BOX issues Area Forecast Discussion (AFD) at Dec 18, 4:10 PM EST426
FXUS61 KBOX 182110
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
410 PM EST Wed Dec 18 2024
.SYNOPSIS...
Increasing clouds early tonight as a fast-moving area of low
pressure progresses through Southern New England this evening
into the pre-dawn hours on Thursday. Rain is expected in many
areas, although a minor accumulation of light snow is
anticipated in the highest elevations. Brief dry weather then
resumes for Thursday. Monitoring an offshore area of low
pressure that could bring additional light precipitation on
Friday. This will be followed by very cold and blustery weather
this weekend into early next week.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
345 PM Update:
Key Messages:
* Rainy conditions develop in most of Southern New England early
tonight, falling at moderate rain rates near south coastal New
England.
* A minor accumulation of wet snow (up to 2") for the Berkshires and
hills in northern Worcester County.
* Precipitation ends west to east by the pre-dawn Thurs hours.
While skies are mostly clear to this point in the afternoon, mid to
high clouds have begun to increase in response to a weak area of low
pressure currently near central WV. This low pressure is expected to
trek ENE through southern NY and coastal CT/RI/Southeast MA through
the evening and overnight. This feature will spread precipitation
into Southern New England; with temps in many areas expected to be
above freezing, the majority of Southern New England will see precip
fall in the form of steady light to at times moderate rains even
with the initial evaporative cooling effects at outset. The highest
elevations in the hills in northern Worcester County and the
Berkshires should be cold enough to support a minor accumulation of
wet snow.
There were overall no substantial changes in forecast guidance that
would require a shift in messaging this event. Precip expected to
begin just after the PM rush hour in western MA and CT, and into
central and eastern portions of Southern New England by mid-evening.
Bulk of the precip will fall during the late evening to early
overnight hours, then start to pull away offshore from west to east
during the late overnight to the predawn hrs.
Overall QPF ranges from a third of an inch in northern MA, and to
around half to two-thirds of an inch, with the heaviest totals over
south-coastal RI and MA closer to the track of the sfc cyclone. As
is usual with events with marginal/borderline temps, there is quite
a bit of difference between the usual 10:1 snow to liquid ratio
snowfall forecast outputs (a few inches higher as it assumes all
precip falls as snow and some of the initial QPF will be lost to
evaporation) vs the positive-snow-depth-change forecast snowfall
guidance. The latter often performs well in these borderline
situations. Given snow growth parameters aren't necessarily
substantial (e.g. weak vertical ascent not lining up with the
dendrite snow crystal growth layer per bufkit x-sections), I've
leaned toward the up to 2 inches of snow forecast similar to the
positive snow depth change graphics. This also carries continuity
with previous forecasts.
Temps mainly in the 30s, with 30-32F readings in the higher terrain
and around 33-37F in the lower elevations/valleys.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
345 PM Update:
Thursday:
High pressure ridge then follows the overnight storm system, and
supplies drier weather with cool advection on NW winds. May see an
increase in high clouds again later in the afternoon but most of the
day should feature abundant sun. Highs Thursday should be pretty
seasonable, in the mid/upper 30s high terrain and in the low to mid
40s elsewhere.
Thursday Night:
1030 mb high pressure cell over Quebec will slowly translate
eastward into the St Lawrence River area through the overnight.
Ridging will still maintain dry weather for the balance of the
evening/overnight. However increasing mid to high level moisture
spreads east from a distant Clipper low pressure system expected to
meander southeast through the Gt Lakes region. So we will see cloud
cover on the increase again for Thurs night. As winds turn onshore
during the overnight, there is some indication for shallow marine
layer of lower level RH to advect back landward towards east coastal
Southern New England. These should just lead to a low cloud layer
into coastal MA but doesn't appear capable of generating any precip.
Lows in low to mid 20s in the interior, mid to upper 20s for the
RI/MA coastal plain, around freezing near the immediate coast, and
mid 30s over the Mid- to Outer Cape.
&&
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Key Messages:
* Light snow accumulation late Fri into Fri night, but closely
watching offshore storm for closer pass and more significant
snowfall in eastern New Eng
* Much colder this weekend into Mon with coldest air Sunday
into Mon
* Moderating temps Tue
Details...
Friday into Saturday...
Next upper level trough in progressive pattern approaches from
the west with a series of shortwaves rotating around the trough
leading to some trough amplification along the coast. Consensus
of the deterministic and ensemble guidance has coastal storm
track south and east of the Benchmark but there has been a
westward shift in the moisture and QPF as storm deepens fairly
rapidly. Will have to watch this closely as a stronger shortwave
could sharpen trough enough to bring more significant QPF north
and west into the Cape/Islands and possibly portions of eastern
MA and this is reflected in 90th percentile of the GEFS
distribution. This is still a low prob scenario but can't be
dismissed yet given latest trends. The more likely scenario is
for some light snow developing later Fri into Fri night,
especially across eastern New Eng as large scale forcing
increases ahead of the upper trough along with moistening of the
column. Ptype likely begins as rain or rain-snow mix near the
coast due to marginal BL temps but will flip to snow as colder
air deepens. Potential for light snow accumulations but could be
a bit more if westward trend continues.
