Area Forecast Discussion
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000
FXUS61 KBOX 191207
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
707 AM EST Thu Jan 19 2017

.SYNOPSIS...
High pressure builds over the region into Friday. Weak disturbances,
one from the southern stream and one from the southern stream, will
race through New England this weekend, ultimately allowing colder
air to drain into parts of Southern New England by Sunday. Developing
low pressure over the Southern Plains will bring rain and possibly
some initial ice to New England Monday, with the rain lingering
into Tuesday. High pressure builds Wednesday with drier air.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

7 AM UPDATE...

High pressure over the Midwest builds east today and brings drier
air. Lots of clouds over Southern New England at sunrise, but with
the drier air moving in, we expect a trend to increasing sun
during the day. Temps at 925 mb suggest max sfc temps in the 40s,
but with sunrise sfc temps in the 30s it is possible a few spots
may be near 50.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Tonight...

A tricky forecast depending on which model solution you wish to
believe. High pressure parked across the region with an attendant
warm-dry inversion up around H9 is well agreed upon per consensus.
What differs is the amount of low level moisture beneath, as to
whether we`ll get low clouds and/or fog.

Interrogating SREF probabilities, confidence is weighed more N/W
in regards to low cloud / fog threats where snow pack resides that
will have likely melted to some degree earlier in the day despite
some uncertainty on cloud cover as highs are forecast around the
low 40s.

So believe we`ll be dealing with a combination of both: low clouds
below 1 kft agl with subsequently the higher elevations becoming
socked in with fog, and perhaps with some cold air drainage into
the valleys we`ll see fog.

A first guess, ran with a local fogger tool procedure wherever the
dewpoint depression was 1 or less and winds were at or near calm.
This put out patchy fog in the CT River Valley and across the high
terrain of the Berkshires.

Will monitor it closely. May have to make some adjustments depending
on how conditions evolve during the day, perhaps if it ends up being
drier with lower dewpoints. Have left sky cover as scattered to
broken for the time being. May need to lean more overcast N/W.

Friday...

Ridge of high pressure in place and again a tricky forecast. The
inversion remains stout around H9 as suggested by some of the high-
res guidance. However the SREF guidance as of 19.03z seemingly
suggests that we mix out.

Will lean with scattered to broken cloud decks during the day
mostly. However, will begin to see return S flow associated with
the ascending branch of the warm-moist conveyor belt in tandem
with a S stream shortwave trying to break through the stout ridge
across the NE CONUS. Isentropic over-running along a warm frontal
boundary lifting NE through the Mid-Atlantic, subsequent moisture
looks to dry out as it approaches.

Will keep the forecast period dry with only a slight chance for
far SW Hartford County CT. Sprinkles if anything. Otherwise
another mild day with highs around the low 40s with light S winds.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Big Picture...

Large scale pattern initially shows trough in the West and ridge in
the East. Shortwave ejecting from Pacific NW storm Friday digs as it
sweeps across the Southern USA over the weekend. This carves out a
negative-tilt trough over the Eastern USA by Monday. This trough
then slowly pulls out to the east early next week.  The remainder of
the Pacific NW storm digs into the desert Southwest as high pressure
rebuilds behind the eastern storm. This rebuilds a semblance of the
trough-west/ridge east pattern, but with lower/colder heights.

The GFS and ECMWF mass fields remain similar through Sunday, then
diverge on the details Sunday night through Wednesday. The position
of the East Coast storm is closer to the coast and slower on the
ECMWF, while the GFS and GGEM are faster in moving it past on
Tuesday. As such, forecast confidence is good through Sunday, then
low early next week. Expect confidence to remain low until the first
part of the Pacific storm moves ashore Friday night or Saturday.

Details...

Friday night through Sunday...

High pressure ridge builds over New England. Even with this in
place, two shortwaves move through the region. The first, a southern
stream system, slides through on Friday night. Cross sections of
relative humidity show deep moisture will be over CT and aided by
marginal lift at best. Will mention chance pops over CT and parts of
RI and Western MA.  A second shortwave moves south from Quebec on
Saturday night. This will bring limited moisture and no support for
lift. The alignment on this system resembles a backdoor cold front.
Expect some clouds but no pcpn Saturday night. The main effect will
be for colder air to settle south into New England.

Cross sections show moisture lingering below 850 mb through the
weekend. So expect a best-case of partly cloudy, with periods of
mostly cloudy skies.

Sunday night-Monday-Tuesday...

