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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 221503
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1103 AM EDT Mon Jul 22 2019

.SYNOPSIS...
A stationary front will remain off the south coast today. Two
waves of low pressure will develop and move along the front,
bringing showers and thunderstorms this afternoon and evening,
followed by a second around Tuesday. The rain will fall heavily
at times. The low and cold front move offshore Tuesday night,
followed by a period of more comfortable rain free summer
weather from Wednesday through the weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

1100 AM update...

Minor adjustment to the rainfall timing this morning. Scattered
rain showers will continue this morning into the afternoon, when
the heavier rain and better thunderstorm chances arrive. At this
time the stalled front is sitting offshore to our south and is
expected to slowly move north today as a warm front, reaching
generally the CT/MA border. This would put CT, RI, southeast MA
in the warm sector with dewpoints reaching the upper 60s to low
70s. It brings with it increased instability on the order of
1000-2000 J/kg MUCAPE, greatest along the south coast. Shear
parameters are also good for some organization and prolonged
updrafts. 0-6km bulk shear values will be around 40-50kt in the
vicinity with low level shear (0-1km) of 25-35kts. Hi-res
guidance has progged the area of greatest updraft helicity (a
good indicator of severe weather) over CT into RI, and this is
where we would expect the best chance of strong to severe storms
this afternoon/evening. The 12Z run has even further increased
this high helicity potential, further increasing confidence
that we may see rotating storms. The best chance of severe
storms will likely be a window from 6pm to midnight, but can`t
rule out a storm outside of that. The greatest risk looks to be
from damaging winds, but given the amount of spin in the lower
atmosphere, the tornado threat is non-zero.

Today...

     Slight Risk for Severe Thunderstorms This Afternoon...

Short range models were in good agreement in keeping the front
pretty much stationary across the southern waters through most
of the day. So will see bands of mid and upper clouds crossing
the sky through around midday. Expect temps to rise through the
70s to lower 80s in the normally warmer areas, but could hold in
the lower-mid 70s across the higher terrain.

As an H5 long wave trough digs S across the Great Lakes during
the day, winds will back to SW across the northeast U.S. Also
noting a developing short wave moving into western areas between
21Z and 00Z Tue. Also noting another good influx of deep layer
moisture as PWATs increase to around 2.2 inches by this evening
in the mid layer SW wind flow.

K indices increase to the lower to mid 30s across the entire
region, as well as the TQ values rising to the upper teens to
around 20 by this evening. Even with the front to the S of the
region, will still see dewpoints rise through the 60s, and will
reach to around 70 across N CT/RI into SE Mass during the
afternoon.

With the first of two low pressure areas developing along the
stationary front, which will waver toward the region late in the
day, will see showers and scattered thunderstorms to develop
from W-E this afternoon.

With these factors, along with the potential for vertical shear
to develop as the short wave approaches from the W later this
afternoon, could see multicells develop that may produce severe
conditions.

SPC has placed N CT into RI under a Slight Risk for Severe
Thunderstorms this afternoon. The primary threat will be for
strong to possibly damaging wind gusts, but can not rule a
slight chance for hail. With increasing PWATS, could also see
locally heavy rainfall develop in any convection.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Slight Risk for Severe Thunderstorms continues into This
Evening...

Tonight...
Showers and thunderstorms will continue through around 02Z-03Z,
some of which may still produce strong winds and heavy
rainfall, should start to see these storms shift slowly S
overnight. Winds shift to light N across inland areas as another
boundary shifts across the region with the exiting short wave.

The short wave will shift E, but precip will linger mainly
across central and southern areas through the night, though the
convection should weaken.

Temperatures will bottom out in the 60s.

Tuesday...
Another short wave approaches during the day as PWATs remain in
the 2 inch range. More instability will shift across the
region, though not quite as strong as today, but could see
another round of heavy rainfall as showers and scattered
thunderstorms return, especially across central and southern
areas.

May see conditions slowly improve across western areas during
the afternoon, but scattered showers may linger.

Expect high temps will be in the 70s.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Big Picture...

Upper ridge builds over the Western USA while a trough digs
over the Eastern USA. We note the models are substantially
slower with the eastern trough axis, now moving through New
England on Thursday, rather than the Wednesday passage in
yesterday`s runs. It is also slower to move out, lingering until
Saturday morning. Strong low level jet leading the trough moves
over Southern New England early in the week, then shifts well
offshore Tuesday night.

Heights at 500 mb over our area will be lower than seasonal normal
heights Wednesday, but trend to normal Thursday and above normal by
the weekend. Based on this we would expect seasonably mild
temperatures Wednesday, trending above normal Friday through Sunday.

Model mass and thermal fields show some differences Friday and
Saturday, but are otherwise similar through Sunday. Forecast
confidence is between moderate and high.

Concerns...

Tuesday night... Trough digging over the Great Lakes with a
progressive axis and a 135 kt upper jet. Stalled cold front remains
just south of New England. Right entrance region of the upper jet is
over Southern New England. This generates the last of a series of
waves on the front Tuesday night, and that wave moves past us by
morning. If this wave is close enough, it may generate a few showers
near the South Coast, especially over the Cape and Islands. The
remainder of the region should be dry. Cloud-level moisture lingers
during the night, so expect mainly cloudy skies. Depending on the
position of the front offshore, there may be some clearing in
Western MA/Western CT overnight.

