BOX issues Area Forecast Discussion (AFD) at Dec 19, 12:31 PM EST591
FXUS61 KBOX 191731
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
1231 PM EST Thu Dec 19 2024
.SYNOPSIS...
Rain exits the region early this morning followed by briefly
drier conditions for the rest of today. An offshore coastal
storm is likely to bring light snow accumulations late Friday
into Saturday followed by very cold weather into early next
week. The weather pattern becomes unsettled again as we head
toward Christmas Day but temperatures will be moderating.
&&
.NEAR TERM /THROUGH TONIGHT/...
Clear skies abound this morning with breezy conditions as gusts
have come up to 20 to 25 mph across SNE while temps sit in the
low 40s. Forecast remains on track.
Previous Discussion...
As of 3:30 AM the back edge of the precipitation shield was in
the vicinity of I-95, rain continues for the next two or three
hours per latest high-res guidance for areas on Cape Cod and the
Island. Otherwise a dry morning for most with temperatures
around the middle to upper 30s.
As for this afternoon it is seasonable and dry. Highs reach into
the upper 30s and the lower 40s. 1030mb surface high pressure
builds in from the west and lends to a quiet albeit breezy day
due to northwest flow and good mixing. Gusts are generally
between 15 and 25 mph, with an apparent temperature in upper 20s
to the mid-30s.
&&
.SHORT TERM /FRIDAY/...
Updated: 3:45 AM
Key Messages:
* Dry and seasonably cold tonight, followed by a period of
unsettled weather Friday into the start of the weekend.
Tonight: 1030mb high pressure system over southeast Canada
leads to a dry and quiet night across southern New England.
Winds are light and generally from the north/northeast. Despite
the surface high to our north, do have increased moisture aloft
advecting in from the southwest, leading to increased mid and
high level clouds. Nighttime low temperatures are seasonably
cold in the 20s and low-30s at the coast. As mentioned by
previous forecaster, there are indications of shallow marine
layer advecting into east coastal Southern New England for
Friday morning. These should lead to a low cloud layer into
coastal MA and perhaps a very isolated flurry Friday morning as
indicated on the 00z HRRR/NAM3KM.
Friday: Mid-level trough and robust shortwave move across
southern Ohio River Valley and ejects off the coast of the
Carolinas come late morning. The developing surface low pressure
system moves northeast and remains south and east of the
benchmark into the afternoon hours. But with onshore flow and
convergence aloft cannot rule out a few snow/rain showers. There
is uncertainty with how the storm evolves overnight into
Saturday morning, more on that in the next section below. As for
temperatures, with the high to the north will have below normal
temperatures in the low and middle 30s with the coastal plain
in the low 40s.
&&
.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Updated: 3:45 AM
Key Messages:
* Mainly light snow accumulations Friday afternoon into
Saturday but some uncertainty remains
* Arctic airmass moves in Sat night into Mon with dry
conditions
* Moderating temps Tue into Wed which may be accompanied by
snow or rain but confidence is low
Details...
Friday night into Saturday...
Fairly amplified upper level trough moves off the coast Fri
night but brunt of the shortwave energy will pass well offshore.
As a result the coastal low is expected to remain well south
and east of the Benchmark but there is still some uncertainty
with how far west the heavier QPF gets. Consensus of the
deterministic and ensemble guidance favors bulk of heavier QPF
remaining offshore. 12km NAM is much more bullish on heavier QPF
and snowfall while 3km NAM is rather muted. 90th percentile of
GEFS distribution which is a worst case scenario also favors a
further W solution with potential for advisory level snowfall.
But this is a low probability outcome with more likely scenario
being a light QPF event as large scale forcing ahead of the
upper level trough with moistening of the column will lead to
periods of light snow Fri night, possibly some rain early along
the coast before cooling BL allows for a change to snow.
Potential for more QPF Cape/Islands but a chunk of this QPF will
likely be wasted due to marginal BL temps and potential for
mixing with rain initially. Overall, looking at accumulations of
1 to 3 inches with highest amounts across interior eastern MA,
but have to continue to monitor latest trends as can't
completely dismiss a more significant impact.
Then a secondary shortwave passage on Saturday with deep
moisture plume will likely bring scattered snow showers across
SNE, especially interior. Below normal temps Sat with highs
upper 20s to mid 30s.
Saturday night through Monday...
Clearing Sat night behind the departing trough with lots of
sunshine Sunday and Monday as strong high pres with very dry
airmass settles over New Eng. Exception will be over the outer
Cape where clouds and ocean effect snow showers may develop Sat
night and Sunday as cold air aloft moves over warmer SST with a
NNW low level trajectory. Turning much colder Sat night with
lows dropping into the single numbers and teens. Wind chills
early Sunday morning bottom out to zero to -10F in the interior
and zero to +5F along the coast. Temps well below normal Sun and
Mon with coldest day Sunday and coldest night Sun night under
favorable radiational cooling conditions. Looking at lows in the
single numbers region wide, except teens along immediate coast.
Tuesday and Wednesday...
Forecast confidence decreases as global guidance and ensembles
differ on timing and amplitude of Pacific energy approaching the
region. Temps expected to moderate and expect a period of
unsettled conditions but timing is very much uncertain. Some
wintry precip is possible. &&
.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
This afternoon: High confidence.
VFR. NW wind 10-20 kt with a few higher gusts.
Tonight: High confidence.
Mainly VFR, with some MVFR east toward 12z in light RA/SN.
Light N-NE wind.
Friday...Moderate confidence.
Lowering cigs to MVFR in light RA/SN, very scattered in the
morning, more widespread toward the evening especially over
southeast MA. Some minor accums possible west of I-95 and 128
in MA. NE wind 8-15 kt.
Friday night...Moderate Confidence.
MVFR/IFR in scattered RA/SN, mainly over SE MA and RI terminals.
Preciip exits toward 12Z with some improvement to VFR overnight
in the interior.
KBOS TAF...High confidence in trends.
KBDL TAF...High confidence in trends.
Outlook /Saturday through Monday/...
Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHSN.
Saturday Night 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 through today for
the outer ocean waters off southern New England.
Today: High confidence.
Early morning rain will give way to dry and gusty conditions.
Northwest wind 12 to 18 knots and gusts up to 25 knots, seas
between 4 and 6 feet.
Tonight: High confidence.
NW winds lighten and then shift to NE around 10-15 kt. Seas
between 2 and 4 feet.
Friday: Moderate confidence.
Surface low pressure system moves up the coast and remains out
to seas. 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 7 feet. Will
likely need to reissue a Small Craft Advisory for these
conditions.
Outlook /Friday Night through Monday/...
Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 9 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. Chance of snow showers, chance
of rain showers. Visibility 1 to 3 nm.
Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of snow showers.
Areas of visibility 1 to 3 nm.
Sunday: Winds less than 25 kt. Local rough seas. Chance of snow
showers. Local visibility 1 to 3 nm.
Sunday Night: Winds less than 25 kt. Local rough seas. Slight
chance of snow showers. Local visibility 1 to 3 nm.
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 until 7 PM EST this evening for ANZ250-
254>256.
&&
$$
SYNOPSIS...KJC/Dooley
NEAR TERM...BW/Dooley
SHORT TERM...Dooley
LONG TERM...KJC
AVIATION...BW/Dooley
MARINE...KJC/Dooley
Source:
BOX issues Area Forecast Discussion (AFD) at Dec 19, 12:31 PM EST----------------
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