BOX issues Area Forecast Discussion (AFD) at Dec 22, 9:35 PM EST208
FXUS61 KBOX 230235
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
935 PM EST Sun Dec 22 2024
.SYNOPSIS...
Very cold tonight, with temperatures across the region ranging
from the low teens to single digits below zero. Light ocean
effect snow showers will continue across the Cape and the
Islands tonight, ending Monday morning. Warmer on Monday with
highs in the mid-20s to low 30s. A weak clipper system may bring
very minor snow accumulations late Monday night into Christmas
Eve morning. Otherwise, dry weather dominates the rest of the
Holiday week into next weekend with seasonably chilly
temperatures.
&&
.NEAR TERM /THROUGH MONDAY/...
930 PM update...
The main forecast challenge tonight is the extent of ocean
effect snow showers. Returns are expanding over the ocean and
approaching Cape Cod to coastal Plymouth county. There is quite
a bit of instability over the ocean as temps at the top of the
boundary layer around -14 to -15C leading to delta T of 22-23C
and ocean induced CAPES 400-500 J/kg. Expect snow showers to
continue to develop and favorable low level trajectory around
350 degrees will carry these snow showers to Cape Cod and
possibly portions of coastal Plymouth county as there may be
some low level convergence here from land-sea interface.
Current forecast of a coating to an inch seems reasonable but
low risk for localized accums up to 2 inches over Cape Cod.
The other story for tonight is the frigid temps expected.
Already single numbers across portions of the interior and with
winds dropping to near calm expect subzero lows across western
MA and portions of northern CT. Portions of interior eastern MA
may also drop to zero to slightly below zero. The rest of the
region will see single numbers for lows, except teens
Cape/Islands due to ocean effect clouds and more wind.
Previous discussion...
Key Points
*Coldest air of the season tonight
*Ocean effect light snow showers/flurries parts of the Cape
1040mb Arctic high pressure moves over the region tonight, setting
up ideal radiational cooling conditions under an already cold air
mass. Low temperatures across the region will range from the low
teens next to the coasts to single digits across much of the
interior. Areas that have snow cover will likely drop below zero
tonight. The pressure gradient will relax overnight, allowing the
northerly winds to subside and become calm. With little to no wind,
the feels like temperature wont drop too far below the actual
temperature.
Winds out over the waters tonight wont drop off as quickly as they
do over the land. This will allow the ocean-effect snow showers to
continue into Monday morning over the Cape and Islands. QPF amounts
are very light on the order of one to two-tenths of an inch.
However, snow ratios are near 20:1, so it wont take a lot of QPF to
get a quick coating of snow. Some high-res guidance sources, such as
the WRF-ARW, show some localized 2-4 inches, but this is a bit
bullish considering the dry conditions in the boundary layer.
Overall, a coating to an inch of snow tonight seems most likely over
the Cape and possibly in eastern Plymouth County.
&&
.SHORT TERM /MONDAY NIGHT THROUGH 6 PM MONDAY/...
Surface winds flip to more SSW by late Monday morning, cutting off
any lingering snow showers. 850mb temps recover from the -16C today
to -4C Monday afternoon. Temps will be slow to recover, given the
cold start. High temperatures will likely struggle to top freezing,
with low to mid-20s across western MA. Skies will start mostly
sunny, but high clouds begin to filter in ahead of a shortwave over
the Great Lakes.
&&
.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Key Messages...
* Light snow showers late Mon night-Tue AM...Dusting to 1" for most
* Highs mainly in the 30s Tue/Wed/Thu/Fri & 40+ possible next weekend
* Dry other than very spotty light precipitation into the weekend
* Potential for unsettled weather returns late Sun and/or next Mon
Details...
Monday night and Tuesday...
A weak clipper system will track across northern New England late
Mon night into Tue morning. This will take the best forcing/deeper
moisture to our north. However...there will still be enough forcing
for a period of light snow showers late Mon night into Tue morning
especially near and north of I-90. Given the lack of deep layer
moisture and forcing...thinking most places just see a dusting to 1"
of snow. It is possible that a few spots in far northern MA see up
to 2". Column will be cold enough for light snow showers for most
locations...but boundary may become mild enough for a change to
light rain showers towards the Cape and Islands. Regardless...this
system will not produce much precipitation and pretty much by over
by Tue afternoon. Highs should recover into the 30s by Tue afternoon.
