Area Forecast Discussion
Issued by NWS Boston, MA

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307
FXUS61 KBOX 252018
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
318 PM EST Sun Feb 25 2018

.SYNOPSIS...

Rain will rapidly be ending from west to east through
early this evening...but fog and drizzle will linger a bit longer.
High pressure will bring dry weather with cool nights and mild
afternoons through mid week.  The potential continues for a
significant coastal storm late this week with several possible
weather hazards...but it is not a certainty this far out.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10am update...

The most significant change with this mid morning update is to
drop the E extent of the WWA. Temps/dwpts now rest comfortably
in the mid and upper 30s in these areas, and even with the
development of the meso-frontal low by mid day it is unlikely to
fully mitigate the warm layer expansion sufficiently to re-
introduce freezing temperatures at or just above the sfc. In the
Higher terrain, and portions of W MA/CT this risk does continue,
as mesowest/obs show temps much closer and even still below
freezing in a few spots. Will continue the headlines there for
the time being and reassess at mid day.

Otherwise, temps are a blend of raw 2m temps and latest RAP/LAV
data, which has handled the temps/dwpts best so far this
morning. No significant changes to POPs as precip should rapidly
end from W-E during the afternoon hours.

*/ Previous discussion...

PWATs 2-3SD above normal combined with good low level isentropic
ascent will bring a moderate QPF event with widespread precip
today. Progressive system so much of the precip will be exiting
the region by late afternoon or early evening.

A mixed bag of wintry precip is expected given warm nose aloft and
presence of low level cold air. In fact models are indicating temps
in the 950-925 mb layer cooling to 0 to -2C near the south coast
this morning and down to -5C near the NH border. With warming in
the 850-700 mb layer advancing northward this will set up a good
sleet signal this morning, especially across northern MA. In addition,
with sfc warm front remaining south of New Eng and sfc wave tracking
along the boundary, low level northerly winds will help to lock in
cold air at the surface, especially higher terrain which will increase
threat of icing after warming aloft changes precip to rain.

North of the Mass Pike...

Cold air is initially deep enough for some snow especially toward
NH border and NE MA for at least a few hours after precip onset
before changing to freezing rain and rain. Snow in NE MA may
last until late morning as warmer air aloft coming up from the
SW. An inch or 2 of snow accum is possible, mainly along and
north of route 2 into Essex county with low risk for 3 inches in
portions of northern Essex county. Some minor sleet accum is
also possible down to the Mass Pike region. Regarding freezing
rain, it appears greatest threat for appreciable icing will be
over higher elevations in the Berkshires and Worcester hills
where temps may hold at or below freezing much of the day. We
relied heavily on 3km NAM and WRF-ARW which shows the cold air
over higher terrain. Potential for localized 0.10-0.20" ice
accretion over the Worcester hills and especially the Berkshires
with less than 0.10" elsewhere.

South of the Mass Pike...

Precip will be mostly rain but will likely mix with sleet for a
few hours this morning across northern CT, northern RI and along
the I-95 corridor to BOS. Freezing rain is less of a threat south
of the pike, but may still see a few hours of freezing rain over
higher terrain in northern CT and central MA.

Advisories largely unchanged but we dropped it for coastal Essex
county and SE Middlesex county where temps expected to remain
above freezing with just a mix of rain and sleet.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
Precip will exit eastern New Eng by early evening, otherwise a
dry night expected. Stratus and patchy fog in the evening may
clear overnight, but any clearing may lead to more fog developing.
Patchy black ice will also be a concern overnight with moisture
near the ground and lows dropping to upper 20s to lower 30s away
from the coast.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...

Highlights...

* Dry/mild with above normal temperatures Tue and especially Wed

* Significant coastal storm possible late this week with several
  "potential" weather hazards but it is not a certainty this far out

Details...

Tuesday and Wednesday...

Westerly flow aloft with above normal height fields coupled with a
surface high pressure to the south will keep our weather dry and
mild. High temps should reach into the upper 40s to lower 50s Tue
afternoon.  Even milder temperatures on Wed as the high moves off
the mid Atlantic coast allowing high temps to recover well into the
50s to near 60...away from the cooling marine influence of the south
coast.

Thursday through Saturday...

The potential continues for a significant/long duration coastal
storm late this week...but there remains plenty of uncertainty. This
event is still 4-5 days out in the model world...where track errors
over 150 miles are quite common. Therefore...it really is
impossible to lock in any particular solutions or outcomes.
Nonetheless...there are several ingredients in place that still have
us concerned for the threat of a significant/long duration coastal
storm with several weather hazards possible.

What exactly unfolds late this week will have to do with
timing/strength of many features. A couple main players involve
shortwave energy emerging from the southwest and another across
the north central plains. Their specific strength/timing will
have a large impact on the developing storm that will occur in
the Ohio Valley. A strong -NAO/Greenland Block should prevent
the storm from cutting to our west and force it eastward...at
least secondary coastal development. As previous forecaster
mentioned...a highly anomalous -NAO can sometimes suppress the
storm track well south of our region that is not always the
case. There have been plenty of big storms with very strong
-NAO/s...but a lot of that is a function of where the block sets
up along with where exactly the storm develops and intensifies.

