NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Mar 13, 7:11 PM EST, 49 minutes earlier.
Area forecast discussion, Mar 13, 7:11 PM EST, 49 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
811 PM EDT Mon Mar 13 2023
.SYNOPSIS...
A high impact, long duration, multi-faceted winter storm affects
the region tonight into Wednesday morning, with: heavy wet snow,
strong winds, possible power outages, and minor coastal
flooding. There still remains some uncertainty on exact snow
amounts across the lower elevations, including eastern MA and
Boston. Drier weather returns Thursday into early Friday, along
with milder conditions into Saturday. Scattered showers are
possible late Friday into early Saturday. Then dry weather
returns for the remainder of this weekend.
&&
.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
8:15 PM update...
Warm conveyor belt precip (WCB) well entrenched across the
region, with increasing moist southeast low/mid level jet,
speeds reaching 40-50 kt by 12z in the 850-925 mb layer. Strong
thermal and moisture advection on the nose of this jet will
yield moderate to heavy rain at times overnight. Boundary layer
warm enough to support all rain, except in the higher terrain,
where rain has already flipped over to snow at 8 pm. Although,
initially temps are above freezing, so snow will first only
accumulate on non paved surfaces. But as the night progresses
and snow intensity increases, diabatic cooling via melting will
lower ground temps and snow will accumulate on paved surfaces
across the high terrain, with snow covered roads for the morning
commute. Snow likely moderate to heavy at times overnight given
the strong synoptic forcing in the snow growth region. 12z HREF
has snowfall rates across the Berkshires and Worcester Hills 1-2
inches per hour at times overnight. Seems plausible given the
strong forcing for ascent in the snow growth region combined
with some upslope contribution. Definitely a high impactful
event for the high terrain tonight and Tuesday. Previous
forecast captures this fairly well, so no wholesale changes with
this update. Earlier discussion below.
------------------------------------------------------------------
Still dealing with lots of critical differences in the latest
guidance suite. While gaining confidence in some aspects of the
forecast, still some lingering questions with the precise
details.
Expecting rain to overspread most of southern New England
overnight, with snow mostly confined to elevations above 500
feet. The more significant snowfall overnight will be towards
the Berkshires, and also towards the northern Worcester Hills.
Across the lower elevations, we're looking at mainly rain into
tomorrow morning. It will take some time for snow to begin
accumulating on paved surfaces, but it will eventually.
Above normal low temperatures will lead to a wetter snow
consistency. Thinking snow:liquid ratios will struggle to get
much higher than 10:1 overnight.
&&
.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
This will be the time period of greatest impact from this winter
storm. Latest guidance suite continued to indicate a multi-
faceted storm will impact southern New England. Among the
guidance, we discounted the 12Z NAM, which looked to be a far
western outlier with its track. This forecast was based more on
the idea of the storm staying just offshore of eastern MA. All
the guidance had a little pinwheel effect with the surface low
pressure at some point tomorrow into tomorrow night. The
critical question is where this happens. The main concern for
this will be related to temperatures, which are rather marginal
already. A more westward jog to this low pressure will mean
armer air and less snow towards the coast.
Another complicating factor to the snowfall forecast will be the
timing and track of a dry slot in the dendritic growth zone.
This looks like it will impact southeast MA and portions of RI.
This will limit snowfall simply because the snowflakes
themselves will not be as large or well formed.
After considering all these factors, and a few others not
mentioned above, snowfall accumulations were lowered towards the
immediate coasts. Additional tweaks to accumulations across the
higher terrain, too.
Here is our latest thinking on impacts from this event:
Rain to Wet/Dense Snow and Snow Load Threat...
* Winter Storm Warnings continue where previously posted.
* Winter Storm Watches were converted to Winter Weather
Advisories.
* Snow accumulations will be greater across the higher terrain.
Expecting sharp gradients between the higher ridge tops and
adjoining valleys. Land use type (grass vs pavement), time of
day and how quickly will temperatures support accumulating
snow remain one of the largest uncertainties of this forecast.
Expecting rain to transition to snow during the morning hours
Tuesday. How quickly this happens, and how long a period of
mixing results during this transition, will significantly impact
snow accumulations. This is a very wet storm. Part of the
problem with the snow accumulation forecast is it will not all
fall as snow. Thinking that snow is the predominant weather type
by late afternoon into the evening hours. This will also be the
time where this storm starts to move away from our region
Tuesday night.
However, since we're now in mid-March, pavement temperatures
may not permit much impact. There is potential for low-
visibility snow, but potentially little if any accumulation on
roads at the lower elevations. Most, if not all, areas should
see accumulations around or after the Tuesday PM commute, where
snow looks likely to become heavy (snow rates at or around 1
inch per hour).
Snow is expected to be wet, heavy and dense with a significant snow
load factor that will greatly increase the risk for snow damage as
totals exceed 4 or more inches. Factor that in with strong NE winds
developing later Tuesday morning, and especially Tuesday
night, the potential for tree and powerlines damage will prove
a significant hazard, reflected in the Winter Storm Severity
Index.
