Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 161338
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
938 AM EDT Tue Jul 16 2019

.SYNOPSIS...

High pressure gives us another fair day as it moves off to the
east. But heat and humidity will start to build. The remnants
of Barry will pass just south of southern New England Thursday,
but will bring showers and scattered thunderstorms, some with
local downpours. High pressure builds for Friday through Monday
bringing hot and humid weather. A couple of cold fronts will
bring scattered showers and thunderstorms Sunday and Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

940 am update...

A ridge of high pressure in control will provide for one more
nice and comfortable day. Mostly sunny skies this morning may
give way to partial sunshine during the afternoon with the
development of some fair weather diurnal cumulus clouds.
Nonetheless, still expect plenty of sunshine to push highs into
the 85 to 90 degree range. Dewpoints in the upper 50s to lower
60s will keep humidity levels in check for mid July standards. Sea
breezes developing along the coast will result in temperatures
a few degrees cooler this afternoon along portions of the very
immediate coast. Overall, a beautiful day of weather continues
across the region.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Tonight...
The warm front pushes across the region tonight, though it will
be mainly dry. May see a few showers or even a rumble of
thunder across the E slopes of the Berkshires this evening. Will
continue to see deep layer moisture work across the region as
H85 temps rise to +17C to +18C during the night. A weak short
wave also moves along in the SW flow, so may see scattered
showers and a few thunderstorms develop mainly around or after
midnight across N central and western areas. Noting increasing
instability as well, as K indices increase to the lower 30s and
SLIs around -1. It will be sultry night with temps remaining
mainly in the upper 60s to mid 70s, though a smidge cooler
across the E slopes of the Berkshires.

Wednesday...
As a cold front slowly pushes S out of Quebec, it will push the
remnants of Barry eastward. The front will tend to stall with
the nearly zonal mid level steering flow in place. Instability
continues to increase as CAPEs rise to 700 to 1100 J/kg (highest
across NE CT/RI/interior SE Mass Wed afternoon), TQ values in
the upper teens and K indices in the mid 30s. So, will see
increasing chances for showers and thunderstorms across the
region. Also noting PW values up to 2 to 2.5 inches, so should
see some heavy downpours with some convection. Another warm,
sultry day with dewpoints up to the lower 70s. Will see temps
top off in the 80s once again across the majority of the region,
though it will be a bit cooler across outer Cape Cod and the
islands.

&&

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

Zonal flow across the northern half of the nation while subtropical
high pressure controls the southern half. Remnants of Barry lifting
north will merge into the zonal flow between the Great Lakes and
Ohio Valley, then move east with the zonal flow and crossing
Southern New England Thursday. After it moves through, the
subtropical high builds north over Southern New England for the
weekend. Upper low pressure over the Gulf of Alaska dips south into
the Pacific Northwest and then east with the zonal flow, approaching
the Northeast USA early next week.

Normal 500-mb heights for mid July are 580 to 582 Dm. Forecast
heights are 585 to 588 Dm during late week, then build to 591 Dm
over the weekend. Falling heights Monday. Expect warmer than normal
temperatures through at least the weekend.

General agreement among the model mass and thermal fields through
Friday and possibly Saturday. The Sunday and Monday fields show
small differences but generally favor continued hot temperatures.
Forecast confidence is moderate-high.

Concerns...

Wednesday night to Thursday night...

Northern stream shortwave moves through the zonal flow. This drives
a cold front southeast through Southern New England Wednesday night.
Remnants of Barry merge with the front over the Midwest.

Model consensus shows the front moving through our area early
Thursday morning, then stalling south of New England. The remnants
of Barry then move along the front and pass just south of the
region. Light flow Wednesday night becomes east-northeast Thursday
as the low center passes to our south. This may try to stabilize the
surface layer while convection moves over the top. Instability is
marginal, shear is weak.

Precipitable water values will be 2 to 2.5 inches Wednesday night
and much of Thursday...this is above 2 std deviations above normal.
Values may diminish north of the Mass Pike but even there expect
values over 1.5 inches.

The deep tropical environment suggests showers and scattered
thunderstorms. Moisture values are high, and the moisture extends
deep through the atmosphere. Lift is forecast about 30 mb/hr. This
suggests potential for downpours and potential for street and poor-
drainage flooding. Heaviest rainfall looks to be across parts of CT,
RI and Southeast Mass during the day Thursday.

