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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
505 am EDT Wed Jul 17 2019

Synopsis...
a cold front will approach the region later today, with the
threat of scattered showers and thunderstorms, along with the
potential of heavy rainfall. As the front pushes across tonight,
the remnants of Barry with bring more showers with locally
heavy rainfall, along with scattered thunderstorms which will
continue through most of Thursday. Hot and humid conditions
settle in for Friday through Sunday with heat related headlines
likely. A cold front sweeps through the region Monday with
scattered showers and thunderstorm. High pressure then builds
over New England Tuesday bringing seasonable temperatures and
much lower humidity.

&&

Near term /until 6 PM this evening/...

* a few strong to severe thunderstorms and heavy rainfall with
localized urban & poor drainage flooding possible this afternoon

First round of scattered showers is centered across upstate New York
as seen on the latest NE regional 88d radar imagery. Noting
bands of heavy rain on latest kenx 1-hr precip product east of
kart-ksyr, some with 0.4 to 0.5 in/hour rainfall rates, with
lighter rainfall moving east toward kgfl and kalb.

Extension of light showers lie across the Mid Hudson valley,
which should push into the east slopes of the Berkshires. Still
noting decent subsidence in place across most areas east of the
Berkshires. Dewpoints have been slowly increasing over the last
several hours with the S-SW wind flow in place.

Should see a few showers push into northwest mass by around 10z,
though they should be on the light side. Temps will bottom out
in the lower 70s for most areas, except the upper 60s across the
higher inland terrain and around Nantucket.

Today...

Will see the increasing threat for showers/thunderstorms as a
cold front slowly pushes S out of northern New England. Pops
increase from north-S during the day.

Big threat, however, will be the potential of strong/damaging
winds in any thunderstorms this afternoon into the evening.
CAPES will rise to between 1100 and 1700 j/kg during the
afternoon, with the highest values S of the Mass Pike, along
with k indices in the mid-upper 30s. Storm Prediction Center has placed most of the
region in a slight risk for severe thunderstorms. Feel the best
chance for convection will be between 2 PM and 10 PM. Weak mid
level lapse rates may limit the potential, but 0-6 km shear from
the SW at around 35 kt which will enhance the potential. Will
need to monitor cloud cover, as this may limit the solar
insolation and cut back on some of the convection potential.

Also noting pwats around 2 to 2.3 inches, along with the S-SW
wind flow bringing an influx of low level moisture, so will see
the threat for heavy downpours that may produce localized urban
and poor drainage flooding. At this point, best chance for this
to occur today will be from north CT to near Boston northward into
this evening, then will slowly shift S. May see a few spots
receive 1-2 inch/hour rainfall rates.

It will be sultry day with dewpoints in the upper 60s to lower
70s. Expect temps mainly in the 80s, with a few spots reaching
the lower 90s, mainly across the lower CT valley and interior east
mass. With the high dewpoints, some spots could see heat indices
in the mid to upper 90s.

&&

Short term /6 PM this evening through 6 PM Thursday/...

* a few strong to severe thunderstorms possible this evening
* heavy rainfall with localized urban & poor drainage flooding
possible this evening through Thursday

Tonight and Thursday...

The front will slowly shift S into the region tonight as the
mid level steering flow becomes nearly zonal. Will see winds
shift to east-NE across north mass mainly around midnight or so. Will
continue to see the threat of thunderstorms that may produce
gusty to possibly damaging winds through around 02z, then should
subside somewhat.

However, the threat for heavy rainfall that may produce local
urban/poor drainage flooding will continue, focusing on north CT/Rhode Island
and southeast mass where the best moisture convergence and feed will be
situated. Most short range high res models bring the bullseye
of heavy rainfall across this portion of the region. Along with
this, the nearly zonal flow will bring the remnants of Barry
across central and southern areas overnight through midday
Thursday, enhancing the rainfall even more.

Current forecast quantitative precipitation forecast amounts could be from 1.5 to 2.5 inches
from this afternoon through Thursday, mainly from the Mass Pike
southward.

