Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 202340

Area Forecast Discussion
National Weather Service Boston/Norton MA
740 PM EDT Sat Jul 20 2019

Extreme heat and humidity continues into Sunday with dangerous
heat indices between 100 and 110 degrees during the afternoon
and early evening. The heat and humidity may trigger a few
isolated showers/thunderstorms from late Saturday through
Sunday. Otherwise most locations experience dry weather. Low
pressure then tracks along a frontal boundary into southern New
England Monday and Tuesday, and will be accompanied by showers
and thunderstorms with tropical downpours and strong winds
possible. Eventually much cooler and less humid air moves into
the region late Tue into Wed. Mainly dry Wednesday through
Friday with seasonable temperatures and comfortable humidity



730 PM update...

Focus this evening is on continued warm temperatures, not
expected to dip below the upper 70s/low 80s tonight. Watching
some thunderstorms over eastern NY as they approach. Thinking
is that they should begin to weaken as they approach and we
losing heating with sunset, but the environment is quite
unstable over SNE at the moment so something to watch as they
approach. Hi- res guidance would have the more extreme CAPE
values seen on mesoanalysis (4500 J/kg over SE Mass, more like
3000 J/kg elsewhere) decrease sharply with sunset, weakening
these storms as CIN increases. However, cloud tops continue to
cool and we can`t rule out one of these storms remaining strong
into our western zones.

Previous Discussion...

*** Excessive Heat Warning Continues into Sunday ***
*** Near Record Warmth Tonight ***

Fairly capped environment with 589 dam heights across the region
along with +12C temps at 700 mb. However a few isolated
showers/T-storms have fired in CT along the seabreeze boundary
and this should be the focus for any new convection moving
forward into early this evening. Given the higher dew pt air
here (mid to upper 70s) highest 0-3km CAPEs are across CT/RI and
southeast MA. However with shear values less than 30 kt, storms
will pulse down as quickly as activity pulses upward. Also with
mid level lapse rates only 6C/KM, this will temper updraft

Otherwise remainder of the region remains dry this evening and

Given dew pts are in the low to mid 70s with a few upper 70s,
only expecting temps to cool off into the mid to upper 70s
overnight. Even warmer in the urban centers of Boston,
Providence and Hartford where temps may only fall into the low
80s, which is near record warmth for overnight mins.


415 PM update ...


*** Excessive Heat Warning Continues Sunday ***
*** Near Record Highs Possible ***

Still capped environment Sunday with only subtle height falls
and warm temps persist aloft. Not much cyclonic curvature to the
mid level flow, thus little if any synoptic scale lift. However
deep layer shear increases a bit from 15 kt today to about 25 kt
Sunday afternoon, still a high CAPE low shear environment.
Although with blyr winds becoming west and increasing to 15-20
kt in the afternoon, this will limit surface convergence and
also dry out the boundary layer with dew pts falling into the
mid and upper 60s. So a notch less oppressive tomorrow but this
will also lower CAPE values down to 1000-2000j/kg, plenty of
instability to support deep convection but definitely less
instability than today. Conversely mid level lapse rates more
impressive tomorrow as EML currently over the Great Lakes and OH
Valley advects across SNE. Thus mixed signals for convection
tomorrow but more unfavorable than favorable parameters. As a
result most of the CAMs models suppress convection Sunday.

Otherwise another very hot day with temps perhaps a few degs
higher than today given west winds will increase tomorrow to
15-20 mph, providing more downslope adiabatic warming. So while
temps may be a few degs high tomorrow, dew pts will likely fall
into the upper 60s and lower 70s, so not quite as oppressive
humidity. Nevertheless the combination of highs tomorrow 95-100
coupled with dew pts in the upper 60s to lower 70s yielding heat
indices of 100-110 will continue with current headlines.

Sunday night...

Still very warm but not as oppressive as tonight with slightly
drier airmass over the region. Any isolated late day storms
should dissipate by late evening. Otherwise dry weather



* Showers/thunderstorms likely Mon into Tue with eventually much
  cooler less humid air moving into the region

* Mainly dry with seasonable temperatures Wed through Saturday with
  comfortable humidity


Monday and Tuesday...

The big idea for Monday and Tuesday will be a very wet storm system
which moves through bringing heavy rainfall and some scattered
thunderstorms to the region. Beginning Monday mid level flow
amplifies, digging a deep trough over the eastern US with a large
ridge over the intermountain west. As this deepening trough
approaches southern New England flow becomes southwest pulling in
deep, moisture rich air ahead of it (PWATs near 2"). The front
stalled south of the coast which brought some relief from the heat
and humidity will lift north again on Monday putting a portion of
SNE in the warm sector, though there is still considerable
uncertainty in just how far north that moves. Best chance now would
be that it stays along or south of the MA Pike, and this is where
we`ll see the best instability and chance for thunderstorms Monday
afternoon and overnight. With instability on the order of 1500 J/kg
CAPE and good deep layer shear (40-45 kts) there is potential for a
few strong to severe thunderstorms in the warm sector. However,  the
greatest risk will likely be from heavy rain and potential flooding
as detailed below.

