Lexington Weather

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 180802
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
402 AM EDT Sun Aug 18 2019

.SYNOPSIS...
Very warm and humid weather will persist through midweek, along
with showers or thunderstorms at times. A cold front will cross
the region later in the week, followed by cooler and drier
conditions.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
3 AM Update:

Last of showers was moving off Cape Ann while other showers/storms
were moving SE off Long Island NY. Airmass has stabilized for 
the time being so no additional activity is expected this
morning.

Plentiful low level moisture and light winds has led to areas of
dense fog, especially along RI/MA South Coast including Cape Cod
and Islands. We will issue a Dense Fog Advisory for these areas
through 9 AM, since observations and webcams indicate the dense
fog is fairly widespread. Elsewhere, fog will be more patchy in
nature, mainly affecting areas where it rained last night.

Weak front drops SE across region early this morning, then
returns N as a warm front this afternoon. Cross sections
indicate fog/low clouds will burn off quickly by mid to late
morning, except early this afternoon on parts of Cape Cod and
Nantucket. Models show potential for good instability this
afternoon as temperatures rise well into 80s and dewpoints hold
in lower 70s. MLCAPE should reach as high as 2000 J/kg but mid
level lapse rates are weak and 0-3km shear is a bit lower than
yesterday, only around 20kt. While main synoptic scale forcing
will be located to our west near the approaching short wave,
combination of instability and some lift is expected to lead to
scattered showers and storms, especially west of Worcester
County. Individual pulse storms may be able to form small bows
or line segments with potential for isolated wind damage in
stronger storms, as well as localized minor flooding. HREF shows
this scenario well, keeping higher probs for stronger updraft
helicity to our west.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
3 AM Update:

As we saw last night, showers and storms will linger into
evening due to lingering instability, although threat of strong
or isolated severe storms should diminish quickly around sunset.
Another warm and muggy night is ahead along with return of low
clouds and fog, which may be dense again along South Coast, Cape
Cod, and Islands. Lows only fall back into mid 60s to lower 70s.

Next short wave and weakening front approach SNE Monday. Temps
aloft become a bit warmer which supports a hot and humid day
with highs in 80s to around 90, and heat index values of up to
105. Later shifts will need to consider Heat Advisories for
parts of area.

While this will also lead to higher CAPE values, these
warmer temps aloft should help keep a lid on organized shower
or thunderstorm activity, although we should still see a few
storms try to develop later in the day. Despite weak mid level
lapse rates and weak shear, higher CAPE may lead to a few strong
storms.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
18/00Z guidance remains relatively consistent with prior runs.
Still need to work out some timing issues. In general, as has
been discussed previously, a series of mid level shortwaves will
traverse over southern New England through mid week. With a 
very humid airmass in places, will be looking at the risk for 
some showers and thunderstorms at times. Some showers, and any 
thunderstorms, will be capable of localized downpours.

The combination of above normal temperatures and very humid
conditions will pose a risk for heat-related illnesses Tuesday,
too. Heat Advisories may need to be continued. 

A cold front will finally cross our region sometime Thursday
into Friday. Once this front passes, high pressure should take
over. Humidity will become much more comfortable Friday into
Saturday with slightly below normal temperatures expected. 

&&

.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...Moderate confidence.

Widespread IFR/LIFR through sunrise in low clouds and fog.
Conditions improve to VFR 12Z-15Z except closer to 18Z on Cape
Cod and Islands. Scattered showers and thunderstorms develop
during afternoon, mainly along and NW of a line from BDL-ORH-
LWM. 

Return to IFR/LIFR in low clouds and fog tonight, especially in
RI and E MA, as showers/storms subside. Repeat for Monday with
improvement to VFR by 15Z-18Z then another round of scattered
showers and thunderstorms.

KBOS Terminal...Moderate confidence in TAF. Conditions should 
lower to LIFR for a few hours before sunrise.

KBDL Terminal...Moderate confidence in TAF. Timing of IFR
conditions may be too soon.

Outlook /Monday Night through Thursday/...Moderate confidence. 

Monday Night: VFR. Slight chance SHRA with isolated TSRA.

Tuesday: VFR. Chance SHRA.

Tuesday Night: VFR. Chance SHRA with isolated TSRA.

Wednesday: Mainly VFR, with local MVFR possible. Chance SHRA
with isolated TSRA.

Wednesday Night through Thursday: VFR. Chance SHRA with 
isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence. 

Light S/SW flow and flat seas will prevail through Monday as a 
series of fronts weaken as they reach the coastal waters.
Pattern favors the usual increase in winds and seas this 
afternoon and Monday afternoon on nearshore south coastal 
waters with gusts of 15-20kt and seas locally 2 or 3 ft.

Main concern will be areas of dense fog this morning and again
late tonight and Monday morning.

Outlook /Monday Night through Thursday/...Moderate confidence. 

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers with isolated 
thunderstorms, patchy fog. 

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers. 

Tuesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers with isolated thunderstorms. 

Wednesday through Wednesday Night: Winds less than 25 kt. Areas
of seas approaching 5 ft. Chance of rain showers with isolated
thunderstorms. 

Thursday: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers with isolated thunderstorms. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 9 AM EDT this morning for MAZ020>024.
RI...Dense Fog Advisory until 9 AM EDT this morning for RIZ004>008.
MARINE...None.

&&

$$
SYNOPSIS...Belk/JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...Belk
AVIATION...Belk/JWD
MARINE...Belk/JWD