Strong to Damaging Wind Threat...
* High Wind Warnings continue along the entire E MA coast. Peak
of these winds appears to be Tuesday afternoon into the
overnight.
* Strong winds will also impact areas outside of the High Wind
Warning areas, potentially to Advisory level. However, these
strong NE winds will be messaged in Winter Storm products.
This threat is the one with the greatest confidence, specific
to timing and the peak gust potential. As our surface low
pressure strengthens to below 980 mb near Mass Bay or Cape Cod,
strong winds will lash the eastern coastline, with gusts as
high as 65 mph in the North Shore/Cape Ann vicinity. Damaging
gusts up to 60 mph should affect the South Shore and into Cape
Cod and the Islands. Damage to trees and powerlines, and
potentially to structures, will be greatest in the High Wind
Warning areas.
Gusts inland should range between 40 to 55 mph Tuesday evening, with
the lower gusts further west one goes. This will add to the
potential for damage inland especially at terrain in combo with
heavy dense snow.
Coastal Flooding Threat...
* Coastal Flood Advisories in effect for east coastal MA for the
Tuesday evening high tide. Widespread minor flooding is
expected, along with beach erosion.
See the Coastal Flooding section for more details.
Dangerous Marine Conditions Threat...
* Gale and Storm Warnings continue.
See the Marine section for more details.
&&
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights:
* Other than scattered rain/snow showers Wed, trending drier but
blustery and cool
* Dry and milder Thu into early Fri
* Showers likely late Fri into early Saturday
* Dry but cooler Sunday and Monday
Wednesday...
Intense occluded cyclone east of Cape Cod will slowly advect
farther offshore during Wed. This departing ~985 mb low,
combined with 1025 mb high over the OH valley advecting east,
will result in tight a pressure gradient and support NW winds
gusting up to 25-35 mph, except up to 40 mph across the hilly
terrain of CT and MA. This may impact power restoration from any
outages Tue/Tue night. This combined with highs only in the
upper 30s to lower 40s, will yield a blustery and chilly day.
Cold cyclonic flow aloft will yield mostly cloudy conditions,
along with scattered rain/snow showers, more numerous in the
upslope regions of the Berkshires and Worcester Hills.
Thursday/Friday...
Chilly and blustery start Thu morning, but as ridge axis advects
across the area, winds diminish Thu afternoon. Morning sunshine
may give way to increasing afternoon clouds, as moisture spills
over the ridge. Milder, with highs 45-50, except low 40s across
the Worcester Hills and Berkshires where snowpack exist. Normal
highs are in the low to mid 40s, so slightly above normal. It
will feel pleasant given decreasing winds. Friday starts dry,
but latest model trends have next upstream trough advecting
eastward faster than previous guidance. Therefore, risk of
showers in the afternoon and especially late in the day and at
night, as a warm front approaches. Mild, especially across CT/RI
and southeast MA, where warm sector airmass may yield highs in
the low 50s, except mid to upper 40s across Worcester Hills and
Berkshires given snowpack. Light winds in the morning, becoming
south in the afternoon. Once possible issue, with snow melting
during the day, then subfreezing temps at night, especially
outside the urban areas, could be some icy spots each night and
early morning.
Weekend...
Short wave and associated cold front move through Saturday
morning with scattered showers, then should experience a
drying trend Sat afternoon as dry slot moves across the area.
Remaining mild, as cold air lags well to the west over the Great
Lakes, where upper low is located. Highs should manage to climb
into the low and mid 50s, except upper 40s across the Worcester
Hills and Berkshires where snow cover will still exist.
Trailing short wave delivers CAA later Sunday into Monday.
Diurnal clouds Sunday likely give way to more sunshine Monday.
Highs Sunday in the 40s, except upper 30s high terrain where
snow cover likely lingers. &&
.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
815 PM update...
No major changes to the 00z TAFs. Rain, heavy times overnight
with snow across the high elevations of CT/MA. Snow here will be
heavy at times overnight with accumulations expected at elevated
terminals, such as KORH, including IFR/LIFR conditions. ESE
winds increasing overnight and become NE toward morning. Earlier
discussion below.
--------------------------------------------------------------------
High confidence in trends, moderate confidence in the timing
details, especially for precipitation type.
Light rain slowly spreads in from SW to NE this afternoon and
evening, with mostly light snow across the hills, but little if
any accumulation on pavement.
Conditions lower to IFR tonight in rain and fog. Areas of LIFR
across higher terrain, where snow is expected, and may be heavy
at times. Rain changes over to snow across lower elevations
later Tue morning, and along coast Tue afternoon with widespread
IFR conditions.
E winds increase overnight, then shift NE and strengthen further
Tuesday. Strongest winds along the immediate coast. Conditions
start to improve Tuesday night.
Forecast Storm Total Snowfall for TAFs (4 PM Mon forecast):
KACK...0.2 inch
KBAF...3.3 inches
KBDL...2.0 inches
KBED...6.5 inches
KBOS...1.9 inches
KFMH...1.7 inches
KHYA...0.4 inch
KORH...15.3 inches
KPVD...2.5 inches
KBOS Terminal...High confidence. Rain expected through Tue
morning, probably seeing a changeover to snow around 18z.
