Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 201728
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1228 PM EST Mon Nov 20 2017

.SYNOPSIS...
Low pressure in eastern Quebec provides blustery and chilly
conditions across southern New England today. The high moves
south of the region Tue as winds shift to the southwest along
with milder temperatures. A cold front approaches the region
Tuesday night, and will combine with moisture working up the
eastern seaboard to bring showers across the region through
Wednesday. Dry and cold conditions expected Wednesday night
through the end of the week. A cold front may bring showers
Friday night into Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
7am update...
Forecast generally remains on track this morning although
temps/dwpts are running a bit higher than previous forecast
suggested. So will update these to bring them up to current
trends. Also noted that daytime mixing is reasonably efficient
given the shortwave rotating through, which suggests highs a bit
warmer as well. Otherwise, breezy conditions persist until this
wave is fully exiting the region.

Previous Discussion...

Fairly high amplitude mid level mean trough moving through the
region this morning with 500 mb temps down to about -36C over
Lake Ontario per early morning SPC mesoanalysis. This
instability combined with cyclonic flow over the northeast
yielding periods of cloudiness across southern New England.
Embedded with this large scale flow are a few lake effect snow
streamers from NY state traversing CT into RI with another band
over northwest MA.

00z NAM and other hi res guidance capturing these narrow lake
effect bands very well, so leaned on this guid for the morning
hours. Not expecting much snow but a few of these bands may
briefly lower vsby and leave a quick dusting or coating behind.
However most locations will remain dry. By afternoon model low
level streamlines indicates trajectory will shift into northwest
MA. Thus any snow shower/flurries this afternoon will be
focused over this region.

Otherwise a mix of sun and clouds today, chilly with blustery
W-NW winds up to 35 mph. Colder than normal with highs only
40-45 except only mid to upper 30s across the high terrain. It
will feel even colder give the gusty W-NW winds. Normal highs
for 11/20 should be in the upper 40s to lower 50s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
330 AM update...

Tonight...

Mid level mean trough kicks eastward with height rises
overspreading the region as mean ridge pushes toward New
England. Anticyclonic flow will provide dry weather. Some
mid/high clouds may clip western MA as warm front lifts
northward across NY state into VT. This transition/airmass
change will result in temps not as cold as this morning along
with less wind, with winds shifting from WNW to SSW. So coolest
temps likely this evening and then temps leveling off or rising
toward morning especially along the coast.

Tuesday...

Short wave ridging across New England as next northern stream
trough dives into the Great lakes. Thus dry weather expected.
Tightening SSW pgrad as high pres moves offshore as cold front
enters eastern Lakes. A robust low level SSW jet of 40-45 kt
develops over the area Tue. Given WAA pattern blyr not fully
mixed but model sndgs indicate up to 35 kt/40 mph possible. So a
windy day but this low level WAA pattern will provide mild
conditions as 925 mb temps warm to about +5C, supporting highs
in the mid to upper 50s, except low 50s across the high terrain.
About 5-8 degs warmer than normal. Should be a mostly sunny day
as well.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Leftover SW wind gusts Tuesday evening ahead of approaching
  cold front
* Atlantic moisture will stream across the region as the cold
  front pushes across late Tuesday night and Wednesday
* Dry but chilly for Thanksgiving Day and Friday
* A cold front may bring some showers Fri night into early next
  weekend

Overview...

Continued progressive mid level steering pattern noting during
most of this forecast period. However, will see amplified but
progressive H5 trough dig across the eastern U.S. Tuesday night
into Wednesday. Most models still signaling low pressure
developing off the SE coast and will combine with H5 short wave
approaching New England to bring tropical moisture up the coast
late Tue night and Wednesday.

As the front pushes offshore Wed night, will see mainly dry
weather but colder than average temperatures for Thanksgiving
and Friday as high pressure ridge builds in. Models showing some
timing and track differences of another low that looks to pass
S and E of Cape Cod Thu night into Fri as another H5 trough digs
across the eastern seaboard. Strong low pressure across southern
Canada will bring another cold front moving across late Fri
night or Sat that may linger further into next weekend.

