Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 280742
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
342 AM EDT Tue Mar 28 2017

.SYNOPSIS...
Wet weather continues into the overnight hours tonight. Some
improvement expected on Wednesday. High pressure brings dry and
cool weather Wednesday night and Thursday. Low pressure from
the Ohio Valley passes south of New England over the weekend.
This brings a mix of precipitation Friday and Saturday, followed
by dry weather Sunday and Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Weak Nly flow continues to impact the region even as the lvls
below about H7 remain fully saturated. This has led mainly to
widespread low clouds with a mix of modest fog/DZ during the
early morning hours. One issue of note, which will need to be
monitored through sunrise is the risk for a few spots to dip
near or below freezing mainly along and N of the RT 2 corridor
of Mass. The Nly flow has allowed dwpts to continue to drop
through the overnight hours and are just now reaching freezing.
This trend may continue and may require a last minute SPS for
isolated freezing spots on area roadways, which could remain wet
from yesterday`s rains/damp conditions. The Nly flow also
continues to keep vsbys in fog higher than dense fog advisory
thresholds.

With this forecast update, am concerned that the warm front
currently draped from NY state across and to the S of New
England may in fact struggle to fully lift N of the region
today. The trapped moisture and low clouds will limit early AM
warming, weak cyclogenesis will continue across the OH valley
then offshore of NJ/Delmarva late today, and there is a lack of
forcing in the form of a S LLJ. Therefore, will keep the warmest
temps mainly W and S of the region. If the front is able to
move into S New England in many capacity temps could easily run
into the 50s, but for now have capped highs mainly in the mid
40s. There is a modest CAD signature in the mass fields that
also supports this thinking with latest model updates.

The damp conditions will remain as moisture mainly in the lowest
400mb or so remains, exacerbated by marine Ely marine flow
developing with the gradient this morning. Dry air aloft
suggests that within the weak overruning supplied by the warm
front the continued risk for DZ and fog will linger until more
widespread rain develops in proximity to the second developing
frontal wave. Expecting DZ/SHRA to give way to widespread RA
late this afternoon and evening as the frontal wave approaches
from the SW and column PWATs increase to around 1.00 inches.
Given the expectation that the warm front remains to the S and
W, have focused highest QPF along and S of the Mass Pike and
predominantly along the S coast into the early overnight.
Widespread QPF is mainly 0.25 to 0.5 inches but some locations
across the S and SE portions of the area may see 0.5-1.00
inches before it fully finishes.

The risk for TS is limited somewhat but there are indications of
some elevated instability in the form of TT near 50 and
conditionally unstable lapse rates late today and this evening.
Therefore, will continue to include this risk in the wx.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Tonight...
Frontal wave once again shifts E of the region during the
overnight hours. This will once again allow a mix of cold
advection and drier air to entrain from W-E especially from
about 03Z to 09Z. Rainfall gradually dissipates, but it may take
some time for the lowest lvl moisture to fully erode. Therefore,
some fog and low clouds may linger beyond the precip ending
times. May need to monitor for a few spots of patchy dense fog
as a result. Otherwise, another mild/damp night for the most
part, but cooling will occur within a few hours of sunrise such
that min temps could drop back into the mid 30s.

Tomorrow...
Finally a break from the prolonged damp conditions. Drier air
will entrain through the entire column through the day allowing
skies to clear and sunshine to poke through. Although some cold
advection clouds may increase through the peak afternoon
heating. H85 temps, although cooling are still near 0C by early
afternoon. Therefore, highs could easily still reach the upper
40s and low 50s especially where enough sunshine is observed.
Breezy with NNW flow increasing through the day, gusts could
reach 25-30 mph at times.

&&

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

A rather flat longwave pattern with individual shortwaves moving
east through that flow. One shortwave moves off through the
Maritimes Wednesday night.  Another ejects out of the Southwest USA
and crosses New England Friday-Saturday.

Models continue to change run-to-run, leaving low confidence in the
details.

Details...

Wednesday night-Thursday...

Surface low pressure passes south of Nova Scotia as high pressure
builds in from Canada. Strong pressure gradient between the systems
will maintain north breezes along the eastern MA coastline with
lighter winds farther inland. The resulting cold advection should be
sufficient to maintain mixing from at least 950 mb where winds will
be 30-35 knots. This will mean gusty winds in at least eastern
Massachusetts early in the night, diminishing overnight as the
pressure gradient diminishes.

