Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 200801
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
301 AM EST Mon Feb 20 2017

.SYNOPSIS...

A brief return to seasonably cool weather occurs today through
Tuesday with high pressure in control.  Strong low pressure
developing across the Central Plains slowly lifting towards the
Great Lakes will result in an extended period of  unseasonably mild
temperatures Wednesday through Saturday. The cold front will cross
the region on Saturday, likely bring some rain followed by more
seasonable temps returning by Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

Mid level trough axis amplifies east of southern New England as
additional shortwave energy crosses the region.  This will result in
dry northwest flow today, but decent cold air advection will allow
850T drop to between -8C and -10C by afternoon.  Column is fairly
dry so plenty of sunshine.  This combined with a rather mild start
should allow highs in the upper 30s across the higher terrain...to
mainly the lower to middle 40s elsewhere. Bufkit soundings show 20
to 25 mph wind gusts developing later this morning and afternoon as
mixing increases.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

Tonight...

Large high pressure builds across southern New England tonight.  The
result will be mainly clear skies, except for some ocean effect
clouds across the Cape/Nantucket as winds become northerly.
Otherwise, light winds/mainly clear skies elsewhere should allow for
overnight lows in the teens across the outlying locations with lower
to middle 20s in the urban heat islands of downtown
Boston/Providence.

Tuesday...

High pressure overhead Tuesday morning will slowly shift offshore by
late in the afternoon.  Plenty of sun expected during the
morning...but some mid/high level cloudiness should move across the
region from west to east during the afternoon.  Mid level
temperatures will be warming as height fields rise, but a chilly
start and relatively weak mixing will keep high temps mainly in the
lower to middle 40s.  Weak pressure gradient should allow sea
breezes to develop along the eastern MA coast and may keep highs in
the upper 30s for those locations.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Seasonably cool Tuesday
* Light rain/pockets of freezing rain Tue night into Wed
* Thursday looks to be our warmest day
* More likely of widespread rain Friday night into Saturday

Overview and model preferences...
The initially amplified and complex synoptic pattern across the
E half of NOAM will gradually trend toward a more zonal
(although with SW-NE orientation) toward the end of the week.
This is mainly a response to an arctic influenced wave which
should remain to our N as it meets the building heights within
ridging across the northeast and Maritimes. Still noting good
agreement amongst both ensemble and deterministic guidance that
positive 500hPa height anomalies remain through the long term,
suggesting the influence of warmer air and a quieter pattern
overall. This is indeed the case amongst guidance save for a
stronger wave arriving by late Fri-Sat. Given there is fair
agreement across the board, will continue with the consensus
blend used over the last few updates until some of the subtle,
mesoscale features are better resolved.

Details...

Tue...
Quiet wx remains thanks to high pres which will crest early Tue
morning. Nearly isothermal profile with both H92 and H85 temps
between 0C and +3C suggests warmer temps, in the mid 40s for the
most part. Southerly flow with increasing moisture will yield
more clouds through the latter portions of the day.

Tue night into Wed...
The differences in timing of a weak warm frontal passage and
its attendant overrunning continue with the 19.12Z guidance
update. GFS continues to be the faster model, but also the
weakest and driest while, out of the operational runs, ECMWF/CMC
are slower and slightly more robust. Noted a modest trend
toward the latter solution in both GEFS/ECENS members, so
timing/impacts will be more reflective of the ECMWF/CMC.

Modest overrunning begins overnight and continues through the
first few hours of daylight Wed. Isothermal profiles remain,
with low lvl temps mainly near 0C to +3C. Bulk of the moisture
(PWATs only around 0.7 inches) relegated to the lowest 200hPa.
Initial wet-bulb processes could yield a mix of sleet/wet snow,
but this will shift to mainly rain. Pockets of freezing rain
possible, especially N Central and W MA, as temps may drop near
or below freezing before precip onset, and wetbulbs will only be
lower. Not expecting much in the way of ice, but enough that
overnight, early AM commutes could be slippery on untreated
roadways.

