Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 250216

Area Forecast Discussion
National Weather Service Boston/Norton MA
1016 PM EDT Mon Sep 24 2018

An approaching warm front will bring a soaking/heavy rain to the
region Tuesday with some poor drainage street flooding issues
possible. The steady rain should wind down Tuesday evening...but
scattered showers and a few thunderstorms will be possible through
the night with locally heavy downpours.  Warmer and humid weather
returns Wednesday with scattered showers/ thunderstorms in the
afternoon and early night. High pressure brings cooler less
humid weather for Thursday through Saturday. A cold front moves
through on Sunday with a chance of showers. Dry weather returns



1015 PM Update...

Forecast is generally on track...following some of the hi-res
guidance have sped up the timing of the leading edge of precip
slightly, but won`t have a big impact. The line which will usher
in our precip sheild is currently along the mid Jersey coast
moving slowly northward. Generally still expecting it to reach
western zones of the southern CT valley by about 09Z and
spreading northeast to the MA coast by 13-14Z. Adjusted cigs
down to MVFR for much of western/central MA as a mid deck of
clouds lingers ahead of the main leading edge that overspreads
the region with low-mid clouds by mid morning. Winds still out
of the northeast but will shift to southeast everywhere by late
morning as the warm front approaches.

Previous discussion...

Warm air aloft is moving up the
Appalachians, with increasing lift generating an area of rain
over PA which is moving northeast. Trends from both AWIPS and
manual means bring the leading pcpn to the CT Valley 08Z- 09Z.
This is within 1-2 hours of the existing forecast. We will
maintain the same timing with this update. Bottom line is the
same... rain in the west developing late tonight and expanding
east. One change in the forecast has been to expand the overcast
across most areas this evening, then across far NE Mass and
Cape Cod during the night.

Based on 7 PM temps, expect most low temps to be in the low to
mid 50s, but still possible that far northern MA touches the
upper 40s.



* Heavy Rain Tue with poor drainage street flooding issues possible
* Low risk for a strong t-storm late Tue night near the south coast



An approaching warm front will team up with Pwats rising to 2+ SD
above normal...while the southerly LLJ reaches 2-3+ standard
deviations above normal.  This will set the stage for a widespread
soaking/heavy rainfall on Tuesday as the cool dome at the surface
will provide a good over running situation for the high pwat
plume/strong low level jet.  Rainfall amounts of 1 to 2 inches
anticipated for much of the region...except lower amounts across far
southeast New England.  There may be some localized 3"+ amounts
across interior southern New England...where strongest 850 mb low
level jet is positioned.  The last two model runs of the ECMWF and a
few of the HREF members show this potential.

Overall...this heavy rain should result in typical nuisance poor
drainage street flooding issues.  While we can not rule out
localized more significant flooding if those 3"+ inch amounts
occur...appears to localized and confidence is not high enough to
issue a Flood Watch at this time.  Nonetheless...this needs to be

As for should remain rather cool through lunch
time with readings mainly in the 50s to the lower 60s.  The mildest
of the readings will be near the south coast.  Temperatures will
probably rise into the 60s in most locations by evening...except the
CT River Valley.

Tuesday night...

The steady/heavy rain should exit the eastern New England coast
during the evening as the strongest forcing exits the region.
However...Pwats will remain between 1.75 and 2 inches along with a
modest low level jet.  While precipitation will become more
scattered in nature...showers with localized downpours will remain
possible.  In fact...there might be a rumble or two of thunder.

The one thing will have to watch is an area near the south coast
towards daybreak Wednesday.  A 30 to 40 knot low level jet with
dewpoints approaching 70 is always a concern this time of year.
Some of the high resolution guidance indicates the low risk for a
few strong thunderstorms near the south coast between 6z and 12z.
Certainly plausible given strength of the LLJ and climatology.  The
NAM is the most concerning given it has 1000+ J/KG of Cape...while
other guidance is less.

Temperatures should rise Tue night across much of the region and
actually may be near 70 in many locations by 12z Wednesday.
However...most high resolution guidance continues to show this being
a slower process across the CT River Valley which seems reasonable
given climatology.  Where low level cool air is trapped for a
time...will have to watch for locally dense fog.


Big Picture...

Ridges off the Pacific and Atlantic coasts with arctic low pressure
digging over Hudsons Bay and generating a trough over the Plains.
The Pacific high actually evolves into a Rex Block, diverting the
airflow up to Alaska and the Arctic coast before turning south and
bringing colder air into the Northern Plains. The Rex block
eventually breaks over the weekend. Meanwhile the Atlantic high
shifts west over the Southern USA over the weekend and early next
week bringing warmer air.

Shortwaves will move through the flow around the Central USA trough.
One moves up the St Lawrence Valley Wednesday, another shifts north
away from us and through Quebec Friday night-Saturday, and a third
moves through Northern Maine on Sunday.

Colder air moving south from Canada...warmer air moving over the
Southern USA. This suggests a zone of strong temperature contrast
developing over the Northern USA.



