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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
931 am EDT Tue Jul 23 2019

another round of showers and embedded thunderstorms impact the
region this morning into early this afternoon, focused across
Rhode Island and southeast Massachusetts. Lingering showers are
expected tonight into early Wednesday across the southeast New
England coast. Mostly dry, seasonable weather late week into
the weekend, with a return of more substantial heat and humidity
early next week. Potential wash out of the mugginess late next


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

* Flash Flood Watch into early afternoon across Rhode Island/southeast Massachusetts

930 am update...

Quite an anomalous pattern for late July with high amplitude
trough across gt lakes to Tennessee Valley with tropical moisture
advecting north across southern New England with 2 inch pwats along the South
Coast. Frontal wave along the coast providing strong low level
convergence combined with upper divergence from right entrance
region of 110 kt upper jet resulting in area of heavy showers
and embedded T-storms moving east-northeast from southern CT and Li. This
activity will move across Rhode Island and southeast Massachusetts through midday. Localized
rainfall amounts of 1-2 inches/per hour is possible with
potential for localized flash flooding of urban areas.

Best instability is south of Long Island but can't rule out a
few T-storms, along the immediate South Coast and islands.
Robust low level jet providing strong low level shear so low
risk for a rotating storm, mainly over the south coastal waters
and islands.

Heavy showers will exit Rhode Island and southeast Massachusetts around midday and the
cape/islands early afternoon. However, a few spot showers will
remain possible through the afternoon. Temps in the mid 70s on
the cape/islands will drop into the upper 60s this afternoon
after the low moves by and winds shift to north. Brief wind gusts to
20-30 mph are possible over the cape/islands this afternoon.
Elsewhere, temps will remain mostly between 65 and 70 degrees
but possibly reach into the lower 70s in the CT valley by late
afternoon if there are any breaks of sun.


Short term /6 PM this evening through 6 am Wednesday/...

The cold front will be well south of our region tonight. There
will be another wave of low pressure, but this will track
further southeast then this morning/S. So the bulk of the
showers will be confined to the southeast New England coast,
particularly the cape/islands. Elsewhere, think dry weather is
on tap for tonight except for a spot shower or two. Low
temperatures will range from the upper 50s to the middle 60s.


Long term /Wednesday through Monday/...
*/ highlights ...

- coastal rains lingering Wednesday
- overall dry and seasonable late week into the weekend
- build up heat and humidity early next week
- potential late week washout of the mugginess

*/ overview ...

Brief relief, but Summer heat will return. The Four-Corners monsoon
ridge persisting as the North Pacific pattern oscillates emphasized by
positive wpo / epo / pna trends signaling an onshore fast North Pacific
jet squeezed between Alaska low north of a NE Pacific high. Diagnosing 850 mb
temperatures, return of warmer than average conditions aloft mixing
down to the surface by the end of July. Conveyor belt motions from
the mild Pacific and hot desert SW conus, trends coinciding with CPC
6-10 / 8-14 day outlooks. For now, lingering 500 mb trof deamplifying
through the week. Seasonable, comfortable conditions initially, a
gradual return of heat and humidity while monitoring for upstream
weak wave impulses through the mid-latitude near-flat flow into the
end of July. Some isolated to scattered shower chances but the over-
all pattern looking dry with the absence of more potent mid to upper
level synoptics (like that presently). Break down the details below.

*/ Details ...

Wednesday ...

Coastal rains lingering. Mid-upper trof and accompanying synoptics
slow to lift up against the North Atlantic ridge / Bermuda high. Gradual
deamplification as warmer air / higher heights prevail out ahead of
additional mid-latitude energy through an evolving flattening flow.
Chance to likely pops further southeast closer to the weak wave low along
the now offshore cold front beneath 300 mb jet dynamics aloft. Cooler
airmass aloft while drier air entrenching at the surface, lean with
seasonable temperatures around the low 80s, however 2m temperatures
expected to be somewhat dependent on cloud cover. Warmest conditions
look to occur over west areas of the CT River Valley. Light north/west winds
despite the offshore cold front, possibility of sea-breezes along
the coast.

Thursday into the weekend ...

Dry, seasonable conditions, however can't rule out an isolated spot
shower. Despite drier conditions with low to mid level low thetae,
favorable lapse rates along with very weak mid level impulses, with
anabatic flow along the high terrain, can't rule out a spot shower.
The better chance Friday and Saturday, Berkshires preferred, however
light winds, can't rule out activity along midday sea-breezes. Low
confidence as there are signals suggesting prevailing subsidence.
Heat and humidity gradually rebounding throughout as we see return S
flow late.

Early next week ...

Heat and humidity continuing. Subsidence ridge prevailing between
upstream synoptics up against tropical energy creeping north. May not be
till the end of the week that we'll wash out the mugginess with a
sweeping cold front. More potent synoptics can only be emphasized if
energy is downsheared out of Canada ahead of re-emerging synoptics
over the Gulf of Alaska. Low confidence forecast.


Aviation /13z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through tonight/ ... moderate confidence.

12z update ...

Today ...
widespread MVFR-IFR with -ra, embedded rain/+ra with possible tsra,
the greater risk over southeast-coastal terminals. With rain/+ra a greater
likelihood of IFR perhaps LIFR. Activity exits from midday into
early afternoon. Accompanying burst of north/NE winds sustained
around 10 to 15 kts with gusts up to 25 kts. Spot showers
towards evening.

perhaps some partial clearing W, however another batch of -ra is
anticipated around midnight into the early morning hours of
Wednesday. MVFR-IFR lingering especially along and southeast of bdl-lwm.
Brief west flow with -ra turning north late.

Kbos terminal...moderate confidence in taf.

Kbdl terminal...moderate confidence in taf.

Outlook /Wednesday through Saturday/ ... high confidence.

Tuesday: MVFR/IFR conditions possible. Breezy. Widespread shra,
patchy br.

Tuesday night: mainly VFR, with local IFR possible. Slight
chance rain showers.

Wednesday: VFR. Slight chance rain showers.

Wednesday night through saturday: VFR.


forecaster confidence levels...

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

Short term /through tonight/...high confidence.

northeast wind gusts of 20 to 25 knots expected behind the wave
of low pressure this morning. This will also result in 3 to 6
foot seas building across many open waters and Small Craft Advisory headlines are
in effect for many of our waters. The other issue will be heavy
showers and embedded thunderstorms through early afternoon. A
few of the storms may be strong across our southern waters and a
waterspout can not be ruled out.

NE wind gusts will diminish as pressure gradient weakens. However,
we will need to keep small craft headlines in effect for our
southern waters as a result of lingering seas.

Outlook /Wednesday through Saturday/...high confidence.

Tuesday: winds less than 25 kt. Seas up to 5 ft. Widespread
rain showers, scattered thunderstorms, patchy fog. Areas of
visibility 1 to 3 nm.

Tuesday night: winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, patchy fog.

Wednesday: winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.

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

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

Thursday night through saturday: winds less than 25 kt.


Box watches/warnings/advisories...
Massachusetts...Flash Flood Watch until 2 PM EDT this afternoon for maz017>024.
Rhode Island...Flash Flood Watch until 2 PM EDT this afternoon for riz001>008.
Marine...Small Craft Advisory until 8 PM EDT this evening for anz231-
Small Craft Advisory until 4 PM EDT this afternoon for anz233-
Small Craft Advisory until 2 am EDT Wednesday for anz235-237.
Small Craft Advisory until 2 am EDT Wednesday for anz254.
Small Craft Advisory until 8 am EDT Wednesday for anz255-256.


near term...kjc

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