National Weather Service National Headquarters

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 140154
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
954 PM EDT Mon Aug 13 2018

.SYNOPSIS...
Scattered showers along with areas of drizzle and fog overnight
as a warm front approaches the region. The front slowly lifts
north Tue, turning warmer and more humid along with scattered
afternoon and evening showers and thunderstorms. Two weak cool
fronts move across the region Wed and Thu providing slightly
less humid and drier weather. Showers and thunderstorms may be
on the increase Friday and Saturday as another front moves
through. High pressure over Quebec brings drier and less humid
weather Sunday and Monday.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
945 PM update...

As per previous discussion, water vapor imagery shows pronounced
dry slot from the NJ coast northward to the Berkshires. The back
edge of the bulk of showers extended from Springfield to
Plymouth at 945 PM and this will continue to lift north over the
next few hours. Areas of drizzle and fog will continue
overnight with the moist easterly flow.

The dry slot also adds convective instability. Strong
thunderstorms were near Philadelphia and we will be monitoring
their progress. Additional showers, albeit more isolated, were
still developing over Long Island Sound. So, there will still be
some scattered showers during the overnight period across
southern New England. Will maintain chance PoPs.

Low pressure off the northern NJ coast was slowly starting to
lift the warm front slightly northward. Winds have shifted from
ENE to SE at Block Island, Martha`s Vineyard, and over Nantucket
Sound (per ferry report) as of 945 PM.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tuesday...

Low clouds and areas of dense fog in the morning as the warm
front slowly lifts north thru the region. By midday model
guidance has warm front vicinity of the Mass pike with Hi Res
guidance developing convection along this boundary especially
the HRW NSSL. Given dry slot will be over the region at midday
this should be low top convection and will limit severity of
convection given shallow updrafts. However always concerning for
rotating T-storms with warm front draped across the region this
time of year (warm ocean not a mitigating factor) combined with
1000-2000 j/kg of CAPE. As warm front lifts north entire region
warm sectors Tue afternoon, first CT/RI and southeast MA then
northern MA later in the day.

It will be much warmer than today with highs Tue 80-85 and feel
muggy with dew pts in the low 70s. As mentioned above this will
yield SB capes of 1000-2000j/kg. Midday/early afternoon
convection will be very isolated/widely scattered and probably
focused near warm front. Then later in the day as the mid level
low approaches, cold pool aloft with -10C at 500 mb combined
with increasing cyclonic flow should support widely scattered
storms across western MA/CT. Good deep layer shear in the
morning and afternoon exits the area by late afternoon/evening,
so instability and shear do not overlap. Thus main threat with
late day storms will be hail with cold pool along with heavy
rainers as wind field weakens and PWATs recover from about 1.25
inches midday to 1.7 inches late in the day over western MA/CT.

Tuesday night...

Isolated to widely scattered showers as mid level low becomes an
open wave moving across the region. However activity should not
be widespread, so not a washout expected. Still prefrontal
airmass over the area so remaining warm and humid.

&&

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

The jet stream is in zonal flow across Canada while subtropical high
remains in control across the southern 2/3 of the 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 will be our pattern for most of the week. A closed low
over the Appalachians reaches northeast to join with the jet
flow, crossing New England late Tues/early Wed as it does so. A
shortwave in the jet flow reaches down as it approaches New
England Friday, then passes overhead during the weekend.

Model mass fields are similar through Friday. Differences from the
Pacific Northwest work their way across the country during the
weekend. Confidence remains high through Friday, then moderate
Saturday through next Monday.

Concerns...

Wednesday-Thursday possible convection...

Ejecting upper low and cold pool linger in the vicinity Wednesday
morning, but move off to the east by afternoon. A shortwave sweeps
across northern New England Thursday with weak curvature extending
over Southern New England. Lots of moisture through an extended
layer Wednesday, then limited moisture Thursday. Lifted Index is sub-
zero much of the time, with PW values 1-1.5 inches. SREF shows a
probability of CAPE at 1000+ at 50-70 pct during Wednesday
afternoon. This holds a chance of showers/tstms Wednesday afternoon,
but likely rain-free Thursday.

Friday-Saturday_Sunday convection...

Shortwave from the Pacific Northwest approaches through the Great
Lakes Friday and crosses New England over the weekend. SBCAPE shows
favorable values over Nrn CT and Wrn/Centrl Mass Friday afternoon,
shifting east Friday night/Saturday morning. PW values rise back to
around 2 inches Thursday night/Friday, indicating return of the
higher humidity as well as increased potential for downpours in any
convection.

Sunday-Monday weather...

Weak high pressure then builds across eastern Canada. This should
bring fair weather and less humid air to us for Sunday and
Monday. Main concern is the position of the high center, with
resulting northeast low level flow. The flow off the water may
bring increased sky cover and less warm temperatures than
forecast.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

945 PM update...

Overnight...

IFR/MVFR in scattered showers. Lowest conditions in the
Worcester Hills. Areas of fog/drizzle. Lowest visibilities
will be over RI and Eastern MA, the Worcester Hills, and east
slopes of the Berkshires. High confidence.

Tuesday...

Drizzle and fog in LIFR/IFR in the morning lifts to MVFR/VFR in
the afternoon. Two rounds of showers/T-storms possible, first
round in the early afternoon along the warm front lifting north
thru MA. Then 2nd round late in the day and evening ahead of
approaching cold front western MA/CT. High confidence on trends
but lower on exact details.

Tuesday night...

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.

KBOS Terminal...IFR/LIFR tonight into Tue morning ahead of
approaching warm front. Showers transition to drizzle. Wind
shift from SE to SW Tue aftn with improving conditions. High
confidence on trend but lower on exact details.

KBDL Terminal...MVFR in showers and isolated thunder transitions
to IFR/LIFR tonight in fog and drizzle. Improving to MVFR/VFR
Tue afternoon but late day showers/T-storms likely. High
confidence on trend but lower on exact details.

Outlook /Wednesday through Saturday/...

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

Wednesday Night: VFR.

Thursday through Thursday Night: Mainly VFR, with local MVFR
possible in patchy fog.

Friday through Friday Night: Mainly VFR, with areas MVFR
possible. Chance SHRA, chance TSRA.

Saturday: Mainly VFR. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

945 PM update...

Overnight...warm front moves across the southern waters.
Scattered showers. Areas of drizzle and patchy dense fog.

Tuesday...warm front slowly lifts north thru the region with
wind shift from SE to SSW as the day progresses from south to
north. Drizzle and patchy dense fog reduce vsbys in the morning,
then improving in the afternoon along with risk for a few
showers/Tstms.

Tuesday night...SSW winds continue along with patchy fog and
isolated/widely scattered showers/tstms.

Outlook /Wednesday through Saturday/...

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft.

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft.

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

Friday: Winds less than 25 kt. Seas less than 5 ft. Slight
chance of rain showers.

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

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

&&

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

&&

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