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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1133 PM EDT Sat Jul 20 2019

Synopsis...
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
levels.

&&

Near term /until 6 am Sunday morning/...

*** excessive heat warning continues through Sunday ***
*** near record warmth tonight ***

11 PM update...

A few isolated showers and thunderstorms crossed north central CT
as well as from eastern Hampden County across southern Worcester
County after 2330z. Quite a bit of lightning developed around
0015z or so as it moved into SW Worcester County near Brimfield
and Charlton, mainly along Route 20. This cell strengthened as
it crossed Oxford and Auburn heading for Sutton by around
0045z. The storm did strengthen briefly as it crossed S central
Worcester County which prompted the severe.

Another area of showers/thunderstorms moved out of east central New York
and Berkshire County into Franklin and Hampshire counties, but
the convection remained below warning criteria. Noted wind gusts
up to 27 kt at kore as the storms crossed there. The convection
continued to weaken as the precip moved across northwest mass, though
a few thunderstorms did cross central and East Franklin County
through around 02z, but finally dissipated.

Scattered showers continued across northwest and west mass at 03z, but
should dissipate by around 05z or so.

Expect sultry conditions across the region overnight as
dewpoints remain mainly in the lower-mid 70s, though the
dewpoint was 77 degrees at kbos at 03z. Temps overnight will
bottom out in the mid 70s to lower 80s, warmest in the urban
centers as well as eastern interior mass.

Have updated to bring conditions current.

Previous discussion...

Given dewpts 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 lows.

&&

Short term /6 am Sunday morning through Sunday night/...

415 PM update ...

Sunday...

*** 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 Ohio
Valley advects across southern New England. 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
prevails.

&&

Long term /Monday through Saturday/...

Highlights...

* 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

Details...

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
southern New England 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 Massachusetts 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 low level jet means we have potential to see a
prolonged period(s) of heavy rain from late Monday through
Tuesday. Wpc currently has western Massachusetts/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 cloud cover 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.

&&

Aviation /03z Sunday through Thursday/...
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.

03z update...

VFR and mainly dry. May see spotty MVFR conditions in patchy fog
or any leftover showers, which will cross north central and west mass
through around 05z, then should dissipate. May also see MVFR
conditions across Outer Cape cod and Nantucket in fog through
12z. Patchy MVFR in patchy fog also the interior valleys. Light
west-SW winds inland, then SW up to 10 kt along the immediate
coast.

Sunday and Sunday night...moderate confidence.

More of the same. VFR conditions 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. Rain showers likely, chance thunderstorms and rain.

Monday night: mainly VFR, with local IFR possible. Breezy. Rain showers
likely, chance thunderstorms and rain.

Tuesday: mainly VFR, with local MVFR possible. Breezy. Chance
rain showers.

Tuesday night through thursday: VFR.

&&

Marine...
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.

11 PM update...

Overnight...mainly light S-SW winds, except 10-15 kt with gusts
up to 20 kt across the southern waters to east of Cape Cod. Patchy
fog near and offshore of Cape Cod and the islands.

Sunday and Sunday night...S-SW winds becoming west-northwest 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
thunderstorms.

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.

&&

Climate...
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)

&&

Box watches/warnings/advisories...
CT...air quality alert until 11 PM EDT this evening for ctz002>004.
Excessive heat warning until 8 PM EDT Sunday for ctz002>004.
Massachusetts...air quality alert until 11 PM EDT this evening for maz017>024.
Air quality alert from 11 am to 11 PM EDT Sunday for
maz020>024.
Excessive heat warning until 8 PM EDT Sunday for maz002>021-
026.
Heat advisory until 8 PM EDT Sunday for maz022-023.
Rhode Island...air quality alert until 11 PM EDT this evening for riz001>008.
Air quality alert from 11 am to 11 PM EDT Sunday for
riz003>008.
Excessive heat warning until 8 PM EDT Sunday for riz001>007.
Marine...none.

&&

$$
Synopsis...nocera/bw
near term...evt/bw
short term...nocera

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