Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 261110

Area Forecast Discussion
National Weather Service Boston/Norton MA
710 AM EDT Fri Apr 26 2019

A warm front today will bring periods of rain and drizzle. A
more stout cold front will sweep the region tonight with
widespread rain and embedded heavy showers, possible
thunderstorms. The front departs early Saturday morning followed
by drier and cooler air across the region on gusty west winds.
Low pressure tracks near the south coast Sunday bringing a
period of rain to the region. High pressure provides a brief
period of dry weather Monday before yet another area of low
pressure brings the threat of rain Monday night into Tuesday.
Dry weather may return the middle of next week.



7 AM update ...

Not much change from previous forecast. Low pressure over Lake
Erie this morning with its attending warm front south of New
England. WAA pattern will yield periods of rain and drizzle
today along with cool temps as boundary remains south of the
region. Thus highs only in the upper 40s to lower 50s. Heavier
rain holds off until this evening and overnight. Previous
forecast captures this well so no major changes with this
update. Earlier discussion below.


Today ...

Widespread showers continue to overspread S New England, AM commute
nuisance outcomes. Isentropic upslope beneath increasing diffluence
aloft and negatively tilting H5 trof. Preceding a deepening E Great
Lakes surface low with leading warm front kinked per isallobaric /
ageostrophic onshore flow off the cooler Gulf of ME waters prior.
Subsequent low-level inversion beneath a stout H9 warm nose brought
about by increasing, convergent low-level flow ushering higher dew-
point, sub-tropical thetaE axis N, becoming conditionally unstable,
with precipitable waters exceeding an inch. Accompanying elevated
instability, convective indices meeting or exceeding thresholds, as
the column aloft saturates with freezing levels rise to 10 kft and
H85 dewpoints rise up around 10C towards the latter-half of today.

So overall, anticipating scattered becoming widespread showers that
towards the back edge of warm frontal ascent has the potential to be
moderate to heavy, with precipitation efficiency (think larger rain-
drops via collision and coalescence) and some rumbles of thunder. Go
with numerous to widespread wording, high categorical PoPs. Believe
there will only be nuisance water ponding issues.

As to winds, momentum transfer mix-down limitations, however surface
pressure falls emerging especially with triple-point low development
(more on that later) can`t rule out that gradient / isallobaric flow
will contribute to E/SE gusts around 25 to 30 mph, higher confidence
along the immediate S/SE coast, Cape and Islands. Will watch closely
as 2m temperatures and dewpoints rise towards evening and there is
the possibility of a brief warm-sector towards sunset, perhaps some
opportunity towards boundary-layer destabilization (i.e., values in
the 60s would allow near-neutral surface-H9 lapse rates). Odds are
that with pressure falls developing around NYC late that weak cold
air damming remains in place, the surface warm front still S of our
region at sunset.



Tonight ...

Widespread rain band with embedded heavier showers and thunderstorm
chances. Backing, occluding low beneath a negatively-tilting H5 trof
over S Quebec. The triple-point low coming to fruition across S New
England. Attendant sweeping frontal boundaries with preceding crux
of convergent low-level inflow, deep-layer subtropical moisture with
+2 standard deviation precipitable waters exceeding 1.5 inches. The
saturated conditionally unstable profile undergoing frontogenetical
lift beneath continued broad diffluence aloft as convective indices
are met or exceeded along a narrow sub-tropical, high thetaE plume.
Majority of outcomes around midnight.

Greatest concern is the influx of low-level warm, moist conditions
that`ll potentially erode the low-level inversion, contributing to
enhanced instability and greater threat of faster momentum mix-down
from aloft, aside from precipitation drag from expected moderate to
heavy rains, possible thunderstorms. SREF / CIPS analogs support the
threat of thunderstorms around the I-95 corridor and points SE with
the threat of 30-40 mph wind gusts over SE New England, coincident
with the warm sector preceding the triple-point low, the threat of
rotating storms signaled by strong 0-1 km helicity / low significant
tornado parameter. All the while, rainfall amounts are forecast to
exceed 1-inch, with locally higher amounts up to 2-inches possible.

So for winds, recognize cooler ocean waters, stable surface profile.
Tricky forecast, but given gradient / isallobaric flow, strong wind
profile aloft, and precip-drag implications, can`t rule out the 30
to 40 mph S gusts along the S/SE coast and across the waters. Have
gale watches on the waters (see MARINE section).

Rainfall, definite sweeping rain band with high confidence of heavy
rain elements and/or thunderstorms immediately ahead of the triple-
point low. Highest rainfall amounts with low-level convergence, more
so along S/E slopes of high terrain. Higher confidence thunderstorm
activity and gusty S winds across SE New England especially along
the coast. Preference to HREF trends and SREF / CIPS Analog signals.
Rainfall contributing to mainstem CT river issues and sharp rises on
smaller streams.

Clearing out into the early morning hours. Lingering shower activity
as winds turn blustery out of the W. A non-diurnal trend prior but
then temperatures dropping ever so slightly before daybreak. Lows
around the upper 40s as an initial cooler airmass begins to build in



Highlights ...

* Active weather pattern with chance of rain every other day
* Temperatures at or cooler than normal

Synoptic Overview ...

Energetic northern stream jet results in an active weather pattern
for southern New England with the threat of rain just about every
other day (Sunday then Mon ngt/Tue MA and again Wed or Thu) this
forecast period. Given amplitude of northern stream temperatures
will be running at or cooler than normal much of this time period.

