624
FXUS61 KBOX 211935
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
335 PM EDT Sat Jun 21 2025
.SYNOPSIS...
A round of scattered showers and thunderstorms could develop Sunday
morning, with an isolated severe storm possible Sunday afternoon. A
multi-day stretch of extreme heat and humidity begins Sunday
afternoon and continues through at least Tuesday, with little to no
opportunity for thunderstorms. The heat and humidity then ease into
the latter half of the workweek, with daily chances at showers and
thunderstorms returning.
&&
.NEAR TERM /THROUGH SUNDAY/...
Key Messages:
* Scattered showers and t-storms may impact the region Sun
morning
* First day of extended period of dangerous heat and humidity
starts Sunday. Highs into the 90s with heat indices 100-105F
* Monitoring potential t-storm development in the afternoon. It
likely remains dry but any isolated storm that develops would
likely become severe
The main forecast concern for late tonight and Sun morning is
the MCS which is forecast to move southward from SE Canada later
tonight along the periphery of the building mid level heat
ridge. This MCS will be driven by a convective shortwave and
strong 850 mb jet within a tropical theta-e plume and will move
southward within NW flow aloft. The latest CAMs are slower with
the MCS and don`t move it into northern New Eng until after 06z,
and differ on how much it weakens as it approaches SNE toward
12z. HRRR is most aggressive with potential severe QLCS rolling
southward through SNE Sun morning, while other hi-res sources
show weakening. Global guidance also indicate decaying MCS Sun
morning. There is minimal surface instability overnight into
early Sun, but we have an EML which is contributing to enough
elevated instability to support a few showers and t-storms in
the morning. We think severe threat is low in the morning due to
low level CIN and effective shear in the inflow layer is
considerably less than 0-6km shear. Still can`t rule out an
isolated strong to severe storm if they make into SNE but
confidence in specific outcomes is rather low.
Then we will have to closely monitor potential storm
development Sun afternoon. EML with mid level lapse rates 7-8
C/km combined with heat and humidity will result in an explosive
convective environment with CAPES potentially 3000-4000 J/kg.
Effective shear is weakening but still around 30 kt in the
afternoon. The question is will there be any convection under
the building ridge as we will have a cap in place. Odds favor it
remaining dry with large scale subsidence in place but if the
cap is broken any storm that develops would quickly become
severe in this environment with wind being the primary threat.
The biggest concern would be any leftover boundaries from
morning convection. The best chance for an isolated severe storm
in the afternoon would be across CT and western MA where best
moisture as there are signs of some drier air trying to move
into eastern MA.
Dangerous heat and humidity will be building on Sunday under the
ridge. Any morning convection and cloud cover could have some
impact on temps but we expect sunshine quickly developing after
the convective debris moves out. 850 mb temps warming to 19-21C
in the afternoon and this should result in highs into the low
and mid 90s. Dewpoints will be tricky as gusty W winds
developing in the afternoon within a well mixed boundary layer
could result in dewpoints a bit lower than currently forecast.
But given tropical plume of moisture in place we should see
dewpoints reach lower 70s in most locations resulting in peak
heat indices 100-105F. But it is not out of the question some
locations remain in the 60s for dewpoints. Gusty SW winds in the
morning will become W in the afternoon with soundings
supporting gusts to 25-35 mph.
&&
.SHORT TERM /SUNDAY NIGHT/...
Warm and humid night with dry conditions. A weak backdoor front on
the eastern periphery of the mid level ridge will be pushing through
eastern MA overnight with a wind shift to NE. Lows will be mostly in
the low-mid 70s, but cooler 60s possible in portions of eastern MA
and Cape Cod.
&&
.LONG TERM /MONDAY THROUGH SATURDAY/...
Key Messages:
* Dangerous heat and humidity continues Mon and Tue, with daytime
high temperatures approaching the lower 100s away from the coast
Tue. Peak "feels like" temperatures up to 110 degrees!
* Gradually cooling off Wed through Fri, although shower and storm
threats then return.
Have high confidence in hot conditions persisting Monday and
Tuesday. NAEFS temperature anomalies are 3-4 standard deviations
above normal. The big question= will be the humidity. Subtle changes
there will significantly impact the "feels like" temperatures. Right
now, expecting dew points 65-70 degrees F for most of the region.
This will result in peak "feels like" temperatures well over 90 both
days away from the immediate coast. Some locations may approach peak
values of 110 degrees! Tue is still expected to be the hottest day
of this stretch.
The heat looks to break some Wednesday, before returning to more
normal summer temperatures late next week.
As for precipitation, the strong ridge beneath the heat dome should
maintain rain-free weather into Wednesday. Some afternoon showers or
thunderstorms are then possible Wednesday afternoon and evening. A
greater risk for showers and thunderstorms Thursday into Saturday as
a frontal boundary lingers nearby.
&&
.AVIATION /20Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18z TAF Update:
Through Tonight: High confidence.
VFR with areas of LLWS developing after 06z as low level jet
moves over the region. Any t-storm activity should remain to
the north through 12z. Light SW wind tonight increasing to 8-15
kt after 09z. Gusts to 20 kt possible.
Sunday: Moderate confidence.
Weakening MCS may move south across SNE in the morning. If it
holds together it would bring a brief period of showers and
t-storms 12-15z with low risk for brief gusty wind. Otherwise,
VFR. Will have to monitor for an isolated storm in the
afternoon which would have potential to quickly become severe
but confidence in storm development is low. SW gusts to 20-25 kt
in the morning becoming W with gusts 25-30 kt in the afternoon.
Sunday Night: High confidence.
VFR. Light NW wind becoming NE across eastern MA after 06z.
KBOS Terminal...High confidence in TAF.
KBDL Terminal...High confidence in TAF.
Outlook /Monday through Thursday/...
Monday through Tuesday Night: VFR.
Wednesday: VFR. Slight chance SHRA, slight chance TSRA.
Wednesday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, slight chance TSRA.
Thursday: Slight chance SHRA, slight chance TSRA.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Through Sunday Night...High confidence.
Increasing SW winds late tonight with gusts to 25-30 kt developing
later Sun morning into the afternoon. SCA issues for all waters.
Winds quickly diminish Sun evening and become light NE late Sun
night.
Main concern for mariners in this period is the potential for a few
t-storms affecting the waters Sunday morning.
Outlook /Monday through Thursday/...
Monday through Wednesday Night: Winds less than 25 kt.
Thursday: Winds less than 25 kt. Slight chance of rain showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
CTZ002>004.
MA...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
MAZ003>007-010>021-026.
Heat Advisory from 11 AM Sunday to 8 PM EDT Tuesday for MAZ002-
008-009.
RI...Extreme Heat Warning from 11 AM Sunday to 8 PM EDT Tuesday for
RIZ001>007.
Air Quality Alert from 11 AM to 11 PM EDT Sunday for
RIZ006>008.
MARINE...Small Craft Advisory from 8 AM to 6 PM EDT Sunday for ANZ230-
231-236-250-251.
Small Craft Advisory from 8 AM to 8 PM EDT Sunday for
ANZ232>235-237-254.
Small Craft Advisory from 8 AM Sunday to 2 AM EDT Monday for
ANZ255-256.
&&
$$
SYNOPSIS...Belk/KJC
NEAR TERM...KJC
SHORT TERM...KJC
LONG TERM...Belk
AVIATION...Belk/KJC
MARINE...Belk/KJC