NWS Forecast Discussion - Boston Area

Forecast Discussion for BOX NWS Office
000
FXUS61 KBOX 190016
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
716 PM EST Thu Jan 18 2018

.SYNOPSIS...
Dry and seasonably cold conditions into Friday, then a warming
trend this weekend with continued dry weather. A storm system
will affect the region sometime late Monday into Tuesday with
mostly rain and coastal wind, but there is a risk of some wintry
mix/ice across the interior. Warm weather Tuesday is replaced
by blustery, dry and seasonably cold weather Wednesday and
Thursday.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
715 PM update...

Marine stratus still hovering from eastern Essex County, MA
southward to just east of Boston and southward to the outer
Cape Cod and recently, to Nantucket. Other low clouds were
banked up along the slopes of the Berkshires and northern
Worcester Hills. A renegade patch of low clouds moved
southward from Worcester and was dissipating over western RI.
Mesoscale models show the clouds remaining near the Berkshires
and the ocean cloudiness lifting eastward after midnight.
Winds were still gusting up to 20 mph along the immediate
east coast of MA.

Previous discussion...

Deamplifying H5 impulse sweeping across the region overnight.
Accompanying ascent with differential vorticity advection and
parent jet streak. Over-running response but atmosphere fairly
void of moisture given precipitable waters up around 0.2 inches.
Green`s and White`s likely to see some snow potentially
extending as far S as the Berkshires as flurries. Otherwise
scattered to broken cloud decks while monitoring low-level
stratus lingering along the high terrain and along the Outer
Cape. With clouds anticipated, leaning away from coldest of
guidance, that being MET/MAV.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday...

Departing H5 vortmax with accompanying enhancement to the low-
level wind profile. Mixing out to around H9 through the day,
there`s the possibility of some breezy W winds during the
morning, otherwise relaxing through the remainder of the day
beneath a follow-up brief, weak ridge of high pressure. Boundary
layer lapse rates remaining well mixed, beneath a dry inversion,
and anticipating temperatures to warm into the mid and upper 30s,
with snow melt, can`t rule out some few to scattered pancake
cumulus.

Friday night...

Continued energy and moisture accelerating through the pseudo-
zonal flow, sagging S into the Great Lakes region. Up against
high pressure over the SE CONUS, an amplified gradient wind
response with accompanying speed max at H925. Warmer, dry air
pushing out of the SW, warming within H9-7 apparent within model
forecast soundings, limitations on mixing down to the surface.
Thus expecting a gradient wind response with strongest winds
potentially in excess of 30 mph along the S-coast, across the
Cape and Islands. Keep it dry but also mild with the winds. Lows
down in the 20s for most locations.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
340 PM update...

Highlights...

* Dry and mild this weekend, possibly low 50s Saturday!

* Another storm will bring mostly rain and gusty coastal winds late
  Mon into Tue with a risk of some interior mixed precip/ice

This Weekend...

Confluent flow aloft across New England provides dry weather to the
region this weekend. Low pres tracking across southeast Canada
combined with high pres over the southeast states yields a tight
westerly pgrad over the area. Warm air just above the surface with
925 mb temps +2C to +4C. However model soundings indicate a stout
subsidence inversion limiting blyr mixing. Thus how much warm air
aloft mixes down to the surface? Given the tight westerly pgradient
and associated downsloping winds/adiabatic warming looks to be an
overachieving temp setup. Model 2-meter temps and MOS guid continue
to trend warmer with each run.  ECMWF MOS and GFS MOS are the
warmest guidance sources and will lean in this direction. Thus will
go warmer than guid Sat with highs in the upper 40s to lower 50s,
warmest in the coastal plain of RI and southeast MA. Although cooler
along the immediate south coast and islands given WSW winds coming
off the chilly near shore waters.

Regarding winds Sat, GFS model soundings suggest G35kt possible.
Pgrad supports a windy so could see a few G40mph but gusts 15 to 30
mph more common. Another nice day Sunday however winds will be much
less along with temps not quite as mild, 45-50. Still 10-15 degs
above normal. Turning colder Sunday night as a backdoor front slips
across the area as 1037 mb high builds southeast into Ontario and
Quebec.

