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FXUS61 KBOX 101831
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
231 PM EDT Mon Aug 10 2020

.SYNOPSIS...

High pressure offshore to the south will bring a flow of hot and
humid air into Southern New England today through Wednesday.
Pop up showers and storms possible today and Tuesday, though
most locations will remain dry. Increasing chances for showers
and storms on Wednesday and Thursday. Seasonable temperatures,
dry conditions and much lower humidity levels return Friday into
the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

2 PM Update...

Temperatures still too cool based on observations, so bumped
temperatures up to the 95th percentile of guidance. Remainder of
the forecast is on track. Still could see a few pop up
shower/storms this afternoon due to strong heating.

10 AM Update...

Bumped up dew points and temperatures across the region.
Forecast was a tad bit too low compared to observations. Upped
the temperatures to the 90th percentile of guidance and dew
points to roughly the 85th percentile (looked good in comparison
to the current obs). Results in heat indices in getting into the
mid to upper 90s.

On top of this increased the wind speed and gusts along the
south coast toward the HRRRE as it has done well with the sea
breeze the past couple of days. Will need too keep an eye on
the breeze as it advances north/northwestward as it may be a
focal point for convection developing this afternoon as
indicated by the last few runs of the HRRR.

7 AM Update:

Heat Advisory still remains valid with no changes to those
headlines. However did make a few adjustments to the forecast
per trends and latest guidance.

First was to increase sky cover a bit and add patchy fog to
parts of the interior. Nighttime microphysics RGB imagery and
METARs showing that pooling moisture in light/calm winds has
generated a larger covg of stratus and mist. Should be short
lived though (thru mid morning) as winds pick up and strong
sunshine erodes any fog and/or low stratus.

Next was to increase PoPs a bit after 17z with mention of
isolated to scattered TSRA. Likely to be under subsidence for
the morning/early-aftn hrs with shortwave ridging in place.
However already a few showers popping up in west-central NY on
the western end of the shortwave ridge axis. It's definitely a
weakly-forced situation today which reduces forecast confidence
a bit. But tough to ignore signal in past couple HRRR runs, the
NSSL WRF and NAM-3km in depicting scattered storms off the
Berkshires and Catskills in NY moving SE into northern
CT/central MA into northern RI, with more isolated coverage
possible on sea- breezes. CAPEs look fairly low, but given the
strong heating expected today, it's possible a stronger storm
could produce locally gusty winds. Too remote a chance to
include gusty wind wording in the grids but won't totally waive
off the need for an SPS or two if storms can get going today.
Timing looks to be after 17z until sunset.

Should be even warmer than yesterday areawide. Boston again runs
the risk of seeing a sea-breeze which could keep temps from
rising too fast, but should again reach the 90 degree mark
today...either if the sea breeze stays just offshore or once
the winds flip back to synoptic SW late in the day. If Windsor
Locks reaches 90 degrees or warmer today, it'd be the 3rd
straight day of 90+ highs, making it a "heat wave" for BDL.

Previous discussion...

High pressure across the Southeast USA and West Atlantic ocean
brings southwest winds at all levels to Southern New England. This
will mean a warming trend surface and aloft. Daytime heating will
cause a mixed layer to at least 850-mb and possibly to 800-mb. The
mixing will tap temps equiv to 17-18C at 850-mb, supporting max sfc
temps of 90 to 95. That will be the target range of max temps.
Southwest winds will be onshore along the South Coast, keeping max
temps a few degrees cooler...generally in the 80s.

One concern for the afternoon is convection. Widely scattered
convection fired yesterday. Conditions are similar today, with a
moist layer in the upper part of the mixing and dry air at mid-
levels. PW values between 1 inch and 1.4 inches is not impressive
for the summer, but may be enough to support a shower. CAPE, per the
GFS, may reach 500-700 J/Kg and possibly a little higher. LI
values will be near minus 3. Convective models are so-so...two
show no development, another two show showers in CT. The
forecast will continue to advertise widely scattered
showers/storms. Probability remains low, about 15-24 pct, and
subsidence from the ridge suggests a fair amount of sunshine.