Otherwise, improvement on Wed outside of some lingering clouds.
Warming of H92 temps to nearly +6C to +8C by 00Z Thu suggests
temps will rapidly rebound and may even approach the low 50s in
spots especially if enough afternoon sunshine is observed.

Thu and Fri...
Modest clipper will move through N New England Thu. Although
little to no impact expected (outside of some clouds), the brief
increase in low-lvl temps in response, combined with a W
component to the mean flow, may yield the warmest day of the
week. H92 temps average +10C to +12C (as a comparison, today`s
H92 temperatures are closer to +6C to +8C. Remaining snowpack,
additional clouds may limit mixing somewhat, but all conditions
point toward a day with widespread 60s possible. Ensemble
probabilities are supportive of this thinking. Some slight
cooling on Fri, especially with increasing clouds ahead of a
warm front suggests highs mostly in the 50s at best. Timing of
overrunning precip looks to hold until Fri night thanks to
initial mesoscale ridging possible.

Next weekend...
Sharper wave currently wrapped up in the Aleutian vortex will
be forced E and may start taking on a neutral or even negative
tilt as it arrives in the OH Valley. Good agreement that parent
low pres remains an inside runner, but it will have some
connection to Gulf of Mexico moisture with PWATS approaching
1.3+ inches (3+ standard deviations above normal). Within the
warm sector (Saturday) LLJ 40-05 kt, TT values near 50 and mid
lvl lapse rates steeper than moist-adiabatic, therefore, will
need to watch for pockets of heavy rainfall at times along with
this feature passage. Can`t even rule out a rumble of thunder or
two if conditions line up. Conditions improve Sun, with a
return to much more seasonable cool temps following in the wake
of this stronger system.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Today...High confidence. Mainly VFR but some brief marginal
MVFR CIGS are possible with the highest risk across Cape Cod.
Northwest winds will gust to 20 knots later this morning and
afternoon with up to 25 knots across portions of the Cape/Nantucket.

Tonight and Tuesday...High confidence. VFR except some marginal MVFR
CIGS possible into Tuesday morning across portions of Cape Cod.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tue night into Wed...Moderate confidence.
Overnight low-moderate risk for some MVFR/IFR CIGS along with
occasional MVFR vsbys in light rainfall. Low risk for some light
icing with light freezing rain, mainly Worcester Hills and
Western MA. Improvement to VFR gradually after sunrise on Wed.
Winds shift to the W-SW.

Thu and Fri...High confidence.
Mainly VFR. Low risk for some mixed MVFR CIGS by late day Fri,
but these may hold off until Fri night.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Today...High confidence. Northwest wind gusts of 20 to 30 knots
in the cold air advection pattern. The strongest of those winds
gusts will occur across our eastern waters. Small craft
headlines posted for all waters except Boston Harbor and
Narragansett Bay.

Tonight...High confidence.  Lingering marginal small craft wind
gusts/seas across our eastern waters this evening should diminish
after midnight as high pressure builds in from the west.

Tuesday...High confidence.  High pressure overhead will result in
light winds and flat seas.

Outlook /Tuesday Night through Friday/...

Tue into Wed...Moderate confidence.
Quiet boating wx remains in place on Tue. Accompanied by some
light rain early Wed morning, W-SW wind gusts may reach near the
25 kt threshold for small craft advisories, but then diminish by
about mid day on Wed. Seas remain generally 5 ft or less, so
there is a low risk that a brief period of small craft
advisories may be needed Tue night into Wed morning.

Thu and Fri...High confidence.
Generally quiet boating weather even as winds shift from mainly
W-SW to E by Fri.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ231-
     232.
     Small Craft Advisory until 1 PM EST this afternoon for
     ANZ233>235-237.
     Small Craft Advisory until 1 AM EST Tuesday for ANZ250-254-255.
     Small Craft Advisory until 9 AM EST this morning for ANZ251.
     Small Craft Advisory until 3 PM EST this afternoon for ANZ256.

&&

$$
SYNOPSIS...Frank
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Doody
AVIATION...Frank/Doody
MARINE...Frank/Doody



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