Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 211727

Area Forecast Discussion
National Weather Service Taunton MA
1227 PM EST Tue Nov 21 2017

Mild this afternoon as winds pick up out of the southwest. A
cold front approaches the region tonight, and will combine with
moisture working up the eastern seaboard to bring a period of
rain through Wednesday morning, especially SE New England. Dry
and colder conditions expected Wednesday night through the end
of this week. Milder with a few showers possible Saturday as a
cold front approaches, then blustery and colder weather follows
Sunday into Monday.


Building southern stream ridge along the eastern seaboard, while
northern stream trough digs over the Great Lakes. This points to
an increasing southwest flow and warm advection.

Cross sections show mixing to 950 mb on the 12Z NAM and 06Z GFS.
Temperatures at that level are equiv to -2C to -4C at 850 mb,
which would support max temps in the 50s. Morning soundings from
Chatham and Upton NY suggest potential to mix as high as 900 mb,
but resulting in a similar max temp potential. Winds in the
mixed layer show 25 knots in observed data and 30-35 knots in
forecast data.

So no changes in the forecast...a sunny day with patchy cirrus.
Max temps in the 50s, possibly 60 in spots. Southwest winds
gusting to 30 knots/35 mph and possibly to 35 knots/40 mph.


Dry weather to start tonight ahead of approaching cold front
from the West. High pressure to the east of the region will help
increase southerly low level moisture off the ocean. This flow
will increase low level moisture and thus some isolated showers
after 2 AM across the south coast.

Upper level shortwave will help push the front closer to the
region during the early morning hours. While moisture appears to
be meager with this front, the focus is more on the coastal low
which will develop off the coast of the Carolinas. Moisture
from this low will stream into the region resulting in PWAT
values increasing to 1 inch which is 1-2 STD above normal.
Appears that the 2 systems will remain split from each other,
however the interactions between the upper and low level jets
will help increase lift across the region. In fact, some model
guidance suggest strong lift around daybreak which could lead to
heavy downpours which will impact the Wed morning commute.
Still a spread in where the heavy axis will be located with the
EC and RGEM on the western envelope compared to Hi- res guidance
and NAM. The GFS continues to the eastern outlier and thus
trended away from it. Right now appears the the axis of heavier
rainfall will occur across the I-95 corridor and points
eastward. However this axis could still shift if the two streams
do meet up, of if coastal low trends farther eastward.
Regardless, looks like a good slug of rain with QPF amounts
around 0.5-1 inch. These amounts could also increase within any
thunderstorms. Went ahead and inserted a slight chc of thunder
as dewpoints reach into the 50s and models continue to show some
surface Cape. Highest confidence will be across the Cape and
the Islands.

Conditions will quickly improve from west to east during the
day on Wednesday. In fact, once the cold front moves through the
region, dry air will quickly usher into the region resulting in
clearing skies and dry weather. CAA will spill into the area
resulting in temperatures to fall during the day. NW winds will
also be on the increase as wind gusts increase to 25-30 MPH by
the afternoon.



* Mainly dry and cool Thanksgiving Day and Friday
* Milder with a chance of showers Saturday
* Blustery and colder Sunday/Monday

21/00Z guidance suite remains in rather good agreement on the
overall pattern through this portion of the forecast. In
general, looking at a persistent mid level trough lingering near
our region, although the amplitude and axis varies with time as
various shortwaves move through the flow. This should mean
temperatures trending below normal, especially late this weekend
into early next week.

Dry weather likely to persist into Friday, despite a cold front
getting close to the northern MA border Friday. The most
supportive moisture and dynamics for showers are expected to
remain over northern New England. Our next chance for
precipitation looks to be sometime this weekend. It`s not
looking like a great risk, as humidity levels are somewhat
lacking. However, there are a couple of fronts to move through,
so this potential will need to be watched over the coming days.
Colder air arriving Sunday into Monday could generate some
ocean-effect showers toward the coast.


Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Through tonight...
VFR through at least early morning, with MVFR CIGS and RA
arriving mainly after 09Z from the S. Areas W of a line from
IJD-ORH-MHT may keep VFR CIGS through sunrise. Winds remain SW,
with gusts 25-35 kt through this evening then gradually
dissipating overnight.

Mixed MVFR/IFR CIGS through the day tomorrow with RA and VSBYS
1-3SM at times, especially E of the same line (IJD-ORH-MHT).
Areas W may remain mixed MVFR/VFR with less RA. The rain clears,
and rapid improvement to VFR occurs after 18Z from W-E as winds
shift to the NW.

Tomorrow night...
VFR. NW flow around 10 kt.

KBOS Terminal...High confidence in overall TAF trends, timing of
lower categories early AM may be off by an hour or two. Clearing
occurs mainly after 20Z tomorrow.

KBDL Terminal...Moderate confidence in TAF. Uncertainty in MVFR
CIGS as CIGS could remain low VFR and bulk of the RA remains E.

Outlook /Thursday through Saturday/...

Thanksgiving Day through Friday: VFR.

Friday Night: VFR. Breezy.

Saturday: VFR. Breezy. Slight chance SHRA.


Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

This afternoon...Building seas across the region as low level
winds increase ahead of an approaching cold front. Southwest
wind gusts 30 to 35 knots with potential for 40 knots...this
will mean borderline gales possible. No changes to the existing
gale headlines...winds increasing this afternoon.

Tonight into Wednesday...Southwesterly gales will continue but
subside across the near shore waters. SCA will be needed as seas
will remain above 5 feet and gusts near 30kts. Approaching cold
front will bring widespread rain for the waters and limit vsbys
by Wed morning. Front will sweep through Wed afternoon
switching winds to the NW with gusts near 30 kts by Wed evening.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Seas up to 5 ft.

Thanksgiving Day: Winds less than 25 kt. Local rough seas.

Thursday Night through Friday Night: Winds less than 25 kt.
Seas up to 5 ft.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers.


MARINE...Gale Warning until midnight EST tonight for ANZ231>235-237-250-
     Gale Warning until 6 PM EST this evening for ANZ230-236.


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