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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
348 am EDT sun Jul 21 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 near or across
southern New England Monday and Tuesday, and will be
accompanied by showers and thunderstorms with tropical
downpours, possibly some strong wind gusts. High pressure builds
in by Wednesday with seasonable temperatures and comfortable
humidity levels the remainder of the week.

&&

Near term /until 6 am this 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 this morning through 6 PM Monday/...
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 night through Saturday/...

Big picture...

Upper ridge builds over the western USA while a trough digs over the
eastern USA. Also of note is a closed low over the Gulf of Alaska
which edges toward the Pacific northwest coast. The Gulf of Alaska
low ejects a shortwave that moves over the top of the western ridge
and east across Canada. This kicks the eastern trough through New
England Wednesday. The subtropical high over the southern USA then
starts rebuilding north late in the week.

Heights at 500 mb over our area lower below seasonal normal heights
Tuesday. As the trough moves off to the East, Heights then rebuild
to normal Wednesday-Thursday and above normal Friday-Saturday. Based
on this we would expect a cooling trend Tuesday, then a warming
trend to normal or slightly above normal wedensday through Saturday.

General big-picture agreement among the model mass and thermal
fields. The ECMWF, ggem and GFS all show the shortwave crossing
Canada passing north of New England. The timing is different, with
the GFS fastest and European model (ecmwf) slowest. Forecast confidence is between
moderate and high.

Concerns...

Monday night and Tuesday...

Upper trough sweeps southeast from eastern Canada. Supporting upper
jet moves ahead of the trough and over New England, with our area
under the right entrance region. Southwest flow of the upper trough
will draw high humidity air back over the top of the stalled front
to our south. Precipitable water values are forecast to climb back
to 2+ inches over a deep column of the atmosphere. The right
entrance region of the jet will generate deep lift.

As for convection, stability parameters are borderline with the most
favorable values along the South Coast, while shear values are
strong. The high moisture values support potential for local
downpours. Low level southwest winds reach 35 ktjust offshore, and
if the coastal low moves up across srn New England then these could
move over US. This would bring potential for strong wind gusts with
any thunderstorms. Based on this, we expect a period of showers and
scattered thunderstorms until the surface low and the upper dynamics
move past our area Tuesday night.

Wednesday through Saturday...

Upper trough moves overhead Wednesday. The airmass dries out, but
moisture lingers at 800-700 mb. Expect a few clouds Wednesday.
Otherwise, high pressure builds over the region with drier air.
Mixing reaches to at least 850 mb each day and possibly to 800 mb.
Temperatures at 850 mb reach 11c Wednesday, and build to 14-16c
Friday-Saturday. This would support sfc Max temps near 80f Wednesday
and in the mid to upper 80s Friday-Saturday.

&&

Aviation /08z Sunday through Thursday/...
forecaster confidence levels...

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

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

Monday night: mainly MVFR, with areas IFR possible. Shra, tsra,
patchy br.

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

Tuesday night: mainly VFR, with local IFR possible. Slight
chance shra, patchy br.

Wednesday: VFR.

Wednesday night: mainly VFR, with local IFR possible in patchy br.

Thursday: VFR.

&&

Marine...
forecaster confidence levels...

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

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

Monday night: winds less than 25 kt. Seas locally approaching
5 ft. Rain showers, chance of thunderstorms. Local visibility 1
to 3 nm.

Tuesday: winds less than 25 kt. Areas of seas approaching 5 ft.
Rain showers, chance of thunderstorms.

Tuesday night: winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers, patchy fog. Areas of
visibility 1 to 3 nm.

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

Wednesday night: winds less than 25 kt. Local visibility 1 to
3 nm.

Thursday: winds less than 25 kt.

&&

Climate...

Min temps/record high mins for Saturday July 20:

Bos: 80 degrees, ties 80 in 1977
bdl: 77 degrees, breaks 73 in 1977
pvd: 77 degrees, breaks 76 in 1977
orh: 75 degrees, breaks 72 in 1977

High temps/records for Saturday July 20:

Bos: 97 degrees, did not break 99 in 1991
bdl: 98 degrees, did not break 100 in 1991
pvd: 94 degrees, did not break 101 in 1991
orh: 90 degrees, did not break 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 77 observed, breaks 76 in 2013
bdl 76 observed, ties 76 in 2005
pvd 79 observed, breaks 76 in 2013
orh 76 observed, ties 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...excessive heat warning until 8 PM EDT this evening for
ctz002>004.
Massachusetts...air quality alert from 11 am this morning to 11 PM EDT this
evening for maz020>024.
Excessive heat warning until 8 PM EDT this evening for
maz002>021-026.
Heat advisory until 8 PM EDT this evening for maz022-023.
Rhode Island...air quality alert from 11 am this morning to 11 PM EDT this
evening for riz003>008.
Excessive heat warning until 8 PM EDT this evening for
riz001>007.
Marine...none.

&&

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

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