The main concern for weather takes place during this period. Low
pressure takes shape over the Southern Plains, sweeps to the
Southeast coast, then moves up the coast and spreads precipitation
into New England Sunday night or Monday.

High pressure building south Sunday night will bring the typical
signal for cold air damming. Low level ageostropic flow is currently
less revealing. It shows strong drainage flow Sunday night, but from
the NW rather than the usual direction of North. Meanwhile, the
building of the ridge over Gaspee and New Brunswick is in a location
that could block the northward movement of any coastal system from
the south.

Because of the high, and because the upper low is forecast to become
negative tilted Monday, we have slowed the onset of pcpn over our
area. Even so, we indicate chance pops Sunday night and the best
pops for Monday and Tuesday.  Temperature profiles suggest that
Western and Central MA and parts of Northern CT may see a period of
freezing rain or sleet Sunday night before milder air starts working
in on Monday. Eastern MA and RI will be facing an increasing East
wind which should keep temps in the coastal plain above freezing.

Also expecting a windy period. The low level jet will shift north up
the coast Monday. Winds in the core of the jet are forecast to reach
60 knots. So coastal areas could be seeing surface gusts of 30 to 40
knots. Most at risk would be Cape Cod and Nantucket.

As the surface system moves up, it will nudge warmer air into our
area. We expect temperatures to rise much of Monday night, with
coastal plain temps reaching the lower 50s Tuesday while the
interior is in the 40s.

The storm moves off to the northeast Tuesday night. At this point,
expect precipitation to wind down, although with lingering clouds
and drizzle much of the night.

Wednesday...

High pressure builds over the region with sunshine and drier air.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday/...Moderate Confidence.

9z update...

Today...IFR cigs in Southeast MA and patchy spots elsewhere. Also
patchy MVFR cigs in the west and north. Light fog lingers in a few
spots, but most areas are at VFR or will be there this morning.
High pressure builds in during the day. Partial clearing has
already moved into the Greater NYC area, and this should spread
across Southern New England by afternoon.

Tonight...Monitoring for the potential development of MVFR-LIFR
cigs and vsbys more so across N/W MA and CT terminals, including
the CT River Valley. Low confidence at this time. Will keep it
mostly VFR for the time being. Light winds.

Friday...Possibly some MVFR-LIFR conditions in the morning though
with uncertainty given the low confidence forecast earlier. Should
at least see increasing mid to high level clouds out of the SW
during the day, lowering down to low-end VFR. A possible sprinkle
into central CT. Light S winds.

KBOS TAF...Moderate confidence. Conditions gradually improving
toward daybreak. Exact timing uncertain.

KBDL TAF...Moderate confidence. Conditions gradually improving
late tonight. Exact timing uncertain.

Outlook /Friday Night through Monday/... Moderate confidence.

Friday night through Sunday...

Plenty of low level moisture for clouds, but not much for any
precipitation. Areas of MVFR cigs, but unrestricted vsbys.  Winds
from the south Friday morning shift out of the west by Saturday and
then out of the east Sunday.

Sunday night through Monday...

Ceiling lowering to IFR/LIFR. Rain and possible freezing rain inland
Monday, with rain along the coast. Vsbys becoming 2 to 4 miles in
rain and fog. East winds increase with potential gusts to 35 knots
Monday.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday/...Moderate Confidence.

Winds have diminished, so small craft advisories were adjusted to
account for hazardous seas. The headline will continue through
Friday morning by which point the presence of building high
pressure and light winds should allow seas to subside below
advisory thresholds.

Outlook /Friday Night through Monday/...Moderate confidence.

Friday night through Sunday...

Variable wind becomes WSW but with speeds less than 20 knots. Winds
shift Sunday from the ENE with speeds near 20 knots. Seas remain 4
feet or less.

Sunday night-Monday...

Rain moves up from the south as a deepening coastal storm approaches
and moves up against high pressure over the Maritimes. This will
bring increasing winds from the east. Winds at 2000 feet above the
surface will reach 60 knots, bringing a potential for 35-40 knots at
the surface. This wind will build seas, with heights up to 15 feet
possible on the outer waters.  Rain and fog will bring occasional
poor vsbys on the waters.  Based on expected winds, a gale
watch/warning may eventually be needed for this time period.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 1 PM EST Friday
     for ANZ250-254-255.
     Small Craft Advisory for hazardous seas until 10 AM EST Friday
     for ANZ251.
     Small Craft Advisory for hazardous seas until 6 AM EST Friday
     for ANZ256.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Sipprell
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell



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