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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 022347
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
747 PM EDT Tue Jun 2 2020

.SYNOPSIS...
Variable cloudiness with a few passing showers possible tonight
as warm front traverses Southern New England, but many will
stay dry. A weak surface low pressure may produce a period of
showers and thunderstorms Wednesday afternoon mainly south of
the Massachusetts Turnpike and southwest of Interstate 495. Dry
and warm on Thursday, then an approaching cold front will bring
a chance of showers and thunderstorms Friday, with a better
chance on Saturday. Cooler conditions follow Sunday with a risk
of a shower, then dry and seasonably warm weather early next
week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

730 PM Update...

Forecast is largely on track. Main change was to adjust the PoPs
for the next few hours as a line of showers move in from New
Hampshire into Eastern MA and RI. Even though radar is showing
some 30 dBZ reflectivity, a lot of it is falling from virga with
dew point depressions between 15 to 20 degrees, especially away
from the immediate coast. Otherwise, brought near- term
parameters more in line with current obs.

PREVIOUS DISCUSSION...

Variable cloudiness continues with less further east and more across
the interior. Sea breezes have kept nearshore areas in the low 60s,
and while it`s been a struggle to break free of mid-level clouds
from this morning across the CT Valley and western MA, much of the
interior is seeing current temps in the upper 60s to low 70s.

Synoptic feature of interest remains a WNW/ESE oriented warm front
that trails from north of the Great Lakes through central NY and the
mid-Atlantic region. This feature is forecast to progress east with
at least a greater increase in clouds as the night progresses.
Models are continuing to struggle with indicating where any
southeastward-moving showers may develop. Seems there are two real
favored general areas: one across central NY that could progress
into the CT Valley area overnight tonight, with another weaker area
of showers now across the northern Adirondacks that could bring
light showers toward the North Shore/Route 2 corridor late tonight.
There remains some weak elevated instability in forecast soundings
over eastern NY into western CT and that could lead to a rumble of
thunder perhaps into Hartford County and part of the Berkshires.
However, seems that progged most-unstable CAPE values were a little
lower and given fairly weak forcing I`ve left prospects for thunder
out of the forecast. Rising moisture levels with weaker southwest
winds may allow a window for fog or mist to develop across the South
Coast, Cape and Islands overnight into early Wednesday morning,
though visbys unlikely to reach into the dense fog level. Lows will
be several degrees warmer than last night, with values in the mid to
upper 50s.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday:

Early-morning overcast skies with areas of mist or fog along the
South Coast, Cape Cod and the Islands should give way to some
cloud breaks by mid to late morning.

We`ll then have to watch the progression/evolution of a weak mid-
level disturbance currently contributing to active convection over
western MN, which still remains source of significant uncertainty to
the forecast especially for later in the day Wednesday. Today`s 12z
synoptic models have shifted the warm front a bit further
north/east. What`s less clear is where this disturbance aloft tracks
and the northern extent of potential thunder chances. Northward
position and location of showers/thunder has been difficult to
pinpoint with this weak disturbance aloft; this feature potentially
struggling with model-gridscale convective feedback issues. Today`s
models have shifted further north and east has resulted in raising a
somewhat greater possibility of thunder a bit more, at least
from Windsor Locks to Cranston south and west, during the mid-
afternoon to early evening (approx. 2 to 7 PM). Forecast
soundings show advection of steeper mid-level lapse rates, which
range from around 6-6.5 C/km in the GFS to about 1 degree C/km
greater on the more robust NAM in the 700-500 mb layer. With
rising dewpoints, it leads to most-unstable CAPE values around
1000 J/kg across northern CT into central RI. There is also a
healthy amount of effective bulk shear into this instability
layer at around 40 kt, less so further north. But as is often
the case with these situations, both models show a fairly good
cap to convection around 800 mb that will also have to be eroded
before any storms were to get going. There are also only a few
CAMs which show any particularly robust convection into northern
CT into RI, and hardly any at all near/north of the Mass Pike.

For the mid to late afternoon hours, at the moment I do think
showers with embedded rumbles of thunder are the more likely outcome
for areas such as Hartford to Windsor Locks east-southeast, with
rain further north and east. If the capping should prove weaker or
the shortwave tracks a bit further north and more favorably through
Southern New England, a few stronger storms embedded in a larger
swath of rain could ensue, possibly capable of hail or localized
wind gusts given the shear/instability space. Given the uncertainty
described in the track of the initiating disturbance and the degree
of capping, however, how strong storms may get and how far
north/east any storms make it is something that will need to be
refined. While signals in the CAMs seem to be pointing more toward
the lower Hudson Valley/Poconos area southward for potential
stronger/severe storms (where SPC has delineated higher severe probs
in the Day-2 Outlook), did note SPC did bring lower-end severe
probabilities - 5%/Marginal Risk - northward into Litchfield and
Fairfield counties in CT. Highs in the 70s to lower 80s for many,
given southwest winds keeping sea breezes to the immediate coast.

Wednesday Night:

Any lingering showers or storms clear early Wednesday night. Frontal
boundary shifts southward with west winds under building weak high
pressure permitting drier weather. Gradually decreasing sky cover
towards mostly clear to partly cloudy by daybreak. Lows mid 50s to
lower-mid 60s.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Dry and warm Thursday
* Few showers/t-storm possible Fri; better chance t-storms Sat
* Cooler Sun with continued risk of a few showers
* Dry and seasonably warm Mon/Tue

Thursday...