Daily Details ...

Saturday ...

Cold front and associated showers exit the region roughly 5 am to 8
am west to east. Then mid level dry slot and post frontal airmass
overspreads the region. Partial clearing expected mid to late
morning. However clouds likely fill back in given cold pool aloft
combined with cyclonic flow continuing. Can`t rule out a few brief
showers given this setup but by no means a washout Saturday
afternoon as most places remain dry. Mild start to the day which
will help highs reach to 55-60, with upper 40s to lower 50s in the
high terrain. However turning cooler in the afternoon as cold pool (-
22C at 500 mb) aloft drifts overhead. In fact sufficient cooling
that any isolated showers late in the day could be mixed with snow
showers across the high terrain as 850 mb temps fall to about -4C
and 925 mb -1C! Dry and cool Saturday night with short wave ridging
advecting across the region. Gusty NW winds diminish overnight.

Sunday ...

00z models have trended more amplified with northern stream short
wave exiting the Great Lakes and as it tracks across southern New
England. Still timing differences with onset of rain but models
agree the day begins dry and this makes some sense given the more
amplified solution. Rain looks to overspread the area late morning
into the early afternoon from west to east with a period of moderate
rain possible late afternoon/early evening given trough
amplification and steepening mid level lapse rates. As this frontal
wave intensifies NE winds will increase and evolve into a chilly
rain late Sunday. In fact some of the guidance suggest there may be
enough cold air on the backside for rain to possible briefly mix
with some wet snow flakes before ending.

Next Week ...

Good model agreement on dry weather Monday behind departing Sunday
evening frontal wave. In fact Mon could be the pick of the week with
sunshine, light winds and highs 55-60. Active weather pattern
continues with next chance of rain Mon ngt into Tue. Lots of
uncertainty for Wed and Thu as EC is more robust with northern
stream, driving frontal boundary well south of New England mid week
yielding dry cool weather. However GFS not as amplified so frontal
boundary remains closer to the region with threat of wet weather
returning Wed. The 50 member EC ensembles support the more amplified
solution from its operational run so will hedge toward a period of
dry weather mid week.


Forecaster Confidence Levels...

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

Short Term /through Friday Night/ ... Moderate confidence.

7 AM update ...

No major changes from previous TAFs. Earlier discussion below.


Today ...

CIGs lowering MVFR-IFR with lowest conditions over E/SE New England
and high terrain late. -RA/RA overspreading the region with embedded
+RA. Attendant MVFR VSBY concerns with RA while areas of IFR-LIFR FG
encroach on the S coast brought about via increasing S/SE winds with
gusts 20 to 30 kts especially along the immediate shore.

Tonight ...

MVFR-IFR CIGs and VSBYs with widespread RA band, embedded +RA with
chance TSRA through roughly 8z, improving thereafter. Accompanying S
winds gusting around 30 kts for the immediate S/SE coast and higher
terrain. LLWS threats with around 45 to 55 kts 2 kft agl. Conditions
improving after 8z towards VFR as winds shift W, becoming blustery.

KBOS Terminal...
CIGs lower and thicken through morning, MVFR/IFR enveloping the
terminal during the latter half of the day. Blustery S winds and
potential LLWS threats along with +RA and chance TSRA around 2-6z.

KBDL Terminal...
RA across the terminal, will increasing in coverage as CIGs lower,
possible VSBY concerns with RA/+RA going into tonight. Conditions
improving 6-8z with winds shifting W.

Outlook /Saturday through Tuesday/...

Saturday: VFR. Windy with local gusts up to 30 kt. Slight chance

Saturday Night: VFR. Breezy.

Sunday: Mainly MVFR, with areas VFR possible. RA likely.

Sunday Night: Mainly VFR, with local MVFR possible. Windy with
local gusts up to 30 kt. Slight chance RA.

Monday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance RA.

Monday Night: Mainly MVFR, with areas VFR possible. Chance RA.

Tuesday: Mainly MVFR, with local VFR possible. Chance RA.


Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

Warm front approaching the waters today ahead of which E/SE winds
prevail, the threat of 20 to 30 kt gusts and SMALL CRAFT ADVISORY
conditions. Waves building around 6 to 8 feet especially for E/SE
Atlantic-exposed waters.

GALE WATCH for E/SE waters preceding a cold front anticipated to
sweep the waters overnight with a widespread band of rain along
with embedded heavier showers, chance thunderstorms. Waves build
over the S waters while maintaining over the E waters around 6
to 8 feet, around 3 to 5 for inner sounds.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 10 ft.

Saturday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 10 ft.

Sunday: Winds less than 25 kt. Local rough seas. Chance of

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Chance of rain.

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

Monday Night: Winds less than 25 kt. Chance of rain.

Tuesday: Winds less than 25 kt. Rain likely.


MARINE...Gale Watch from this evening through Saturday morning for
     Small Craft Advisory until 10 PM EDT this evening for ANZ231-
     Small Craft Advisory until 8 PM EDT Saturday for ANZ233-234.
     Small Craft Advisory from 2 PM this afternoon to 5 AM EDT
     Saturday for ANZ230.
     Small Craft Advisory from 11 AM this morning to 8 PM EDT
     Saturday for ANZ236-251.
     Gale Watch from this evening through late tonight for ANZ235-
     Gale Watch from this evening through Saturday afternoon for


NEAR TERM...Nocera/Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera
MARINE...Nocera/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.