With east-NE winds behind the slowly moving cold front, low level
moisture will feed off the ocean, allowing the precip to linger
through the majority of Thursday from S of Boston across southeast
mass into Rhode Island and NE CT, but should see some diminishing precip
further north and west during the afternoon.

Highs on Thursday will mainly be in the 70s.

&&

Long term /Thursday night through Tuesday/...

Big picture...

Zonal flow across the northern half of the USA while subtropical
high pressure controls the southern half. Remnants of Barry in the
zonal flow will move east of southern New England by Friday. Early
next week the subtropical high builds north over the western USA,
while a trough digs over the northeast USA.

Normal 500-mb heights for mid July are 580 to 582 dm. Forecast
heights build to 591 dm over the weekend, then trend to normal or a
little lower early next week as the upper trough digs in. From this,
we expect above-normal temperatures Friday-Saturday-Sunday, then
expect warmer than normal temperatures through the weekend, then a
trend to near normal temperatures early next week.

General agreement among the model mass and thermal fields through
the weekend. The fields early next week show a similar general
pattern with small differences. Overall forecast confidence is
moderate-high, but confidence in timing of the cold fronts Sunday-
Monday is low-moderate.

Concerns...

Thursday night and Friday...

Remnants of Barry pass south of southern New England Thursday night.
Winds are initially from the east and northeast, then turn from the
north after the remnants move past. The east flow is a wet pattern
is a wet pattern, especially in eastern Massachusetts and potentially in
central Massachusetts and Rhode Island. Expect continued showers and scattered
thunderstorms the first half of the night, diminishing to no
precipitation in western Massachusetts and northern CT after midnight. Precipitable water
values remain at 2 inches or more across Rhode Island and eastern Massachusetts the first
half of the night, then diminish overnight. This means continued
potential for downpours in this area through midnight, then
diminishing showers after midnight.

Cross sections show lingering low-level moisture for clouds over all
of the region Friday morning. So expect clouds to start Friday, and
possibly some drizzle or widely scattered showers in coastal Massachusetts
Friday morning. But expect drying at cloud-level and warming temps
aloft suppressing further shower development...700 mb temps climb
above 10c...and so we should see clearing during the day. With dew
points in the 60s, expect min temps Thursday night in the 60s.
Increasing sun Friday will allow mixing to 850 mb, tapping
temperatures of 18-20c which would support Max sfc temps of upper
80s to mid 90s. We bumped guidance values up 3-5f to line up this
these values. The temperatures and humidity will generate heat
index values in the 90s.

Friday night through Sunday...

High pressure builds surface and aloft, cresting over southern New
England Saturday and then pulling south on Sunday. Temps at 700 mb
crest about 13c Saturday, which should suppress anything that tries
to form a shower. Precipitable water values fluctuate, but stay close to 2 inches
through the weekend. So the airmass will have a good supply of
moisture in it and surface dew points 70f to 75f. With dew points at
that level, expect overnight min temps Friday night and Saturday
night will be in the 70s to near 80, warm and muggy both nights.
Mixing to 850 mb will tap temperatures of 21-23c, supporting Max sfc
temps of 95-100f. Heat index values will be above 100f most places
and above 105f in some inland areas.

Sunday starts quite warm aloft, but with the relaxing ridge we
should see temps aloft lowering with time. Temps at 700 mb start the
day above 10c, but lower just below by late in the day. This heat
dome should suppress convection for at least the first half of the
day. The question is whether the heat aloft diminishes enough to
allow showers/tstms later in the day. Best chance for this would be
across the northern half of Massachusetts. We have chance pops in
this northern area during the afternoon/evening. Temps at 850 mb are
forecast at 20-22c, which translates to another day well into the
90s at the surface. This brings the heat index to 95-100, similar to
Friday.

Monday-Tuesday...

Cold front sweeps south from Canada Monday, with potential for
showers/tstms. Precipitable water values linger between 1.5 and 2 inches until the
front moves through, showing potential for downpours. High
pressure then moves in behind the front Tuesday, bringing rain-
free and less humid air.

