580
FXUS61 KBOX 171124
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
724 AM EDT Fri Jul 17 2026
.WHAT HAS CHANGED...
Risk for excessive rainfall and potential street flooding
has increased Saturday and Saturday night for the South Coast.
The severe weather risk level has also increased for Saturday
and Saturday night.
&&
.KEY MESSAGES...
- Drier and seasonable with better air quality today, but smoke
could return again tonight/overnight.
- Very muggy Saturday with showers and thunderstorms.
Torrential rainfall is possible Saturday late afternoon to
overnight mainly along the RI/MA South Coast, with a risk for
severe weather too.
- Drying out with much less humidity Sunday into early next
week, but our weather turns active again by mid next week.
&&
.DISCUSSION...
KEY MESSAGE 1...Drier and seasonable with better air quality
today, but smoke could return again tonight/overnight.
Cyclonic flow aloft governs Southern New England today and brings a
seasonably cooler but more refreshing airmass to our area today. As
of early Friday morning, there were still some areas reporting smoke
or haze in CT - although air quality indices, while still in the
unhealthy for sensitive groups range, were slowly improving as
northerly flow takes hold. Latest smoke guidance from the HRRR-Smoke
and RRFS shows the more concentrated area of smoke gradually
progresses to our south into southern NY and PA. More on that to
follow, though. Today`s probably the pick of the workweek with
better visibilities and generally good air quality, to go along with
full sun and comfortable humidity levels as higher dewpoints last
couple days mix out. Potential risk for seabreezes late in the day
as NW sfc winds ease and PBL flow decreases to around 10 kt. Highs
in the lower to mid 80s.
For tonight, we get into a lower-level southerly return flow. As
that occurs, we`ll be starting to moisten up again in modest SW flow
but also start to get into the smoke layer that had been to our
southwest again per near-surface HRRR-Smoke/REFS progs. BUFKIT
profiles show a pretty good inversion developing. If we do get
the smoke layer to return, with the nature of inversions
limiting mixing, those outside later tonight and overnight could
smell the smoke and/or lead to degraded air quality again. May
not be as extensive as it was over the last couple days though.
KEY MESSAGE 2...Very muggy Saturday with showers and thunderstorms.
Torrential rainfall is possible Saturday late afternoon to
overnight mainly along the RI/MA South Coast, with a risk for
severe weather too.
Active weather returns for Saturday, and especially late in the day
into the overnight. After a day of comfortable humidities Friday, we
then turn absolutely tropical Saturday. This occurs as a plume of
rich moisture sourced from the northeast Gulf and Carolina coast
advects northeastward, along/ahead of a seasonably-strong frontal
system working its way across the eastern Great Lakes with rather
strong low to mid-level windfields for mid-July (upwards of 45 kt at
925 mb by Sat night), enhancing low-level and deep layer shear
profiles. Main potential hazards include thunderstorms capable of
heavy downpours and possible street and flash flooding where
repeated storms develop, as well as a lesser risk for severe
weather. We`ll talk about each below.
On the hydro front...Strong integrated water vapor transport fields
are progged especially for southeast New England Saturday evening,
as precipitable water values rise to around 2-2.25", about two
standard deviations above mid-July climatology. Warm- cloud-
depths also push upwards of 13,000 ft. These big-picture factors
favor highly efficient downpours. Placement of maximum QPF is
still uncertain and pretty scattershot, but the best potential
for excessive rainfall and possible street and urbanized
flooding seems poised for CT-RI-southeast MA during the
evening/overnight, where the above described ingredients are
maximized. Cloud-layer flow aloft is pretty strong so individual
storms should be moving along vs sitting in one place. We`re
also not totally in the range of the mesoscale guidance either;
that really precludes a Flood Watch, but later shifts could
consider one for the southern coast once there`s better
agreement and the mesoscale guidance is reviewed. Some of the
larger cities in RI (Providence, Cranston, Warwick) and
southeast MA (New Bedford, Fall River) as we know tend to be
vulnerable to flash flooding when rain rates turn excessive.