Deeper moisture and higher PWATs get pushed to the south behind the
cold front. Result will be lots of sunshine and warm temps.
Soundings show a deep boundary layer to 850 mb and above. 850 mb
temps 12-14C will support highs in the low/mid 80s, except 70s along
the immediate south coast. Comfortable humidity levels with
dewpoints in the 50s, except lower 60s near the south coast.

A weak mid level shortwave approaching from the SW may bring a few
showers or an isolated t-storm Thu night, mainly south of the Pike.

Friday into Saturday...

There is some uncertainty with respect to moisture return and higher
PWATs on Fri which will have an impact on instability. Given flat
flow aloft with better forcing well back across the Gt Lakes,
thinking any convection will be widely scattered at best with very
marginal instability and weak mid level lapse rates. Most of the day
will likely be dry with partly to mostly sunny skies and very warm
temps as 850 mb temps increase to 15-16C. Highs Fri will reach
mid/upper 80s away from the south coast. More humid as well as
dewpoints climb into the 60s.

Better chance of showers/t-storms Sat as mid level trough and
attendant cold front approach from the Gt Lakes with better forcing
and modest instability. CAPES possibly increasing to 1000-2000 J/kg.
Can`t rule out a few strong storms with severe wx possible as wind
field and deep layer shear increase with the approach of the mid
level trough. More cloud cover expected Sat but highs should reach
upper 70s and lower 80s. If more sun, temps would likely get into
mid 80s.

Sunday into Tuesday...

High amplitude trough sweeps across New Eng Sun with cooler temps
and risk of a few showers. Then it looks dry and seasonable early
next week as New Eng will be under NW flow aloft and sandwiched
between strong ridge across the midwest and upper trough to the NE.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

2330z TAF Update:

Through 03z: High confidence with VFR conditions. Chance of
light showers affecting the Eastern terminals through 02z. Light
SW winds of up to 5 kt except up to 10 kt on the coast.

Later tonight: Moderate to high confidence.

Mainly VFR, but some MVFR conditions possible after midnight
mainly for CEF-BDL-BAF with possible showers. Confidence on how
far east these showers extend is fairly low and could stay far
enough west. May also see some patchy mist or fog with scattered
IFR visibility towards the Cape and the Islands as moisture
levels start to rise. But confidence in location and coverage of
fog formation is relatively low. Will have to further monitor
later tonight. Light SW winds.

Wednesday: Moderate confidence.

Any IFR-LIFR mist/fog South Coast, Cape Cod and the Islands
gives way to BKN VFR ceilings all TAFs. Where there is greater
uncertainty in the forecast is Wednesday afternoon into early
evening, with the potential for SHRA/-TSRA as a disturbance
moves through or south of Southern New England. Biggest
challenge is how far north might thunder (were any to develop)
might make it. Think best chance for thunder is mainly south and
west of the TAFs, but could make it as far north as BDL-PVD and
perhaps FMH. Best chance sub-VFR conditions in that corridor.
WSW winds of 5 to 15 knots with some gusts into the lower 20
knots possible during the afternoon.

Wednesday Night: High confidence.

Clearing to VFR early. W winds decreasing to 5-8 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF thru 03z, trends
moderate confidence overnight into Wed.

Outlook /Thursday through Sunday/...

Thursday: VFR.

Thursday Night: VFR. Slight chance SHRA.

Friday: VFR. Slight chance SHRA, isolated TSRA.

Friday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA, slight chance TSRA.

Saturday: VFR. Chance SHRA, slight chance TSRA.

Saturday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Sunday: Mainly VFR, with areas MVFR possible. Slight chance
SHRA.

&&

.MARINE...

730PM Update...

Fairly quiet weather on the waters as seas remain under 4 ft
and winds less than 20 kt. For tonight, high pressure will give
way to a weak warm front with winds becoming SW. Rising moisture
could allow for areas of fog to develop on the southern waters
overnight. Visibilities between 1-3 miles, as low as one-half
mile possible.

A stronger disturbance moving southeastward Wednesday will
allow for southwest winds to increase but still only lead to
lower-20 kt gusts. Seas may get close to SCA criteria for
southern/southeastern waters and that may need to be considered
later but confidence wasn`t high enough to hoist at the time.
Rain/embedded thunder possible from Block Island Sound eastward
to the waters south of Nantucket/MVY late in the day. Improving
conditions with lighter west winds and lower seas for Wednesday
night.

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Local rough seas.

Thursday Night through Friday: Winds less than 25 kt. Areas of
seas approaching 5 ft. Slight chance of rain showers.

Friday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers, slight chance of thunderstorms.

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, slight chance of thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers.

Sunday: Winds less than 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...High Rip Current Risk from 6 AM EDT Wednesday through
     Wednesday evening for RIZ008.
MARINE...None.

&&

$$
SYNOPSIS...KJC/Loconto
NEAR TERM...Loconto/Chai
SHORT TERM...Loconto
LONG TERM...KJC
AVIATION...KJC/Loconto
MARINE...KJC/Loconto