113
FXUS61 KBOX 202340
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
740 PM EDT Mon Jul 20 2026

.WHAT HAS CHANGED...
Gale Watches continue for Tuesday afternoon through Wednesday
evening. High Surf Advisory issued for the south-facing coasts
from Wednesday morning through Thursday evening.

&&

.KEY MESSAGES...

- Chances for soaking downpours and severe weather return
  Tuesday evening through early Wednesday morning.

- Limited risk for severe weather Wednesday.

- Drying out Thursday into the latter part of the week
  with seasonable temperatures and lower humidity into the
  weekend.

&&

.DISCUSSION...
KEY MESSAGE 1...Chances for soaking downpours and severe
weather return Tuesday evening through early Wednesday morning.

As the dominate high pressure system pushes offshore, a low
pressure system pushes east through Ontario and the trailing
fronts sweep through New England Tuesday into Wednesday. A near
60kt LLJ develops Tuesday into Wednesday, ushering in dewpoints
near or slightly above 70F and high PWATs on the order of
2.0"-2.25" across much of southern New England.

As the frontal system approaches Tuesday, showers and
thunderstorms will begin to form primarily later in the
afternoon/evening hours. While the greatest risk for severe
thunderstorms continues to be to our west, a surge of SBCAPE
1200-1500 J/kg combined with increasing K-indices and 0-3km
helicity values will support some risk for straight-line winds
within any thunderstorms that do form. Can`t rule out a few
brief spin-ups, but the risk remains on the lower side.

The main concern continues to be torrential rainfall given how
water- loaded these storms could be, especially along the South
Coast, Cape, and Islands. While model soundings indicate mid-
level winds around 30-40kts, some training of showers and
thunderstorms cannot be ruled out. The latest ensemble guidance
has nudged up the 24 hour QPF probs of 1"+ to near 80% along the
South Coast, with 2"+ probs remaining near 40-50% (especially
along the Cape and Islands). Latest Flash Flood and experimental
Urban Rain Rate guidances are showing increasing probabilities
that Providence and surrounding urban areas could approach the
10-year ARI early Wednesday morning... near 2.3" in 2 hours or
2.75" in 3 hours. After working with neighboring offices,
highest confidence in heavier rain bands setting up remains over
the Mid-Atlantic/NYC locations, prompting a Flood Watch for
those areas; however, later shifts may need to expand that Flood
Watch further up the I-95 corridor.

KEY MESSAGE 2...Limited risk for severe weather Wednesday.

Rain continues into Wednesday, with the higher chances and
totals towards the Cape and Islands Wednesday morning into the
afternoon. Ensembles continue to have higher probabilities for
1"+ totals towards the Cape and Islands and along the south
coast heading into Wednesday afternoon and evening, with low
probs of 2"+ over the same areas though with more discrepancies
among the guidance.

The main thing to keep an eye on will be a limited severe
weather risk over southern New England Wednesday given the
frontal passage. Guidance shows the cold front not pushing
offshore until Thursday morning. By 00z Thursday, the front may
be draped over the I-95 corridor. If anything, this would aid in
an increased severe risk for the interior. Global guidance
indicates the midlevels drying out relatively quickly behind the
front while the latest NAM is slower to clear that moisture
out. Something to monitor will be how much instability can be
developed behind the front. NAM guidance has around 2100 J/kg of
MLCAPE over much of the interior Wednesday afternoon and
evening (other guidance has values around 1500-2000 J/kg as
well), and dewpoints increase to the low 70s. Given these
conditions, there could be a risk for severe thunderstorms for
Wednesday afternoon. Will continue to monitor this risk in
coming updates.

KEY MESSAGE 3...Drying out Thursday into the latter part of the
week with seasonable temperatures and lower humidity into the
weekend.

The cold front exits the region by Thursday morning, leaving
drier and seasonable conditions in its wake. With the front, the
upper level trough centered over Quebec also moves off to the
northeast. High pressure starts to build in over southern New
England from the Great Lakes, continuing the drier conditions
into the weekend. Temperatures aloft fall slightly, with 925 mb
temperatures falling to around 16C for much of the second half
of the week. Notably, Wednesday night will likely be cooler as
925 mb temperatures may fall to as low as 10C over the interior.
Lows there could get into the low 50s. 850 mb temperatures are
expected to mostly be around 10C as well through the back half
of the week. Even with these reduced temperatures, the continued
high sun angle will keep temperatures from falling too low,
keeping them seasonable. Conditions remain dry and seasonable
until around Sunday when the flow shifts more S again,
increasing humidity. A surface low may also pass just offshore,
but any impacts from it -- if any at all -- are not certain at
this time.

&&

.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:

Through Tonight...Moderate confidence.

VFR. Sea breezes are continuing to shift back offshore this
evening, so nearshore winds (north and south shore, BOS) should
become light S/SSW through the rest of tonight. Light SSW winds
prevail across the rest of the region tonight as well. Some
high-res guidance continues to hint at the possibility for MVFR
in late night/early morning fog over the interior tonight, but
confidence is too low to include in TAFs.

Tuesday...Moderate confidence.

General deterioration to MVFR-IFR ceilings by late afternoon,
with risk for showers and possible storms after 18z from W to E.
Better potential for storms Tue night. Winds become SE to S and
increase to 10-15 kt with gusts 20-25 kt.

Tuesday Night...Moderate confidence.

MVFR/IFR possible in any SHRA or TSRA. Rain overspreads southern
New England heading into Tue night with chances for TSRA across
the region. Included PROB30s for SHRA at this time, though TSRA
will likely be needed in future TAF issuances for more terminals
across the region. Winds increase to around 10-15 kt sustained
particularly over SE MA, Cape & Islands. Gusts 20-25 kt remain
possible through the nighttime period in these areas as well.

KBOS Terminal...Moderate confidence in TAF. TSRA possible
towards end of current TAF period, however exact timing for TSRA
is still somewhat uncertain, along with if TSRA makes it this
far east.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: Mainly VFR, with local MVFR possible. Windy with
gusts up to 30 kt. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. Windy with local gusts up to 30 kt.
Chance SHRA.

Thursday through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

As the frontal system approach and the 60 kt LLJ form over the
Cape/Islands Tuesday, SSW-ly winds will pick up to near Gale
force. Gale Watches continue Tuesday evening into Wednesday,
especially across the southern and southeastern waters, and
later shifts may need to upgrade to Gale Warnings. Latest round
of guidance is indicating marginal Gales, but rough seas 10-12
ft. Near-shore waves along the southern coast, Islands, and
outer Cape are likely in the 7-10 ft, and warranted High Surf
Advisories starting early Wednesday and lingering late into
Thursday. Strong Rip Currents and waves near 6 ft are possible,
especially across the outer Cape, Thursday as the cold front
stalls out near the coast, prompting the later ending to the
High Surf Advisory.

Outlook /Wednesday through Saturday/...

Wednesday: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 13 ft. Chance of rain showers, slight
chance of thunderstorms.

Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 12 ft. Rain showers,
slight chance of thunderstorms. Local visibility 1 to 3 nm.

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, slight chance of thunderstorms.

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

Friday through Friday Night: Winds less than 25 kt. Seas up to
5 ft. Slight chance of rain showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday
     for MAZ020>024.
RI...High Surf Advisory from 6 AM Wednesday to 8 PM EDT Thursday
     for RIZ006>008.
MARINE...Gale Watch from Tuesday afternoon through Wednesday evening
     for ANZ231>235-250-254>256-280>283.

&&

$$

DISCUSSION...Hrencecin/McMinn
AVIATION...Hrencecin/McMinn
MARINE...Hrencecin/McMinn