Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 240225
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1025 PM EDT Thu Mar 23 2017

.SYNOPSIS...
High pressure along the eastern seaboard will move offshore
later tonight bring windy and milder conditions Friday. A cold
front will push south across the region Saturday, then stall
south of New England early next week. Weak low pressure waves
forecast to push along this front, with the potential for
periods of rain and mixed precipitation. Another stronger front
may approach around the middle of next week.

&&

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

1025 PM Update...

A few high level clouds were starting to arrive in southern New
England. However it will be a while longer before we would need
to worry about precipitation 10 PM dew point depressions were
still on the order of 10 to 25 degrees. The mainly clear sky and
very dry air will lead to good radiational cooling for the next
few hours. This will set the stage for mixed precipitation later
tomorrow, so will need to monitor this evolution closely
overnight.

Mainly made tweaks to the forecast to reflect observed trends.
The latest guidance is still rather quick to bring up dew
points. Although the LAMP guidance still appears to have better
timing.

730 PM Update...

Mainly clear skies, except for a few strips of thin cirrus seen
across New England and down the eastern seaboard on latest IR
satellite imagery at 23Z. Leading edge of lower and mid level
clouds working into the eastern Great Lakes and along the
NY/Canadian border, moving steadily SE. Based on the satellite
trends, expect high clouds to approach western areas by around
midnight, then move steadily SE with mid deck clouds pushing in
around or after 07Z-08Z.

Noting light precip moving out of southern Ontario into western
NY state on latest NE regional mosaic 88D radar, though
observations beneath the obs not reporting precip at 23Z. Very
dry airmass across the region, with dewpts in the single digits
either side of zero, so will take quite a while to saturate the
airmass, as dewpts upstream mainly in the 5-10 degree range.
With the much lower dewpts and mainly clear skies for a few
more hours than previously forecast, will need to monitor temps
which could go into freefall with quick radiational cooling once
winds drop off.

Current forecast in pretty good shape, with dry conditions
expected through the night. Needed to update dewpts, which were
too high based on observations. Remainder of forecast brought
current.

Previous Discussion...

We expect radiational cooling the first part of the night,
shutting down later tonight as the clouds thicken. Dew points
start in the single numbers below zero and climb into the teens
by late night. We favored going a couple of degrees below
guidance with a range from the high single numbers in the
interior to near 20 along the coastline. Most areas would be in
the teens.

&&

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

High pressure offshore will provide a flow of milder air. Winds
above the surface will climb to 30-40 knots by midday. That will
provide a flow of moist air with precipitable water values
reaching near an inch by Friday evening. Weak overrunning
created by this southwest flow will create clouds and light
precipitation starting in the morning. Temperatures will be cold
enough at the start for snow, but quickly moderate to rain by
afternoon if not sooner. The transition period may feature some
sleet.

The best chance for this precipitation will be in Northern and
Western Mass. But frozen amounts will be light, with any
snow/sleet accumulation less than an inch.

Have high confidence in the overall trend, but only moderate
confidence in the precipitation types.

The strong southwest low level jet will also provide the
momentum for gusty winds, especially during the afternoon when
mixing is greatest. It still appears the greatest risk for gusts
over 30 mph to be along the Boston-Providence corridor and
southeast from there.

Max temperatures should make a 5-10 degree rise from today, but
values in the mixed layer Friday are supportive of highs in the
low to mid 40s so still below March normals.

Friday night...

Clouds and a southwest flow of milder air will keep
temperatures from falling too much, and most of that should be
in the evening. Temps may rise a little overnight. We maintained
a chance of showers in far Northern Mass, but low values for
pops.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights...

* Seasonal temperatures on Saturday with periods of light rain
* Unsettled weather at times Sunday-Tuesday with some rain,
  sleet and/or freezing rain possible during the overnight
  periods
* Low pressure and a cold front push across around mid week,
  then improving conditions around next Thursday

Overview...

While the 12Z operational model suite and ensembles continue to
signal an active weather pattern across the northeast U.S.
during most of this timeframe, noting some improving conditions
toward the end of the long term. Still noting a mainly amplified
but progressive steering pattern through most of the long term
period.

