NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Jan 31, 10:57 PM EST, 1 hour 3 minutes earlier.
Area forecast discussion, Jan 31, 10:57 PM EST, 1 hour 3 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
1057 PM EST Sun Jan 31 2021
.SYNOPSIS...
High pressure provides dry and cold weather tonight. A coastal
storm will impact our area Monday into Tuesday, with heavy
snow, strong winds and coastal flooding. High pressure returns
with seasonable temperatures Thursday and a chance of rain
Friday and Saturday.
&&
.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
1030 PM Update...
Radar reflectivity shows a band of light showers moving into CT
and Western MA. As surface high centered just east of Hudson
Bay slowly weakens and lifts north, this would allow light snow
to overspread the area through the overnight hours. With temps
in the teens to mid 20s, a coating to an inch of snow is
possible by daybreak. Nonetheless, with the primary low still
over the Ohio valley and a secondary low just getting its act
together off Southern VA, it will not be till the morning hours
when we start to see the beginning of more significant snowfall
given that we still need time to saturate the lower levels and
for the better dynamics to approach our area. Otherwise, expect
increasing clouds and temps to stabilize or rise slightly
overnight.
Main change to the near-term forecast is to raise PoPs to
account for the light snow moving in.
PREVIOUS DISCUSSION...
430 pm update...
Cold airmass remains in place late this afternoon with temps
15-20, 20-25 over southeast MA including Cape Cod and the
Islands. Airmass remains dry with dew pts in the single digits,
except teens across Cape Cod and the Islands given light NE
flow. Temps will fall this evening given light winds, very dry
airmass and only high thin clouds. However temps will level off
second half of the night as clouds lower and thicken along with
increasing NE winds. Light snow likely arrives very late tonight
toward daybreak Monday across Southwest MA (Springfield), much
of CT and possibly into Southwest RI. Could see some ocean
effect snow showers/flurries into Boston and the north shore
towards morning.
&&
.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
* Major Winter Storm with Heavy Snow and hourly rates of 1-2"
likely Monday afternoon & evening *
* Widespread Snowfall Totals of around a foot, less along the
coast and more across the Worcester Hills and east slopes of
the Berkshires *
* High Wind Warning Cape Cod & the Islands including Block
Island *
* Coastal Flood Watch for entire East Coast of MA including Cape
Cod and the Islands *
Amplifying East Coast Trough yields cyclogenesis off the DE
coast tonight and Monday, then becoming vertically stacked and
tracking between Nantucket and 40N/70W benchmark Tue. Very cold
airmass currently over the region this evening, sets the stage
for an impressive front end thump of heavy snow, before a
changeover to rain and wintry mix across Eastern CT/RI and
Eastern MA Monday evening and overnight. Travel will be
extremely difficult Monday afternoon and evening across
MA/RI/CT.
Below are the main hazard types:
Heavy snow...
As mentioned above very cold/dry airmass over the region this
evening will set the stage for an impressive front end thump of
heavy snow Monday afternoon and evening. Very strong easterly
jet at 850 mb of 5 standard deviations will provide an Atlantic
fire hose of moisture to overrun the cold/dry airmass currently
over the region. This strong forcing for ascent will be enhanced
by mid level instability Monday afternoon and evening as dry
slot approaches. This entrainment of mid level dry air will
steepen mid level lapse rates and result in 1-2" hourly snowfall
rates Mon afternoon and evening. HREF supports this with high
probs of 1-2" hourly rates. Even the HREF hourly mean is 1.5-2.0
inches. That combined with instability aloft, would not be
surprised if 3+ inch hourly rates for a time in the strongest
snow bands, along with a low risk of TSSN! Nevertheless, travel
will be extremely difficult Monday afternoon and evening given
these hourly snowfall rates. Given the evaporative cooling
potential initially, followed the colder guidance here. Although
given low level temps are marginal across Eastern CT/RI/MA,
followed the lower SLR from the NBM which are less than 10-1.
Although this may be offset early on as snow growth is very
favorable 18z-00z. Thus heavy wet snow across this region will
increase the risk for snow loading issues/tree damage and power
outages.
