Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 111105
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
705 AM EDT Tue Aug 11 2020

.SYNOPSIS...

Oppressive heat and humidity will continue through Wednesday, though
today will be the hottest of the two days. A cold front will
sag southward across Southern New England tonight into
Wednesday, bringing showers and storms. Rather warm but mainly
dry weather follows Thursday and Friday behind the cold frontal
passage. High pressure building over the Canadian Maritimes will
bring a return to cooler/seasonable temperatures this weekend.
The overall dry weather pattern looks to continue through at
least next Monday.

&&

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

705 AM Update...

Scattered pockets of low clouds across portions of the CT River
Valley into southeast MA will burnoff over the next few hours.
Previous forecast captures this well and no significant changes
ave been made to this update.

A hot, humid and hazy day in store for Southern New England. Today
should prove to be the hottest and most oppressive day of this
stretch of hot/humid weather. Elevated to potentially dangerous heat
index values are anticipated.

Shortwave ridging aloft initially over Southern New England early
this morning, ahead of a trough over eastern MI associated with
yesterday`s severe convection that evolved across the Midwest.
Surface ridge of high pressure SW of Nantucket is allowing for
continued SW winds to tap in higher moisture/dewpoints. Upper 60s to
mid 70s dewpoints were already fairly common early this morning, and
while some of these dewpoints should decrease a bit in the afternoon
with mixing, it will still be quite sticky today.

The Heat Advisory remains valid with no changes made with the early-
morning update. However the trend for today is for less cloud cover
and more limited prospects for thunderstorms. Ambient air temps are
expected to rise even higher today than yesterday given more sun
than clouds. 925/850 mb temps are similar or about a degree warmer
than yesterday. Given the warm start, mostly sunny conditions and
that most of the climate sites reached 90-plus degrees (even
Worcester), I bring highs today in the mid to upper 90s for most
lower-elevation interior locales and upper 80s for the South Coast,
Cape and Islands. This change resulted in higher maximum heat index
values ranging from the upper 90s to low 100s. It`s not out of the
question that a few locations could approach or meet Excessive Heat
Warning criteria heat indices for a time. The best chance looks to
be the Merrimack and CT Valleys and into SE MA. Envisioned coverage
being no more than isolated precluded upgrading any areas at this
time, however. Despite the onshore SW winds, higher dewpoints near
the South Coast and the Cape could bring Barnstable County into heat
advisory levels but too limited here as well. Only sort of modest
break from the heat will be that SW winds may become somewhat
breezy, though it may just blow the humid/hot air around versus
provide relief.

Secondary to the heat/high heat indices will be the chances for
storms today. Though the heat and higher dewpoints should produce
afternoon CAPE values around 1000-1500 J/kg, am not optimistic
much if any storms develop this afternoon. The issue is that
there`s really nothing to trigger any convection with SW non-
convergent flow. Even shallow cumulus may be hard to come by.
CAMs provide really mixed signals before the evening (more on
that below). Most should be dry for the afternoon, but one area
that could be a focus for isolated storms is along the Route 2
corridor into the Merrimack Valley. There is a subtle wind shift
that could be enough of a trigger in those areas between mid-
afternoon to early evening. Weak flow aloft supports pulse-type
storms that could produce heavy downpours, as well as localized
strong wind gusts given robust low-level lapse rates achieved
through strong daytime heating.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...

Tonight:

We start to get into somewhat stronger 500 mb height falls by early
this evening, as a cold front from eastern NY slowly approaches the
Berkshires. Higher-res models blossom thunderstorms along it across
central NY, and most of these shift eastward into western MA/western
CT between 00-03z (after sunset). Though a couple storms could still
be strong, given the time of day and the lack of better mid-level
lapse rates to sustain robust convection through the evening, expect
storms to be generally weakening on approach. Uncertain how far east
storms may make it before the weakening trend begins, but better
prospects for storms looks to be west of a FIT-ORH-IJD line. Carried
25-30 PoPs for scattered storms, then slight chances later in the
overnight.

With increasing clouds and continued though lighter southwest winds,
tonight has the potential to be even warmer than this overnight was.
And it certainly will be another muggy night. I bring lows up to the
70s for most, with upper 70s not out of the question in the metro
areas.

Wednesday:

The cool front will continue to be slowly progressing southeast into
Wednesday. This will provide a focus for showers and thunderstorms,
which should tend to shift southeastward from roughly Boston-
Worcester southward into CT/RI and SE MA. CAPE values look more
limited than today, mainly from lack of greater heating. However
parameters favoring heavy downpours, including deep warm cloud
depths (12.5-13kft) and PWAT values around 1.8-2" should allow for
any storms to be efficient rain producers. Some potential for
localized urban or poor drainage flooding could ensue if storms
train over the same areas, especially near/south of the frontal
zone.

