National Weather Service Forecast Discussion

Forecast Discussion for BOX NWS Office
FXUS61 KBOX 210258

Area Forecast Discussion
National Weather Service Taunton MA
958 PM EST Mon Nov 20 2017

High pressure moves south of the region Tue with milder temperatures
as winds shift to the southwest. A cold front approaches the region
Tuesday night, and will combine with moisture working up the eastern
seaboard to bring a period of rain through Wednesday morning,
especially SE New England. Dry and cold conditions expected
Wednesday night through the end of the week. Milder with a few
showers possible Saturday as a cold front approaches, then blustery
and colder weather follows Sunday and especially Monday.



1000 PM Update...

Minor changes to the ongoing forecast tonight. Temperatures have
remained several degrees warmer than prev thought, thus have
increased them to align with current conditions. Cloud cover
across the region has kept temps from radiating out thus the
warmer start. However clouds are moving to the north and east
thus skies will clear which will help temps drop. However, 925
mb temps are warming as warm front approaches from the south.
Dewpoints are also beginning to rise thus temps may not bottom
out as prev thought. Went ahead and raised overnight temps a
few degrees because of the above factors.

Otherwise gusty winds across the higher terrain and near the
waters will begin to diminish as inversion sets-up. However
closer to dawn tomorrow mixing will increase and another round
of breezy conditions tomorrow.

Previous Discussion...
Area of low/mid level moisture assocd with developing warm
advection resulting in some high based SC/AC moving across SNE.
These clouds expected to lift to the north overnight followed by
clearing skies. Winds still gusty in some locations but will
diminish overnight as pressure gradient weakens and decoupling
from the PBL occurs. This should allow for some radiational
cooling during the overnight. The return flow does suggest a
slight increase in sfc dwpts overnight, but this will be
countered by the cooling. Therefore, will still see mins dipping
into the 20s and low-mid 30s across the region.


Weak mid lvl ridge and attendant sfc high pres center will begin
to pass to the E of the region during the morning tomorrow. This
will act to enhance the S-SW flow through the day, leading to
increased low lvl moisture and rising dwpts/low lvl temps. H95
temps are progged to reach near +5C by late afternoon, which
should allow high temps to rebound nicely into the low-mid 50s
with a few spots in the upper 50s to near 60 by mid afternoon.
Column remains dry under the ridge`s influence, so expecting
a quiet day in terms of precip. The only fly in the ointment
will be another round of gusty winds, this time mainly S-SW. H92
LLJ hovers near 35-40 kt, and the limited mixing beneath the
continued inversion should limit mixing to this height.
Therefore, feel most gusts/sustained winds will remain below
wind advisory thresholds, but it will be breezy nonetheless as
mixing should still yield gusts 25-35mph.

Tomorrow night...
Area of weak low lvl convergence develops ahead of approaching
cold front associated with sharpening shortwave approaching from
the W during the overnight hours. Column begins dry, with PWATs
generally at or below normal values through at least midnight
local, so feel the development of any precip will be mainly in
the morning hours, especially closest to daybreak as low pres
develops near the mid atlantic. A slight increase in moisture,
with PWATs then approaching 1 std deviation above normal occurs
as the convergence develops in the form of an inverted trof
linked to the coastal low well to the S. An area of rainfall
will develop along this axis and shift into S New England from
the S through sunrise. Note there is still some uncertainty on
exactly where this axis sets up, but current data suggests a
focus from Westerly to Boston and points SE, with lower risk to
the NW. POP gradient reflects this thinking. Leaned away from
GFS/NAM, which continue to be easterly outliers and mainly
offshore and added more weight to the more amplified ECMWF which
remains consistent.

With the continued influence of low-mid lvl moisture manifesting
as increasing clouds and rising dwpts overnight mins will be
mild compared to previous nights, mainly in the low 40s E of a
line from Hartford to Beverly, then mid thirties further inland,
where clouds will take longest to influence.



* Morning rain Wed, especially SE New Eng, then clearing afternoon
* Mainly dry and cool Thanksgiving Day and Friday
* Milder with a chance of showers Sat
* Blustery and colder Sun/Mon


Models are in good agreement on the overall pattern in the extended
period. High latitude blocking persists over Greenland to near the
North Pole which results in the southward displacement of the Polar
Vortex over SE Canada. Series of northern stream shortwaves rotating
around this vortex will swing through New Eng on Wed, Thu/Thu night
and again Sun/Mon. Result will be an overall cool pattern with temps
averaging below normal. The coldest period will be late Sun/Mon as
deep trough settles over New Eng but a brief warm up to milder than
normal temps Sat ahead of this deepening trough. The extended period
looks mostly dry with exception of a period of rain Wed morning,
mainly SE New Eng as southern stream moisture links up with mid
level trough, with another round of showers possible Sat as
amplifying trough and attending cold front approaches.