&&

Aviation /09z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Thursday/...moderate confidence.

Through 12z...
mainly VFR. S-SW winds at 5-10 kts, with gusts up to 20 kt
across S coastal terminals. May see patchy MVFR-IFR visibilities in
patchy fog at times along S coastal terminals from about 07z to
11z. Spotty showers across north central and west mass with brief MVFR
conditions.

Today...
scattered rain showers/isold thunderstorms and rain developing from north-S during the day, some
that may produce +ra and possibly gusty winds. IFR-LIFR
conditions with any +ra. Lower confidence on exact timing of
impacts. West-SW winds 5-10 kt, gusting up to 20 kt at times along
the S coast. Scattered-broken cigs becoming overcast late.

Tonight...
scattered-likely rain showers/chance thunderstorms and rain through most of the night. Continued +ra
threat with IFR-LIFR conditions. West-SW winds 10 kt or less,
shifting to east-NE from Cape Ann around 04z S to near Outer Cape
cod toward daybreak. Ceilings lowering to IFR-LIFR with local IFR
visibilities in patchy fog along the S coast, Cape Cod and the islands
and possibly into the CT valley around or after midnight. Overcast
conditions overall.

Thursday...
Cat rain showers across NE CT/RI/S coastal mass Thu morning, with likely
rain showers mainly near/S of the Mass Pike along with scattered thunderstorms and rain.
Conditions slowly improve, but scattered rain showers/isold thunderstorms and rain may linger
inland, with better shot for rain showers/thunderstorms and rain across east mass/Rhode Island with
onshore flow lingering. East-NE winds around 10 kt with some gusts
up to 20 kt along east coastal terminals.

Kbos terminal...
high taf confidence this morning, then moderate confidence this
afternoon and tonight. May see a few thunderstorms and rain this afternoon and
evening, along with possible +ra and gusty winds.

Kbdl terminal...
high taf confidence this morning, then moderate confidence
this afternoon and tonight. Main concern will be for a few thunderstorms in the vicinity
this afternoon into tonight with possible +ra and gusty winds.

Outlook /Thursday night through Sunday/...high confidence.

Thursday night: mainly VFR, with local IFR possible. Slight
chance rain showers.

Friday through Saturday night: VFR.

Sunday: VFR. Slight chance rain showers.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday/...high confidence.

Through today...
high pressure pushes further offshore this morning, along with
an approaching cold front will result in SW winds of 10 to 15
knots with gusts up to 20 knots across the southern waters.
However, both wind gusts and seas should remain below Small
Craft Advisory thresholds. Potential for a few strong
thunderstorms with localized strong winds later today into this
evening, especially across the near shore waters.

Tonight...
a cold front will push S during the night, with SW winds
shifting to east-NE across the eastern waters to Outer Cape cod,
but will remain SW across the southern waters. Showers and
isolated thunderstorms continue, which will be locally heavy at
times. Areas of fog develop, mainly across the southern near
shore waters to east of Cape Cod.

Thursday...
winds gradually shift to east-southeast across the southern waters during
the day, with east-NE winds 10-15 kt across the eastern waters.
Gusts up to 20 kt Thu morning on the eastern waters. Threat for
showers with locally heavy rainfall, along with a few
thunderstorms continues mainly from about Boston Harbor
southward, though will start to diminish during the afternoon.

Outlook /Thursday night through Sunday/...high confidence.

Thursday night: winds less than 25 kt. Slight chance of rain
showers, slight chance of thunderstorms.

Friday: winds less than 25 kt.

Friday night: winds less than 25 kt. Seas locally approaching
5 ft.

Saturday through Saturday night: winds less than 25 kt. Areas
of seas approaching 5 ft.

Sunday: winds less than 25 kt.

&&

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 highest min temps for Saturday July 20:

Bos: 75 in 2016
bdl: 73 in 1975
pvd: 77 in 1983
orh: 72 in 2013

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 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.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...wtb/evt
near term...evt
short term...evt

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