Monday/Tuesday...

Pacific short wave marches across the country with an influx of Gulf
and Atlantic moisture as it approaches from the southwest with PWATs
surging up to +2 SD Tue morning across southern New England. Fairly
robust wind anomalies within this moisture plume with GEFS ensembles
offering +2 SD 850 mb and 925 mb winds.  ECMWF more amplified than
the GFS but the GEFS lends some support toward the stronger ECMWF.
Thus will base this portion of the forecast toward a ECMWF/GEFS
blend.

Meanwhile, shallow cold air bleeds southward into southern New
England Monday behind the backdoor cold front and 1038 mb high
pressure builds from Quebec into Maine and New Brunswick. This will
provide a cold air damming (CAD) setup for especially northern MA
later Monday and possibly lingering into early Tue morning. Thus
potential for a period of mixed precip/ice across this region.

Elsewhere expecting a chilly windswept rain Monday night into Tue
morning with increasing ESE winds off the cool near shore waters
ahead of a strong cold front and possible triple pt low. ECMWF and
GFS both have 925 mb southerly jet up to 65-70 kt crossing southern
New England Tue. In addition, it`s aways out but some of the
deterministic guid has mid level lapse rates up to 6-7C/KM along and
ahead of the frontal boundary. Thus could be some elevated
convection which would enhance the threats of strong winds aloft
reaching the surface and locally heavy rain. Both ensembles and
deterministic guid have 1+ of rainfall possible. As for temps,
possibly another 50+ day especially in the coastal plain where warm
sector has best chance of advancing but also temps may jump into the
50s behind the front as post frontal winds scour out any leftover
shallow cold air from Mon night/Tue morning.

Wednesday/Thursday...

Post frontal airmass is colder with 850 mb temps around -8C on both
GEFS and ECENS ensembles, which is seasonably cool for this time of
year. So expecting highs at or slightly above normal. Although it
will probably feel a bit colder given gusty WNW post frontal winds.
Dry weather expected Wed and Thu.

&&

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

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

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

Tonight... VFR. BKN MVFR likely to hang just offshore of
E-coastal MA but impact the Cape and Islands with a more N/NE
onshore flow, eroding as the winds become north towards Friday
morning. BKN-OVC MVFR along the slopes of the Berkshires.
Otherwise, increasing SCT- BKN high clouds towards evening,
eroding into Friday morning. W/NW winds prevailing.

Friday...
VFR. Brief period of breezy W winds as SCT-BKN high clouds
erode. FEW-SCT 035 possible during the day.

Friday night...
VFR. W winds on the increase towards Saturday morning. Mainly a
gradient wind with the potential of sustained values up to 25 kts,
gusts up to 30 kts.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: VFR. Windy with gusts to 30 kt.

Saturday Night: VFR. Breezy.

Sunday through Sunday Night: VFR.

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

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

Tuesday: Mainly MVFR, with local IFR possible. Breezy. SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

715 PM update...

SMALL CRAFT ADVISORIES remain in effect over the southern and
southeastern outer waters. Winds were still gusting to 20 to 25
kt there and seas were near 5 ft. Winds and seas should
gradually diminish to below SCA levels overnight. Continued W
winds through Friday. Greater concern is on the Friday night
into Saturday morning period with increasing sustained winds
potentially up to 25 kts, gusts up to 30 kts. Headlines may need
to be renewed and there is a low risk of gales.

Outlook /Saturday through Tuesday/...

Saturday: Low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

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

Sunday Night: Winds less than 25 kt.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Rain likely.

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 9 ft. Rain showers.

&&

.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...Small Craft Advisory for hazardous seas until 6 AM EST Friday
     for ANZ254>256.

&&

$$
SYNOPSIS...Nocera/Sipprell
NEAR TERM...Field/Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera
AVIATION...Field/Nocera/Sipprell
MARINE...Field/Nocera/Sipprell
HYDROLOGY...WFO BOX Staff

NWS BOX Office Area Forecast Discussion