Area Forecast Discussion
Issued by NWS Boston, MA

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279
FXUS61 KBOX 170752
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
352 AM EDT Tue Apr 17 2018

.SYNOPSIS...
An upper level system will keep unsettled conditions across the
region through mid week. A low pressure area will produce a
period of chilly rain potentially mixing with wet snow during
Thursday. Lingering rain showers Thursday night may mix with or
change to wet snow across northern Massachusetts. The weekend
looks to be dry but with more below normal temperatures.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Cold front moves off the east coast of Mass first thing this
morning. Winds turn from the west, bringing in drier air at the
surface.

Upper low will be overhead of New England today, with a -24C
cold core. Cross sections show a deep moist layer reaching to at
least 600 mb. Low level lapse rates reach 9C/Km, while mid level
lapse rates are around 5.5 to 6C per Km.

Satellite images show lots of clouds. If any breaks develop this
morning, the sunshine working on the cool temps aloft should
generate clouds, leaving us with mostly cloudy skies. The
shortwave and instability may also generate widely scattered
showers.

Mixed layer is forecast to reach 850 mb. Temps at the top of the
layer at -5C to -7C suggest max sfc temps in the mid 40s
northwest to lower 50s coastal plain. Winds in the mixed layer
reach 20 to 25 knots, so expect gusty west winds late morning
and afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Tonight...

Second shortwave rotates around the upper low and across
Southern New England during the first half of the night. Deep
moisture lingers overhead as this takes place. Expect mostly
cloudy skies with widely scattered showers through midnight,
then partial clearing late. We stayed close to guidance min temp
forecast in the 30s.

Wednesday...

Another shortwave moves through during the day. This one will be
supported by a 100-knot jet, but with less moisture aloft and
most of that will be shallow. Expect a partly cloudy day with
mixing again reaching 850 mb. Temps at that level will be a
couple of degrees higher than today, about -3C to -4C.  This
supports max temps in the upper 40s to mid 50s.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights...

* Rain, potentially mixing with wet snow on Thu.
* The coming weekend looks to be dry with gradually warming
  temps.

Overview and model preferences...
As previous forecaster noted, amplified spring pattern with
several weak cutoffs linked to the N stream continue to impact
the region through the mid term. However, am noting that a
gradual mid and upper lvl height rises on the periphery of the
modest S stream jet persist through much of the mid and long
term. Therefore, following a secondary wave mid-late week (most
impacts felt on Thu) the rising heights should allow enhance
synoptic ridging in its wake yielding a drier and milder
stretch. During this stretch, temps should generally remain at
or above seasonal normal values. Synoptic differences in
guidance through the mid and long term are relatively limited in
magnitude. While there are track differences for deepening low
pres Thu, they are close. Should the S track favored by
ECMWF/CMC/UKMET and several GEFS and ECENS members, there could
be a bit more cold air for mixed precip in the higher terrain.
The S track is also more amplified and therefore less
progressive, thanks to more residence time over the waters.

Details...

Wed night into Thu...
Developing cutoff born of both Pacific and Nunavut energy will
be moving quickly W-E across the NE, at which point it will
become absorbed in the stalled cutoff near Labrador. Attending
sfc low pres gradually deepens, into the 990s by the time it
arrives in S new England. As mentioned above there are still
differences in the track which may ultimately determine the
final QPF totals. However, PWATs on both the deterministic and
ensemble members peak just below 1.00in, or only about 1 std
deviation above normal. LLJ energy is also modest. Therefore,
regardless of track this low pres passage will not be as dynamic
of the previous, with final QPF values mainly between
0.5-1.00in. Soundings are initially cold for some light wet
snow, with the possibility of some minor accumulations (mainly
on grassy surfaces/cars) in the higher terrain. This occurs
again as precip ends Thu night. Ensemble probabilities are
relatively high for at least 1.0in of accumulation before any
transition.

Fri into Sat...
Given the reinforcement of the cutoff in the Maritimes by the
wave moving through Thu, this will slow the progression of the
trof E of the region. Although drier air will be shifting in
from the NW, temps will remain cool as H85 temps drop to an
average around -4C and H92 near +1C. By Sat, upstream ridging
has full control but with little mid lvl temp changes. Mixing
will make up some of the difference with temps trending back
toward normal.

Early next week...
Continued impact from ridging, as N stream attempts to dive S
again upstream allows for gradual warming throughout the column.
Mainly dry with temperatures warmer than normal are expected.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

Today...High confidence.

Lingering MVFR/IFR cigs leading the overnight cold front, but
conditions improve to VFR after the cold front moves through.
Expect cloud bases 3500-5000 feet during the late morning/afternoon.
A few isolated rain showers are possible during this time, mixed
with snow over the higher elevations. West-southwest winds will
gust 15-20 knots most places, and 20-25 knots Cape Cod/Islands.

Tonight and Wednesday...High confidence.

VFR. Winds gusting 20 to 25 knots Wednesday.

KBOS Terminal... High confidence in TAF. MVFR early morning
ahead of a cold front crossing the region. Winds shift from the
WSW behind the front, and ceilings improve to VFR. This happens
by 10Z/6 AM. Winds gust 20-25 knots this afternoon.

KBDL Terminal...High confidence in TAF. VFR. Winds gust 20-25
knots this afternoon.

Outlook /Wednesday Night through Saturday/...Moderate confidence.

Wednesday Night: Starting VFR trending to mix MVFR/IFR early
morning. Chance RA, chance SN.

Thursday: MVFR/IFR conditions expected. RA likely, SN possible.

Thursday Night: Mainly MVFR, with areas IFR possible. Breezy.
Chance RA, chance SN.

Friday: Mainly MVFR to start, with VFR thereafter. Breezy.
Slight chance SHRA.

Friday Night through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Today... High confidence.

Southerly winds turn from the west-southwest with gusts to 25
knots. Rough seas were already in place early, and with the
expected winds we expect only a slow subsiding through the day.
Small Craft Advisory continues for most areas.

Tonight and Wednesday... High confidence.

Winds diminish a little tonight, but then return to similar 25
knot gusts on Wednesday. Seas linger at or above 5 feet on the
outer waters and RI waters, but subside below 5 feet on several
nearshore waters including Mass Bay and Cape Cod Bay. Small
Craft Advisory will continue for most waters.

Outlook /Wednesday Night through Saturday/...Moderate

Wednesday Night: Winds less than 25 kt. Local rough seas.
Slight chance of rain. Small craft advisory for seas needed.

Thursday: Winds less than 25 kt. Local rough seas. Chance of
rain. Small craft advisory for seas needed.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of rain showers,
chance of rain.

Friday: Winds less than 25 kt. Local rough seas. Slight chance
of rain showers. Small craft advisory for seas needed.

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Saturday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 6 PM EDT Wednesday for ANZ232>234.
     Small Craft Advisory until 8 PM EDT this evening for ANZ231.
     Small Craft Advisory until 6 PM EDT this evening for ANZ236.
     Small Craft Advisory until 6 PM EDT Wednesday for ANZ235-237-
     250-254>256.
     Small Craft Advisory until 5 AM EDT Wednesday for ANZ251.

&&

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



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