LexMAWeather

Area Forecast Discussion
for Lexington, MA

        
148 
FXUS61 KBOX 270756
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
356 AM EDT Thu Jul 27 2017

.SYNOPSIS...
High pressure moves offshore today giving way to more humid 
conditions which will be accompanied by scattered showers and 
isolated thunderstorms which may persist into this evening. 
A drying trend likely develops late tonight into Fri as a cold 
front moves through the region and then stalls near the south 
coast Fri. A coastal storm will push off the mid Atlantic coast 
and track south of New England this weekend, which will bring 
showers and a few thunderstorms, along with gusty onshore winds 
with cool temperatures this weekend especially close to the
south coast. The coastal storm may linger south of New England 
into early next week with cool and unsettled conditions 
persisting.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Approaching mid level shortwave from the NW will bring a modest
instability burst into New Eng today as K indices spike into the
low/mid 30s with deepening moisture and PWAT plume exceeding 1.5
inches. Global and Hi-res guidance signaling a cluster of
showers and a few t-storms moving into SNE from the west this  
afternoon. Instability is marginal with SBCAPES 500-1000 J/kg 
and mid level lapse rates are poor so risk for strong to severe
storms is low, but some of the hi-res guidance indicating
potential for localized heavy rainfall. Greatest threat will be
in the interior north and west of the I-95 corridor where better 
instability, but do expect some showers making it to the coastal
plain later this afternoon. 

Cloud cover and shower potential will limit heating with highs
mostly in the low to mid 70s and could see temps fall into the
60s in showers over the interior high terrain. Increasing 
humidity as dewpoints climb into the low and mid 60s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Tonight...
Mid level shortwave and attending cold front move into the 
region which may help to sustain convection into the evening 
before drier air pushes in from the north. Trend will be for 
drying conditions overnight as the shortwave exits and K indices
drop off. It will remain humid as low level drier air lags to
the north given slow movement of front across SNE so areas of 
low clouds and fog may develop. Mild night with lows in the 
60s.

Friday...
Anomalous closed low digs SE across the Great Lakes with 
confluent flow across New England likely keeping deeper 
moisture to the south with mostly dry conditions. However, 
can't rule out an isold afternoon shower/t-storm as marginal 
instability develops, mainly south of the Pike and particularly 
across RI and SE MA where stalled cold front may provide a focus
for convection. Expect partly sunny skies developing after any 
morning stratus/fog burns off. 850 mb temps 12-14C support highs
reaching into the 80s, except cooler along the coast as sea 
breezes develop.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Risk continues for showers and a few thunderstorms possible Fri 
  night and Sat, best chance S coast
* Gusty E-NE winds possible along coastal areas, highest along S
  coast, Cape Cod and the islands Sat 
* Dry conditions return early next week, but timing uncertain 

Overview and model preferences...
Per latest 00Z upper air obs, it's somewhat difficult to 
believe that what is currently a very innocuous wave along the 
Canadian border with North Dakota/Minnesota will be one of the 
main components to the weekend forecast. This weak wave, 
deepening in response to upstream convection, looks to phase 
with a Hudson valley wave rotating into Quebec. What occurs 
after the phasing is where most significant disagreement begins.
The N cutoff shifts E allowing the the remnant longwave trof 
along the E seaboard to meander. GFS has this longwave cutting 
off completely, and hence is the slowest to shift the pattern. 
ECMWF/NAM are more progressive with ensembles falling on either 
side. For this update, given GFS seems to be an outlier, will 
lean more on the more progressive ECMWF solution with this 
update. All-in-all with stalled front to the S, a slow frontal 
low pres associated with this wave will pivot, mainly SE of the 
40/70 benchmark Sat, and slow as the system continues to dig. 
This brings about a prolonged period of NE flow off the Gulf of 
Maine and how long it last is based on the depth of the longwave
trof. Using the ECMWF timing, will release it from the throes 
of this feature by early in the work week, but this remains 
somewhat uncertain. Beyond this timing, a period of ridging is 
likely, as the longwave trof weakens and the pattern reloads.

Details...

Fri night into Sun...
Convection firing along a stalled warm front draped from the 
mid Atlantic, although there remains the uncertainty of 
convective feedback, will assist in generating a low pres wave 
which will traverse the front slowly late Fri night into Sat. 
This allows for the development of NE flow, drawing air across 
the Gulf of Maine where SSTs rest between the upper 50s and low 
60s. The combination of cooling flow and modest increase in 
moisture should lead to increased cloud cover through the 
period, culminating in temps remaining below normal. PWATs, 
closely associated with the low pres hover near 2.00 inches and 
ensemble probabilities keep the core of this higher moisture 
mainly offshore. Combine this with the influence of high pres 
from the NW under anticyclonic flow, and expect a very tight 
gradient in QPF and therefore, POPs as well. Will be tightening 
the POP/QPF gradient somewhat with this update based on the 
ECMWF/NAM blending suggested above. However, along S coastal 
areas, some heavy rain is possible given the higher PWATs and 
conditionally unstable soundings. Low risk for 1.00+ inches of 
rain, but very uncertain on this axis as it could remain 
entirely offshore. Overall, a period of potentially 
gray/cool/damp weather, with the highest risk for heavy rain 
along the S coast. 

