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FXUS61 KBOX 192251
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
651 PM EDT Thu Jul 19 2018

.SYNOPSIS...
High pressure will bring dry weather with pleasant temperatures
into Saturday afternoon. Low pressure moves up the coast Saturday
night into Sunday morning, bringing showers and thunderstorms with
heavy rainfall and gusty winds to southern New England.
Bermuda high pressure then builds west toward southern New
England and directs tropical humidity and warm temperatures for
next week. Areas of heavy showers will accompany the heat and
humidity next week, especially across western MA/CT.

&&

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

7 PM update...

High pressure overhead. Satellite shows mainly clear skies with
one or two patches of clouds. Surface obs show light wind. Dew
points 45 to 55, except a smidge higher along the South Coast.

No changes to the forecast. Clear skies and light wind will
allow radiational cooling. Min temps should reach the 50s, with
readings around 60 in a few warm spots. A few cold spots may
touch 45-50.

&&

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

Another beautiful day is on tap for the region as high pressure
remains on control. It will be a few degrees warmer than
today with plenty of sunshine. Highs should top out in the
middle 80s in many locations away from the immediate coast.
Humidity levels will remain comfortable with light winds, so
another great day for outdoor activities.

Friday night...

High pressure slowly slides east of the region, but dry weather
will still prevail. Low temps should still drop down into the
50s in many locations with light winds. Will have to watch for
some low clouds/fog patches approaching the southeast New
England coast toward daybreak...depending on the amount of low
level moisture return by that time.

&&

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

Longwave pattern is zonal through the weekend with contour heights
well above normal. This evolves next week with a ridge building over
the Western USA, while a trough digs over the Northeast. Contour
heights over our area will be normal to a little above normal.

Shortwave pattern has a few more details. Shortwave trough over the
upper Miss Rvr Valley digs over the Eastern USA Friday and forms a
closed low. This low is blocked from further eastward movement due
to stationary high pressure over the Atlantic Ocean southeast of the
Maritimes. The result is a south-to-north flow along the USA East
Coast during later Friday and the weekend. Model consensus has
formed around yesterday's ECMWF in showing coastal shortwave moving
through this flow, reaching Southern New England later Saturday
night. Upper ridge over the Atlantic then shifts west a little,
turning our inflow more from the southeast for early next week.

During all of this time, the northern jet stream will remain well to
our north in Canada. Toward midweek next week, a shortwave in the
flow will dig over the Northeast USA...in the longwave trough. The
jet associated with this shortwave will generate increasing lift to
our north and west through midweek, some of which will move over our
area during midweek.

Model mass and themal fields are in good agreement through Tuesday,
then diverge a little in handling the shortwave during midweek next.
Confidence is moderate to high through the weekend and early next
week, then moderate for midweek-next.

Concerns...

Saturday...

Moisture cross sections show increasing moisture at the cirrus
level, and to a lesser extent below 850 mb. Expect some increase
in sky cover, especially during the afternoon, but otherwise a
dry day. Mixing reaches to at least 900 mb, possibly to 850 mb.
Temperatures in the mixed layer support max temps mid 70s to
around 80 in the 900 mb case, and lower 80s in the 850 mb case.

Saturday night-Tuesday...

Coastal shortwave moving north through the flow generates a surface
low pressure area off the Carolinas, which moves north on Saturday
and crosses Southern New England Sunday morning. Expect southeast
winds ahead of the low, south winds behind the low. Low level winds
aloft on both the GFS and ECMWF show 40 knots, with winds near 50
knots at 850 mb but in different locations. We opted to forecast
winds near the Cape and Islands gusting at 30-32 knots. Later shifts
will adjust this as the wind forecasts come into line.

Humidity increases during this time, with dewpoints climbing through
the 60s in the southeast flow and peaking in the low to mid 70s
after winds turn out of the south. Precipitable water values start
Saturday night below 1 inch and increase to 2.25 inches by Sunday
morning, then diminish to 1.5 inches Sunday afternoon. Strong lift
generated by the upper jet will create widespread showers as the
coastal low approaches, and subzero LI values suggest potential for
scattered thunder. Showers diminish after the coastal low moves
past, but the moist south flow should maintain scattered showers,
even areas of drizzle. Given the high PW values, locally heavy
downpours are again expected. This will mean a good potential for
street/poor drainage flooding Saturday night and Sunday.

SREF 0-1Km Helicity forecasts 50-100 values during this time, which
is below the usual threshold of 150 but high enough to recommend
monitoring for rotating tstm potential.

Atlantic ridge tries to build back in Monday-Tuesday, which could
push the moist inflow farther west, especially on Tuesday. Even so,
the airmass remains unstable with LIs -2 to -4, and PW values return
to around 2 inches. We will maintain chance pops for showers/tstms
during this time and continue to mention locally heavy downpours.
Overall, a warm and humid period.

Wednesday-Thursday...

Upper shortwave moves over the Great Lakes Wednesday and lingers to
our west Thursday. PW values of 2 inches are across NY and Nrn New
England Wednesday, but move back across our area Wed night and
Thursday. Continue the warmth and humidity with scattered
showers/tstms.

&&

.AVIATION /00Z 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/...

Tonight and Friday...High confidence in VFR, other than perhaps
a few hours of very localized patchy ground fog toward
daybreak in the typically prone locations.

Friday night...High confidence. Mainly VFR conditions.
However...will need to watch for some low clouds and fog
approaching the Cape/Islands towards daybreak Saturday...but
that is uncertain and dependent on the moisture return by that
time.

KBOS Terminal...High confidence in TAF. ESE seas breeze shifts
to more SE between 21z and 23z, finally turning to the SSW by 1z
or 2z.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: VFR. Slight chance SHRA.

Saturday Night: MVFR/IFR conditions possible. Windy with local
gusts to 30 kt. SHRA likely, chance TSRA. Low vsbys in heavy
downpours.

Sunday: MVFR/IFR conditions possible. Windy with local gusts to
30 kt. Chance SHRA, slight chance TSRA, patchy DZ.

Sunday Night: MVFR/IFR conditions possible. Breezy. Chance
SHRA, slight chance TSRA, patchy DZ, patchy BR.

Monday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, slight chance TSRA, patchy DZ.

Monday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA, slight chance TSRA, patchy DZ.

Tuesday: VFR. Breezy. Chance SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Tonight through Friday night...High confidence. High pressure in
control will result in winds/seas remaining below small craft
thresholds. Good visibility too, but some fog is possible across
our southern waters toward daybreak Saturday depending on the
amount of low level moisture return by that time.

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers.

Saturday Night: Low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Local rough seas. Rain showers, locally
heavy downpours possible, chance of thunderstorms.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas. Chance of rain showers,
locally heavy downpours possible, slight chance of
thunderstorms, patchy drizzle.

Sunday Night through Monday: Winds less than 25 kt. Areas of
rough seas. Chance of rain showers, slight chance of
thunderstorms, patchy drizzle.

Monday Night: Winds less than 25 kt. Local rough seas. Slight
chance of rain showers, isolated thunderstorms, patchy drizzle.

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers, slight chance of thunderstorms.

&&

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

&&

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