Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 142306
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
706 PM EDT Tue Aug 14 2018

.SYNOPSIS...
Low pressure and its attending cold front will move across the
region tonight along with scattered showers and thunderstorms
with locally heavy rain and gusty winds. The front exits
offshore Wednesday with a drying trend during the afternoon
along with warm but slightly less humid conditions. It remains very
warm Thursday along with slightly less humid conditions.
Tropical humidity returns Friday into Saturday with increasing
risk of showers and thunderstorms as a cold front approaches.
Cooler weather follows for Sunday and Monday, then tropical
weather returns yet again on Tuesday.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

7 PM update...

Closed upper low over Eastern New York is spinning as it drifts
east. Cloud and shower bands are rotating around the center of
the low. Expect a diminishing trend with the existing showers at
press time. A few tstms/heavy downpours moving through the east
slopes of the Litchfield/Berkshire Hills. This should continue
to be a concern through 10 PM. Lingering showers/thunder in the
Merrimack Valley will move into NH 8-9 PM.

The upper low itself will move overhead during the night
bringing a period of clouds to all areas overnight. This is
already starting in our western sections, and should spread over
the coastal plain around or shortly after midnight.

Previous discussion...

Mid level low over northeast PA into central NY providing strong
cyclonic flow aloft into our region and combining with warm
sector airmass over southern New England with ML capes up to
1500 J/KG. This is supporting a few strong storms across the
region at 4 pm. 80kt upper level jet streak also providing deep
layer shear up to 35 kt across the area in addition to forcing
for ascent in LFQ. So a few strong storms will contain gusty to
perhaps damaging winds along with heavy downpours/flash flood
threat with dew pts in the 70s (76 at MVY) and PWATs up to 1.6
inches or so. Few storms have been rotating as meso low
beginning to form over the region, so this combined with low
LCLs very low risk of a brief weak tornado.

Storms farther southwest over Hudson Valley southward to NYC
into NJ beneath cold pool and mid level low. This activity
slides east this evening and overnight with a downward trend
after sunset. However cold pool aloft with steepening lapse
rates combined with shear from upper level jet streak, some
convection will survive overnight, however coverage will be more
isolated to widely scattered.

Otherwise warm sector airmass will provide a warm and muggy
night with lows only falling back to 65-70, low 70s south coast
including Cape Cod and Islands. Patchy fog possible given humid
airmass and especially where heavy rain has fallen today.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday...

Upper trough and frontal boundary still over the region at 12z,
thus scattered showers and isolated T-storm especially over
southeast MA. As the trough and frontal boundary move offshore
height rises thru the day along with flow becoming more
anticyclonic will support a drying trend along with slightly
less humidity. Overall a real nice summer afternoon although it
will be very warm with highs in the mid to upper 80s.

Wednesday night...

Ridging provides dry and warm conditions. Some patchy fog
possible but not widespread as airmass not quite as humid as
today with dew pts falling into the 60s.

&&

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

Zonal flow jet stream across Canada while subtropical high remains
in control across the southern USA. The USA northern tier is in
between, with a lighter west-east flow overhead while most of the
jet energy stays to our north. This pattern lingers until the
weekend. At that time high pressure builds over the Western USA,
which in turns allows a trough to dig over the Great Lakes/Midwest.
West Atlantic high remains off our coast.

For us, this means a couple of shortwaves moving east across New
England Friday through Sunday. After that, our upper flow turns from
the southwest for early next week as the Midwest trough digs.
That would trend us back into the tropical air again on Tuesday.

Model mass fields are similar through Sunday. Differences arise from
the trough digging from Canada into the Midwest, although these
scenarios all bring a southwest flow to our area. Confidence remains
high through Friday, then moderate over the weekend and low early
next week.

Concerns...

Friday-Saturday convection...

Shortwave on the southern edge of the zonal flow approaches New
England Friday and crosses the region Saturday. Jet support is
mostly to our west during Friday, but moves over Southern New
England Friday evening through Saturday morning. Low level winds are
forecast 10-20kts, while mid level forecasts top out around 35
knots. CAPE is forecast to build to 1500-2500 J/Kg...the NAM even
suggests a core of 3000 J/Kg. Theta-e ridge builds over the Hudson
Valley Friday afternoon and moves east across our area by evening.
GFS/ECMWF show 0-1km Helicity at 100-200 during the
afternoon/evening, mostly across northern Mass. Precipitable Water
builds to 2-2.25 inches by Friday.

This suggests increased risk of showers and thunder. The high
moisture values suggest local downpours. The winds are borderline
for damaging convection, but combined with the stability parameters
and helicity this suggests potential for strong wind gusts and hail
in any convection...this will need to be monitored.

Timing the departure of the upper trough and surface cold front
holds less confidence, but current data supports convection during
the first half of the day, shifting offshore during
afternoon/evening.

Sunday night-Monday southern offshore system...

The next shortwave sweeps east from the Great Lakes, forming a
surface low over/near Virginia on Sunday. This moves east-northeast
Sunday night with the surface low passing to our south Monday.

Weak high pressure then builds across eastern Canada. This should
bring fair weather to us for Sunday and Monday. Main concern is the
position of the high center, with resulting northeast low level flow
and its effect on temps and sky cover. This has potential to bring
showers to our area Sunday night/Monday.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Tonight...

Scattered showers and isolated thunder from west to east in a
mix of VFR/MVFR. Moderate confidence given some uncertainty on
areal coverage of convection.

Wednesday...

Any morning IFR/MVFR improves to VFR by mid to late morning.
Scattered showers/storms focused over southeast MA in the
morning then moving offshore. Can`t rule out an isolated shower
during the afternoon but VFR conditions prevail. High
confidence.

Wednesday night...

VFR and dry weather. High confidence.

KBOS Terminal...very tricky forecast regarding wind direction
and cigs/vsbys. Thinking around or after sunset once surface
heating ceases, onshore winds will become more S-SW allowing
cigs/vsbys to improve to MVFR/VFR. Scattered showers/storms thru
21z then diminishing. More isolated overnight into Wed morning.

KBDL Terminal...VFR but MVFR at times in scattered
showers/storms. Heavy rain and gusty winds may concern.

Outlook /Thursday through Sunday/...

Thursday: VFR.

Thursday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, isolated TSRA, patchy BR.

Friday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, slight chance TSRA.

Friday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SHRA, slight chance TSRA, patchy BR.

Saturday: Mainly VFR, with local IFR possible. Chance SHRA,
slight chance TSRA.

Saturday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA, patchy BR.

Sunday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Tonight...SSW winds increasing 15-20 kt with gusts to 25 kt
south waters of MA/RI where SCA remains posted. Tropical airmass
yields patchy dense fog at times along with heavy downpours in
isolated showers/T-storm.

Wednesday...SW winds early becoming WSW with frontal passage.
Scattered morning showers gives way to a drying trend during the
afternoon.

Wednesday night...light west winds. Dry weather and good vsby.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt.

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Local visibility 1 to 3 nm.

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, slight chance of thunderstorms.

Friday Night through Saturday: Winds less than 25 kt. Areas of
seas approaching 5 ft. Chance of rain showers, slight chance of
thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers, isolated
thunderstorms.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 6 PM EDT Wednesday for ANZ235-255-
     256.

&&

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


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