That said, to bring more awareness, coordinated with WPC`s
Excessive Rainfall desk to increase the excessive rainfall
threat level to Slight (Level 2 of 5) along the southern third
of Southern New England; the risk looks lower further north away
from the southern coast.
Severe weather risk was also increased into the Marginal to Slight
risk levels. With weak lapse rates, instability values are pretty
meager with low normalized CAPE values typical of tropical-like
settings. In fact the higher values take place during the nighttime
hours as dewpoints rise. But the low-level and deep layer shear
profiles are quite strong, and 0-1 km SRH values nearly to 300
units are notable. 0-3 km CAPE values can aid in these really
muggy environments, which are around 50-100 J/kg, not especially
off the charts but notable. It`s probably more a secondary risk
to the heavy rain/possible flooding but wet downbursts and/or a
spin-up tornado could develop if enough low level instability
and low- level shear can match up.
KEY MESSAGE 3...Drying out with much less humidity Sunday into early
next week, but our weather turns active again by mid next week.
Drier and much less humid weather returns again for Sunday into
early next week, with seasonable temperatures. We then look to
turn active again around midweek, with the global guidance
offering another strong frontal system coming out of the
northern Plains and interacting with Gulf moisture.
&&
.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
12z TAF Update:
Today: Overall high confidence, but moderate on seabreeze
potential.
VFR. NW winds around 8-12 kt, though they weaken by early
afternoon. A risk for seabreezes at BOS and PVD but it would be
later in the day (after 19z) if the NWlys can slacken off
enough to bring the seabreeze onshore.
Tonight: Moderate confidence.
VFR conditions should generally prevail, but FU looks to return
tonight in light SW flow, earliest south and west and latest
north and east. Kept visbys at 6 SM. Light SW winds.
Saturday: Moderate confidence.
VFR initially, some areas of FU around in the morning but
coverage should decrease. Categories decrease to MVFR-IFR levels
in showers and thunderstorms. Heavy downpours could reduce
visibility, especially later into Saturday from HFD-PVD- PYM
line south. SW winds increase to 10-15 kt with gusts 25-30 kt.
KBOS Terminal...Moderate confidence in TAF. VFR today with
decreasing NW winds; risk for late-day seabreeze. Smoke could
return again tonight but may not restrict visibility much.
KBDL Terminal...High confidence in TAF. VFR today. Smoke could
return again tonight but may not restrict visibility much.
Outlook /Saturday Night through Tuesday/...
Saturday Night: Mainly VFR, with local IFR possible. Windy with
local gusts up to 30 kt. SHRA, chance TSRA.
Sunday: VFR. Breezy.
Sunday Night through Monday Night: VFR.
Tuesday: VFR. Breezy. Chance SHRA, chance TSRA.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Through Saturday: High confidence.
Winds and seas remain below small craft advisory thresholds
through tonight. NW winds around 10-15 kt, easing and becoming
light SW tonight. Seas 3 ft or less all waters. Visbys locally
reduced along the far southern offshore waters but generally
good visibility elsewhere.
SW gusts increase to around 25-30 kt Saturday, with a low chance
of Gale force gusts along the eastern/southeast waters Saturday
evening. Marine headlines likely. Showers and thunderstorms,
some of which could reduce visibility.
Outlook /Saturday Night through Tuesday/...
Saturday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Rain showers,
chance of thunderstorms. Local visibility 1 to 3 nm.
Sunday: Winds less than 25 kt. Areas of rough seas.
Sunday Night: Winds less than 25 kt. Local rough seas.
Monday: Winds less than 25 kt. Seas up to 5 ft.
Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft.
Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
$$
DISCUSSION...Loconto
AVIATION...McMinn
MARINE...Loconto
NWS BOX Office Area Forecast Discussion
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