BOX issues Area Forecast Discussion (AFD) at Dec 18, 1:40 PM EST594
FXUS61 KBOX 181840
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
140 PM EST Wed Dec 18 2024
.SYNOPSIS...
High pressure brings drier weather with a period of milder than
normal temperatures today. A fast-moving area of low pressure
progresses through Southern New England this evening into the
pre-dawn hours on Thursday, bringing mainly rain with minor
accumulations of light snow 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 /THROUGH TONIGHT/...
1000 AM Update:
Forecast still holding up pretty well to this point with no need
for significant change. Will reassess forecast for tonight once
the 12z guidance is in.
Previous discussion:
Key Messages:
* Sunshine to start, though clouds are increasing throughout
the day, albeit dry with highs in the 40s.
* Fast moving low pressure system brings widespread rainfall
and minor snow accumulation the higher elevation of northern
and western Massachusetts.
Much colder this morning, as compared to Tuesday morning when
much of the region was sitting in the 50s. This morning, quite
different as temperatures are roughly 15 to 25 degrees colder.
This afternoon our high temperatures rebound to the upper 40s to
near 50F for areas in southeast Massachusetts and coastal Rhode
Islands. As you move north and west temperatures further inland
only reach the upper 30s and low-40s. Still, these afternoon
highs are near to above average for mid-December.
A fast moving low pressure system exits off the Mid-Atlantic
coast early this evening and races to the northeast, nearing the
coast of Nova Scotia by Thursday morning. As the low races
away, it will draw down colder air from northern New England,
overnight lows fall to the upper 20s and low 30s for the
interior and mid to upper-30s for the coastal plain.
We are anticipating the rain and elevation snow to arrive late
in the afternoon to early evening. Latest high-res guidance has
pushed back the onset, thinking western areas are between 5pm
and 7pm while eastern areas are 7pm to 10pm. PWATs increase to
0.75" (northwest) to 1.25" (southeast), leads to a healthy drink
of water for the area and anticipate widespread 0.25" to 0.75"
with localized 1.0" for southern Rhode Island and south coast of
Massachusetts. FWIW, the HREF 24 hour probability-matched mean
trends towards higher amounts for Washington County, RI (a.k.a.
South County) 1.0" up to 1.5".
Northern and northwest Massachusetts there will be just enough
cold air to have a snow/rain mix. City of Worcester, a few wet
snow flakes mix in, but do not expect any accumulation.
Accumulations in the county will be confined to areas along and
north of Route 2 and those amounts should be minimal, a coating
to as much as an inch for the towns along the border with New
Hampshire. The east slopes of the Berkshires have minimal
accumulations too, generally a coating to an inch, with limited
areas AOA 1000 feet could see as much as two inches. Showers
exit the region early Thursday, with the high-res guidance
suggesting it moves off shore between 1am and 3am, with the
outer Cape and Nantucket closer to 5am.
&&
.SHORT TERM /THURSDAY/...
Key Messages:
* Dry, seasonable and breeze.
Seasonable and dry on Thursday, highs are in the upper 30s to
the low-40s. Surface high pressure builds in and lends to a
quiet albeit breezy day due to northwest flow and deep mixing of
the boundary layer. Should have gusts between 15 and 25 mph,
with an apparent temperature in the low and middle 30s.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
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 /18Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18z TAF Update:
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 /Thursday Night through Sunday/...
Thursday Night: VFR.
Friday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SN, chance RA.
Friday Night: MVFR/IFR conditions possible. Breezy. Chance SN.
Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHSN.
Saturday Night through Sunday: VFR. Breezy.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Key Messages:
* Small Craft Advisory late Wednesday night through Thursday
for the outer ocean waters off southern New England.
Today...High confidence.
Fairly quiet weather with surface high pressure shifting over
the waters. Near calm wind this morning gives way to increasing
southerly flow, wind speeds are 10 to 15 knots, and seas are
between 2 and 4 ft.
Tonight into Thursday...High confidence.
Fast moving low pressure system moves off the Mid-Atlantic
coast this evening and quickly races to the northeast, nearing
Nova Scotia by Thursday morning. This progressive system will
bring widespread rain from early this evening and ending before
sunrise Thursday.
Southerly wind overnight veers, becoming northwesterly
Thursday. Gusting 20 to 25 knots and seas building 4 to 6 feet.
Outlook /Thursday Night through Sunday/...
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.
Friday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain.
Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 9 ft. Rain likely, snow
likely. Local visibility 1 to 3 nm.
Saturday: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 9 ft. Chance of snow
showers. Areas of 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 showers.
Local visibility 1 to 3 nm.
Sunday: Winds less than 25 kt. Local rough seas. Slight chance
of snow showers.
&&
.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...KJC/Dooley
NEAR TERM...Loconto/Dooley
SHORT TERM...Dooley
LONG TERM...KJC
AVIATION...Loconto/KP
MARINE...Loconto/KP
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 18, 1:40 PM EST----------------
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