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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
352 am EST Mon Nov 18 2019

Synopsis...
a coastal storm will bring some rain and strong coastal winds
today, with scattered light icing just northwest of the Boston
to Providence corridor. The rain should change to a bit of snow
and ice across the interior high terrain of Massachusetts late
tonight and Tuesday morning. Mainly dry and seasonably chilly
weather follows Tuesday afternoon into Wednesday morning, then
a disturbance may bring a few showers Wednesday afternoon and
evening. Dry with a warming trend Thursday into early Friday
before a cold front brings a risk for light showers Friday
afternoon. Colder air filters back in for Friday night into
Saturday. Another storm system may affect the region Saturday
night into Sunday, though details are highly uncertain.

&&

Near term /until 6 PM this evening/...

* risk of light icing just northwest of the Boston to Providence
corridor toward daybreak and into the morning.

Coastal low pres has trended a bit further south as it crosses
70w well south of the benchmark this morning before lifting north-northeast
during the day. As a result, steadiest rainfall will be
confined to eastern Massachusetts today and it will be a struggle to get
much precip further back into the interior.

Dry air aloft is persisting early this morning and just getting
some light returns moving west from the ocean so spotty
drizzle or freezing drizzle is expected toward daybreak into the
morning. Coastal front has slipped past bos and temps have
dropped to the upper 30s. The front will remain near or a bit
east of the I-95 corridor this morning. Temps along and west of
I-95 are near or slightly above freezing but dewpoints are in
the 20s. So any light precip may allow enough wet bulb cooling
for temps to fall a degree or 2 which would create problems with
any light drizzle that develops. While precip in the interior
will be spotty this morning, there is a risk for some very light
icing during the morning commute in the interior to the I-95
corridor as it only takes a trace of ice to result in
hazardous/icy travel. As a result will keep advisories in place.
Temps should rise above freezing this afternoon but may hold
near freezing in the interior Massachusetts higher terrain, especially the
Berkshires so have extended the advisory into tonight here as
another round of precip develops from the west tonight.

The moisture begins to deepen across cape/islands toward 12z
then spreads across eastern Massachusetts through the morning. This is when
some steadier bands of rain will develop and continuing into
the afternoon. The deeper moisture never really gets into
western new eng so any precip here during the day will be light
and spotty.

Regarding wind potential, the magnitude of the low level jet
weakens slightly and remains focused across cape/islands into
the afternoon. Wind gusts will be 35-45 mph for the cape/islands
with strongest gusts up to 50 mph likely confined to Nantucket
and possibly the Outer Cape.

Regarding coastal flood potential, see tides/coastal flooding
section below.

&&

Short term /6 PM this evening through 6 am Tuesday/...

* another round of precip moving in from the west tonight
changing to snow over higher elevations

Coastal storm will move toward the Maritimes tonight with
steadier precip lifting north of the region. However, a robust
mid level shortwave from the Ohio Valley will lift into southern New England later
tonight. Increasing large scale forcing combined with deepening
moisture will result in another round of precip developing and
moving into southern New England from the west. Greatest coverage later tonight
will be in western new eng. Initially, precip will be rain but
temps over the higher terrain will be near 32 so can't rule out
pockets of freezing rain light icing, especially in the
Berkshires where advisory will continue into tonight. As heights
fall and cold air deepens precip should mix with then change to
sleet and snow, especially over the higher elevations. Snow
accum of a coating to an inch is possible with a low risk for up
to 2 inches in the Berkshires.

&&

Long term /Tuesday through Sunday/...
highlights...

* rain to wet snow in interior Massachusetts/CT tues am thru midday; slushy
coating-1" of snow with limited if any impact. Stays as light rain
eastern MA/RI.

* Distant coastal low may spread light rain or snow showers
eastern/northeastern counties & the waters later Wed-early Wed
nite. Breezy conditions on eastern/southeastern waters.

* Dry/seasonable weather under high pres for thurs.

* Milder temps Fri ahead of a cold front that brings light showers
and colder air Fri night.

* Monitoring for another frontal system for the weekend, though
uncertainty is large.

Details...

Tuesday:

Potent, secondary mid-level shortwave trough will be racing north/neward
through southern New England Tuesday. Ongoing band of rain showers
owing to qg ascent and mid-level diffluence related to the trough
should be ongoing from the overnight - with the model-quantitative precipitation forecast Max from
the Hudson Valley eastward thru the Berkshires into the CT/Pioneer
Valley and northern Worcester County by 12z Tuesday, and starting to
trend toward less quantitative precipitation forecast/drier across our northern CT/northwest Rhode Island areas.
Lesser precip amounts anticipated over the remainder of Rhode Island and into
the East Coast of Massachusetts where relative humidity in the column isn't as deep as across
our western areas - given good consistency among the global model
quantitative precipitation forecast forecasts as well as the sref, am generally discounting the NAM-
based guidance which is on the higher/wetter end of the quantitative precipitation forecast envelope
for these areas. Most of the precip will have lifted into Vermont/New Hampshire by
noon, with decreasing pops thereafter toward dry thru aftn.

