Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182011
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
411 PM EDT Mon Jun 18 2018

.SYNOPSIS...

Heat and humidity will come to an end tonight as a cold front from
the northwest pushes through. A few strong thunderstorms are
possible especially across western MA and northern CT. A much nicer
day is on tap for Tuesday behind the cold front with plenty of
sunshine and much lower humidity. Low pressure passes off the South
Coast Wednesday, with a chance of showers along the coast.
High pressure from Canada brings dry weather Thursday and Friday. a
cold front them brings a chance of showers over the weekend,
followed by dry weather early next week.

&&

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

This afternoon and first half of the night...

Heat...Southern New England remains on the edge of the upper level
ridge anchored over the Great Lakes. This has resulted in any cloud
debris from earlier convection to ride along the ridge and right
over southern New England. These clouds have been quite thick
especially during the heating of the day. Thus forecasted high temps
have not been realized, esp across eastern MA. Went ahead and
knocked them down a few degrees earlier today, and have brought it
more in line to ongoing conditions. Dewpoints continue to rise this
afternoon with several sites now 70F. With the warm temps and
increasing dewpoints we have seen heat index values in the low to
upper 90s across the CT River Valley, and lower Merrimack. Went ahead
and dropped Norfolk and Suffolk counties as they have been just shy
and the clouds will limit any more heating. Have seen a few 100F
heat indices across BDL and BAF so will keep heat advisory in that
region for the rest of the day. Will leave it for portions of
Middlesex and Essex as there values are on the edge.

Convection...Showers and thunderstorms continue to fire up across
northern New England and upstate NY this afternoon. Better moisture
and lift are still just outside of the region as of 300 PM. Per SPC
meso-analysis the higher K values are starting to push into the
region and with approaching pre frontal trough in MSAS MSLP should
start seeing a few storms to pop-up here in the next two hours.
Overall threat for severe weather still seems a bit marginal as the
environment is not favorable. Mid-level lapse rates are still poor
with just 5-5.5 C/km over the region with 0-6km shear near 25-35
kts. Cape values have increased to over 2000 J/kg thanks to the high
dewpoints, but these values will only last for a few more hours as
CIN will begin to mix into the atmosphere and we lose heating of the
day. Still have to watch for the occasional strong to severe storm
as directional shear is pretty good per BUFKIT sounding profile.
Best location for any severe weather will be across western MA and
northern CT which had the best heating today and highest probs from
HREF, HRRR and SREF. Gusty winds and heavy rainfall are still the
main threats for today. Cannot rule out a microburst potential, but
believe that the low level lapse rates marginal (7.5 C/km) and DCAPE
above 1000 J/kg. Heavy rainfall is most likely as PWAT values are
over 1.5-1.8 inches and will continue to increase into tonight.

Late Tonight...

Approaching cold front from the north will swing through the
overnight hours. Guidance continues to indicate that severe
potential will be less as the atmosphere will struggle to
recuperate due to ongoing/upcoming convection. However still cannot
rule out a rumble of thunder overnight as K values are still high
and showalters drop to be 0C. Still some lingering MU Cape, combined
with the high PWATs, warm cloud processes and increasing LLJ, may
have to watch for heavy rainfall. Especially as the dewpoints
continue to increase overnight and pool up against the front. Precip
should wind down after 2-5 AM due to FROPA. Still expect a muggy
night on tap as the true dry air wont work its way into the region
until Tuesday.

Aside from the precip, will have to watch for fog and stratus across
the south coast. Persistent south/southwest flow will continue
overnight and with high dewpoints over the cooler waters could
develop areas of fog. This is supported by the NARRE-TL and HRRR.
Best region for low vsbys will be the Cape and Islands, as well as
the immediate south coast.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...

Tomorrow...

A few lingering showers early Tuesday as a cold front finally pushes
offshore. Dewpoints and K values will be quickly dropping in the
late morning into afternoon and thus expect clearing skies and dry
weather. Northwest winds will be gusty in the morning before high
pressure moves into the area. Gusts to near 20-25 MPH are possible.
Otherwise a pleasant weather day.

&&

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

Longwave pattern shows above normal heights across the western and
southern USA with a persistant trough over the Great Lakes and
Northeast USA. Northern stream jet flows from far northern Canada
southeast across New England eventually moving north of us by the
weekend. A shortwave embedded in this flow moves across New England
Wed night or Thursday. A weaker southern flow across the central USA
carries a closed low from the Northern Rockies to the Western Great
Lakes. The low then ejects into the northern stream and crosses New
England during the weekend.

