Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 220209
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1009 PM EDT Tue Aug 21 2018

.SYNOPSIS...
A low pressure system will bring two rounds of scattered showers
and embedded thunderstorms. The first very late tonight into
Wednesday morning, then a second round Wednesday afternoon along
with the return of high humidity. High pressure will then
follow, bringing mainly dry and comfortable weather Thursday
and Friday. Dry weather will likely persist through the weekend
although it will turn a bit warmer. Very warm to hot
temperatures along with humid conditions may return by early
next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

1005 pm update...

Some clear skies still hanging on over eastern MA into northeast
RI. However this will be very short lived as strong WAA pattern
evolving over the northeast, as 100 kt upper level jet streak
entering the Great Lakes (per SPC mesoanalysis) ejects 2.3 inch PWAT
airmass from the Mid Atlc northward into New England. Still
expecting showers with embedded T-storms and heavy downpours to
overspread the region 06z to 12z west to east across the region.
Heavy downpours will impact morning commute but heavy rain may
not be as widespread as earlier thought. Anyway, previous
forecast remains on track. Earlier discussion below.

==================================================================

Dry weather persists this evening, but low clouds will continue
to increase in coverage and may see some fog develop too with
cooling boundary layer/increasing low level moisture.

Appears the threat for showers and embedded t-storms will be
mainly after 2 or 3 am across interior southern New England and
near daybreak along the coastal plain. Main concern continues to
be brief heavy rain/localized street flooding which may impact
part of the morning commute. Odds favor storms being sub-severe
as surface instability appears to lag behind warm front/higher
low level helicity. Nonetheless...given we have a modest LLJ/70+
dewpoints working northward along with a warm front can not
rule out the low risk for an isolated severe t-storm/very
localized wind damage. While unlikely, this does include the
low risk for a brief tornado/waterspout.

Previous Discussion:

Overnight, the main concerns will be the progress of a low
pressure and its warm front. Have high confidence in this low
pressure moving through the Great Lakes into Canada. have lower
confidence in the timing of a warm front. Many members of the
21/12Z guidance suite suggest this front could move into
northern New England before daybreak. Thinking is this likely a
bit too quick. Have more confidence in a warm frontal passage in
the 8-11 AM Wednesday time range.

This could be key to weather outcomes. The higher dew points
will be south of this warm front. These higher dew points will
provide the fuel for local downpours, as well as lowering cloud
bases. Some of the higher resolution short-term guidance suggest
high values of shear in the 0-1 km layer. Thus, still need to
be wary of rotating thunderstorms.

A caveat to this will be the time of day. Instability should not
be very high overnight, but would be a little higher if our
region can tap into the higher dew points south of the warm
front. At this point, have much more confidence in the potential
for locally heavy rainfall.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...

* A few strong to severe storms possible
* Localized street flooding possible, too

Wednesday...

Once our region firmly gets into the warm sector, looking at a
strong southerly low level jet to help recharge the instability
for Wednesday afternoon. Current timing for a cold front is
during the afternoon and evening. The quicker sunshine develops
in the morning, the greater the risk for strong to severe
thunderstorms during the afternoon and evening.

Otherwise, still expecting another day of higher heat and
humidity. This should start to an end later Wednesday night, as
drier air arrives behind a cold front.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Dry and comfortable weather Thu into the weekend
* Very warm to hot/humid weather likely returns early next week

Thursday and Friday....

Large high pressure will be in control of our weather Thursday
and Friday. This will bring rather cool nights with lows mainly
in the 50s...but some upper 40s possible in the normally coolest
outlying locations. Highs should be in the upper 70s to the
lower 80s with plenty of sunshine/low humidity.

Saturday and Sunday...

The high pressure system slowly moves off the coast this
weekend. However, mainly dry weather will likely prevail with
the lack of synoptic scale forcing. Temperatures will also
moderate a bit, mainly looking at highs in the lower to middle
80s.

Monday and Tuesday...

Upper level ridge to our south will pump a west southwest flow
of unseasonable warmth/humidity into southern New England early
next week. This is supported by most long range guidance, but
uncertainty in regards to just how above normal temperatures
get. The potential exists for a return of 90+ degree heat. There
probably will also be at least some risk for a few
thunderstorms, but low confidence especially in regards to
timing.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

10 PM update...

No major changes with TAFs. Still expecting showers with
embedded thunder to overspread the area 06z to 12z west to east.
Earlier discussion below.

================================================================

Tonight...Moderate confidence. Low end MVFR-IFR become
widespread yet again but timing remains uncertain. LIFR
conditions are possible too particularly along portions of the
coast/high terrain in low clouds and fog. Showers and embedded
t-storms with locally heavy rain overspread the region from
southwest to northeast between 6z and 12z. LLWS possible toward
daybreak.

Wednesday...Moderate confidence. Widespread SHRA with embedded
TSRA should end during the morning from west to east.
Additional, but more hit and miss showers and t-storms expected
to develop during the afternoon. Lower cigs/vsbys should improve
to perhaps mainly VFR-MVFR by mid to late afternoon, but that will
be outside of any additional scattered showers and thunderstorms.

Wednesday Night...

Mainly VFR.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Thursday through Sunday/...

Thursday through Saturday Night: VFR.

Sunday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...High confidence.

Low risk for a brief waterspout late tonight as a warm front
moves across the waters. Vsby lowered at times in morning fog
and scattered thunderstorms, then improving before second round
of thunderstorms in the afternoon.

Some risk for Small Craft Advisories along the southern outer
coastal waters due to marginal 25 kt gusts and 5 ft seas.

Outlook /Thursday through Sunday/...

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

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

Friday through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from noon to 8 PM EDT Wednesday for
     ANZ232-233.
     Small Craft Advisory from noon Wednesday to 11 AM EDT Thursday
     for ANZ235-237.
     Small Craft Advisory from 9 AM Wednesday to 7 AM EDT Thursday
     for ANZ250.
     Small Craft Advisory from 9 AM Wednesday to 6 PM EDT Thursday
     for ANZ254>256.

&&

$$
SYNOPSIS...Belk/Frank
NEAR TERM...Belk/Frank/Nocera
SHORT TERM...Belk
LONG TERM...Frank
AVIATION...Belk/Frank/Nocera
MARINE...Belk/Frank


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