As for snow totals, given increase qpf from 12z guidance and
incorporating the hi res guidance to capture snow shadowing in
the CT River of MA given downslope off the Worcester Hills,
most locations 8-12 inches, including southeast MA. Just north
and west of Providence to Boston, mesoscale banding, good snow
growth and longer duration of snow before possible changeover
will yield a large area of 12-18 inches. This encompasses much
of Eastern CT- Western/Northern RI into the Worcester Hills and
into Metro West Boston. 6-10 inches in the CT River Valley of
Western MA and then increasing 12-18 inches along the east
slopes of the Berks. This area along with the Worcester Hills,
there is a low prob of isolated 18+ if elevated convection
develops. 2-6" possible for the Cape and Islands, with closer to
6" for the upper Cape near the canal.
Strong winds...
Low level jet at 925 mb is 70-75 kt across the region late
Monday into Monday night. Model soundings support gusts up to
50 kt/58 mph across Cape Cod and the Islands including Block
Island. Given increasing trends we converted the high wind watch
to a warning. A few inches of heavy wet snow combined with winds
of this magnitude, tree damage and power outages are possible.
Winds of this magnitude will impact Cape Ann as well, and this
is covered by the Winter Storm Warning.
&&
.LONG TERM /TUESDAY THROUGH SUNDAY/...
Highlights...
* Light scattered snow/rain showers continue Tue
* Drier with widely scattered snow showers Wed
* Dry Thursday and beginning a warming trend.
* Unsettled weather returns Friday into the weekend
Details...
Tuesday through Thursday...
A secondary low develops over the Gulf of Maine on Tuesday as the
low to our south slowly begins to lift to the northeast. The best
dynamics and moisture will be removed from the region by sunrise
Tuesday. So, while the upper level low lingers overhead and the sfc
low is slow to exit, excessive dryness above 700 mb will keep precip
light and scattered in nature. By Wednesday the surface low lifts
out of the area. This means even less precipitation than Tuesday,
but given the cyclonic flow and cold pool aloft, widely scattered
snow showers may continue through the day. When it comes to winds,
Tuesday morning will be the gustiest, especially over
eastern/northeastern MA and the waters as that low level jet exits
to the northeast through the day. Late Tuesday and Wednesday winds
come down but will be a bit breezy simply thanks to the pressure
gradient of the passing low.
By Thursday everyone dries out beneath surface high pressure and mid
level ridging. Plenty of sun and warming temperatures are expected
thanks to warm air advection in the low levels...at 850 mb
temperatures finally rise above 0C. Highs should be back in the
upper 30s and low 40s.
Friday through Sunday...
Warming up substantially late week into the weekend under SW flow on
the back side of that ridge. This will bring highs well into the
40s, potentially even the upper 40s! This comes with rain ahead of
the next mid level trough which approaches Friday into the weekend.
Still a good deal of uncertainty with the details of who sees
precipitation and when, but unsettled weather returns for some
portion of the far extended forecast.
&&
.AVIATION /04Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels.
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent
04Z update...
Through 12Z...
VFR ceilings lowering to MVFR after midnight. Localized IFR
possible where light snow is falling. Significant snowfall does
not arrive till after 12z.
Monday...
MVFR early, then trending to IFR with LIFR in the afternoon from
south to north as heavy snow overspreads the region. 17z-00z
heavy snow will result in +SN with hourly snowfall rates of 1-2
inches, possibly higher! Strong NE winds toward 00z along the
south coast including Cape Cod and the Islands.
Monday night...
IFR with LIFR in heavy snow (+SN) with hourly snowfall rates of
1-2" per hour (possibly higher) from 00z to 05z, then lifting
northward and exiting the region. Strong NE winds 00z-06z with
significant blowing and drifting especially in the Worcester
Hills. Significant low-level wind shear expected to be a factor
after 00Z.
KBOS TAF...High confidence in TAF. Preliminary snowfall totals
6-10 inches.
KBDL TAF...High confidence in TAF. Preliminary snowfall totals
8-12 inches.
Outlook /Tuesday through Friday/...
Tuesday: Mainly IFR, with local MVFR possible. Strong winds
with local gusts up to 45 kt. Chance SN, chance RA.
Tuesday Night: Mainly IFR, with areas MVFR possible. Windy with
local gusts up to 30 kt. Chance SHSN.
Wednesday: MVFR/IFR conditions possible. Breezy. Slight chance
SHSN.
Wednesday Night through Thursday: VFR. Breezy.
Thursday Night: VFR. Chance SHSN.
Friday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
chance SHSN.
&&
.MARINE...
* Storm Warning for most waters Monday afternoon and night *
Tonight...light winds but increasing toward morning as low pres
develops off DE coast. Dry weather and good vsby, although snow
may enter the RI waters toward Monday morning.