Expect temperatures to run close to or below normals through
this period.

Details...

Tuesday night and Wednesday...High confidence.

High pres at the surface and aloft moves off the eastern
seaboard Tue night. Rather strong pressure gradient on western
periphery of the high. Low level jet at 45-50 kt from H95 to
H925 Tue evening across S coastal MA and S RI along with the
leftover low level lapse rates (on order of 7-8C/km) through
around midnight may be enough for some SW wind gusts up to 30-35
kt or a bit higher. During the early morning hours, will see
pres gradient between the high and approaching front relax and
the low level jet move offshore.

Noting inverted troughing off the Carolina coast with S-SE
winds bringing surge of low level moisture up the coast on the
backside of the high, then gets caught up in the S-SW flow ahead
of approaching cold front. Models continue to signal weak
tropical or subtropical low form off the FL coast which will
ride NE in the low level flow.

The moisture plume will enhance the precip across eastern areas
as the low passes E of the 40N/70W benchmark. Showers will move
into S coastal areas late Tue night with the best QPF amounts
moving in Wednesday, with the highest amounts forecast along E
coastal areas on the order of 0.4 to 0.7 inches.

The cold front pushes through late Wed/Wed evening, with rain
possibly changing over to snow showers across the E slopes of
the Berkshires before ending. Not expecting much in the way of
accumulations there. The colder air works in Wed night as skies
become mostly clear, though some clouds linger along S coastal
areas.

Thanksgiving and Friday...High confidence.

Behind the front on Wednesday, CAA will move back into the
region for Thursday. High pressure ridging builds across the
region with a dry but cool day expected for Thanksgiving.

Another mid level short wave moves across Thu night and Friday,
but the moisture remains N of the region so expect to see
continued dry and cool conditions.

Friday night through Sunday...Low to moderate confidence.

Expect dry conditions Fri night as high pressure moves east.
May see a dry cold front move across, then stall S of the
region. Low pressure moves across central Canada, with its
associated front approaching. This may bring a chance of light
mixed precip to the E slopes of the Berkshires after midnight
then will change over to rain Sat morning as the precip
progresses eastward. Not a lot of confidence on the timing of
this front, and whether precip may linger into Sat night. Looks
like another shot of cold air works in late next weekend but
mainly dry conditions seen for now.

&&

.AVIATION /17Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Tuesday/...High confidence.

Through tonight...
VFR. A FEW-SCT clouds 040-060. W winds continue to gust 25-35 kt
at times through 22Z, then beginning to diminish overnight and
shift to the SW.

Tomorrow...
VFR. Increasing SSW wind gusts 25-35 kt through the
day.

Tomorrow night...
Mainly VFR although a few patchy near MVFR clouds are possible
toward early Wed morning. Some light SHRA also possible mainly
across W/Central MA/RI.  SSW winds around 10 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Wednesday: Mainly VFR, with areas MVFR possible. Breezy. SHRA
likely.

Wednesday Night: VFR. Breezy.

Thanksgiving Day through Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Tuesday/...high forecast confidence.

640 AM update...

Today...

Marginal W-NW gales with gusts up to 35 kt. A few brief
rain/snow showers through mid morning, otherwise mainly dry
weather prevails.

Tonight...

WNW winds slacken and shift from WNW to SSW as high pres slides
south of New England. Dry weather and good vsby prevail.

Tuesday...

SSW winds may approach gale force especially near shore.
Otherwise dry weather and good vsby prevail. SSW winds will
provide a long fetch of wind along the south coast and will
yield building seas.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Chance of rain
showers.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of seas approaching 5 ft. Rain showers.

Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Local rough seas. Chance of rain
showers.

Thanksgiving Day through Friday: Winds less than 25 kt. Seas up
to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning until 6 PM EST this evening for ANZ231>235-237-
     250-251-254>256.
     Small Craft Advisory until 6 PM EST this evening for ANZ230-
     236.

&&

$$
SYNOPSIS...Nocera/EVT
NEAR TERM...Nocera/Doody
SHORT TERM...Nocera
LONG TERM...EVT
AVIATION...Doody/EVT
MARINE...Nocera/EVT


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