The last of the remnants moves east of our area Thursday night,
and drier air moves in above 700 mb. This should wind down the
showers. However, a lingering surface flow out of the east may
hold low clouds and widely scattered showers into Friday
morning, especially across Eastern Mass.

Friday through Monday...

Note that the ECMWF and GGEM hold onto showers Friday morning in
Eastern Mass before ending precipitation. Could be widely scattered
showers lingering into the start of Friday. All models play with
showers to our west and east Friday, but little or nothing over us.
They also show a weak surface trough moving through. At the same
time, we see the upper ridge building over the Northeast USA Friday
and lingering over us Saturday. Temps at 700 mb are expected to
reach above 10C Friday afternoon, peak around 13C Saturday, and
linger above 10C until late on Sunday. Tend to favor a rain-
free forecast for Friday and Saturday. Cold front then works
through on Sunday and may support a few showers/tstms, mainly in
Western MA.

Mixing reaches to 850 mb each day. Temps at that level are at least
18-20C, and data for Saturday suggests 20-22C. So sfc max temps will
be in the 90s each day, and could approach 100F on Saturday. Dew
points will be 70 to 75 in this period, leading to heat index
values of 95 to 103F Friday and Sunday...and 100 to 105F on
Saturday. Heat Advisory headlines may be needed Friday to
Sunday.

A second cold front approaches on Monday with another round of
shower/tstms especially in the west.

&&

.AVIATION /14Z 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/...High confidence.

Today...VFR. Light/variable winds early shifting to S-SW 5-10
kt. Gusts to around 15 kt across S coastal terminals during the
afternoon. The exception will be portions of the immediate
eastern MA coast, where southeast sea breezes are anticipated.

Tonight...Mainly VFR. May see patchy fog develop after midnight
across N central/W Mass with local MVFR-IFR VSBYS. Widely SCT
SHRA/Isold thunder possible across W Mass into N Central CT
after midnight, slowly shifts E toward daybreak.

Wednesday...Mainly VFR through midday. Conditions lower to MVFR
as areas of SHRA/SCT TSRA move in across N and central areas
during Wed afternoon, which may include KBOS-KORH terminals.

KBOS Terminal...High confidence in TAF through tonight, then
moderate confidence Wednesday.

KBDL Terminal...High confidence in TAF through tonight, then
moderate confidence Wednesday.

Outlook /Wednesday Night through Saturday/...High confidence.

Wednesday Night: MVFR in clouds and fog. SHRA likely, chance
TSRA.

Thursday through Thursday Night: MVFR in clouds and fog. Chance
SHRA, slight chance TSRA.

Friday: VFR. Slight chance SHRA, isolated TSRA.

Friday Night and Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Today...S-SW winds around 10 kt with gusts 15-20 kt. Seas 3
feet or less. Good visibility.

Tonight and Wednesday...SW wind 10 to 15 kt. Gusts up to 20 kt
on the southern outer waters. Seas 4 ft or less. Visibility
lowering to 3-5 NM in areas of showers Wednesday afternoon, with
a chance for thunderstorms.

Outlook /Wednesday Night through Saturday/...High confidence.

Wednesday Night: Winds less than 25 kt. Rain showers likely,
chance of thunderstorms.

Thursday through Thursday Night: Winds less than 25 kt. Chance
of rain showers, slight chance of thunderstorms.

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

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

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

&&

.CLIMATE...
Record highs for Wednesday July 17 are as follows:

BOS: 98F in 1977 & 1999
BDL: 97F in 1999
PVD: 97F in 1977 & 1999
ORH: 93F in 1900

Record highs for Saturday July 20:

BOS: 99 in 1991
BDL: 100 in 1991
PVD: 101 in 1991
ORH: 92 in 1901

Record highest min temps for Saturday July 20:

BOS: 75 in 2016
BDL: 73 in 1975
PVD: 77 in 1983
ORH: 72 in 2013

Record highest min temps for Sunday July 21:

BOS: 81 in 1991
BDL: 74 in 1977
PVD: 77 in 1980
ORH: 73 in 1991

&&

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

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...Frank
SHORT TERM...EVT
LONG TERM...WTB
AVIATION...Frank/EVT/EVT
MARINE...WTB/EVT
CLIMATE...Staff


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