Christmas Through Next Weekend...
A quiet and mainly dry stretch of weather from Christmas through
next weekend other than perhaps some very brief spotty/light
precipitation near the coast. This mid level ridging will build into
the region from the west with above normal height fields.
However...a large 1035-1040 mb high pressure system over Quebec will
offset this with some shallow cool air oozing down from the north.
Highs will mainly be in the 30s through Fri with lows in the teens
to middle 20s for most locations. The high pressure system may move
far enough east allowing for a better risk of more widespread 40+
highs by next weekend.
Given the upper level ridging building into the region...dry weather
will dominate other than perhaps very brief spotty light
precipitation near the coast for a time with some low level
moisture. Otherwise...the main risk for precipitation will not come
until late Sun and/or Mon. Ptype would favor rain but perhaps some
snow/ice at the onset in the interior depending on how much residual
cold air is left.
&&
.AVIATION /02Z MONDAY THROUGH FRIDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
00z TAF Update...
Tonight...High confidence.
VFR except for some periods of MVFR conditions...mainly the
outer-Cape from ocean effect cloudiness and light snow
showers/flurries. This activity may impact KHYA for a time and
perhaps KACK...but a lower risk there. NW winds continue to
diminish and become light/calm overnight.
Monday...High confidence.
VFR, light northerly winds in the AM, turning light southerly
in the PM.
KBOS TAF...High confidence in TAF.
KBDL TAF...High confidence in TAF.
Outlook /Monday Night through Friday/...
Monday Night: Mainly VFR, with areas IFR possible. Chance SHSN.
Tuesday: Mainly VFR, with local IFR possible. Chance SHSN.
Tuesday Night through Christmas Day: VFR. Breezy.
Wednesday Night: Mainly VFR, with local MVFR possible.
Thursday: VFR.
Thursday Night: Mainly VFR, with areas IFR possible. Slight
chance SHSN, slight chance PL.
Friday: VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Updated: 3:30 PM
Key Messages:
* Small Craft Advisory in effect eastern outer-waters overnight
* Light icing from freezing spray for the waters surrounding
Nantucket through Monday morning
Tonight and Monday...High confidence.
Winds gradually decrease overnight, but still remain northerly
around 10 knots. With very cold temperatures overnight, light
freezing spray is likely on the near shore waters overnight.
Ocean effect snow showers are also likely in and around Cape Cod
and the Islands through Monday morning. Seas decrease under 5ft
by daybreak Monday with winds turning light southerly by Monday
afternoon.
Outlook /Monday Night through Friday/...
Monday Night: Winds less than 25 kt. Chance of rain showers,
chance of snow showers. Local visibility 1 to 3 nm.
Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers. Local visibility 1 to 3 nm.
Tuesday Night: Winds less than 25 kt.
Christmas Day: Winds less than 25 kt. Seas locally approaching
5 ft. Local visibility 1 to 3 nm.
Wednesday Night: Winds less than 25 kt. Slight chance of rain
showers. Local visibility 1 to 3 nm.
Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.
Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
&&
.CLIMATE...
Record Lowest Maximum Temperature: 12/22
BOS: 17F (1989)
BDL: 11F (1989)
PVD: 17F (1989)
ORH: 9F (1989)
Record Lowest Minimum Temperature: 12/23
BOS: -12F (1883)
BDL: -5F (1989)
PVD: 2F (1989)
ORH: -2F (1989)
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 4 AM EST Monday for ANZ250-254.
Small Craft Advisory until 10 PM EST this evening for ANZ255.
&&
$$
SYNOPSIS...Frank/KP
NEAR TERM...KJC/KP
SHORT TERM...KP
LONG TERM...Frank
AVIATION...Frank/KP
MARINE...Frank/KP
CLIMATE...Dooley
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 22, 9:35 PM EST----------------
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