Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 200001
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
701 PM EST Fri Jan 19 2018

.SYNOPSIS...
A warming trend this weekend with continued dry weather. A storm
system will bring the potential for heavy rain and strong coastal
winds late Monday night into Tuesday. There is, however, an
additional risk of some wintry mix/ice across the interior late
Monday into Tue morning. Blustery, dry and seasonably cold
weather follows Wednesday through Friday.

&&

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

7 pm update...

Forecast remains generally on track. Main near term issue will
be cloud cover. Looks like stratocumulus clouds courtesy of
trapped moisture under an inversion will likely persist to
around midnight or perhaps a little later. During the early
morning hours, anticipate a drier air mass aloft as the depth of
warmer air increases across southern New England. The warm
advection will likely offset radiational cooling overnight and
result in temperatures holding fairly steady or even rising a
couple of degrees.

4 pm update...

Wave clouds filling back in. Low level moisture which has remained
trapped beneath a stout dry inversion around H9 continues to advect
S/E with the light steering current. Locations mainly N of the MA-
pike have found themselves underneath a blanket while folks to the
S have seen a day of abundant sunshine. Yet overall highs over much
of S New England have warmed up around the low 30s, some locations
around 35 degrees.

Expecting low clouds to linger as temperatures drop towards midnight.
Watching the clearing line out of Upstate NY as warmer, drier air
begins to push in out of the W/SW, increasing which is discussed
in much further detail below. Temperatures are anticipated to stabilize,
slightly warming in some spots. Looking at temperatures to drop towards
nighttime lows around the mid to upper 20s.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Through Saturday...

Digging N-stream energy out of the Arctic reaches of Canada,
capturing and stretching energy out of a sweeping H5 trof axis
into the W CONUS, parent surface low continues to deepen across
the N Great Lakes region into the St. Lawrence River Valley down
to around 990 mb up against high pressure over the SE CONUS up
to 1030 mb. A tightening pressure gradient emerges along with a
subsequent jet streak, a gradient wind threat exists around the
Saturday morning timeframe.

H925 westerly winds 45 to 55 mph, the height of which is around
Saturday morning. Accompanying stout inversion, strengthening,
with pronounced warming through the H8-9 layer. Limitations on
the potential mix down of faster momentum to the surface, however,
if the boundary layer is able to mix up to H95 / 1200 feet agl,
then there is the possibility of getting wind gusts up around
40 to 45 mph.

However, considering the cold, icy ocean, and snow pack ground,
expect outcomes to mainly be gradient driven with gusts on the
order of +5 to the sustained wind. Sustained around 30 mph, gusts
around 35 mph possible along the S-coast, Cape and Islands, but
again, colder conditions right at the surface are likely to limit
mixing.

Warm air advection proceeding aloft along with abundant sunshine,
temperatures warming towards the low 50s through the day as the
height of the winds aloft pass through, it is only then that
gusts 20 to 30 mph are possible across much of S New England
aside from the faster winds along the S/SE coast, both of which
diminish towards sundown.

GALE WARNINGS for the waters but no WIND ADVISORY headlines given
such a marginal event. If any one location has the best chance
of seeing wind advisory criteria with sustained winds over 30
mph it would be Nantucket.

Saturday night...

Main surface cold front pushing S into the region, eroding beneath
high pressure. Light winds expected with filtering mid to high level
clouds across the region within the zonal pressure gradient aloft.
Stout inversion building across the region down to H95, with higher
dewpoint air lingering along with the likely boundary layer mixing
of snowmelt and now cold air advection proceeding, expecting to seeing
low clouds develop towards Sunday morning. Keeping it mild with lows
around the upper 20s to low 30s for the timeframe.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Highlights...

* Dry and mild Sunday along with light winds
* Risk of a wintry mix/ice western MA late Mon into Tue morning
* A period of heavy rain and strong coastal winds likely late Mon
  night into Tue
* Dry, Seasonably cold Wed/Thu & Friday

Synoptic Overview...

Winter break? It looks that way given the lack of high latitude
blocking combined with subtropical ridge emerging over FL from time
to time will result in a fast/progressive northern stream jet of
modest amplitude across the CONUS. Thus temperatures likely running
at or slightly above normal the next 7-10 days. In addition given
the progressive nature of the northern stream and streams appearing
to be unphased, excessive precip events appear unlikely.

Sunday...

Dry/quiet weather with confluent mid level flow over the region
providing plenty of subsidence. Thus sunshine, light winds and a
mild airmass will support highs in the mid to upper 40s. Leaned
toward the milder EC MOS here. It will feel even milder given the
light winds and full sunshine. Not too shabby for Jan 21st.

Monday...

Turning colder as 1038 mb high over southern James Bay advects into
southern Quebec with cold air bleeding southward into New England.
This will set the stage for a wintry precip event for interior
southern New England as warm advection from an approaching closed
low rides up and over a cold dome spreading southward. Monday will
begin dry but the GFS and its ensembles (GEFS) have precip
overspreading the region during the afternoon. New 12z EC now
introduces some light qpf late in the day across CT, west-central
MA. We will lean toward a model blend which brings chance pops
across the area Mon aftn. EC more amplified/colder solution than
GFS/GEFS with high probability of mixed precip and ice across the
interior Mon ngt into Tue morning. EC ensembles support the
operational run with 50% probability of 32 degs or colder Mon ngt
all the way to the CT/MA border. Thus will hedge the forecast toward
this colder scenario especially given the cold air damming (CAD)
signature with 1038 mb high over Quebec. Could be some impact here
given it doesn`t take much ice to cause problems.