Noting a fast flow aloft to start this weekend while cutoff H5
low sits across the central Mississippi valley. This will keep
weak low pres waves moving across the northeast while a front
wavers across southern New England. The cutoff low will lift NE
while it opens up to a short wave over the weekend into early
next week, while large high pressure builds southward out of
Quebec across Maine and N NH. Some question as to whether the
eastern extent of the stationary front over the region will
shift south for a time during the weekend. At this point, the
ridging may push a bit too far E to bring somewhat drier
conditions.

The cutoff low will bring another shot of lift and moisture
and, with some marginally cooler air trying to work in from the
passing ridge to the E, may see a wintry mix of snow, sleet
and/or freezing rain Sun night into early Mon morning. This
should pass quickly as yet another H5 short wave works E from
the central U.S early next week. Another surface low and
associated cold front should shift sometime in the Tue-Wed
timeframe, though wide model solution spread lends to lower
timing confidence.

May see improving conditions push in around late Wed or Thu,
but again exact timing is still very much in question.

Details...

Saturday-Saturday night...Moderate confidence.

Will be a mild day on Saturday as H85 temps start off from +5C
to +8C. However, as high pressure builds SE out of Quebec into N
NH and Maine during the day. Will see periods of rain develop
during the day along an advancing cold front that will sink S of
the region. Temps will top off near seasonal normals, from the
mid-upper 40s across N Mass, ranging to the mid 50s across S
coastal areas.

Saturday night-Sunday...Moderate confidence.

As temps fall steadily behind the front, readings will approach
the freezing mark across N central and W Mass into far N CT.
Partial thickness pattern suggests that precip will change over
to sleet/freezing rain mainly across the higher terrain of N
central and W Mass, with some snow also mixed in for a time
overnight. QPF will be light, generally 0.1 inches or less, but
any icing could be problematic. Will continue to monitor this
closely.

Temps will slowly rise, so any mixed precip should change back
to light rain. However, temps may not reach freezing until late
morning across some of the higher terrain of the Worcester hills
and Berkshires near the MA/NH border. Another shot of steadier
precip will push out of NY state during the afternoon. It will
be a cooler day with highs only in the mid-upper 30s well inland
to the lower 40s across the coastal plain.

Sunday night-Monday...Moderate confidence.

Will see yet another shot of QPF and lift move across the
region during this timeframe. Also noting increasing PWATs in
the S-SW flow aloft, on order of 0.8 to 1 inch. QPF amounts
through 18Z up to 0.5 to 0.6 inches are possible, with the best
shot between 06Z-12Z Mon but mainly S of where the colder temps
will be. However, this is still another more problematic
situation for possible winter weather headlines, especially for
portions of central and western Mass into far N CT. Have
highlighted this in the hazardous weather outlook.

Temps will again slowly rise during Mon morning, but precip
will continue through the day. Expect readings to top off from
around 40 degrees across the higher terrain to the upper 40s
across S coastal areas as milder air starts to push toward the
region.

Monday night through Thursday...Low confidence.

Noting somewhat better general forecast agreement through most
of this timeframe, but timing and track of specific features
still in question.

Could see another round of mixed precip late Mon night mainly
around the Route 2 area of the Worcester hills and Berkshires
with marginally cold temps with some residual cold air damming,
which should not last too long as temps rise through the 40s on
Tuesday. May see a brief break in the precip as well as a weak
ridge builds across.

Another H5 short wave/surface low will shift NE Tue night/Wed.
Again, appears another round of rain will move in ahead of
associated cold front. Some model spread with the frontal
passage, with faster timing shown on the ECMWF, and somewhat
slower on the 12Z GFS. Held on to chance POPs through midday
Wed, then should start to see improving conditions Wed
afternoon/night.

As ridging develops at the surface and aloft across the Great
Lakes/western Quebec by 12Z Thursday, looks like a dry but cool
day.

&&

.AVIATION /02Z 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/...

Tonight...VFR. High confidence.

NW winds becoming light and variable or calm for a time tonight.
Winds then become light south to southwest late tonight. Area
of high clouds pushes into western areas by around 03Z-04Z,
steadily shifting SE.