High pressure builds over New England Thursday, bringing fair skies
and mixing to between 850 and 900 mb. Temps at these levels support
max sfc temps in the 40s.

Friday-Saturday...

Questions continue with end-of-week system. General model agreement
on surface low moving up the Ohio Valley and then jumping to the Mid
Atlantic coast Friday night, passing south of New England Saturday
or Saturday night. Projected low level winds show 25-30 knot
southeast jet from NJ through Eastern PA and Central NY at 12Z
Friday. Light south flow into Western CT and Western MA. Meanwhile
model QPF shows measurable pcpn almost all the way to Boston.
We passed on a straight importing of the model values in favor
of limiting morning POPs and QPF to areas Worcester and
west...then spreading east to the rest of our area during the
afternoon/evening. Timing and track of the surface low and
resulting northeast winds supports likely pops across all of
Southern New England. Precipitable water values reach .75 to
1.00 inches over our area.

Temperatures and resulting precip type are also a lingering
question. High pressure departing through the Maritimes provides a
small cold air damming signal, both in the pressure pattern and a 25
knot low level ageostropic flow. Surface winds off the Gulf of Maine
should keep coastal areas above freezing, and daytime temps may
nudge above freezing each day. But interior locations may at least
have a rain/snow mix and may see a period of sleet or freezing rain
especially Friday night and Saturday morning.

Sunday-Monday...

High pressure builds in with dry weather. Building heights and
warming temps aloft suggest Monday will be several degrees milder
than Sunday.

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

Through 18Z today...High confidence.
Mainly IFR/LIFR with CIGS below 800ft (except for some interior
sites which are currently MVFR but will too be dropping this
morning). The IFR/MVFR conditions continue with a mix of
fog/drizzle and low vsbys into the early afternoon hours. Mainly
E flow.

Late today into tonight...High confidence.
Mix of IFR/LIFR continues but with more of a mix of RA/fog
rather than drizzle. Rain tapers off from W to E 03Z-09Z. After
rain ends a period of IFR/LIFR in fog lingers afterwords.

Wed...High confidence.
Improving conditions through sunrise with VFR everywhere by 15Z.
NNW winds. Gusts to 20-25 kt at times.

KBOS TAF...High Confidence. IFR to LIFR through much of the
period.

KBDL TAF...High Confidence. MVFR gives way to IFR and LIFR
through much of the period.

Outlook /Wednesday night through Saturday/...

Wednesday night-Thursday...

VFR as high pressure builds over the region.  Strong gusty North
winds along the eastern MA coast including BOS early at night with
gusts 30-35 knots.  Winds diminishing overnight and on Thursday.

Friday-Saturday...

MVFR lowering to IFR in the afternoon, then IFR/LIFR cigs/vsbys
Friday night and Saturday. Rain expected but with a period of sleet
and freezing rain possible inland Friday night-Saturday. East-
southeast winds Friday become Northeast by Saturday.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

Today and tonight...High confidence.
Mainly quiet boating weather as winds and seas remain below
small craft advisory thresholds. These winds will shift, mainly
E today, then veer around to the NW by early tomorrow morning.
Fog/drizzle along with showers may lead to low visibilities at
times. Low risk for a modest thunderstorm on the waters late
today through the early overnight hours, especially on the
southern waters.

Wednesday...High confidence.
NW flow increases with gusts 25-30 kt at times through the day
and seas on the E waters increasing to 5-7ft by late Wed
afternoon. Small Craft Advisories will be needed.

Outlook /Wednesday night through Saturday/...

Wednesday night...Low-moderate confidence.

Low confidence for low-end north gales along the eastern waters.
Otherwise moderate confidence for north winds gusting 25-30 knots,
diminishing overnight. Seas 5 to 8 feet, mainly on the eastern and
southeast outer waters. A Small Craft Advisory will be needed.

Thursday...High confidence.

Diminishing north wind with speeds near 25 knots during the morning.
Seas 5 to 8 feet on the eastern outer waters diminishing through the
day.  Lingering small craft advisory will be needed in these areas.

Friday-Saturday...Moderate-high confidence.

Midwest weather system moves to the Mid Atlantic coast Friday and
passes south of New England Saturday.  Increasing southeast winds
during Friday but speeds remain below 25 knots until Friday night.
Winds turn from the northeast Saturday and from north Saturday night
with speeds 25-30 knots.  Seas build Friday night and Saturday with
heights 5 to 7 feet. Small craft advisory will be needed.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

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


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