Fairly robust mid level trough and attending cold front moves into
New Eng. Southern stream moisture assocd with a sfc low to the south
and inverted trof lifts north into SNE and interacts with mid level
trough approaching from the west and right entrance region of the
upper jet. Result is an area of rain which overspreads SNE,
especially SE New Eng in the morning. There is some uncertainty
with extent of rainfall and QPF amounts in the east.
International models are much wetter than US models as
ECMWF/GGEM and UKMET to a lesser extent indicate up to an inch
of rain across SE New Eng while GFS/NAM less than 0.25".
Favorable jet dynamics with right entrance region of 150 kt
upper jet combined with modest low level jet across SE New Eng
within deep moisture plume with PWATs nearing 1". EPS ensembles
also support the deterministic guidance with high probs of 0.50"
SE New Eng. As a result we blended the ECMWF with less robust
UKMET. Expect a period of widespread rainfall Wed morning in the
coastal plain, especially SE MA, locally heavy at times. A
period of light rain may briefly get into the CT valley but
lower confidence this far west. After fropa, rapid drying moves
in from the west with clearing skies from west to east during
the afternoon.

Thursday through Saturday...

High pres in control Thu/Fri with dry weather and below normal
temps. Sunshine both days, but some ocean effect clouds may impact
Cape Cod Thursday. Then increasing SW flow ahead of next amplifying
trough will bring milder temps for Sat. The warm advection will
result in clouds increasing and cant rule out a few showers as the
cold front approaches.

Sunday and Monday...

Strong cold front moves through Sunday followed by blustery and
colder conditions. The core of the cold air aloft settles into the
region Monday which will be the coldest day of the extended period.
Ensemble mean 850 mb temps -10 to -12C should keep temps in the 30s
Mon. Mainly dry weather but can`t rule out a few flurries or snow
showers, especially interior higher terrain as mid level trough
moves overhead.


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.

Through tonight...
VFR. SCT-BKN cigs 060-080 will lift to the north after midnight
with clearing skies. Gusty winds diminishing.

VFR. Increasing SSW wind gusts 25-35 kt through the

Tomorrow night...
Mainly VFR although a few patchy near MVFR clouds are possible
toward early Wed morning. Some light SHRA also possible mainly
across W/Central MA/RI.  SSW winds around 10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday: Moderate confidence. Areas of MVFR/IFR possible in rain,
especially SE New Eng in the morning. Improving conditions in the

Wednesday Night: High confidence. VFR. Gusts to 20-25 kt

Thanksgiving Day through Friday Night: High confidence. VFR.

Saturday: Mainly VFR, with areas MVFR possible. Windy with local
gusts to 30 kt. Chance SHRA.


Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...high forecast confidence.

Through the overnight hours...
One more impulse of stronger winds approaching Gales this
evening, so have lengthened gales on the outer waters through
the evening hours. Seas will continue to diminish, albeit
slowly.  Small craft advisories will need to follow Gales.

Tomorrow and tomorrow night...
Although seas on the east waters may drop below 5 ft by early
morning, as winds shift to the S-SW gusts will once again
increase, especially after sunrise to 25-30 kt, so small craft
advisories will continue. There is a risk for Gales for a period
during the late afternoon and evening hours, which may require
another set of Gale warnings, but in any case, small craft
advisories at least will linger into tomorrow night, especially
as seas increase in response to the S-SW flow.

Outlook /Wednesday through Saturday/...High confidence.

Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Vsbys reduced in rain during the morning, improving in the

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

Thanksgiving Day: Winds less than 15 kt. Local rough seas.

Thursday Night through Friday: Winds less than 25 kt. Seas up
to 5 ft.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft.

Saturday: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Areas of rough seas. Chance of rain


MARINE...Gale Warning from noon Tuesday to midnight EST Tuesday night
     for ANZ231>235-237-250-254>256.
     Small Craft Advisory until 10 PM EST this evening for ANZ230-
     Small Craft Advisory until 3 AM EST Wednesday for ANZ251.



NWS BOX Office Area Forecast Discussion