Second concern is the heat. Dew points are expected to be 68 to 72,
which combined with the expected temperatures will bring heat index
values in the low to mid 90s, with a broad area from the CT Valley
through Nrn RI into Eastern MA showing mid 90s. With conditions
Tuesday expected to be warmer, two-day heat advisory conditions
still look on target. The Heat Advisory starting today will continue.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight...

Any showers/storms dissipate in the evening with the loss of
heating. Expect rain-free weather but with continued high humidity
with dew points around 70. The high dew points will support fog
formation mainly along the South Coast. Min temps will be 68 to 73.

Tuesday...

Repeat performance on Tuesday, with lots of solar heating causing
mixing to between 800-850 mb. Temps at that level will be a little
higher than today, about 19-20C, which will support max sfc temps in
the mid 90s. Dew points will continue in the upper 60s and lower
70s, with heat index values inland in the mid to upper 90s. The Heat
Advisory continues through the day.

There will be another moist layer in the upper part of the mixed
layer, and similar instability. The forecast will again feature
widely scattered showers/storms, mostly across the interior and
North Shore.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Very warm with high humidity levels leads to continued elevated
heat indices on Wed.

* Slow-moving front Wed into Thurs brings solid chances for showers
and storms. While not everywhere gets wet, localized downpours
possible.

* Seasonable temps with much lower humidity levels return Fri into
the weekend.

Details...

Tuesday Night through Thursday:

Will still be contending with elevated heat indices at least on Wed.
By Wed and Thurs, a slow-moving cool front acting on anomalously-
high PWAT air will favor solid chances for showers and t-storms,
ones which could bring localized downpours.

The cool front will still be making its way across eastern New York
by Tues night into early Wed, with a continued SW feed of moisture
leading to a very warm and muggy night. There will be more in the
way of clouds into Wed as the cool front makes its way into the
Berkshires. But the combination of temps in the mid 80s to low 90s
and dewpoints in the low to mid 70s should still produce heat
indices in the mid/upper 90s on Wed along and south of the Mass Pike
(excluding the South Coast/Cape and Islands). It's possible the
existing Heat Advisory may need to be extended into Wed in later
updates, but will wait another cycle of model runs before making
that determination. The cool front will be more or less bisecting
Southern New England early on Thurs, and while it will still be warm
and muggy south of the boundary (mainly our CT, RI and SE MA areas),
lower high temps and prospects for showers/storms should permit sub-
Advisory heat indices.

While not everyone gets wet, given good consistency have raised
PoPs Wed into Thurs as high as lower-Likely (about 60%). On
Wed, best chances for rains are mainly north and west of the
Mass Pike but could extend as far south as I-95. The front will
continue to provide enough lift to keep showers and/or storms
going and/or re-developing into Wed evening for most of the
area. By Thurs, best chances for rains become more focused in CT
eastward into SE MA and the Cape and Islands, with lesser PoPs
north of the MA/RI/CT border. Model soundings each day don't
reveal much instability (tall, skinny CAPE profiles) and flow
aloft is weak. Localized downpours look to be more the threat
versus severe, particularly given deep warm-cloud depths between
12-13 kft offer a signal for efficient downpours, PWAT values
~2", and indications for re-developing storms along a slowing
front. Still unclear at this point if and which areas would be
of greater concern for hydro. Localized issues could pop up in
urban areas, other areas possible but TBD depending on prior
day(s) rain. Many could use the rain, but just not all at once
or in the same areas.

Thursday Night into Friday:

Recent trends in ECMWF/GFS guidance indicate the front will have
pushed far enough south Thursday night to allow for drier conditions
to develop, though dissipating showers should be the trend thru
midnight across CT, southern RI into the South Coast/Cape and
adjacent waters. By Friday, northwesterly 500 mb flow should keep
the effective frontal position near Long Island, with a surface high
building into ME and allowing for a period of cooler, much drier and
more stable onshore flow to begin. May be some spotty light showers
on the western periphery of the ridge in the lee of the Litchfield
Hills and into the Berkshires but not a washout.

