Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 270826
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
426 AM EDT Tue Jul 27 2021

.SYNOPSIS...

It will be very warm today ahead of a cold front that will bring
scattered showers and thunderstorms this evening into the overnight
hours. Some showers may linger Wednesday morning and it will be
quite a bit cooler than today. Another period of unsettled weather
is expected sometime Thursday into early Friday. Dry and pleasant
weather is expected to start this weekend.

&&

.NEAR TERM /UNTIL 4 PM THIS AFTERNOON/...

425 AM Update...

* Very Warm This Afternoon with mainly dry weather through 4 or 5 PM

The HRRR model indicates that the worst of the western wildfire
smoke plume that impacted the region yesterday will push offshore
early this morning. The model does indicate a second pulse of smoke
that may impact portions of the region especially east and northeast
MA later this afternoon, but not nearly to the extent of yesterday.

Otherwise, plenty of sunshine today and westerly flow ahead of a
cold front will allow for a very warm afternoon. This coupled with
925T near 23C should allow for highs mainly in the upper 80s to near
90 with a few locales perhaps hitting the lower 90s. It will be
somewhat humid too, but not quite as bad as it could be this time of
year with dewpoints generally in the lower 60s. Mainly dry weather
should prevail through 4 or even 5 pm given that the forcing will
still be to the northwest of our region with the cold front.

&&

.SHORT TERM /4 PM THIS AFTERNOON THROUGH 6 PM WEDNESDAY/...

Highlights...

* Scattered showers & t-storms mainly after 5 or 6 PM this evening
* Overall Severe Weather Threat Appears Limited/Isolated
* Some lingering showers into Wed morning with much cooler temps

Tonight...

As the cold front works into our region, we should see scattered
showers and thunderstorms enter western MA after 5 or 6 pm in
association with the cold front. The dynamics are quite impressive
for this time of year with 0 to 6 km on the order of 35 to 45 knots.
However, given the late arrival of the cold front and waning
instability the severe weather threat will probably be
limited/isolated. If surface dewpoints do not exceed 65 degrees,
MLCapes will probably be held to between 800 and 1500 J/KG. That
being said; steep low level lapse rates and strong dynamics may
allow for an isolated damaging wind threat this evening. The
greatest risk for this will be across western MA, where the activity
will arrive the earliest.

It probably will take until after 8 or 9 pm for the scattered
showers and thunderstorms to reach the Boston to Providence
corridor. We do expect this activity to survive to the coast this
evening given the strong dynamics and forcing along the cold front.
Again, the severe weather threat will be limited/isolated with less
than ideal instability. However, given the impressive 0-6 km shear
for this time of year isolated pockets of damaging wind gusts can
not be ruled out through the evening hours.

After midnight; we may see another area of showers and a few
embedded thunderstorms develop behind the cold front. This in
response to a surface wave that developing along the sagging frontal
boundary. Low temperatures will mainly be in the 60s.

Wednesday...

Some showers will linger into Wed morning given the surface wave/ana-
frontal forcing behind the cold front. The bulk of the showers will
probably be over by Wed afternoon as drier mid level air works into
the region, but a few showers will remain possible across the
interior. Cloudy skies in the morning will probably give way to
partial sunshine Wed afternoon, but a much cooler airmass behind the
front will keep high temperatures generally in the middle to upper
70s. In fact, portions of the eastern MA coast may only see high
temps in the lower 70s given the NE winds.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

* Cooler than normal temperatures through early next week.
* Multiple frontal passages provide chance for showers and
  thunderstorms with periods of dry weather in between.
* The weekend looks to feature mainly dry and pleasant conditions.

Details:

We come under the influence of a persistent upper level trough
through early next week. Not surprisingly, the CPC has 70 percent
probability of below normal temperatures for our area in the 6 to 10
day outlook. Normal highs for the last week of July are in the low
to mid 80s. In general, expect highs to be 5 to 10 degrees below
normal on most days with lower humidity to boot. So talk about a
taste of early fall! Precipitation wise, there is less confidence
but the latest CPC 6 to 10 day outlook has 40 percent probability of
below normal precipitation. This is good news for communities hoping
to catch a break from the anomalously wet pattern. All four of our
climate sites have recorded the top 5 wettest July in station
history. BOS 9.54 inches (2nd), PVD 6.93 inches (3rd), BDL 9.87
inches (3rd) and ORH 12.86 inches (1st). Multiple frontal passages
will provide the chances for showers and thunderstorms with periods
of dry conditions sandwiched in between.