As the upper trough digs a surface low is generated along the
stalled frontal boundary, strengthening over eastern PA on Monday
before moving into New England, potentially as a double barrel
low. This would bring two rounds of very heavy rain to the
region. Plenty of moisture is present as mentioned earlier. This
with approaching frontal convergence, the RRQ of a 110kt 300mb
jet, and a 30-35kt LLJ means we have potential to see a
prolonged period(s) of heavy rain from late Monday through
Tuesday. WPC currently has western MA/CT in a Slight Risk of
Excessive Rainfall, with a Marginal Risk for the rest of the
area. We`ll have to monitor for the potential of flooding with
this system given deep layer moisture and strong unidirectional
shear. The low should lift out late Tuesday bringing rain to an
end. Cooler and drier air moves in behind the system which along
with cloudcover should keep high temps on Tuesday in the 70s.

Wednesday through Saturday...

A cooler, more seasonable, and drier period is on tap for mid week
into the weekend. The upper trough moves overhead with building high
pressure at the surface. 850mb temps around 10C temperatures will
make surface temps feel cool relative to this weekend, in the mid
80s; should feel drier as well, with PWATs <1" and dewpoints in the
50s and low 60s.


Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...Moderate to high confidence.

00z update ...

VFR and mainly dry. Any shower/T-storm will be isolated and
short lived, with the best chance in western MA/CT. Modest WSW
winds. Patchy fog in MVFR possible later tonight Cape Cod and
Islands including well inland valleys.

Sunday/Sunday night ... moderate confidence.

More of the same VFR with any T-storms very isolated in areal
coverage. West winds a bit more than today 15-20 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...High confidence.

Monday: VFR. SHRA likely, chance TSRA.

Monday Night: Mainly VFR, with local IFR possible. Breezy. SHRA
likely, chance TSRA.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance

Tuesday Night through Thursday: VFR.


Forecaster Confidence Levels...

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

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

415 PM update ...

Tonight ... mainly light to modest SSW winds. Patchy fog
possible overnight. Any isolated T-storms remain close to shore.

Sunday/Sunday night ... SSW winds becoming WNW late Sunday into
Sunday night. Isolated T-storm possible. Patchy fog possible
Sunday morning but improving.

Outlook /Monday through Thursday/...High confidence.

Monday: Winds less than 25 kt. Chance of rain showers, slight
chance of thunderstorms.

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Rain showers likely, chance of thunderstorms.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Chance of rain showers, chance of

Tuesday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers.

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: Winds less than 25 kt.


Record highest min temps for Saturday July 20:

BOS: 80 in 1977
BDL: 73 in 1977
PVD: 76 in 1977
ORH: 72 in 1977

Record highs for Saturday July 20:

BOS: 99 in 1991
BDL: 100 in 1991
PVD: 101 in 1991
ORH: 95 in 1991

Record highest min temps for Sunday July 21:

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

Record highest max temps for Sunday July 21

BOS: 102 in 1977
BDL: 101 in 1991
PVD: 102 in 1991
ORH:  95 in 1926

Last Occurrence of 100F or higher temperatures
BOS: 7/22/2011 (103F)
BDL: 7/18/2012 (100F)
PVD: 7/22/2011 (101F)
ORH: 7/4/1911 (102F)

Notable high dew points in past years for July 20 and July 21:

Saturday July 20

BOS 76 in 2013 (last of multiple years)
BDL 76 in 2005
PVD 76 in 2013 (last of multiple years)
ORH 76 in 2013

Sunday July 21

BOS 76 in 1994
BDL 78 in 1972
PVD 81 in 1977 (tied for highest all-time at PVD)
ORH 76 in 2011 (last of multiple years)


CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004.
     Excessive Heat Warning until 8 PM EDT Sunday for CTZ002>004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ017>024.
     Air Quality Alert from 11 AM to 11 PM EDT Sunday for
     Excessive Heat Warning until 8 PM EDT Sunday for MAZ002>021-
     Heat Advisory until 8 PM EDT Sunday for MAZ022-023.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008.
     Air Quality Alert from 11 AM to 11 PM EDT Sunday for
     Excessive Heat Warning until 8 PM EDT Sunday for RIZ001>007.


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