Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 270227
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1027 PM EDT Thu Apr 26 2018

.SYNOPSIS...
A weak high pressure ridge will bring a return to mainly dry
weather later this morning through tonight with seasonable
temperatures. Another fast moving low pressure system will bring
a period of widespread showers to the region Friday afternoon
and evening. Milder temperatures return Saturday, but a cold
front will bring a brief shot of cooler air to the region Sunday
into Monday. A significant warming trend will occur Tuesday
into Thursday with dry weather expected.

&&

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

10 PM Update...

Noting leftover low clouds across N central and NW Mass on
latest GOES-East Nighttime Microphysics and latest CIGS
observations. These clouds are tending to dissipate from S-N,
but still linger across portions of the E slopes of the
Berkshires toward the vicinity of KORE. Otherwise, skies were
mostly clear. Also seeing leading band of high clouds filtering
NE out of NY state/PA, ahead of the next approaching system.
The mid and high clouds increase as they push NE overnight.

W-NW winds are slowly diminishing, though still seeing some
gusts up to 15-20 kt across portions of E Mass, up to 26 kt at
KORH at 02Z. Winds should continue to diminish as weak ridging
builds in overnight, then back to light S-SW.

Temps were mainly in the 50s across most areas at 02Z, though
down to the mid-upper 40s along the immediate S coast as well as
at KORH. Agree with previous forecaster about potential for
patchy ground fog across portions of the sheltered interior
valleys in combination with today`s rainfall and temps falling
close to dewpts.

Have updated near term to bring conditions current.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday into Friday night...

Late day into evening moderate, possibly heavy rain at times. S-
stream shortwave through a broad mid-level trof pattern with high
pressure situated E. Capturing sub-tropical, anomalous moist pwat
plume in excess of 1-inch into S New England fetched via strong
southerlies. Forced ascent beneath broader diffluence, focus along a
low-level warm front parent to a weak surface low reflection. Both
convergent and isentropic upsloping low-level jet into the boundary,
frontogenesis and lift, both deep column moisture and lift apparent.
Indications of steep lapse rates, moist adiabatic, conditionally
unstable environment with negative showalter indications.

Altogether a moderate to possibly heavy rain event with a potential
rumble of thunder. Greatest concern for PM commute over SE New
England with reduced visibility, ponding of water on roadways.

Some divergence on axis of heavy rainfall from high-res models as
well as timing, centering around 2p (18z), GFS faster than other
available guidance. Will lean with HREF trends with bullseye on
SE New England of generally 0.50 to 0.75 inches, locally heavier
amounts at or in excess of 1-inch, especially those areas observing
convection.

Follow-up weak frontal boundary as the surface low passes overnight.
Becoming diffuse beneath increasing subsidence, mid-level ridging,
hardly much airmass change, low level moisture becomes trapped below
the inversion. Diurnal cooling, visibility issues look certain thru
Saturday morning with some light shower activity lingering.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights...

* Scattered showers Sunday and Monday
* Dry and warmer Tuesday through Thursday

SW flow prevails Saturday ahead of cold front. After morning
fog/low clouds burn off, especially near coast, partial sunshine
should help temperatures reach 60s to around 70 away from South
Coast. May see few showers near Berkshires Sat afternoon/evening,
otherwise dry weather expected.

On larger scale, cutoff low over Great Lakes gradually heads
east and crosses New England Sunday and Monday, which should
bring diurnal clouds and scattered showers to much of SNE.
Colder air aloft means highs will remain in 50s to near 60 both
days.

Warming trend gets underway Tuesday through Thursday as upper
ridge builds over Gulf of Mexico and off southeast coast. Looks
like typical summertime pattern with prevailing SW flow and dry
weather. Good agreement among GFS and EC ensembles with rather
high probabilities of 80 degree temperatures Wednesday and
Thursday. Showers/possible thunderstorms with next cold front
should hold off until after Thursday.

&&

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

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

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

Tonight...

VFR. W winds, gradually diminishing, becoming light. Watching
interior sheltered valleys, dense forests, whether shallow IFR
fog will develop. Isolated. Otherwise mid-high CIGs increasing
after midnight.

Friday into Friday Night...

Rain in by 16-18z, conditions deteriorating towards IFR-LIFR.
Heaviest around 21z with a risk TSRA over E/SE coastal New
England terminals. S/SE winds, potential gusts up to 20 kts,
especially SE coast with an additional risk of SW LLWS at 40
kts. Areas of FG developing into evening continuing overnight
with light winds. Potential beginning trend by early Saturday
morning of conditions improving.

KBOS Terminal...VFR with W winds, gusts up to 20 kts, becoming
light overnight. Mid-upper CIGs increasing towards Friday
morning. -RA/RA in by 18-19z.

KBDL Terminal...VFR with W winds, gusts up to 20 kts, becoming
light overnight. Mid-upper CIGs increasing towards Friday
morning. -RA/RA in by 17-18z.

Outlook /Saturday through Tuesday/...Moderate to High confidence.

Saturday: VFR. Slight chance SHRA.

Saturday Night: VFR. Chance SHRA.

Sunday: VFR. Breezy. Slight chance SHRA.

Sunday Night through Monday: VFR. Breezy.

Monday Night through Tuesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Rest of today...
SW winds shifting W towards late, last of fog / low visibility
pushing out. Gusts up to 25 kts possible with improving conditions.

Tonight...
W winds gusting to 25 kt early then diminishing. Seas will
slowly subside but remain at or above 5 ft. Good visibility.

Friday into Friday Night...
Rain moving in towards noon, lower visibility with moderate,
possibly heavy outcomes in addition to a low risk of thunder.
Increasing S winds, peaking around late afternoon into evening,
waves building 5 to 7 feet with low visibility that`ll likely
continue with areas of fog into Saturday morning.

Outlook /Saturday through Tuesday/...Moderate confidence.

Saturday: Winds less than 25 kt. Seas up to 5 ft.

Saturday Night through Sunday: Winds less than 25 kt. Areas of
seas approaching 5 ft. Slight chance of rain showers.

Sunday Night through Monday: Winds less than 25 kt. Areas of
seas approaching 5 ft.

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

Tuesday: Winds less than 25 kt.

&&

.EQUIPMENT...
The NOAA Weather Radio transmitter serving Providence is out of
service due to phone line problems. The phone company has prioritized
the issues and is working to get the weather broadcast back on the
air as soon as possible. The transmitter serving Hyannis is back
in service.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 1 AM EDT Friday for ANZ233-234.
     Small Craft Advisory until 6 AM EDT Saturday for ANZ235-237-
     250-254>256.
     Small Craft Advisory until 8 AM EDT Friday for ANZ251.

&&

$$
SYNOPSIS...Sipprell/JWD
NEAR TERM...EVT
SHORT TERM...Sipprell/JWD
LONG TERM...JWD
AVIATION...KJC/Sipprell/EVT/JWD
MARINE...Sipprell/EVT/JWD
EQUIPMENT...Staff


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