It does look like the weekend would feature mostly dry and pleasant
conditions, with Saturday looking like the pick of the weekend, with
cool and crisp conditions. In fact, if the winds decouple overnight
Friday, some locations could drop into the 40s on Saturday morning
given that the core of the cold air looks to be crossing Northern
New England. Temperatures at 500mb drop to -15C, which is near the
minimum moving average for this time of the year according to the
SPC sounding climatology for Chatham, MA. So talk about a taste of
fall! The only kink in the forecast is whether we could see a return
of wildfire smoke.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...

Forecaster Confidence Levels.

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

06Z Update...

Today...High Confidence. MVFR to IFR visibility in fog and smoke
early this morning should improve to mainly VFR by mid morning.
Mainly VFR conditions should continue through 20 or 21z. While some
more wildfire smoke will be possible this afternoon; do not think we
will see the widespread reduction in visibility that occurred
yesterday. SW winds mainly between 5 to 10 knots, but some localized
sea breezes will be possible along the eastern MA coast this
afternoon.

Tonight...Moderate Confidence. A cluster of scattered showers and
thunderstorms will work into western MA after 21z or 22z and push
towards this coast later this evening. Isolated strong wind gusts
will be possible with the activity. Some more showers and a few
embedded thunderstorms may develop after midnight. This will result
in MVFR to localized IFR conditions with this activity. In addition,
low clouds will move in overnight resulting in more MVFR & localized
IFR conditions overnight. SW winds shift to the N at 5 to 10 knots.

Wednesday...Moderate Confidence. MVFR conditions will dominate
during the morning with some lingering showers. Improvement to
mainly VFR should occur during the afternoon. NNE winds of 5 to 10
knots, but a bit stronger along portions of the coast.

KBOS Terminal...Moderate Confidence. Main issue is the likelihood of
an ESE sea breeze developing this afternoon. The main threat for
scattered showers & t-storms in the vicinity of the terminal should
not be until after 00z.

KBDL Terminal...High Confidence. Main risk for scattered showers &
thunderstorms in the vicinity of the terminal will not be until
after 23z or 00z.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR.

Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA.

Thursday Night: Mainly MVFR, with local IFR possible. Chance
SHRA.

Friday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance SHRA.

Friday Night through Saturday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels.

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

Today through Wednesday...High Confidence. Winds/Seas should
generally remain below small craft advisory except for perhaps a
brief time Wed morning. Southerly flow of 5 to 15 knots into
early this evening will shift to the NNE overnight into
Wednesday. Biggest concern will be a brief surge of NNE 20 to 25
knot wind gusts and choppy seas Wednesday morning into the mid
afternoon hours. Conditions probably remain just below small
craft thresholds, but it may be close for a short time.

/Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.

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

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

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

&&

.HYDROLOGY...
No active river flood warnings are in effect for rivers and
waterways across southern New England.

However, after having a tremendous amount of rainfall from early
to mid July, many rivers and streams are running much higher
than normal. This is resulting in swift currents that would not
typically be seen in the summer. These swift currents can catch
those swimming or tubing/kayaking on area waterways off guard.
There have been 3 total reports of drownings in the last week on
the Farmington River in CT and Squannacook River in MA given
the higher than usual flows and swift currents.

Those with interests swimming, tubing or kayaking on area
rivers and waterways should be aware that swift currents due to
high flows are occurring. This can result in dangerous
conditions, and it is generally discouraged to swim, inner-tube,
or kayak until flows decrease.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...Air Quality Alert until noon EDT today for RIZ001>008.
MARINE...None.

&&

$$
SYNOPSIS...Frank/Chai
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Chai
AVIATION...Frank/Chai
MARINE...Frank/Chai
HYDROLOGY...Staff


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