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Office: BOX

FXUS61 KBOX 162323
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
723 PM EDT Mon Jul 16 2018

.SYNOPSIS...
Isolated showers and thunderstorms dissipate this evening, otherwise
mainly dry and muggy weather the rest of tonight.  A cold front will
bring a period of widespread showers and thunderstorms Tuesday
afternoon and night with localized torrential rainfall. Other
than perhaps some lingering showers early Wednesday, mainly dry
weather is anticipated for the rest of the work week with warm
afternoons but lower humidity. Unsettled weather may return
sometime next weekend along with increasing humidity.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
7pm update...
Initially, convection formed along the differential heating
provided by the higher terrain in interior MA/CT. However, once
the sea breeze began to penetrate inland, and outflow boundaries
were formed, these proved to be the focus for realizing the
1500-2000j/kg of ML CAPE across MA. Storms have been self
dissipating given a lack of shear to tilt updrafts. Most storms
topping out around 35kft or lower.

With the loss of diurnal support, storms are likely to
dissipate, but there remains enough elevated instability to
maintain some spotty SHRA/TSRA through the late evening hours.
However, as the gradient flow out of the SSW continues to yield
a marine influenced airmass they should continue to drop off
toward the early AM hours.

Overnight looks to be more of a stratus rather than fog night,
as mixing near the sfc may limit the ability for dense fog to
form. At this time, only expecting dense fog in the usual areas,
Cape/Islands through the evening, before its likely to be pushed
offshore in these locations as well.

Lack of synoptic scale forcing and upper level ridging has kept the
weather dry in most locations this afternoon.  However, MLCapes on
the order of 1000 to 1500 J/KG and some subtle low level convergence
has allowed isolated showers/thunderstorms to develop.  While the
areal coverage of the storms was rather limited...the activity was
slow moving and producing very localized torrential rainfall.  We
should see this activity wind down this evening with the loss of
daytime heating and lack of synoptic support.

Otherwise, mainly dry weather prevails the rest of the night but it
will be quite muggy.  Given the high dewpoint air in place low
temperatures should only bottom out in the 70 to 75 degree range.
Given the cooling boundary layer...we do expect some low clouds to
overspread the region from southeast to northwest as the night wears
along. The other big question is how much fog do we see overnight. A
fog bank has been flirting with the eastern MA coast much of the
day. While it is possible it may briefly advance further onshore
early this evening...winds will be shifting to more of a south
southwest direction.  Therefore...if this does occur expect it to be
short-lived.  So while we may see patchy fog develop overnight in
many locations...feel the widespread fog will generally be confined
to the south coast, Cape and Islands given the south to southwest
wind direction.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
* Localized urban flash flooding possible Tue afternoon/night
* Isolated severe thunderstorms possible Tue afternoon/evening

Tuesday and Tuesday night...

Active weather on tap for the region as a cold front approaches from
the west.  This will tap an anomalous airmass in place with a
southerly LLJ/Pwats around 2+ standard deviations above normal.
There is plenty of forcing along the front so expect a period of
widespread showers and thunderstorms Tuesday afternoon and night.
The bulk of the activity should come to an end across the interior
by midnight and wind down along the coastal plain by daybreak.

The two main concerns revolve around a localized flash flooding
threat along with the potential for a few severe thunderstorms.  We
will discuss them both below.

1) Heavy Rain/Localized Urban Flash Flood Potential:

Pwats in excess of 2 inches along with 70+ dewpoints, strong
forcing, and modest instability has us concerned for the potential
of localized mainly urban flash flooding.  We also have southwest
flow which is parallel to the cold front and can allow for training
of the showers and thunderstorms.  This will result in the potential
for torrential rainfall and perhaps localized rainfall amounts of 2+
inches in one hour.  If these amounts are realized over some of our
urban centers...the threat is there for localized urban flash
flooding. The greatest forcing/instability appears to be along and
especially northwest of the Boston to Providence corridor.  The next
shift may have to consider a Flash Flood Watch if confidence
increases. Regardless...pockets of heavy rainfall will certainly
result in poor drainage street flooding with the main time frame of
concern Tuesday afternoon and night.

2) Severe Weather Potential:

The threat exists for a few severe thunderstorms Tuesday afternoon
and evening...with the primary threat being locally damaging
straight line wind gusts.  Appears there will be enough time to
destabilize...despite some clouds for 1500 to 2500 J/KG of Cape to
develop. The main area of concern is northwest of the Boston to
Providence corridor...with the highest risk across western and
northern MA where guidance indicates better shear/updraft helicity.
There also will be some low level helicity...so can not rule out a
rotating thunderstorm or two.  While we do see the potential for
isolated pockets of wind damage...there are some factors that we
think will prevent this from being a widespread severe weather
outbreak.  The 0 to 6 km shear is 25 to 35 knots which is okay...but
probably not enough for widespread severe weather.  Another limiting
factor is poor mid level lapse rates...but 70+ dewpoints will be
able to compensate for that to some degree.  All in all...a few
strong to severe thunderstorms seem like a reasonable bet mainly to
the northwest of a Boston to Providence corridor.

