BOX issues Area Forecast Discussion (AFD) at Dec 20, 6:30 PM EST795
FXUS61 KBOX 202330
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
630 PM EST Fri Dec 20 2024
.SYNOPSIS...
An offshore coastal storm will continue to bring light snow
accumulations this afternoon through the early morning hours of
Saturday followed by very cold weather into early next week. A weak
clipper system moves through Christmas Eve and will likely
bring light snow accumulations to the region. Temperatures
slowly moderate through the week back into the 30s and 40s
&&
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
630 PM update...
Still seeing ocean enhanced snow bands moving westward across
Boston and SW suburbs, while heavier snow gradually shifts to
SE coastal MA and Cape/Islands as deepening low pres makes its
closest approach tracking just SE of the Benchmark. Additional
snow accum of 1-3 inches is expected across much of eastern MA
tonight, with locally up to 4 inches possible on the upper Cape
and portions of eastern Norfolk and northern Plymouth counties
where enhanced bands continue to rotate in from the ocean. The
snow will begin to taper off around midnight along I-95 corridor
3-4 am over the Cape/Islands.
Previous discussion...
Key Messages...
* Light to moderate snow will impact much of SNE this evening into
the overnight hours dropping 2 to 5 inches of snow.
The big picture for the next 12 hours is that we'll haver periods of
rain and snow continuing for the next several hours, then
transitioning to all snow with sunset and falling temperatures.
Since this morning's update snow totals have been increased, mainly
for eastern MA where some banding is setting up on top of ocean
effect snow.
We're dealing with two mechanisms for snow production today (which
made for a unique radar look earlier as we had snow showers
converging from east and west). The bulk of the snow so far
experienced for those north of the coastal front (currently draped
from Quincy to Providence) is from ocean effect snow showers as the
cold air flows over the relatively warm waters. This same phenomenon
has brought rain showers to the Cape and southeast MA all day as
well. The pesky thing to nail down as usual is just how far
inland/northwest the coastal front would make it today, and guidance
certainly was too far north which is why we've had more hours of
snow than expected in the Boston/metro west area this afternoon. As
temperatures begin to drop with sunset that front will collapse
south and rain will flip to snow over southeast MA including Cape
Cod.
The other factor with this evening's snow is the precipitation
associated with a deepening coastal low moving up the coast outside
of the benchmark tonight. While the maximum of the moisture
plume/QPF remains offshore guidance did shift west with some of it
which is part of the reason snowfall totals were increased. We
expect an already observed banding of precipitation over
eastern/northeast MA to continue this evening as a band of 700 mb
frontogenesis sets up over southeast MA. We know that historically
the maximum of snowfall typically falls just to the northwest of
these maxima which would be northern RI into northeast MA. The
combination of ocean enhanced snowfall and this deformation axis
will most likely lead to snowfall totals of 2 to 5 inches in this
region. The biggest concern will be for the evening commute where
model soundings suggest strong lift in the dendritic growth zone
which may lead to localized snowfall amounts of up to 1"/hour (30-
40% chance). Elsewhere, even down to the coast Cape Cod could pick 1-
3 inches of snow as well in the ~12 hour window that snow is likely.
This all exits east by sunrise on Saturday.
&&
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Key Messages...
* Drier and colder on Saturday and very cold on Saturday night
Saturday the mid level trough remains overhead but the low quickly
exits east. We'll see breezy NW winds 10 to 15 mph with gusts as
high as 25 mph. Given the cyclonic flow aloft we may see some widely
scattered light snow but mostly just continued cloudcover. In the
afternoon heights will be starting to rise and drier air advected in
on that NW flow so clouds will dissipate through the afternoon and
evening. This will be the transition toward our coldest day of the
season on Sunday, with highs only making it into the upper 20s and
low 30s. The real cold arrives overnight, though, as frigid air
arrives. 850 mb reach around -15C, while at the surface lows will
drop into the single digits (interior) and mid teens (coast). Breezy
winds will make it feel more like single digit negatives to near 0
in the interior.
&&
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Key Messages:
* Arctic Airmass continues to bring highs in the 20s and lows in
the single digits Sunday into Monday
* Gradual warm-up the rest of the week with highs in the 30s and
lows in the 20s
* Weak clipper system moves through Christmas Eve, but confidence
is still low
1040mb high pressure and arctic airmass remain over the region
through Monday. Despite sunny skies, highs will struggle to top 30F,
with most of the region in the low 20s. With blustery NW winds at 10-
15mph, the wind chill factor will make it feel like the single
digits. Overnight lows will likely drop to some of the coldest we
have seen thus far for the season, with lows in the single digits,
possibly below zero in high terrain in NW MA. Winds will decrease
overnight, which will help prevent the wind chill factor from going
far below zero. Surface flow turns southerly on Monday, allowing
high temps to moderate slightly into the upper 20s to low 30s.