Mass fields over the Eastern USA are similar through Saturday, then
diverge. Thermal fields agree on cooler temps Thursday-Friday
followed by warming temps over the weekend, then trending cooler
again early next week. Forecast confidence is high through Friday,
trending to moderate over the weekend.

Concerns...

Tuesday night-Wednesday...

High pressure builds over the region Tuesday night, then shifts
offshore Wednesday. Northern stream shortwave dives southeast into
New England. This shortwave will be supported by a 105 knot jet and
drive a surface cold front toward our area. The jet may also provide
enough upper divergence and lift to support a developing low along
the Mid Atlantic coast late in the day Wednesday or Wednesday night.
W values climb to 1.5 inches along the South Coast Wednesday evening
before drier air pushes south across New England overnight.

Expect dry weather Tuesday night and Wednesday. The Mid Atlantic
system mostly passes offshore, but the upper lift may allow for rain
to graze the South Coast.

Thursday-Friday...

High pressure builds over the region with dry weather and cooler
temperatures.

Saturday through Monday...

Midwest upper low and associated surface system eject into the
Northern stream by Saturday, then cross New England Sunday. Rain out
ahead of the system arrives Saturday, with rain along the cold front
Sunday afternoon or night. PW values increase again, with 2 inch
values possible Saturday night and Sunday.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

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

Before 00z... VFR through early afternoon. Low risk SCT MVFR-
IFR along S-coast in fog/stratus. SCT SHRA / TSRA developing by
20z, mainly interior MA / CT. RA/+RA possible along with IFR
CIGs and VSBYs. W G30-40 kts possible with storms as well.

Tonight...
SCT SHRA / TSRA dissipating, as late as 3z, again mainly interior
MA / CT. Continued RA/+RA threats along with IFR CIGs and VSBYs.
Otherwise VFR. SW winds through early morning, LLWS for S-coast
along with MVFR-LIFR CIGs and VSBYs in fog and stratus, before
winds shift NW by Tuesday morning.

Tuesday...
VFR conditions expected. Breezy out of the NW.

KBOS Terminal...
SHRA / TSRA into terminal roughly 22z. Showers thereafter.

KBDL Terminal...
SCT SHRA / TSRA into the terminal roughly 21z. Showers
thereafter.

Outlook /Tuesday Night through Saturday/...Moderate confidence.

Tuesday Night through Wednesday: VFR.

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

Thursday through 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.

Tonight... Near shore southwest wind gusts should diminish a
bit by late evening...but marginal small craft wind gusts and 3
to 6 foot seas should persist longer over the open waters. An
isolated t-storm or two is possible tonight.

Tuesday...
Lingering small craft swell across our southern most outer-waters
diminishes by late morning. Otherwise...winds/seas should remain
below small craft advisory thresholds.

Outlook /Tuesday Night through Saturday/...Moderate confidence.

Tuesday Night: Winds less than 25 kt. Seas less than 5 feet. Local
visibility 1 to 3 nm in patchy fog.

Wednesday: Winds less than 25 kt. Seas less than 5 feet.

Wednesday Night: Winds less than 25 kt. Seas less than 5 feet. Chance
of rain showers.

Thursday: Winds less than 25 kt. Seas less than 5 feet. Slight
chance of rain showers.

Thursday Night through Friday: Winds less than 25 kt. Seas less
than 5 feet.

&&

.CLIMATE...
Here are the record and forecast highs for Monday June 18...

BOS ... 94 (1907 & 1929) ... Forecast High 95
BDL ... 95 (1957 & 1994) ... Forecast High 96
PVD ... 94 (1929)        ... Forecast High 88
ORH ... 93 (1907 & 1929) ... Forecast High 91

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 6 PM EDT this evening for CTZ002.
     Air Quality Alert until 10 PM EDT this evening for CTZ002>004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ003>007-
     009>016-026.
     Heat Advisory until 6 PM EDT this evening for MAZ003-005-006-
     010-011.
RI...None.
MARINE...Small Craft Advisory until 10 PM EDT this evening for
     ANZ230>237-251.
     Small Craft Advisory until 4 AM EDT Tuesday for ANZ250.
     Small Craft Advisory until 10 AM EDT Tuesday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Dunten
NEAR TERM...Dunten
SHORT TERM...Dunten
LONG TERM...WTB
AVIATION...WTB/Dunten
MARINE...WTB/Dunten
CLIMATE...Staff


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