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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1024 am EDT Mon Jun 18 2018

Synopsis...
excessive heat and humidity is expected this afternoon away from the
cooling marine influence of the South Coast. A cold front approaching
from the west may result in a few strong thunderstorms this afternoon
and evening...mainly across interior southern New England. A much
nicer day is on tap for Tuesday behind the cold front with plenty
of sunshine and much lower humidity. Skirting wet-weather Wednesday
followed by a period of seasonable, dry weather. Return of warmer,
muggier conditions into the weekend with the chance of showers and
thunderstorms. Perhaps a return of seasonable, dry conditions early
next week.

&&

Near term /until 6 PM this evening/...

1030 am update...

Overall trend in the weather remains on track for today and
tonight. Continue to keep an eye on the storm potential latest
12z guidance is just now coming in so will review and make any
changes if needed. Biggest thing was to increase pops as the
cams continue to show precip moving into to the region tonight.

Aside from the storms continue to monitor the heat adv. A lot of
cloud debris across the region which could actually hinder the
high temps for today. Have dropped temps down a few degrees
resulting in low to mid 90s away from the coastline. However,
still expecting high dewpoint air which will still put heat
indices in the upper 90s. Will continue to keep the adv going
for now as there is not enough confidence to lower it at this
time.

Gusty wind this morning will increase out of the southwest.
Expect gusts to near 25-30 miles per hour by the afternoon as we continue
to mix across the region.



Highlights...

* excessive heat/humidity this afternoon northwest of I-95

* isolated strong-severe T-storms with localized heavy rain possible
between 2-10 PM mainly across interior Massachusetts/CT

Details...

1) excessive heat/humidity:

Anomalous airmass was moving into southern New England early this
morning. 850t should rise to between +18c and +20c along with plenty
of sunshine into mid afternoon. This coupled with excellent mixing
on gusty southwest winds should allow high temps to exceed guidance
numbers in many locations. Highs should reach into the middle to
upper 90s to the northwest of the I-95 corridor. Dewpoints rising
well into the 60s should result in afternoon heat index values
around 100 degrees and heat advisories are posted for this region.

Meanwhile...southeast of the I-95 corridor there will be enough of a
marine influence to hold highs in the upper 80s to near 90 with
upper 70s to lower 80s on South Coast beaches. In fact...gusty
southwest winds will result in these high temperatures occurring by
early afternoon and cooling marine air will be advancing northward
later in the day.

2) isolated strong to severe weather potential:

There is uncertainty in regards to the convective potential in
southern New England today....but there is the risk for isolated
strong to severe thunderstorms mainly across interior MA/CT. Most of
the high resolution guidance develops isolated to scattered
convection after 2 or 3 PM across interior southern New England in
association with a pre-frontal trough. We may see another round of
scattered convection...perhaps with a bit more areal coverage after
7 or 8 PM in association with the surface cold front.

The biggest limiting factors are weak mid level lapse rates which
will limit updraft strength/potential and also uncertainty in
regards to the amount of forcing. However...given the anomalous
airmass in place there will be modest instability along with 0 to 6
km shear of 30 to 40 knots. This may be enough for isolated strong
to severe thunderstorms mainly across interior Massachusetts/CT with the main
threat being localized strong-damaging wind gusts. While the
tornado threat is low it can not be completely ruled out either,
because there is decent low level helicity. Pwats 3+ Standard
deviations above normal will result in the potential for any
thunderstorm to produce torrential rain and a very localized street
flooding threat. The strong to severe weather threat appears rather
low across eastern Massachusetts/Rhode Island as forcing will be slower to arrive and a
stabilizing marine layer will be working up from the south lowering
instability.

&&

Short term /6 PM this evening through 6 PM Tuesday/...
late tonight...

While the isolated severe threat should diminish by late
evening...the potential for scattered showers and embedded T-storms
will continue into much of the overnight hours. This a result of
some elevated instability coupled with a cold front moving into a
high pwat environment. We should see the scattered shower/isolated
T-storm threat confine itself to the South Coast toward daybreak.
Low temps will only fall back into the upper 60s to the lower 70s in
most locations and it will be quite muggy for most of the night. Fog
may impact the cape/islands as well given high dewpoint air in place.

Tuesday...

A much more comfortable day is in store for the region on Tuesday
behind the cold front. Much drier air should work in from the
northwest allowing for plenty of sunshine and significantly lower
humidity. High temperatures should top out in the lower to middle
80s...but it will feel refreshing after today/S heat and humidity.