A clipper system drops out of the Northern Great Lakes on Christmas
Eve, which could bring another round of light wintry precipitation.
Uncertainty remains high with this system as models are usually too
quick to kick out strong high-pressure systems. Current ensemble
guidance shows high probs for 1 inch across the region, with low
probs for 3 inches in the high terrain of the Berkshires.
Temperatures Tuesday continue to moderate into the low to mid-30s.
High pressure builds back in from the north for Christmas Day
through the end of the week. High temperatures continue to moderate
into the mid to upper 30s to even low 40s by the end of the week.
&&
.AVIATION /23Z FRIDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
This afternoon...Moderate confidence.
Widespread MVFR cigs across SNE with pockets of IFR in heavier
SN. Rain or mixed rain/snow near the coast, changing to snow
toward evening. Minor accums likely west of I-95 and 128 in MA.
NE wind 8-15 kt with 20+ kt gusts developing near the coast
toward evening.
Tonight...Moderate confidence.
MVFR conditions with areas of IFR SE MA and Cape/Islands. Snow
becoming more focused across eastern MA during the evening, then
gradually moving offshore toward 12z with some improvement to
VFR away from SE New Eng coast. N-NE wind becoming N 10-20 kt
with 25-30 kt gusts along eastern MA coast and Cape/Islands.
Saturday...Moderate confidence.
Mainly VFR cigs, but lower MVFR cigs possible early
Cape/Islands. Scattered snow showers developing which may
reduce vsbys at times, especially interior. NW wind 10-20 kt
with some higher gusts.
Saturday night...Moderate.
VFR. NW winds 5-10 kts gusting to 20 kts.
KBOS TAF...Low to Moderate confidence. Continued light to
moderate snow off of the ocean will reduce vsbys at times below
3 miles through the evening. NNE gusts to 20-25 kt.
KBDL TAF...Moderate confidence. Cigs lowering to MVFR with
light to moderate snow in the afternoon and evening which may
reduce vsbys below 1 mile at times.
Outlook /Sunday through Wednesday/...
Sunday: VFR. Breezy.
Sunday Night through Monday: VFR.
Monday Night: Mainly VFR, with local MVFR possible. Chance SN.
Tuesday: Mainly VFR, with areas MVFR possible. Chance SN,
chance RA.
Tuesday Night: Mainly VFR, with local MVFR possible. Chance SN.
Christmas Day: VFR. Slight chance RA, slight chance SN.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Key Messages:
* Small Craft Advisory in effect through Sunday.
Tonight...Moderate confidence.
Surface low pressure system moves up the coast and remains out to
sea. Increasing east/northeast winds 15 to 20 knots with gusts
up to 30 knots on the outermost waters, nearshore waters gusts
are closer to 25 knots. Seas building 4 to 9 feet for the
waters. Periods of rain and snow tonight, diminishing early
Saturday morning.
Saturday...High confidence.
Low pressure system lifts northeast from southern New England, but
with high pressure building to the west, it will lead to northwest
flow with gusts continuing throughout the day. Have extended the
Small Craft Advisory as a result through the day. Seas remain rough
3 to 6 feet on the southern waters and 5 to 8 feet on the eastern
waters.
Saturday night...High confidence.
High pressure moving overhead. NW winds 15-20 kts gusting to 25
kts. Seas 5-7 ft.
Outlook /Sunday through Wednesday/...
Sunday: Winds less than 25 kt. Seas up to 5 ft. Chance of snow.
Local visibility 1 to 3 nm.
Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of snow.
Monday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of snow.
Monday Night: Winds less than 25 kt. Chance of rain, chance of
snow.
Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Rain likely. Local visibility 1 to 3 nm.
Tuesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain, chance of snow.
Christmas Day: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory until 4 AM EST Saturday for MAZ005>007-
012>022.
RI...Winter Weather Advisory until 4 AM EST Saturday for RIZ001>005-
007.
MARINE...Small Craft Advisory until 7 AM EST Sunday for ANZ231-232-251.
Small Craft Advisory until 7 AM EST Saturday for ANZ233>235-
237.
Small Craft Advisory until 7 PM EST Sunday for ANZ250-254>256.
&&
$$
SYNOPSIS...BW/KP
NEAR TERM...KJC/BW
SHORT TERM...BW
LONG TERM...KP
AVIATION...BW/KP
MARINE...BW/KP
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 20, 6:30 PM EST----------------
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