Shortwave moves through the St Lawrence Valley with a supporting
upper jet of 140-150 kts. Our area is in or close to the right
entrance region of the jet from afternoon through Wednesday night.
Precipitable Water values, after backing off during the morning,
surge back to 2 inches from mid-afternoon through early night.
Stability indices are favorable for convection, primarily from mid
aftn to early night. Model consensus would place cold frontal
passage between 00Z west to 06z east.  Southwest winds at 950 mb
ahead of the front show 25-30 knots, much of which may mix to the
surface in gusts. Temperatures in the mixed layer support sfc temps
of 78-82F. Overall...a warm humid day with gusty SW winds and a
period of showers/scattered tstms.


The upper trough lingers over the Central USA and maintains a
southwest upper flow over New England. As the cold front moves
offshore it will become more parallel with the upper flow which will
slow down its departure.  As such there could be a lingering chance
of early morning showers/tstms along or off the South Coast and
Islands. All of this should move out to sea Thursday afternoon.

Otherwise surface high pressure builds over the region Thursday with
dry weather and cooler temperatures/lower humidity. The high moves
off to the east Friday. Temps aloft equiv to 3-6C at 850 mb will
support max sfc temps in the 60s


Two more shortwaves move through the upper flow during the weekend.
The first will move north through Quebec and away from our area, so
expect its associated cold front to either stall or dissipate by
Saturday morning, leaving us with dry weather. The second shortwave
is closer to the border, but could generate a few showers.


High pressure follows Monday with dry weather. At the end of the
forecast window, a warm front slides east toward Northern New
England and could generate a few showers in our area Monday night.


Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

Tonight...Moderate to high confidence.

Ceilings are around 3000-3500 feet, with a few spots 2500-2900
feet. A dry area over EC NY shows no ceilings. Lower ceilings
are moving up from Central PA, and should move into our area
overnight. Expect an uneven trend to 2000-3000 foot cigs duirng
the night, possibly lower toward morning as rain moves in.
Timing of the rain is 08z-09z in the CT Valley, then to Central
MA and RI toward sunrise.

Tuesday...High confidence. Widespread low end MVFR to IFR
conditions as a widespread soaking rain overspreads most of the
region by 15z...but perhaps a bit later across the Cape/Islands.
The rain will be heavy at times along with patchy fog. ESE wind
gusts of 20 to 30 knots develop on the coast...but inversion
results in lighter winds further inland especially across the
the CT River Valley. LLWS expected to develop during the

Tuesday night...Moderate confidence. Low end MVFR-IFR conditions
persist with localized LIFR conditions in fog. Widespread rain
diminishes during the evening...but scattered showers and a few t-
storms possible overnight with localized downpours. Low risk for a
strong storm or two near the south coast towards 12z.  LLWS expected.

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

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

Outlook /Wednesday through Saturday/...Moderate Confidence.

Wednesday and Wednesday night...

VFR with areas of MVFR cigs/vsbys in showers and thunderstorms,
ending from west to east after midnight.

Thursday 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/...

Tonight...High confidence.  High pressure over Quebec shifts east
towards the Canadian Maritimes.  While NE winds will shift to more
of an E direction...gusts of 20 to 30 knots are still expected over
the open waters.  Small craft headlines continue.

Tuesday...Moderate to high confidence.  Potent SSE low level jet at
925 mb of 40 to 50 knots sweeps across the region in association
with a warm front.  This will certainly bring strong small craft
wind gusts to all waters and perhaps brief marginal Gales.  In
coordination with GYX...we opted to hoist a Gale Watch Tue
afternoon/early evening across our northern waters where there
greatest risk appears. Strong small craft headlines elsewhere.
Widespread rain that will be heavy at times will also reduce vsbys
for mariners.  Seas build back to between 6 and 10 feet across our

Tuesday night...Moderate to high confidence.  Strongest LLJ lifts
northeast of the region as we enter the warm sector.
Therefore...winds should diminish but still expect 20+ knot wind
gusts to persist overnight with the LLJ and seas will remain above
small craft thresholds across most open waters through 12z. Scattered
showers and perhaps a few t-storms along with fog will also reduce
vsbys for mariners.

Outlook /Wednesday through Saturday/...Moderate Confidence.

Wednesday and Wednesday night... Small Craft Advisory may be needed.
Southwest winds gusting to 30 knots, diminishing after midnight.
Seas 5 to 8 feet. Showers, chance of thunderstorms, patchy fog.
Local visibility 1 nm or less.

Thursday and Thursday night... Small Craft Advisory may be needed.
North to Northeast wind gusting 20 to 25 kt early, diminishing
during the afternoon. Seas 5 to 6 feet on the outer waters,
diminishing Thursday night. Chance of a shower early.

Friday and Saturday... Winds less than 20 knots. Seas less than 5
feet Friday and Saturday, except seas near 5 feet on the outer
waters Saturday.


MARINE...Small Craft Advisory until 8 PM EDT Tuesday for ANZ231>234.
     Small Craft Advisory from 6 AM to 5 PM EDT Tuesday for ANZ230-
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ235-237-
     Gale Watch from Tuesday afternoon through Tuesday evening for
     Small Craft Advisory until noon EDT Tuesday for ANZ250-251.


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