Temperatures should limit accumulation on pavement until later
in the day, more likely Tuesday evening.
KBDL Terminal...High confidence. Rain changes over to snow
after sunrise Tuesday morning, but temperatures should limit
accumulation on pavement until Tue afternoon.
Outlook /Wednesday through Saturday/...
Wednesday: Mainly VFR, with areas MVFR possible. Strong winds
with gusts up to 45 kt. Chance SN, slight chance RA.
Wednesday Night: VFR. Strong winds with local gusts up to
40 kt.
Thursday: VFR. Windy with local gusts up to 30 kt.
Thursday Night: VFR. Breezy.
Friday: VFR. Chance RA.
Friday Night: Mainly VFR, with areas MVFR possible. Breezy. RA
likely.
Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance
RA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Overall high confidence.
* Dangerous Boating Conditions for Vessels Tuesday into Wednesday.
* Gale and Storm Warnings posted.
A sub-980 mb cyclone moving up the Atlantic seaboard to coastal
Southern New England will yield hazardous to potentially dangerous
conditions for mariners for Tuesday into much of Wednesday.
E/SE will ramp up tonight into early Tuesday. Period of strongest
gusts is anticipated Tuesday afternoon into early Wednesday, with
gusts 50 to 60 kt on the eastern waters. bays and harbors. Seas on
the outer offshore waters will become rough, with the potential for
20 to 25 ft waves into the eastern and northeast offshore waters.
Gale to strong gale force gusts are expected on the southern waters,
and it is not out of the question gusts could get near storm force
on the waters east of Montauk.
Mariners are encouraged to seek safe harbor given these expected
conditions. Besides that, rain will reduce visibility as well.
Outlook /Wednesday through Saturday/...
Wednesday: Strong winds with gusts up to 45 kt. Rough seas up
to 12 ft. Chance of rain.
Wednesday Night: Strong winds with gusts up to 35 kt. Rough
seas up to 12 ft.
Thursday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of rough seas.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.
Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain.
Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Rain likely.
Saturday: Winds less than 25 kt. Local rough seas. Rain likely.
&&
.TIDES/COASTAL FLOODING...
The Coastal Flood Watch has been replaced with a Coastal Flood
Advisory, as any inundation and erosion should be minor. The
early Wed morning high tide is the most vulnerable tide cycle,
with a storm surge of 2.0 to 2.5 ft. However, with the storm
tracking closer to the coast, onshore ENE winds are of shorter
duration, as winds turn to north by Tue evening, limiting the
fetch and wave action for the Wed morning tide. Also, pressure
rises begin to advect across the area late Tue night/early Wed
morning. This combination has resulted in slightly lower storm
surge than earlier expected. Thus, inundation and erosion is
expected to be in the minor category from Cape Cod northward to
the MA/NH coastline.
The most vulnerable places include Gloucester, Revere, Hull, and
Scituate and perhaps the Bay side of Cape Cod (Sandwich to Dennis).
In addition, the large waves offshore will likely result in minor
beach erosion Tuesday and Wednesday. If water levels were higher,
more significant erosion would be possible, but that's not expected.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 8 AM EDT Wednesday for CTZ002.
Winter Storm Warning until 8 AM EDT Wednesday for CTZ003-004.
MA...Winter Weather Advisory until 8 AM EDT Wednesday for MAZ010-
011.
Winter Storm Warning until 8 AM EDT Wednesday for MAZ002>006-
008-009-012-026.
High Wind Warning from 8 AM Tuesday to 8 AM EDT Wednesday for
MAZ007-015-016-019.
Winter Weather Advisory from 8 AM Tuesday to 8 AM EDT
Wednesday for MAZ007-013>021.
Coastal Flood Advisory from 3 AM to 8 AM EDT Wednesday for
MAZ007-015-016-019-022.
High Wind Warning from 8 AM Tuesday to 2 PM EDT Wednesday for
MAZ022>024.
RI...Winter Storm Warning until 8 AM EDT Wednesday for RIZ001.
Winter Weather Advisory from 8 AM Tuesday to 8 AM EDT
Wednesday for RIZ002>006.
MARINE...Storm Warning until 8 PM EDT Wednesday for ANZ230-231-251.
Storm Warning until 2 AM EDT Thursday for ANZ232.
Gale Warning until 8 PM EDT Wednesday for ANZ233>235-237.
Gale Warning until 2 PM EDT Wednesday for ANZ236.
Storm Warning until 8 AM EDT Thursday for ANZ250-254-255.
Gale Warning until 8 AM EDT Thursday for ANZ256.
&&
SYNOPSIS...Belk/Nocera
NEAR TERM...Belk/Nocera
SHORT TERM...Belk
LONG TERM...Nocera
AVIATION...Belk/Nocera
MARINE...Belk/Nocera
TIDES/COASTAL FLOODING...Nocera