Some snow showers may linger into Sat morning, otherwise drying
out and blustery with temps well below normal for Saturday.
Highs upper 20s to low-mid 30s with lows Sat night single
numbers and teens. Wind chills by Sunday morning will lower to
-5 to +5.
Sunday through Tuesday...
Dry weather expected during this period as strong high pres
(1040 mb) builds into the region before retreating on Tue. Core
of coldest air settles in over SNE Sunday as ensemble mean 850
mb temps bottom out around -15 to -16C. Sunday will be the
coldest day of the season with highs only recovering to upper
teens to mid 20s. While temps aloft begin to moderate Sun night,
favorable radiational cooling as strong high pres builds
overhead will lead to a very cold night with lows in the single
numbers (teens near the immediate coast), then recovering into
the 20s Mon. Temps expected to get close to seasonable levels by
Tue but still slightly below normal.
&&
.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18z TAF Update:
Rest of the Afternoon through 12z Thursday: High confidence
thru 00z, then trends toward moderate confidence.
Through about 22z, VFR conditions although with lowering
ceilings. Thereafter conditions deteriorate, with ceilings
going to MVFR and eventually IFR. Low pressure passing through
coastal Southern New England spreads VFR- MVFR visby rain and
higher elevation snow between 00-03z Thurs. Temps should be
mild enough at all TAF sites such that ORH may be the only site
which sees a mix of rain and snow, with minor (up to 2")
accumulations in the higher elevations. Between 07-12z Thurs,
precip should start to taper off soonest west and later end of
that timeframe over the Cape and Islands along with categories
trending toward improvement (MVFR- VFR range).
Winds will begin SW around 4-8 kt. Areas near and north of PVD
will then see winds become SE around 4-6 kt then NE by midnight
and become NW around 10 kt later-overnight/pre-dawn. For the
Cape and Islands, SW winds will steadily increase this evening
to 12-15 kt with gusts around 20 kt, then shift to NW by the
overnight/pre-dawn hrs.
Thursday: High confidence.
Conditions improve to VFR early, if they haven't done so before
12z Thurs. NW winds around 10 kt, up to 15 kt for the Cape.
Thursday Night: Moderate confidence.
VFR early and most areas should stay in the VFR range. Have to
watch eastern and southeast MA for possible sub-VFR ceilings
after 06z Fri. N winds around 5-10 kt, though will decrease and
shift to NE overnight.
KBOS TAF...High confidence in trends but moderate on timing.
VFR through 00z, then lowering cigs to MVFR-IFR after 02z as
rain develops. Improving to VFR by 12z.
KBDL TAF...High confidence in trends but moderate on timing.
VFR rest of today, then lowering cigs to MVFR-IFR after 00z as
rain develops. Improving to VFR by 09z.
Outlook /Friday through Monday/...
Friday: MVFR/IFR conditions possible. Breezy. Chance SN, chance
RA, slight chance FZRA.
Friday Night: MVFR/IFR conditions possible. Breezy. Chance SN.
Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance
SN.
Saturday Night: VFR. Breezy. Slight chance SN.
Sunday through Sunday Night: VFR. Breezy.
Monday: VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Key Messages:
* Small Craft Advisory continues in effect late tonight through
Thursday for the outer ocean waters off southern New England.
Tonight into Thursday: High confidence.
SW winds 15-20 kt with gusts to 25 kt developing over the
southern outer waters tonight, with building seas to 4 to 6 ft
over the southern waters. For the eastern waters, winds shift to
E/NE around 10-15 kt tonight, then become NW and increase to
15-20 kt through Thurs. Widespread rains develop over the waters
tonight into early Thurs AM which could reduce visbys to 3
miles, trending to dry for Thurs.
Thursday Night: High confidence.
NW winds lighten early Thurs night and then shift to NE around
10-15 kt. Seas 2-4 ft.
Outlook /Friday through Monday/...
Friday: Winds less than 25 kt. Seas up to 5 ft. Rain.
Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Rain, snow likely.
Local visibility 1 to 3 nm.
Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Snow likely, chance of rain.
Visibility 1 to 3 nm.
Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 20 kt. Areas of rough seas. Chance of snow. Areas
of visibility 1 to 3 nm.
Sunday: Winds less than 25 kt. Local rough seas. 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.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 3 AM to 7 PM EST Thursday for ANZ250-
254.
Small Craft Advisory from 10 PM this evening to 7 PM EST
Thursday for ANZ255-256.
&&
$$
SYNOPSIS...Belk/Loconto/KP
NEAR TERM...Loconto
SHORT TERM...Loconto
LONG TERM...KP
AVIATION...Loconto/KP
MARINE...Loconto/KP
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 18, 4:10 PM EST----------------
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