Main question is how soon will rain mix with wet snow in the
interior. Cold thermal advection will be taking place west to east
with an initially warm low-level profile cooling with time through
Tuesday morning. However the cold advection isn't overly strong. In
addition, model 2-M dry and wet-bulb temperatures, however, are in
the 33-35f range, with the NAM being a few degrees cooler. Would
expect rain mixing with wet snow during the Tuesday am commute and
into the remainder of the morning across western Massachusetts into northern
Worcester County southward into the hills in Tolland County and into
Hartford counties in CT. Given the slow cooling column shown in most
models, that snow-liquid ratios should be low and the accumulation
period being pretty short - at the moment i'm not thinking snow
amounts to be anything more than an inch even at elevation, and
think slushy coatings would be more common in lower elevations.

Though 850 mb temps will be falling to values from 0 to -2c,
clearing by afternoon should push highs into the 40s most locales.
Under modest cold advection (850 mb temps fall further into the -2
to -4c range), lows Tuesday night into the upper 20s-mid 30s in the
interior to the mid/upper 30s near the coasts.

Wednesday:

Will be watching an amplifying, eventually closing-off 500 mb low
moving into the waters over or just south of Li sound to around
Nantucket Wednesday, helping to energize a coastal low that passes
well east of the 40n/70w benchmark. Though most of the interior
should be mostly cloudy but dry, some moisture is drawn back
westward into the cooler air supporting some showers around the
second half of Wednesday into Wednesday night at least for the
coastal waters and the East Coast MA, the cape and mvy/Nantucket.
Uncertainty in how far west this moisture extends and p-type -
either rain or snow showers, sensitive to surface temps given that
lapse rates are fairly steep from sfc to 800 mb given cold advection
regime. Both the 00z European model (ecmwf) and Canadian Gem bring some light quantitative precipitation forecast
into at least a part of interior central Massachusetts and northern Rhode Island
Wednesday afternoon/early evening, with the European model (ecmwf) being the furthest
west. Progged quantitative precipitation forecast isn't more than a tenth of an inch in any area, so
not currently looking at a significant precip event. Opted to spread
slight to lower end chance pops approximately from northern Franklin
County in Massachusetts southeast to the islands on northeastward.

Will see north/northwest winds tend to increase with eastern extent and
especially over the eastern/southeastern waters, as gradient
tightens somewhat between the distant coastal low and a building sfc
ridge over the mid-Atlantic/north-central Appalachians. Could see a
need for marine headlines, though at the moment looks sub-gale
level.

Thursday - Thursday night:

Sfc ridge and shortwave ridging aloft shift/build into southern New
England by Thursday, leading to quiet conditions. Returning
southwest flow boosting 850mb temps +2 to +4c allowing for a return
to temperatures into the mid-upper 40s, with a few spot 50s
possible. Warm advective cloudiness associated with a sfc warm front
supports early lows in the lower mid 30s before rising into the
upper 30s/mid 40s by daybreak.

Friday:

Potent northern-stream shortwave trough and associated sub-1000 mb
cyclone moving through the Great Lakes into the St. Lawrence Valley
friday; sfc cold front progresses through New England through the
with a period of frontal showers to result. Differences apparent in
the 00z GFS compared to the international guidance in timing the
front across, and think the GFS is being much too progressive in
phasing a filling closed low with the northern stream trough. This
would result in a slower/wetter solution. The European model (ecmwf)/Canadian Gem
keep these features separate and think this looks more reasonable.
Thus will follow the non-GFS guidance and will show a frontal
passage first part of the day with limited quantitative precipitation forecast amounts. Drying for
the late-aftn/evening period.

Friday looks to be the warmest day with 850 mb temps in the +4 to
+6c range supporting highs before the front arrives in the upper 40s-
mid 50s for most. Strong cold advection on 850 mb winds 35-45 kts
for the evening should push lows into the 20s to mid 30s. Northwest
breezes/gradient flow will likely lead to another period of sub-20f
wind chills for Friday night, especially in the Worcester/Tolland
County hills and into the Pioneer/CT valleys.

Saturday - sunday:

High pressure, dry conditions and cooler than normal temps for
Saturday is pretty well agreed upon by the ECMWF/Gem. Will also see
slackening northerly winds through the day.