Heat Advisory continues into Wednesday, but maximum heat index
values now appear lower and more marginal. The net effect of
partial cloudiness and areas of rain/storms should temper high
temperatures into the mid 80s to low 90s. Dewpoints should be at
their highest on Wednesday though, and that should bring
maximum heat indices in the mid 90s. Less sun might make those
seem somewhat more tolerable than if the sun was fully out. For
now will maintain the heat advisory but we should see lower-end
advisory heat indices.

&&

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

Highlights...

* Still rather warm but less humid Thu/Fri and mainly dry

* Mainly dry/cooler this weekend with seasonable temps

* Overall, dry weather pattern continues into at least Mon

Details...

Thursday and Friday...

A cold front will press towards the south coast Wednesday evening,
bringing an end to most of the scattered showers and thunderstorms.
Generally dry weather will follow Thursday and Friday, but its
possible the front lingers near the south coast over this time.
Therefore, a low risk exists for a few spot showers/t-storms mainly
south of the MA Turnpike Thu and Fri; but regardless expect dry
weather to dominate. It will be somewhat less humid, but enough
heating should push highs well into the 80s just inland from the
coast to perhaps even a few lingering 90 degree readings on Thu.

This Weekend...

The main story this weekend will be surface high pressure dropping
south into the Canadian Maritimes. This will allow for an onshore
flow of cooler/seasonable temperatures across southern New England.
Humidity levels will also be quite comfortable for this time of year.
Given the trough axis will be to our east, expect dry weather to
dominate this weekend, but can not rule out a few spot showers by
Sunday with onshore flow.

Early Next Week...

Temperatures may moderate some by early next week as high pressure
moves east of the Canadian Maritimes, but not to the extent of what
we are currently experiencing. The overall dry weather pattern looks
to persist into at least Mon of next week.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

12Z TAF Update:

Today: High confidence in trends, moderate/high on thunder
coverage.

Any residual IFR/LIFR stratus near the CT River Valley into
southeast MA burns off by 13z to VFR all terminals. TSRA
potential during the mid to late afternoon looks really limited
to nil today, but is still somewhat uncertain; best chance for
TSRA before 00z is NW of BOS (Merrimack Valley/North Shore).
Winds S/SW 6-12 kt, with gusts to 20 kt starting to develop
around 15z. Sea-breeze should stay offshore.

Tonight: Moderate confidence.

VFR initially. A cold front will approach western MA/northern CT
from 00-06z with what should be weakening SCT TSRA along it.
Unclear exactly how far east any evening SHRA/TSRA threat may
encompass but best chance is west of ORH. Ceilings may lower to
sub-VFR initially South Coast/Cape 00-03z, then could overspread
further north to the Mass Pike after 06z. Lowest ceilings (LIFR)
for the Cape/South Coast. SW winds 6-10 kt initially, easing to
4-7 kt by daybreak.

Wednesday: Moderate confidence.

Thinking a mix of flight categories (VFR to IFR) as a slow-
moving and southward-sagging front produces renewed SHRA/psbl
embedded TSRA. Localized downpours possible that could reduce
visby. SW winds 4-8 kt south of the front, WSW/W winds at
similar speeds north of it.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF

Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR. Breezy. Slight chance SHRA, isolated
TSRA.

Thursday: VFR. Slight chance SHRA, isolated TSRA.

Thursday Night through Friday: Mainly VFR, with local MVFR
possible. Slight chance SHRA, isolated TSRA.

Friday Night: VFR.

Saturday: VFR. Breezy.

&&

.MARINE...
Wind and sea conditions should be below small craft advisory
criteria. That said, strengthening SW winds/gusts could get close to
SCA levels later today into early tonight across the northeastern
waters. For this area, wind gusts in the lower 20s-kt with seas
around 4 ft.

Areas of stratus and low clouds will continue early tonight and then
again into tonight. Foggy intervals as well, but stronger winds may
keep visbys in the 1-3 SM range.

Isolated storms possible near the adjacent waters in northeast MA
this afternoon, though coverage is limited. Better chances for
showers and thunderstorms overnight tonight into Wednesday.

Outlook /Wednesday Night through Saturday/...

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

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

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004.
MA...Heat Advisory until 8 PM EDT Wednesday for MAZ003>007-010>021-
     026.
RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>004-006.
MARINE...None.

&&

$$
SYNOPSIS...Loconto/Frank
NEAR TERM...Frank/Loconto
SHORT TERM...Loconto
LONG TERM...Frank
AVIATION...Loconto/Frank
MARINE...Loconto/Frank


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