Monday...NE winds increase to around 50 kt along the southern MA
waters into RI in the afternoon. Heavy snow reduces vsby to less
than a mile and possibly near zero for a time. Seas building to
near 20 ft across the ocean waters!
Monday night...NE winds up to 50 kt across the eastern MA waters
along with 20-25 ft seas! Heavy snow with vsby near zero eastern
MA waters, rain southern waters but trending toward rain all
waters.
Outlook /Tuesday through Friday/...
Tuesday: Low risk for gale force winds with areas of gusts up
to 45 kt. Rough seas up to 22 ft. Chance of rain. Local
visibility 1 nm or less.
Tuesday Night: Low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 14 ft. Chance of rain
showers, chance of snow showers. Visibility 1 to 3 nm.
Wednesday: Winds less than 25 kt. Rough seas up to 10 ft.
Slight chance of rain showers, slight chance of snow showers.
Areas of visibility 1 to 3 nm.
Wednesday Night: Winds less than 25 kt. Areas of rough seas.
Thursday: Winds less than 25 kt. Seas up to 5 ft.
Thursday Night through Friday: Winds less than 25 kt. Areas of
seas approaching 5 ft. Chance of rain showers.
&&
.TIDES/COASTAL FLOODING...
* Coastal Flood Watch Issued For Potential Moderate Flooding *
* Minor to Moderate Beach Erosion Possible *
NE winds increasing to 50 to 60 mph will yield a storm surge of
2-3 ft for Monday night high tide cycle, then again for the Tue
high tide cycle from Plymouth county northward to the NH coast.
Could have minor to perhaps moderate coastal flooding
Nantucket and Marthas Vineyard Monday afternoon, thus included
that into the Watch. Widespread minor flooding is expected for
at least two tide cycles but risk of moderate coastal flooding
which includes a risk structural damage in the most vulnerable
areas such as Plum Island, Salisbury, Scituate, Marshfield, etc.
Expected a storm surge of 2 ft to 3 ft will bring storm tide
for both Mon night and Tue afternoon tide cycles close to 13 ft
MLLW for Boston and near 6 ft MLLW for Nantucket, along with
wave action of 20-25 ft in MA Bay and 10-15 ft east of
Nantucket. Wind direction mainly NE for the Monday night high
tide, then turning more to NNE Tue afternoon high tide.
Given multiple tide cycles will be impacted, minor to moderate beach
erosion is possible. Leaned toward the Steven's Institute surge
guidance, which supports 2-3' storm surge for both tide cycles.
If forecast confidence increases, coastal flood watch will be
upgraded to a warning.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Warning from 7 AM Monday to 5 AM EST Tuesday for
CTZ002>004.
MA...High Wind Warning from noon Monday to 2 AM EST Tuesday for
MAZ022>024.
Winter Weather Advisory from 7 AM to 7 PM EST Monday for
MAZ022-023.
Winter Storm Warning from 7 AM Monday to 1 AM EST Tuesday for
MAZ017>021.
Coastal Flood Watch from Monday evening through Tuesday
afternoon for MAZ007-015-016-019-022.
Winter Storm Warning from 7 AM Monday to 5 AM EST Tuesday for
MAZ002>016-026.
Coastal Flood Watch from Monday afternoon through late Monday
night for MAZ023-024.
RI...High Wind Warning from noon Monday to 2 AM EST Tuesday for
RIZ008.
Winter Weather Advisory from 7 AM to 7 PM EST Monday for
RIZ008.
Winter Storm Warning from 7 AM Monday to 1 AM EST Tuesday for
RIZ002>007.
Winter Storm Warning from 7 AM Monday to 5 AM EST Tuesday for
RIZ001.
MARINE...Storm Warning from noon Monday to 3 AM EST Tuesday for
ANZ232>235-237-254>256.
Gale Warning from 1 PM Monday to 1 PM EST Tuesday for ANZ230.
Storm Warning from 6 PM Monday to 3 AM EST Tuesday for ANZ231.
Gale Warning from noon Monday to 2 AM EST Tuesday for ANZ236.
Storm Warning from 6 PM Monday to 7 AM EST Tuesday for ANZ250-
251.
&&
SYNOPSIS...Nocera/BW
NEAR TERM...Nocera/Chai
SHORT TERM...Nocera
LONG TERM...BW
AVIATION...BW/RM
MARINE...Nocera/BW
TIDES/COASTAL FLOODING...