&&

Long term /Tuesday night through Sunday/...
*/ highlights...

- skirting S-coastal rains Sunday night
- cool, dry late-week pattern
- return of warm, muggy conditions into the weekend
- accompanying chances of showers and thunderstorms

*/ overview...

Back and forth. Rossby wave train evident. A sharpening Aleutian 500 mb
trof, a present downstream ridge-trof-ridge pattern up against the
Baffin Bay gyre. Midweek broadening, pattern breaks, flatter, shifts
E, with Pacific energy rotating towards the gyre, S-stream energy at
times drawn N, accompanying warm, muggy air. But with re-affirmation
of the Aleutian trof, downstream pattern amplifies once again, air-
masses transition.

So a cooler, drier early week pattern beneath northwest flow, accompanying
500 mb trof, energy drawn through amplified via cyclonic digging, parent
cold front around Wednesday night ahead of which some rains could
skirt S New England. Pattern broadening, higher heights, bit more S-
stream dominance over Baffin Bay gyre, a warmer and muggier pattern
towards the weekend. If the upstream Aleutian trof sharpens, down-
stream pattern amplifying, our natural air conditioner could re-
emerge towards late June, otherwise Summer is here. Targets of
opportunity in the discussion below.

*/ Discussion...

Around Wednesday night...

Weak mid-Atlantic energy emerging as 500 mb trof digs around the Baffin
Bay gyre. Hesitant on pops given a weak low up against a dominant
Canadian high. Fast, flat flow at the base with the north-stream vort-
Max digging, left feeling rain will skirt the S-shore, remain mostly
offshore, S New England between the two waves, prevailing sinking
air, less favorable environment. Keep chance pops confined to S
waters, islands.

Thursday through Saturday...

High pressure, likely sea-breezes. Transitioning over S New England
Friday, monitoring radiational cooling, interior fog issues. Leaning
low-end of MOS guidance for nighttime lows. High pressure E, return
S flow ahead of warm front Sunday, marine stratus may return.

Sunday into Monday...

Lifting warm front followed by a weak cold frontal passage. Watching
for showers and thunderstorms. Return of warm, muggy conditions. Too
early on specifics, a low confidence forecast.

&&

Aviation /14z 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 confidence.

Today...
VFR through early afternoon. Low risk scattered MVFR-IFR along S-coast
where SW winds will be gusting upwards of 35 kts. Scattered rain showers / thunderstorms and rain
developing by 18z, mainly interior Massachusetts / CT. Rain/+ra possible along
with IFR cigs and vsbys. West g30-40 kts possible with storms as well.

Tonight...
scattered rain showers / thunderstorms and rain dissipating, as late as 3z, again mainly interior
Massachusetts / CT. Continued rain/+ra threats along with IFR cigs and vsbys.
Otherwise VFR. SW winds through early morning, low level wind shear for S-coast
along with MVFR-LIFR cigs and vsbys, before winds shift northwest by
Tuesday morning.

Tuesday...
VFR conditions expected.

Kbos terminal...
rain showers / thunderstorms and rain into terminal roughly 21z. Scattered nature, will prevail
with thunderstorms in the vicinity.

Kbdl terminal...
scattered rain showers / thunderstorms and rain into the terminal roughly 20z. Will prevail
accordingly with thunderstorms in the vicinity.

Outlook /Tuesday night through Friday/...moderate confidence.

Tuesday: VFR. Breezy. Patchy fog.

Tuesday night: VFR.

Wednesday: breezy.

Wednesday night: slight chance shra, slight chance thunderstorms and rain.

Thursday through friday:

&&

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...
southwest winds increase to between 20 and 30 knots by afternoon.
Strongest of those winds will be near shore with excellent mixing
given daytime heating...so very choppy waters to develop in harbors
and bays. Long southwest fetch will also build sees to between 3
and 6 feet.

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 Friday/...moderate confidence.

Tuesday: winds less than 25 kt. Seas locally approaching 5 ft.
Patchy fog.

Tuesday night through wednesday: winds less than 25 kt.

Wednesday night: winds less than 25 kt. Slight chance of rain
showers, slight chance of thunderstorms.

Thursday through friday: winds less than 25 kt.

&&

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.
Massachusetts...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-
010-011-013>016.
Rhode Island...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...Frank/sipprell
near term...Frank/dunten
short term...Frank

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