover, significant heat
appears unlikely, and temperatures should remain near seasonal
normals.

&&

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

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

00z TAF Update:

Rest of Today...High confidence.

Isolated SHRA/TSRA towards the western terminals persist early
this evening, fading post sunset. Outside of these, VFR
conditions should prevail across the region for the rest of
today. SE winds 5-10 kt through this afternoon and evening.

Tonight...Moderate confidence.

VFR to start, then the general trend is deteriorating ceilings
toward low end VFR/MVFR, with localized IFR/LIFR ceilings
possible. Confidence regarding lower ceilings is still somewhat
more moderate, but generally have them rolling in around 08z in
TAFs. Widely scattered showers possible overnight, though not
enough confidence to include in TAFs at this time. Potential
for widespread precipitation increases for Tuesday. Light S
winds.

Tuesday...Moderate confidence.

Chances for widespread rainfall ramp up Tuesday with lower end
VFR through much of the day, then deteriorating to MVFR/IFR
towards the afternoon. Chances for embedded TSRA are low though
not zero at this time. Breezier SE winds 10-13 kt with gusts to
20 kt possible in the afternoon.

Tuesday Night...Moderate confidence.

IFR/MVFR. The most significant rainfall is expected to
overspread the region heading into Tuesday night-Wednesday
morning. IFR to LIFR ceilings across much of the interior with
low end MVFR eastward. Persistent SE winds 6-10 kt with gusts
to 20 kt possible towards the interior.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday Night through Saturday/...

Wednesday: Mainly MVFR, with areas IFR possible. Breezy. SHRA,
isolated TSRA.

Wednesday Night: Mainly VFR, with local IFR possible. Breezy.
SHRA.

Thursday: Mainly VFR, with areas IFR possible. Breezy. SHRA,
chance TSRA.

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

Friday: VFR. Chance SHRA, slight chance TSRA.

Friday Night: VFR. Slight chance SHRA.

Saturday: Mainly VFR, with local MVFR 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 Tuesday Night: High confidence.

Winds and seas to remain below small craft advisory levels
through the daytime hours on Tuesday. Southerly winds around 10
kt will be increasing to around 15-20 kt Tuesday, with seas
building to 3-4 ft. A few showers are possible on southern
waters during the Tuesday daytime hours.

Small craft advisories have been issued Tuesday evening into
Wednesday as south to southeast winds increase to around 25 to
30 kt and seas in the 4 to 7 ft range, highest offshore. Showers
and embedded thunderstorms can be expected, mainly into
Wednesday.

Outlook /Tuesday Night through Saturday/...

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Rain showers, isolated
thunderstorms.

Wednesday Night through Thursday: Winds less than 25 kt. Seas
up to 5 ft. Chance of rain showers.

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

Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers, slight chance of thunderstorms.

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch from Tuesday afternoon through Wednesday evening
     for CTZ002-003.
MA...Flood Watch from Tuesday afternoon through Wednesday evening
     for MAZ002-003-008>011.
RI...None.
MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Wednesday for
     ANZ230-231-250-251.
     Small Craft Advisory from 8 PM Tuesday to 8 PM EDT Wednesday
     for ANZ232>237-254>256.

&&

$$

DISCUSSION...Loconto/Dooley
AVIATION...Loconto/Dooley/Hrencecin
MARINE...Loconto/Dooley