Tuesday...

How quickly shallow cold air scours out will hinge on timing and
location of triple pt low development. If triple pt low tracks
directly over the region (along with pres falls) shallow cold air
would likely linger across northwest MA much of the day per EC and
its ensembles (ECENS). Thus will have to watch this evolution
closely as could be an extended period of freezing rain into Tue
across northwest MA.

Otherwise warm sector likely overspreads RI and at least southeast
MA with temps surging into the 50s. More importantly will be a
subtropical plume of moisture advecting up the eastern seaboard
ahead of an approaching cold front with PWATs +2SD. Also strong
southerly wind anomalies of up to +2SD within this moisture plume
providing periods of heavy windswept rain. Also could be some
embedded elevated convective elements as mid level lapse rates near
moist adiabatic. Deterministic guidance has qpf ranging from 0.75 to
1.5 inches which is supported by both GEFS and ECENS ensembles with
modest probs of 1 inch potential. System remains progressive which
will limit excessive rainfall potential.

As previous shifts have mentioned strong low level southerly jet of
65-70 kt at 925 mb crosses the region Tue, yielding a wind swept
rain along with the risk of stronger winds aloft reaching the
surface especially in heavier shower/convective activity and in the
warm sector (coastal plain).

As for impacts to river ice with potential rainfall near an inch
next Tue, given dew pts remain at or below 32 degs much of the time
leading up to Tue and then only briefly warming to 45-50 Tue aftn
followed by dew pts falling back into the teens and 20s after Tue
rainfall, not anticipating much melting, thus little impact to river
ice.

Wednesday, Thursday and Friday...

Behind the departing Tue storm system it turns colder but seasonably
cool with temperature anomalies at 850 and 925 mb very close to
normal for late Jan. Mainly dry weather expected as well mid to late
next week.

&&

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

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

Short Term /through Saturday Night/...

Rest of this evening...High confidence. VFR cigs will linger
along and N of the Masspike with some high end MVFR cigs at
times over the higher terrain.

Overnight and Saturday...High confidence. VFR. Low-end VFR CIGs
N of the Masspike expected to thin out during the early morning
hours. Of greater concern is the increasing W winds. Strongest
winds will be along the S-coast and over the Cape / Islands.
Potential 40 kt gusts over ACK around 12-15z Saturday. Roughly
30-40 kts S/SE- coast, 20-30 kts elsewhere. Threat of LLWS
roughly 9-15z with a 45-55 kt SW jet at and just above 2 kft
agl.

Saturday night...Moderate confidence.
W winds diminishing, however low clouds potentially building
across the region. Will keep low- end VFR with this forecast but
there is the potential of IFR CIGs.

KBOS Terminal...High confidence. Low-end VFR CIGs around 4 kft
agl through evening. Increasing W winds overnight into Saturday
with gusts to around 25 kts likely, possibly as high as 30 kts
for an hour or two.

KBDL Terminal...High confidence. Due to the valley location, W
winds will increase but likely remain at or below 25 kts.

Outlook /Sunday through Wednesday/...

Sunday: VFR. Breezy.

Sunday Night: VFR.

Monday: MVFR/IFR conditions possible. Chance RA, chance SN,
slight chance FZRA.

Monday Night: Mainly MVFR, with areas IFR possible. Breezy. RA.

Tuesday: Mainly MVFR, with local IFR possible. Windy with areas
gusts to 30 kt. RA.

Tuesday Night through Wednesday: VFR. Windy with local gusts to
30 kt.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Increasing W winds overnight to gale force. Mainly a gradient
wind event of around 30 kts for the S/SE waters with gusts +5,
GALE WARNINGS posted. Waves building to 10 feet at the height of
the winds centered around the Saturday morning hours, diminishing
gradually thereafter.

Outlook /Sunday through Wednesday/...

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

Sunday Night: Winds less than 25 kt.

Monday: Winds less than 25 kt. Slight chance of rain.

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

Tuesday: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 12 ft. Rain.

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

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 11 ft.

&&

.HYDROLOGY...
A Flood Warning remains in effect for the Connecticut River at
Middle Haddam, where ice jams are causing river fluctuations.

For details on specific area rivers, including observed and
forecast river stages, please visit the Advanced Hydrologic
Prediction Service (AHPS) graphs on our website.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 7 AM to 4 PM EST Saturday for ANZ232>234.
     Small Craft Advisory from 7 AM to 4 PM EST Saturday for ANZ230.
     Small Craft Advisory from 1 AM to 7 PM EST Saturday for ANZ231-
     251.
     Small Craft Advisory from 3 AM to 4 PM EST Saturday for ANZ236.
     Gale Warning from 5 AM to 4 PM EST Saturday for ANZ235-237.
     Gale Warning from 7 AM to 1 PM EST Saturday for ANZ250.
     Gale Warning from 4 AM to 7 PM EST Saturday for ANZ254-255.
     Gale Warning from 4 AM to 5 PM EST Saturday for ANZ256.

&&

$$
SYNOPSIS...Nocera/Sipprell
NEAR TERM...Sipprell/Thompson
SHORT TERM...Sipprell
LONG TERM...Nocera
AVIATION...Nocera/Sipprell/Thompson
MARINE...Nocera/Sipprell
HYDROLOGY...



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