Friday and Friday night...Moderate confidence.

VFR with some MVFR mainly north of the Mass Pike. Increasing
and lowering clouds during the morning. Areas of MVFR
cigs/vsbys in light snow or sleet, with best chance along or
north of Boston and the Mass Pike. All frozen precip should
change to rain during late morning/early afternoon. Southwest
winds will gust 25-30 knots during the day, highest along the
coast and in higher terrain. Winds diminish Friday night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday-Saturday night...Moderate confidence. Periods of light
rain through Saturday with areas of MVFR CIGS and local MVFR-
IFR VSBYS. May see brief period of mix of sleet/freezing rain
overnight Sat night across higher terrain of Central and W Mass
into N CT. NE wind gusts to around 20 kt Sat night across Cape
Cod and the islands.

Sunday-Monday...Low to moderate confidence. Areas of MVFR
CIGS/VSBYS with local IFR conditions through most of the period.
May briefly improve for a time during the daylight hours. Areas
of rain, sleet and/or freezing rain possible late Sun night
into Mon morning, which may linger most of Mon across the higher
terrain. E-SE winds may gust to around 20 kt along the
immediate E coast Monday.

Tuesday...Low confidence. Areas of MVFR CIGS/VSBYS with local
IFR at times. Should see mainly patchy light rain, though can
not rule out brief freezing rain early Tue morning and again
late Tue night with marginally cold temps.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Main change this evening was to post a Gale Watch for the
eastern outer coastal waters. Have moderate confidence in gusts
up to 35 kt across those waters. If confidence grows overnight,
this watch may be converted to a Gale Warning. Just wanted to
highlight the potential for mariners.

Tonight...High confidence.

Diminishing NW winds becoming light SW overnight. Seas near 5
feet on the outer waters east of Massachusetts early, but
subsiding during the night. Seas elsewhere less than 5 feet. A
Small Craft Advisory lingers on those Eastern Outer Waters for
a few more hours until seas subside.

Friday...Moderate confidence.

High pressure moves well offshore, bringing southwest winds to
the waters. Potential for wind gusts 25-30 knots. This will
build seas with 5 foot heights on many of the waters by the
afternoon. Patchy light rain may bring brief visibility
restrictions. Small Craft Advisories are being issued for most
waters. An advisory for the eastern outer waters will be issued
once we move clear of the existing headline.

Friday night...Moderate confidence.

Southwest winds 25 to 30 knots may become west toward morning.
Seas of 5-7 feet will linger through the night on the outer
waters and all southwest-exposed waters such as RI Sound. Small
Craft Advisory will lower on Boston Harbor and Narragansett Bay,
but linger on the remaining waters.

Outlook /Saturday through Tuesday/...

Saturday...High confidence. NW winds gusting to around 25 kt
Sat morning, then diminish and veer to NE by Sat night. Gusts
may approach 25 kt on the southern outer waters Sat night. Seas
5-6 ft on the southern waters early then subside.

Sunday...Moderate confidence. E winds gusting to around 25 kt,
become SE Sun night. Small Crafts may be needed. Seas build to
around 5 ft over the outer waters E and S of Cape Cod and the
southern outer waters.

Monday-Tuesday...Low to moderate confidence. E-NE winds gusting
to 25 kt. Seas building to 4-7 ft over the eastern open waters
Mon night-Tue.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 10 AM Friday to 6 AM EDT Saturday
     for ANZ232>235-237-255-256.
     Small Craft Advisory from noon to 8 PM EDT Friday for ANZ230.
     Small Craft Advisory from 10 AM Friday to 3 AM EDT Saturday
     for ANZ231-251.
     Small Craft Advisory from 10 AM to 10 PM EDT Friday for ANZ236.
     Gale Watch from Friday afternoon through Friday evening for
     ANZ250-254.
     Small Craft Advisory until 2 AM EDT Friday for ANZ254.

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...WTB/Belk/EVT
SHORT TERM...WTB
LONG TERM...EVT
AVIATION...WTB/EVT
MARINE...WTB/Belk/EVT



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