Temps as much as 5-10 degree below normal except for where 
there is a chance for breaks of sun, mainly NW MA. Raw 2m more 
supportive of this than MAV/MET data. Otherwise, risk for some 
gusty NE winds as LLJ is nearly 5 std deviations above normal, 
out of the E. Coastal areas show 40-50 kt E-NE LLJ.

Mon and Tue...
Gradual improvement as the longwave trof slides slowly E 
assuming no cutoff forms. The increased anticyclonic curvature 
and induction of drier air in the form of subsidence should 
allow for more breaks in clouds and a much lower risk in 
precipitation as the higher theta-e ridge/PWATs and strongest 
LLJ shifts further offshore. Filling low pres wave noted as 
well. Moderate NW flow as well, suggest a slight moderation in 
temps at least closer to (but still below) normal as the shallow
cold air should give way to better mixing, reaching H85 where 
average temps run about +12C yielding low-mid 80s, however, 
given the uncertainty, this will be dependent on how quickly the
low pres clears the SE waters.

Wed into late week...
As the longwave trof gives way to resurgence of upstream 
ridging, more zonally oriented flow should allow for a return of
drier with temps near to potentially above normal given current
timing. Still uncertain as front begins to parallel the mid- 
upper lvl flow, and this setup is strongly dependent on the 
timing of the phased wave system this weekend.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Through 12z...VFR. 

Today...Moderate confidence. Mainly VFR cigs but areas of MVFR 
along and north of the Mass Pike. A cluster of showers and sct 
T-storms expected to move into the region from the west later 
this morning into the afternoon with greatest coverage in the
interior, north and west of I-95 corridor.

Tonight...Low confidence due to uncertainty with extent of
developing stratus and patchy fog. May see IFR/MVFR conditions 
developing. Scattered showers and isold t-storms in the evening
should push offshore to the south overnight.

Friday...Moderate confidence. Areas of MVFR/IFR early, 
improving to mainly VFR conditions. Low risk of a few afternoon
showers/t-storms, mainly south of the Mass Pike.

KBOS Terminal...Moderate confidence. A few showers/isold 
t-storm possible in the afternoon and evening but VFR should 
prevail. MVFR cigs likely developing tonight but low confidence
in cigs lowering to IFR late tonight. 

KBDL Terminal...Moderate confidence. A few showers/isold 
t-storm possible in the afternoon and evening but VFR should 
prevail. MVFR cigs likely developing tonight but low confidence
in cigs lowering to IFR late tonight. 

Outlook /Friday Night through Monday/...

Fri night into Sat night...Moderate confidence.
Area of rain/fog and low clouds possible mainly close to the S
coast. This could be coincident with periods of MVFR/IFR
conditions. Meanwhile areas across NW MA and extreme W CT could
stay mainly VFR. Otherwise, winds gusting 25-30 kt possible,
especially across SE MA/Cape Islands and RI out of the NE.

Sun and Mon...Moderate confidence.
Some improvement possible as rain/fog and lower CIGS shift
gradually E of the region, however exact timing uncertain. This
improvement would see more widespread VFR conditions develop
where not already observed. Otherwise, winds still gust 20-25 kt
mainly across SE MA/RI.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

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

Short Term /through Friday/...High confidence.

Today...S/SW winds with gusts up to 20 kt and seas below SCA. 
Late afternoon shower/t-storm possible. 

Tonight...Diminishing SW winds shifting to west overnight. 
Light seas. Patchy fog may reduce vsbys. Evening shower/t-storm
possible. 

Friday...Light winds becoming E/SE in the afternoon with sea
breezes developing nearshore waters. Light seas. 

Outlook /Friday Night through Monday/...

Fri night...Moderate confidence.
E-NE winds begin to build, reaching 20-25 kt by the early Sat AM
hours. This also allows seas to build to 3-4 ft. Some rain/fog
possible, mainly during the late evening early morning.

Sat into Sun...Moderate confidence.
Low pres will slowly pass south of the waters, bringing a period
of rain/fog and potential for occasional thunderstorms. NE
winds, could gust to low end gales mainly Sat and Sat night with
seas 7-9 ft offshore. Gale warnings may ultimately be needed, or
at the very least, high end small craft advisories. 

Mon...Moderate confidence.
N-NE winds continue, but gusts should generally stay around 25
kt with seas still 5-8 ft. Small craft advisories are likely to
continue even as rainfall/fog improves slowly.

&&

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

&&

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