570
FXUS61 KBOX 161757
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
157 PM EDT Tue Jul 16 2024

.SYNOPSIS...
Dangerous heat and humidity will continue through Wednesday with
little nighttime relief expected. Scattered showers and
thunderstorms will be possible both Tuesday and Wednesday. A cold
frontal passage late Wednesday into Thursday will give way to a much
more comfortable airmass to end this work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
930 AM update...

MCV moving through the Gt Lakes will pass to the north later
this afternoon and evening. This will trigger storms across New
York state this afternoon which will move into western New Eng
toward evening. Timing wise it looks like 6-10 pm is the main
window for a few storms moving across SNE, especially interior
MA. Instability and shear are marginal this evening and so
storms should be weakening as they move into SNE past peak
heating, but can`t rule out a few severe storms.

No change to heat headlines. 850 mb temps 20-21C support highs
in the 90s away from immediate south coast, with mid-upper 90s
across the CT valley and eastern MA. Dewpoints during peak
heating should mix out into the upper 60s, with lower 70s across
RI and SE MA. Max heat indices will be 98-103.

Previous discussion...

Heat and humidity continues today much like we saw yesterday but to
a greater magnitude as the airmass warms slightly. 850 mb temps rise
from around 19C to 20-21C and thus we`ll see high temperatures
reaching a few degrees warmer today, into the upper 90s for many.
This, with dewpoints in the 70s will lead to heat indices up to 102F
which is why a Heat Advisory is in effect.

The other weather story of the day is, also similar to Monday, a
round of showers and thunderstorms that will be moving into southern
New England this evening, weakening as they slide east. The driving
force behind these storms is a shortwave rotating around the broader
cyclonic flow over the Great Lakes and into New England. Ahead of
the shortwave today`s heat and humidity will lead to decent
instability (1000-1500 J/kg of CAPE), but with only marginal shear
available and the late time of arrival of the line (6-7pm in western
MA/CT most likely) the favorable window for any severe weather will
be small. Timing of any storms would be in the evening (after 6 pm)
with diminishing chances after sunset. The SPC has placed a Slight
Risk of severe weather (level 2 of 5) over western MA/CT which is
where we`ll have the best chance of seeing damaging winds and hail.
Overnight fog and stratus clouds will return to the south coast and
low temperatures will be very warm, in the mid 70s.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
Wednesday expect more of the same, but take solace in the fact that
this will be our last day of the seemingly endless humidity we`ve
been dealing with of late. While the Heat Advisory continues through
Wednesday, a cold front will be knocking on our door heralding
relief from the oppressive heat. Unfortunately that, along with a
negatively tilted upper trough, will serve as a forcing mechanism
for one final round of strong to severe storms. The SPC this morning
upgraded western/central MA and CT to a Slight Risk (level 2 of 5)
where an earlier arrival (compared to Tue) of the best forcing may
bring a better chance of strong or severe storms. Confidence in
timing of the arrival of the storms is only moderate at this point,
but guidance would indicate storms arriving several hours earlier
than Tuesday, as early as mid afternoon. The environment is more
favorable on Wednesday as well thanks to increased instability
(2000J/kg+ of CAPE) and 0-6km bulk shear (30-40 kts) which should
allow for some sustained updrafts. Given this favorable shear storms
may be more organized and any could produce damaging gusts along
with heavy downpours and localized urban and poor drainage flooding.
After sunset storms should weaken and eventually dissipate during
the overnight hours. Expect one more night of lows in the 70s.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Key Points...

* Dry and seasonal weather Thursday through this weekend

After an extended period of hot, muggy, and unsettled weather, a
major pattern change is store for SNE starting Thursday afternoon. A
cold front will traverse the region late Wednesday into early
Thursday. Showers and thunderstorms could linger into Thursday
morning, otherwise drying out Thursday afternoon with PWATs dropping
to 0.70 inches. Dewpoints will also be falling Thursday afternoon
into the low 60s to even upper 50s and staying there through the
weekend, even into early next week. Temperatures will also be much
more comfortable Thursday and Friday with highs in the low to mid
80s, warming into the upper 80s for the weekend. Overnight lows will
also stay more comfortable in the low to mid 60s through the weekend.

As for precipitation, high pressure and rather dry airmass will drop
down from Canada helping suppress rain and thunderstorm chances
through the weekend. Next chance for precip looks to be early to mid
next week as a weak shortwave exits the Great Lakes.

&&

.AVIATION /18Z TUESDAY THROUGH SATURDAY/...
Forecast Confidence Levels:

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

18z TAF update...

Through 00z...

VFR, but increasing risk of showers/t-storms moving into
western MA 22-00z. Stratus deck may expand across ACK by 00z.
SW gusts to 20+ kt.

Tonight...Moderate confidence.

Weakening area of scattered showers/t-storms expected to move
across interior MA 00-02z. Stratus and patchy fog will develop
across Cape/Islands with IFR-LIFR conditions, otherwise VFR.

Wednesday...Moderate confidence.

Areas of stratus and patchy fog Cape Cod will lift by 13z, but
may linger through much of the day across ACK. Otherwise, mainly
VFR, but a line of showers/t-storms expected to move W to E
across SNE 18-00z with briefly lower conditions and strong wind.

Wednesday night...Moderate confidence.

Weakening showers/t-storms moving to the coast, otherwise
stratus and patchy fog will expand across Cape/Islands and south
coast. Some uncertainty how far north/inland lower cigs will
get.

KBOS TAF...High confidence in TAF. Weakening convection in the
form of a few showers may reach BOS around 02z. Increasing risk
for t-storms toward Wed evening.

KBDL TAF...High confidence in TAF. Moderate risk for a t-storm
00-02z. Increasing risk for t-storms after 21z Wed.

Outlook /Thursday through Saturday/...

Thursday: VFR. Chance SHRA.

Thursday Night through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Wednesday night: High confidence.

A ridge of high pressure off the Atlantic coast will continue
to result in persistent SW winds across the waters through
Wednesday night. There is a risk for thunderstorms on the
northeastern waters through sundown but the better chance for
thunderstorms making it to the waters will be on Wednesday
evening and Wednesday night. We will continue to deal with morning
fog across the southern waters Tuesday morning. Seas will be
reaching 5 ft along the southern waters overnight into
Wednesday.

Outlook /Thursday through Saturday/...

Thursday: Winds less than 25 kt. Local rough seas. Chance of
rain showers, isolated thunderstorms.

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night through Saturday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004.
     Air Quality Alert until 11 PM EDT this evening for CTZ002.
MA...Heat Advisory until 8 PM EDT Wednesday for MAZ003>007-010>021-
     026.
RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>007.
MARINE...Small Craft Advisory from 5 PM this afternoon to 8 AM EDT
     Thursday for ANZ254>256.

&&

$$

SYNOPSIS...BW/KP
NEAR TERM...KJC/BW
SHORT TERM...BW
LONG TERM...KP
AVIATION...KJC/BW
MARINE...BW/KP