While it is way too early to get into specifics about this potential
strong coastal storm...there are several possible weather hazards.
These include significant amounts of rain and/or snow depending on
thermal profiles...which are impossible to say at this point.  If
ptype favored rain there would be a concern for fresh water
flooding.  A period of strong to damaging winds especially along the
coast. The GEFS ensembles are already indicating an easterly LLJ 4+
standard deviations above normal at 12Z Friday...which is quite
unusual to see in an ensemble 120 hours out. Lastly...a
persistent and potentially strong onshore flow may result in
significant coastal flooding/beach erosion during several high
tide cycles along the eastern MA coast given high astro tides.

In a nutshell...the potential hazards listed above are if this storm
comes to fruition but we are still 4 to 5 days out.  If shortwave
timing/strength changes a weaker storm suppressed further to
the south could spare the region many issues. Nonetheless...this
system does have significant potential and bears watching.

Sunday...

Low confidence this far out...but looks like bulk of the
precipitation will have departed with seasonable temperatures.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...

Through 21Z...High confidence.
Mainly a mix between MVFR and upper end IFR with RA/fog and
lowered CIGS. The RA/fog will dissipate initially late
afternoon.

21Z this evening through 10Z Mon morning...Moderate confidence.
Evening and early overnight, expect IFR/LIFR conditions in fog,
especially as winds remain 100-040. These winds will be backing
toward the NW during the early morning hours, which should allow
improvement to MVFR/VFR although timing may be off a bit in TAF.
Some sites, especially those typically prone to fog, may stay
IFR/LIFR longer.

After 10Z and into Mon night...High confidence.
Winds settle NW around 10 kt. Lingering IFR/fog conditions
dissipate through the early AM, with VFR then remaining through
Mon night.

KBOS Terminal...MVFR in RA/BR gives way to some IFR CIGS/Fog
this evening. Once winds shift mainly to the N-NW between 04Z-
06Z, conditions should improve to VFR then remain. High
confidence in these trends, but lower confidence in timing.

KBDL Terminal...Moderate confidence. Lingering RA/BR keeps
conditions mainly MVFR through 21Z. Then with winds shifting N,
then NW late, lingering BR should dissipate, with a trend toward
VFR after. Timing somewhat uncertain, as some lower ground fog
cloud form as CIGS rise.

Outlook /Tuesday through Friday/...

Tuesday through Wednesday...High confidence in mainly VFR
conditions.

Thursday: Increasing threat for MVFR to perhaps even IFR
conditions as the day wears on Thu...but that will depend on the
timing of a developing coastal storm.

Thursday Night and Friday: MVFR-IFR conditions likely. Windy
with gusts to 40 to 50 knots possible along the coast depending
on track/timing/strength of coastal storm. RA and/or SN likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...Moderate confidence.

Today...A period of marginal easterly gales likely across much
of the open waters today as low level jet moves over the waters.
Cold air above the boundary layer will promote decent mixing
this morning but inversion will strengthen during the
afternoon. Rain and fog will reduce vsbys.

Tonight...Winds diminishing and becoming NW during the night.
Speeds dropping below SCA but hazardous seas will continue over
outer eastern waters.

Outlook /Tuesday through Friday/...

***Potentially dangerous storm for Mariners late Thursday into
 Friday with strong winds and high seas which are dependent on
 the track/strength of a potential strong coastal storm***

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday Night: Winds less than 25 kt.

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

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft.

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain.

Thursday Night and Friday: Gale to storm force wind gusts
between 40 and 50 knots may develop depending on track/strength
timing of coastal storm. Seas may approach or exceed 20 feet!

&&

.TIDES/COASTAL FLOODING...

Thursday through Saturday...

The potential exists for significant coastal flooding/beach erosion
over several high tide cycles late this week...particularly along
the eastern MA coast.  While this is far from a certainty given the
potential event is 4 to 6 days in the future...there are several
ingredients in place coupled with high astronomical tides that this
will need to be watched closely.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory until 4 PM EST this afternoon for
     MAZ002>004-008>010-026.
RI...None.
MARINE...Gale Warning until 4 PM EST this afternoon for ANZ231>235-237-
     255-256.
     Small Craft Advisory until 6 PM EST this evening for ANZ230.
     Small Craft Advisory until 4 PM EST this afternoon for ANZ236.
     Gale Warning until 6 PM EST this evening for ANZ250-254.
     Small Craft Advisory until 5 AM EST Monday for ANZ251.

&&

$$
SYNOPSIS...Frank
NEAR TERM...KJC/Doody
SHORT TERM...KJC
LONG TERM...Frank
AVIATION...Doody/Frank
MARINE...Doody/Frank
TIDES/COASTAL FLOODING...Frank



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