Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 191130

Area Forecast Discussion
National Weather Service Boston/Norton MA
630 AM EST Wed Dec 19 2018

Gradually moderating temperatures today and tomorrow with high
pressure sliding to the south. Strong low pressure passing west
of the region will bring heavy rainfall and strong coastal
winds Friday into Friday night. Low pressure departs on Saturday
followed by mainly dry and seasonably cold conditions Sunday
through Christmas Day.



630 AM Update...

Satellite imagery shows patches of cirrus moving from the
Adirondaks across VT/NH/ME as well as parts of Mass. Otherwise
dry weather with light west winds. Clear skies across the rest
of the Northeast USA. The clear skies come from high pressure
over the Eastern USA and resulting subsidence. The current dry
weather forecast continues on track.

Previous discussion...

WV imagery shows a very weak and highly sheared vort max sliding
ESE across New England. This has had some impact, in the form of
a deck of mid and high clouds which formed along the
anticyclonically sheered side of this vort max. This BKN cloud
deck, combined with the slight differential vort-advection has
helped to maintain the sfc pres gradient most of the night,
limiting radiational cooling.

This sheared shortwave will shift N through the AM, with mid lvl
heights rising throughout the day. Sfc high pres slowly slips S
of the region with this ridge building. Warm advection slowly
develops with 00Z H85 temps reaching +5C (an anomaly of 10C).
However, with the ridging both near the sfc and aloft,
subsidence will suppress the mixing well below this value.
Still, with slight warming, expect highs to be warmer than
previous days, mainly in the mid and upper 30s. Otherwise dry
with some mid/upper lvl clouds.


Although high pres slips to the SE, the pres gradient across S
New England remains light. Column moisture, outside of a few
high clouds is limited, so another round of radiational cooling
is likely. Limiting factor will be the continued advection of
higher dwpt air given the return flow. Still, some areas prone
to decoupling could reach the teens. Non-heat island locations
drop into the low-mid 20s.

Mid-upper lvl ridge crest late Thu which suggests continued warm
advection through the day. Although clouds increase, plenty of
sunshine should allow for better mixing than Wed although not
the full scope of the inversion. However, given the continued
advection, highs should reach the mid and upper 40s. Remnant
lower lvl dry air will hold precip shield at bay until the late
evening hours, so expect dry conditions to persist.


Big Picture...

Thursday night shows a ridge over the West Atlantic and the
Maritimes, with a deep trough from the Mississippi Valley to the
Gulf of Mexico. Farther west, another ridge across the Plains and
Western USA and then a shortwave moving ashore in the Pacific

A closed low within the deep central trough will sweep north up the
Appalachians, passing west of New England Saturday. This puts New
England in a mild southerly flow through that time. The flow then
turns from the west as the upper low moves off through Canada. The
long-range models maintain a zonal flow through the early week, but
with differing depictions/timing of the west-coast shortwave moving
through our area Monday/Tuesday.

Mass and thermal fields show fair agreement through Saturday with
differences in detail, then diverge early next week. MSLP fields
agree that the surface low takes an inside track through NY/PA
Friday and Quebec Saturday, putting Southern New England in the
warm air, and swinging a cold front through the region Saturday
morning. All show the zonal upper flow Sunday through Christmas,
but with different timing for the movement of the shortwave.


Thursday night-Friday-Saturday...

As noted, the Midwest deep trough ejects up the Eastern USA Thursday
night and Friday, crossing New England Saturday. The upper flow over
New England becomes more southerly Thursday night with the jet core
to our west but close enough to generate upper divergence starting
after midnight. Meanwhile, low level winds increase after midnight
and create low level convergence. The result is strong lift
developing late Thursday night and continuing through Friday.

Deep moisture moves in Thursday night with PW values in Southern New
England forecast to reach 1.5 inches. As per the SPC Sounding
climatology, 1.5 inches would be at or near record levels for this
time of year. Expect a lot of moisture, and unseasonably high values
of precipitation. Bufkit soundings show deep layer of abv freezing
air, so expect any precip to be in the form of rain and developing
Thursday night after midnight.

Stability is marginal Friday-Friday night. Totals climb to the
mid 40s Friday afternoon, primarily in the warmer areas such as
RI and SE Mass. LI is 0-2, K is 30-35. Could be some rumbles of
thunder, especially in RI and SE Mass.

South to southeast flow builds Thursday night with 60 to 80 knot
winds forecast to move overhead at 850 mb Friday and winds 35 to 40
knots forecast at 1000 mb. The former actually suggests surface
winds could gust to 45 knots.

Strong south-southeast winds will bring temperatures well above
freezing Thursday night and Friday, with values at 950 mb supporting
max temps in the 50s. A few spots could be near 60, especially in
southeast MA and RI.

We continue to expect Southern New England will have rain developing
Thursday night, especially after midnight, and continuing through
Friday. Heaviest rainfall should take place on Friday. Rainfall
should total 1-3 inches, with potential for urban/poor drainage
flooding. Some rivers may approach minor flood levels as all of this
water runs off.  Potential for wind advisories especially where the
warmest temperatures occur, such as in Eastern/Central Mass and RI.

The cold front moves through during Saturday morning and early
afternoon as the surface low pressure passes north of us through
Canada. Expect any rain to end by that point, with drier more
seasonable air moving in later Saturday.

Rest of the period...

Dry. Height contours at 500 mb are forecast at seasonable or a
little below Sunday through Tuesday. This would mean temperatures
trending to seasonably cold levels early next week. Passage of the
shortwave would support a chance of rain/snow showers
Monday/Tuesday, but the lack of agreement on timing brings low
confidence on this occurring.


Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

Through tonight...
VFR. Winds lighten and gradually back to the SW.

VFR but with gradually lowering CIGS through the
afternoon/evening. S-SW winds increasing to around 10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: MVFR/IFR conditions possible. Windy with local
gusts to 30 kt. RA.

Friday: Mainly IFR, with areas MVFR possible. Strong winds with
areas gusts to 40 kt. RA.

Friday Night: MVFR/IFR conditions possible. Windy with gusts to
35 kt. RA.

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance

Saturday Night: VFR. Windy with local gusts to 30 kt.

Sunday: VFR. Breezy.


Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

Seas will continue to diminish through the morning, dropping
below SCA thresholds early. Quiet boating weather will then
continue into Thu, as winds gradually shift around to the S-SW.

Outlook /Thursday Night through Sunday/...

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas locally approaching 5 ft. Chance of

Friday: Moderate risk for gale force winds with local gusts up
to 45 kt. Rough seas up to 16 ft. Rain.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 16 ft. Rain.

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 14 ft. Slight chance of
rain showers.

Saturday Night: Moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Rough seas up to 14 ft.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas.


With southerly low level jet, the south coast will be the main
focus area for possible minor coastal flooding on Fri. The
morning high tide Fri is the higher of the 2 tide cycles, but
wind and surge values will be limited at the time of the high
tide so no flooding is expected with this tide. Strongest
winds/surge may actually coincide with the low tide Fri
afternoon, but timing is not certain and it is possible the
stronger winds may linger and affect the Fri evening high tide.
While the evening high tide is a bit lower, if this occurs, a 2+
ft surge will be possible which may lead to minor flooding
along the shores of Narragansett Bay and Buzzards Bay, and some
splashover or minor flooding along ocean exposed beaches along
the south coast as wave action will become a factor with 10+ ft
seas just off the coast.


MARINE...Gale Watch from late Thursday night through Friday evening for
     Small Craft Advisory until 9 AM EST this morning for ANZ250-


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