High temperatures should reach well into the 80s on Tuesday and it
is possible that a few locations break 90.  This will depend upon
how much partial sunshine is realized during the first part of
Tuesday.  If this occurs its possible that heat index values reach
into the middle 90s for the second day in a row across portions of
the region.  Confidence was not high enough to issue a Heat Advisory
at this point...but later shifts may have to take another look.  Low
temperatures by daybreak Wednesday should be down into the 60s to
near 70 as cooler air begins to work into the region behind the cold
front.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

* Dry and less humid for the rest of the work week
* Showers/storms may return next weekend

Overview...
16.12Z guidance is in relatively good agreement through the
weekend, at which point there are some disagreements with a
deepened trof and development of sub-tropical low pres near
Florida or the Carolinas. While these will gradually converge
through the week, the synoptic pattern is such that a general
consensus can be used for this long term forecast update. The
trof from Tue-Wed will give way to rising mid and upper lvl
heights, an amplification process forced by digging wave into
the N-central CONUS. This trof, as it continues to deepen and
shift E will become the primary driver of wet and unsettled
conditions for the late weekend and early next week.

Temperatures...
H85 temps beginning Wed average around +12C. With continued
warm advection and height rises through Fri, they are expected
to reach around +15C. This suggests a gradually warming trend
through the late week, with seasonably mild highs throughout,
mainly in the mid 80s away from the coastlines, where sea
breezes will keep some areas in the 70s especially Thu, when sfc
pres gradient is weakest. Cooler onshore flow is expected Sun,
with inverted ridging implying flow off the Gulf of Maine.
Humidity increases Sun into Mon as the subtropical moisture
stream is driven N into New England by the upstream trof.
Overall, seasonably summer like with most comfortable lows in
Wed night and Thu night before the humidity increases.

Precipitation...
With rising heights/ridging yield sfc high pres development
beginning Wed, dry wx is expected to prevail into at least Sat
based on mean timing. By Sun/Mon, with increasing PWATs (overall
average is about 2.00in or +1 std deviation above normal).
Given the moisture is of subtropical, or even tropical origins,
periods of heavier rain are possible, which will need to be
monitored. Regarding instability, it will be dependent on how
deep within a developing warm sector S New England will get, but
with deep S flow delivering marine layer, this may inhibit
destabilization. Overall, dry through early in the weekend, with
a shift toward wet, unsettled conditions late weekend and early
next week.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

No significant changes with the 00Z forecast update...

Tonight...Moderate confidence.
Mainly VFR this evening...but should see MVFR-IFR ceilings
advect in from southeast to northwest as the night wears along
and boundary layer cools. Lowest ceilings most likely across
southeast New England. Timing is uncertain...but this first
should occur by early evening across the Cape/Islands and work
northwest. Most of the widespread fog will likely be confined to
the south coast, Cape, and Islands with the southwest winds
expected.

Tuesday and Tuesday night...Moderate confidence. Improvement to
MVFR to even VFR conditions should occur in most locations by
Tuesday afternoon except lower cigs/vsbys may prevail across
portions of the Cape and Islands. However, a period of
widespread showers and thunderstorms with locally torrential
rainfall are expected mainly Tuesday afternoon and night. A few
of the storms may result in localized strong wind gusts. IFR to
even LIFR conditions can be expected in the heavier showers and
thunderstorms. Lastly...a period of southwest wind gusts around
25 knots are anticipated along the coastal plain Tuesday
afternoon with good mixing near the land. Improvement should
occur from west to east after midnight.

KBOS Terminal...Moderate confidence in TAF. Still need to watch fog
bank offshore that will flirt with the coast through early
evening...but should be come less of an issue after that time as
winds shift to more of a southerly direction.

KBDL Terminal...Moderate to high confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday through Thursday: VFR.

Thursday Night: Mainly VFR, with local IFR possible.

Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Patchy BR.

Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Tonight...High confidence.  Winds and sea should remain below small
craft advisory thresholds.  Low risk that an isolated thunderstorm
impacts a small section of our near shore waters this evening.

Tuesday and Tuesday night...High confidence.  Near shore southwest
wind gusts near 25 knots are expected to develop Tuesday afternoon
with good mixing ahead of the approaching cold front.  We have
hoisted a small craft advisory for this region.  In addition...long
southwest fetch will result in seas building to between 3 and 5 feet
Tuesday into Tuesday evening across the outer waters...where we have
also posted headlines. A few strong thunderstorms are possible late
Tue into Tue night mainly across our near shore waters north of
Boston.

Outlook /Wednesday through Saturday/...

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

Wednesday Night: Winds less than 25 kt. Seas up to 5 ft.

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

Thursday Night through Friday Night: Winds less than 25 kt.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from noon to 6 PM EDT Tuesday for
     ANZ230>234-236-251.
     Small Craft Advisory from 8 AM to 8 PM EDT Tuesday for ANZ235-
     237.
     Small Craft Advisory from 6 AM to 11 PM EDT Tuesday for ANZ250-
     254>256.

&&

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