Highs Friday in the low to mid 80s with more comfortable humidity
levels (dewpoints in the upper 50s to lower-mid 60s). A welcomed
break from the heat and humidity from earlier in the workweek!

The Weekend into Monday:

Broad high pressure over eastern Maine will ridge southward bringing
850 mb temps in the low-teens (+11 to +13C range) to Southern New
England, with an onshore NE/E flow for at least a part of the
weekend. Should actually lead to a fairly nice weekend with partly
to mostly sunny conditions, low to mid 80s highs and low humidity
levels, though may have greater cloud cover across southwestern
areas.

Uncertainty becomes quite large moving into Sunday and Monday. GFS
shows a low developing on the stalled front to our south, which
returns back northward and delivers what would verbatim be a soaking
rain to much of the region Sunday into part of Mon. ECMWF on the
other hand maintains sfc ridging and tranquil conditions until at
least Mon night/Tues. Obviously that's quite a spread in outcomes
and will stick with NBM guidance until there's better agreement.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

Through 00Z...High confidence

VFR at all locations with generally SW winds around 5-10 kts.
BOS and PVD should see the sea breeze into this evening. Not out
of the question an isolated shower/thunderstorm moves over a
terminal. Have held off on the thunder mention due to
uncertainty in the coverage.

Tonight...High confidence

VFR with light SW winds. Will see stratus and fog developing
across the south coast. Could have MVFR to LIFR conditions with
the lowest anticipated at ACK.

Tuesday...High confidence

VFR with SW winds. Strong heating and the mixed layer growing
should result in some 15-20 kt gusts mid to late in the morning
through the afternoon at most locations. Isolated showers and
storms possible across the interior.

Tuesday night...High confidence

VFR conditions expected with SW winds around 5-10 kts.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF

Outlook /Wednesday through Friday/...

Wednesday: VFR. Chance SHRA, slight chance TSRA.

Wednesday Night: VFR. SHRA likely, chance TSRA.

Thursday: VFR. Chance SHRA, slight chance TSRA.

Thursday Night through Friday: VFR. Slight chance SHRA.

&&

.MARINE...

10 AM Update...

Increased southwest winds speeds and gusts. Could have
borderline Small Craft Advisory gusts (approaching 25 kts)
across Buzzards Bay, Vineyard Sound and Nantucket Sound this
afternoon. Rest of the forecast remains on track.

Previous discussion...

High pressure remains offshore. Expecting generally quiet
weather, although widely scattered showers/storms forming over
land may move off the coast this afternoon.

Stratus/fog may develop tonight, especially along the South
Coast. Visibility may reach 1-3 nm.

Tuesday will be similar to today, with quiet weather except for
the potential of widely scattered afternoon showers/storms.

Waves at or below 4 feet. Winds out of the SW at 10-15 kts with
gusts around 15-20 kts.

Outlook /Tuesday Night through Friday/...

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Patchy fog.

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

Wednesday: Winds less than 25 kt.

Wednesday Night through Thursday: Winds less than 25 kt. Slight
chance of rain showers, isolated thunderstorms.

Thursday Night: Winds less than 25 kt. Chance of rain showers.

Friday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Tuesday for CTZ002>004.
MA...Heat Advisory until 8 PM EDT Tuesday for MAZ003>007-010>018-
026.
RI...Heat Advisory until 8 PM EDT Tuesday for RIZ001>004.
MARINE...None.

&&

$$
SYNOPSIS...WTB/BL/Loconto
NEAR TERM...WTB/BL/Loconto
SHORT TERM...WTB
LONG TERM...Loconto
AVIATION...BL
MARINE...WTB/BL/Loconto
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