Wednesday through Sunday...

Upper trough and a -14C to -16C cold pool lingers overhead through
Friday. High pressure builds in at the surface and aloft during the
period. Meanwhile, model cross-sections show a near constant moist
layer between 850 mb and 700 mb through Sunday. Synoptic lift is
mediocre, but convective parameters are favorable on at least two
days out of Friday-Saturday-Sunday. Currently looks like a dry
period, but we will need to monitor that three-day period for
potential showers/thunder.

Mixing reaches to about 810 mb each day. Temps at 800 mb will be 6-
8C Wednesday-Thursday, warming to 12C by Sunday. The 850-mb equiv
for each will be 11-13C Wednesday-Thursday, warming to 17C Sunday.
So we expect max temps upper 70s and lower 80s Wednesday, trending
to 85-90 Sunday. Dew points 55 NW to 65 SE Wednesday, trending to
the low to mid 60s by the weekend. Expect min temps similar to or a
little higher than the dew points.

&&

.AVIATION /15Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...Moderate to high confidence.

Today...VFR conditions early, but clouds fill in and bases lower
later this morning and afternoon. Showers with isolated
thunderstorms move in from W-E. Areas of MVFR-IFR conditions in
any convection. Potential for strong wind gusts and heavy
rainfall in any thunderstorms from about 18Z through the
remainder of the region, mainly across N CT into RI. Light E-NE
winds, becoming SE along the S coast during the afternoon.

Tonight...Showers and a few thunderstorms linger into early
tonight with areas of MVFR-IFR conditions. Showers linger
through the night across central and southern areas, but
conditions may improve to locally VFR after 06Z across N Central
and W Mass but could still see scattered showers. Light S-SW
winds becoming N after midnight.

Tuesday...Showers remain across the region, with isolated
thunderstorms possibly across NE CT/RI/SE Mass through midday.
Could see heavy downpours as well. Conditions slowly improve
across the N central and western Mass during the afternoon.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...High confidence.

Tuesday Night: Mainly VFR, with areas IFR possible. Slight
chance SHRA, patchy BR.

Wednesday through Thursday: VFR.

Thursday Night: Mainly VFR, with local IFR in Patchy BR.

Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence.

Today...Light/variable winds become E around 10 kt on the
eastern waters and light SE winds on the southern waters this
afternoon. Dry conditions through mid morning, then showers and
isolated thunderstorms move in across the western waters around
midday, filling in over remaining wates this afternoon.
Visibility restrictions in patchy fog. Seas 3 ft or less.

Tonight...Winds become SW up to 15-20 kt this evening on the
southern waters, but remain E-NE on the eastern waters early.
Seas build up to 5 ft on the outer waters as well as BI and RI
sounds, so Small Crafts have been issued. Expect visibility
restrictions in patchy fog and locally heavy downpours. Showers
through the night, but any thunderstorms should end from N-S
during the night, though may linger on the southern waters.

Tuesday...Expect N-NE winds up to around 10 kt. Seas remain up
to 5-6 ft on the southern waters through the day. Visibility
restrictions continue in showers and patchy fog, with local
thunderstorms during the morning along with locally heavy
downpours.

Outlook /Tuesday Night through Friday/...High confidence.

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, patchy fog. Areas of visibility
1 to 3 nm.

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft.

Wednesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Local visibility 1 to 3 nm.

Thursday: Winds less than 25 kt.

Thursday Night: Winds less than 25 kt. Patchy fog. Local
visibility 1 to 3 nm.

Friday: Winds less than 25 kt.

&&

.CLIMATE...
Temperature and Dew Point marks Tied and Set this Weekend

Saturday July 20
BOS High Min Temp for Date 80, Tied 80 in 1977
BOS High Min Temp 80, Tied for 6th for All-Time Minimums
BOS High Dew Point for Date 77, Surpassed 76 in 2013
BDL High Min Temp for Date 77, Broke Record of 73 in 1977
BDL High Min Temp 77, Tied for 5th for All-Time Minimums
BDL High Dew Point for Date 76, Tied 76 in 2005
PVD High Min Temp for Date 77, Broke Record of 76 in 1977
PVD High Min Temp 77, Tied for 25th for All-time Minimums
PVD High Dew Point for Date 79, Surpassed 76 in 2013
ORH High Min Temp for Date 75, Broke Record of 72 in 1977
ORH High Dew Point for Date 73, Tied 73 in 1991

Sunday July 21
BOS High Min Temp for Date 83, Broke Record of 81 in 1991
BOS High Min Temp 83, Tied for 1st All-Time w/ Aug 2 1975
BDL High Min Temp for Date 74, Tied 74 in 1977
BDL High Temp of 100, Last 100 was on July 18 2012
PVD High Min Temp for Date 78, Broke Record of 77 in 1980
PVD High Min Temp 78, Tied for 7th for All-Time Minimums
ORH High Min Temp for Date 73, Tied 73 in 1991

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM to 5 PM EDT Tuesday for ANZ235-
     237.
     Small Craft Advisory from 2 AM to 5 PM EDT Tuesday for ANZ250-
     254.
     Small Craft Advisory from 11 PM this evening to 5 PM EDT
     Tuesday for ANZ255-256.

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...WTB/BW
SHORT TERM...EVT
LONG TERM...WTB
AVIATION...WTB/EVT
MARINE...WTB/EVT
CLIMATE...