Still monitoring for the potential of another surface low pressure
system that may affect the region Saturday night into Sunday, but
variation in guidance becomes large to result in a low-confidence
clouds/precip forecast. These differences are in part related to
varied handling of energy ejecting out of The Four Corners region;
European model (ecmwf) is most bullish with more interaction with northern stream
leading to a more robust precip event, while the Canadian Gem is
more suppressed into the mid-Atlantic with ridging over New England.
Though the GFS is discounted in this period, it shows essentially a
clipper like system for late Sunday into Sunday night. A lot of
uncertainty in the period and will follow a guidance blend to
minimize errors until there's better agreement among the nwp
guidance.

&&

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 tonight/...moderate confidence.

Through 12z...
mainly MVFR cigs with spotty drizzle or interior freezing
drizzle possible toward daybreak. NE gusts 25-35 kt
cape/islands.

Today...
MVFR trending toward areas of IFR with steady rainfall
developing and mainly confined to eastern new eng with spotty
precip further west. Pockets of fzdz or -fzra possible in the
interior this morning. Temps should warm above freezing all
locations this afternoon. NE gusts 30-40 kt cape/islands and north
gusts up to 25 kt back to bos-pvd.

Tonight...
MVFR/IFR conditions. Another round of precip will move in from
the west after midnight. Ptype mostly rain but changing to a
wintry mix with some freezing rain/snow possible over the higher
elevations. Diminishing wind.

Kbos taf...moderate confidence in taf.

Kbdl taf...moderate confidence in taf.

Outlook /Tuesday through Friday/...

Tuesday: mainly VFR, with local IFR possible. Chance ra, patchy
br.

Tuesday night: mainly VFR, with local MVFR possible.

Wednesday: mainly MVFR, with areas VFR possible. Breezy. Slight
chance rain showers.

Wednesday night: mainly VFR, with local MVFR possible. Breezy.

Thursday: VFR. Breezy.

Thursday night: mainly VFR, with areas MVFR possible. Breezy.
Chance shra, slight chance freezing rain.

Friday: mainly MVFR, with areas VFR possible. Breezy. Chance
rain showers.

&&

Marine...
short term /through tonight/...moderate confidence.

Gale warnings continue for all waters. NE gusts 35-45 kt today
with strongest gusts over waters south and east of Cape Cod.
Seas will build up to 15 ft across the eastern outer waters.
Winds and seas diminishing tonight with sub Small Craft Advisory winds by late
tonight. Vsbys reduced in rain and fog, especially over eastern
waters.

Outlook /Tuesday through Friday/...

Tuesday: winds less than 25 kt. Rough seas up to 11 ft.

Tuesday night: winds less than 25 kt. Seas up to 5 ft.

Wednesday: low risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Wednesday night: moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas.

Thursday: winds less than 25 kt. Local rough seas.

Thursday night: winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers.

Friday: moderate risk for Small Craft Advisory winds with local
gusts up to 30 kt. Areas of rough seas. Chance of rain showers.

&&

Tides/coastal flooding...
astronomical high tide at Boston is 10.3 ft MLLW, and 3.7 ft
MLLW at Nantucket Harbor. A storm surge of 1.5 to 2 ft is
expected during the high tide this afternoon with the highest
surge likely at Nantucket where the low level jet axis is
located. The combination of the surge and seas up to 15 ft
offshore will likely result in pockets of splashover or minor
coastal flooding for much of the eastern Massachusetts coast during the
high tide with the greatest risk at Nantucket. Somewhat lesser
risk along the North Shore which will be furthest removed from
the low level jet.

A coastal flood advisory remains in effect for the eastern mass
coast including Nantucket. However, the winds and storm surge
will not be high enough to cause a significant coastal flood
event.

&&

Box watches/warnings/advisories...
CT...Winter Weather Advisory until 9 am EST this morning for
ctz002>004.
Massachusetts...coastal flood advisory from 2 PM to 5 PM EST this afternoon
for maz007-015-016-019-022-024.
Winter Weather Advisory until 9 am EST this morning for maz005-
006-009>014-026.
Winter Weather Advisory until noon EST today for maz002>004-
008.
Wind Advisory from 6 am this morning to 5 PM EST this
afternoon for maz023-024.
Wind Advisory from 7 am this morning to 5 PM EST this
afternoon for maz022.
Rhode Island...Wind Advisory from 6 am this morning to 2 PM EST this
afternoon for riz008.
Winter Weather Advisory until 9 am EST this morning for riz001.
Marine...Gale Warning until 11 PM EST this evening for anz232-254-255.
Gale Warning until 8 PM EST this evening for anz233>235-237-
256.
Gale Warning from 6 am this morning to 6 PM EST this evening
for anz230-236.
Gale Warning from 6 am this morning to 11 PM EST this evening
for anz231.
Gale Warning until midnight EST tonight for anz250.
Gale Warning from 6 am this morning to 10 PM EST this evening
for anz251.

&&

$$
Synopsis...kjc/loconto
near term...kjc
short term...kjc

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