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Office: BOX

FXUS61 KBOX 212335
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
735 PM EDT Sun Oct 21 2018

.SYNOPSIS...
High pressure ridge will build across the region tonight
through Monday, with dry conditions and cold temperatures. A
weather system crosses New England late Monday night and
Tuesday with showers. Colder air then returns Tuesday night
through Friday. A coastal storm may bring rain to the region
Saturday night and Sunday.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
As skies become mostly clear and winds continue to diminish
tonight, expect a rather cold, frosty night. As the airmass
dries out, dewpoints will fall to the upper teens and 20s, which
will also allow temps to fall quickly, mainly around or after
midnight. Expect readings by daybreak to bottom out mainly in
the 20s. The only locations that will remain close to or above
freezing will be along the immediate coastline.

With these conditions overnight, a Freeze Warning is in effect
for the immediate eastern coast of Essex, Suffolk, and Norfolk
counties, as well as southern Plymouth and Newport county.

Low level clouds currently over upstate New York are starting to
slowly move SE into western MA. However, these clouds will
likely dissipate on the eastern slopes of the Berkshires, having
minimal impact on the Connecticut River Valley.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
Monday...
High pressure ridge will build across the region during Monday,
with the large center across the SE U.S. H5 heights will slowly
rise during the day, but even with a lot of sunshine, temps
will remain on the cool side for late October. Temps should top
off in the 50s, except only the mid-upper 40s across the E
slopes of the Berkshires and Worcester hills. Expect light W
winds to begin to back to SW during the day. High clouds may
also begin to filter across the region from midday through the
afternoon.

Monday night...

12Z model suite all signaling some mid level short wave energy
working quickly E-SE out of the Great Lakes in the fast flow
aloft. However, timing the arrival of this energy is in
question. Should see increasing clouds Monday night as the short
wave's surface reflection works toward the eastern Great Lakes
after midnight.

Guidance suggests that some convergence tries to trigger some
shower development along S coastal areas toward midnight, but
there is not a lot of moisture to work with. At this point, may
see scattered showers develop after midnight across Cape Cod and
the islands, with a few spotty showers further inland toward
daybreak. If temps are cold enough across N central and NW Mass,
and the precip does develop, could see some mixed rain/snow
showers. Overnight lows will fall to the lower-mid 30s well
inland, ranging to around 30 at the coast.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Big Picture...

Pattern starts the long-term period with a trough over the Eastern
USA, a ridge over the West, and a closed low over the Gulf of
Alaska. The land features show some progressivity with the eastern
trough moving across New England Wednesday and slowly lifting
northeast through the Maritimes. Shortwave ejects from the Gulf of
Alaska low and follows the northern jet stream across Canada
midweek, then digs south as it phases with a southern stream
shortwave from the Desert Southwest. This phasing creates a deep
trough over the Eastern USA next weekend and turns the general flow
up the East Coast to New England.

Contour heights are below normal through the week, and near normal
over the weekend. This suggests a colder than normal week,
moderating over the weekend.

Mass fields are in general agreement nationally through Wednesday,
and over New England through Thursday. There are differences with
the handling of the Northwest shortwave and its phasing with the
southern shortwave. All models show the trough developing over the
Southeast USA this weekend, suggesting a developing coastal storm,
but with differences that lead to different potential storm tracks.
Confidence is high through Thursday, and low-moderate over the
weekend.

Concerns...

Tuesday...

Upper trough approaches from the Great lakes, supported by a 105 kt
upper jet that is aimed at the New England South Coast. At the
surface a low pressure area over the St Lawrence Valley will swing
both a warm front and then a cold front across our area. Expected
placement of the nose of the upper jet would support coastal surface
low developing either over or just south of Southern New England.
The moisture overhead is limited to 600 mb and lower, but this same
layer shows lift through the layer, especially along the coast. We
will continue to show chance pops for most of our area for this
event. 850 mb temps of 1-3 suggest max temps in the lower 60s, but
clouds much of the day will work against mixing and so max temps
should be a little lower...we went with 50s.

The system moves off through the Maritimes Tuesday night. Sfc
pressure changes are forecast light but positive and should tighten
the gradient enough for a stiff northwest wind.

Wednesday through Friday...

Upper trough overhead Wednesday with cold pool temps as low as -32C.
This will support mixing to 850 mb Wednesday and Thursday. Northwest
winds aloft in the column reach 25 kt each day, so expect gusts to
that speed. Temps aloft at -3C to -5C Wednesday suggests max temps
in the lower 50s, and -7C to -9C Thursday suggests max temps in the
40s...the higher terrain near 40. Moisture remains limited each day,
but enough for clouds and widely scattered showers. Nighttime temps
will trend colder each night as winds trend lighter. Expect temps
around freezing Wednesday night and mid 20s to low 30s Thursday
night.

High pressure moves overhead Friday. This will mean light winds and
clearer skies. Temps aloft suggest max sfc temps in the 40s.

Saturday-Sunday...

Different models, different forecasts.  The GFS shows high pressure
offshore but in control through the day Sat with increasing clouds
but dry weather, then rain at night. The ECMWF shows increasing
clouds with rain CT-RI-SE Mass Sat afternoon. The GGEM shows dry
Saturday and rain Sunday. Only confidence is that there will be wet
weather sometime over the weekend, high uncertainty/low
confidence...take your pick...as to the details.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

Tonight... VFR. Winds diminish away from the coast this
evening, but will linger through around 06Z or so along the
coast before diminishing. W-NW winds slowly shift to SW toward
daybreak.

Monday and Monday night...
VFR conditions. SW winds 5-10 kts. Clouds increase with low
chance of spotty showers especially along the S coast after
midnight.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA.

Tuesday Night through Wednesday: VFR. Breezy. Slight chance
SHRA.

Wednesday Night through Thursday Night: VFR. Breezy.

Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

Through 00Z...
Leftover Gale Warnings through 00Z on the southern outer
waters, otherwise Small Craft Advisories in place.

Tonight...
High pres S of the waters will keep W-NW winds in place.
Leftover gusts up to around 25 kt through most of the night
across the open waters.

Monday and Monday night...
W winds gusting up to 25 kt early Monday on the eastern waters,
then should drop below small craft levels by around midday.
Winds shift to SW by Monday evening, with some gusts to 20 kt
possible. Some visibility restrictions possible in spotty
showers mainly around or after midnight Monday night.

Outlook /Tuesday through Friday/...

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

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

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Slight
chance of rain showers.

Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of seas approaching 5 ft. Slight
chance of rain showers.

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Slight chance of rain showers.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Freeze Warning from 11 PM this evening to 9 AM EDT Monday for
     MAZ007-015-016-021.
RI...Freeze Warning from 11 PM this evening to 9 AM EDT Monday for
     RIZ005-007.
MARINE...Gale Warning until 10 PM EDT this evening for ANZ231-232-251.
     Small Craft Advisory until 3 AM EDT Monday for ANZ233-234.
     Small Craft Advisory until midnight EDT tonight for ANZ230.
     Small Craft Advisory until 10 PM EDT this evening for ANZ236.
     Small Craft Advisory until 2 AM EDT Monday for ANZ235-237.
     Gale Warning until midnight EDT tonight for ANZ250.
     Gale Warning until 11 PM EDT this evening for ANZ254>256.

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...EVT/Correia
SHORT TERM...EVT
LONG TERM...WTB
AVIATION...WTB/EVT/Correia
MARINE...WTB/EVT