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Office: BOX

FXUS61 KBOX 210733

Area Forecast Discussion
National Weather Service Boston/Norton MA
333 AM EDT Tue Aug 21 2018

High pressure over the Canadian Maritimes will result in another day
of mainly dry and pleasant weather. A low pressure system will bring
two rounds of scattered showers and embedded thunderstorms, first
late tonight into Wednesday morning then a second round Wednesday
afternoon along with the return of high humidity. High pressure will
then follow bringing sunny skies and comfortable humidity Thursday
and Friday. The high moves off the coast next weekend but still will
provide mainly dry weather.



330 AM update...

1020 mb maritime high pressure remains anchored into New England
providing another day of stable relatively cool maritime air over
the region. Low clouds will dominate the morning but model low level
RH fields suggest clouds will lift and thin a bit to provide some
breaks of afternoon sunshine. Given subsidence inversion will limit
mixing, shallow cool maritime airmass will be difficult to mix out
and modify. Thus cooler than normal temps today with highs
restricted to about 70-75, except upper 70s across the CT river
valley given increasing distance from the cooling northeast flow off
the ocean. These temps are cooler than normal as the average high
for late August is around 80 degs. Dry weather prevails today the
only exception may be brief spotty drizzle this morning near the
coastline of southeast MA especially Cape Cod and the Islands.



* A few strong to severe storms possible late Tonight/Wed
* Localized street flooding possible too


Dry weather lingers this evening in response to slowly departing
maritime high pressure. However second half of the night low
pressure tracks thru the St Lawrence River Valley with attending
warm front approaching from the southwest. Very strong moisture
advection into southern New England with K indices rising into the
mid 30s and PWATs jumping to over 2 inches by 12z Wed per 00Z
NAM/GFS/GEFS and 12z EC! Good synoptic scale forcing as well as seen
via mid to upper level Qvector convergence, in addition to RRQ of
upper level jet streak approaching the area. Thus combination of
strong moisture advection and synoptic scale forcing supports
widespread rain with locally heavy downpours and possible
urban/street flooding which could impact the morning commute.

Other concern is as warm front approaches low level shear will be
increasing as strengthening low level jet moves across the area.
This increasing low level shear vicinity of approaching warm front
combined with high surface dew pts (rising into the low 70s south
coast) and low LCLs there is a concern for rotating T-storms.
However one limiting factor regarding severe weather is marginal
instability with all guidance sources limiting MU Capes to less than
500 j/kg by 12z Wed. However will just have to watch how this system
evolves. As of now forecast confidence is higher regarding heavy
rain/flood threat than severe storms.


After morning threat of heavy rain/flooding and a few strong to
severe storms associated with warm front exits the area, focus then
shifts on potential afternoon convection associated with approaching
cold front. This portion of the forecast will hinge on how quickly
clouds can give way to breaks of sunshine for atmosphere to
destabilize. All model guidance including 00z EC & GEFS offering
1000-2000 j/kg of CAPE during the afternoon. This instability
combined with around 30-35 kts of deep layer shear and modest
synoptic scale lift from frontal convergence and RRQ of upper level
jet streak, should be more than sufficient for a round of afternoon
convection including the risk for a few strong to severe storms. As
previous forecaster mentioned main concerns will be for strong to
damaging winds in the strongest storms. Collaborated this thinking
with SPC this morning and both of us agreed to increase general T-
storm forecast for SNE to marginal risk Wed.

Otherwise expect a summer-like day with heat and humidity as warm
sector airmass overspreads the region yielding highs in the low 80s
and dew pts in the low to mid 70s!

Also front may be progressive enough that sunshine makes a return
late in the day especially across CT and western-central MA.



2 AM update...

*/ Highlights...

 - Dry, comfortable weather Thursday into Friday
 - Return warmer, humid conditions beginning over the weekend
 - Accompanying chances of showers and thunderstorms

*/ Discussion...

Warmer than average temperatures forecast towards September. Latent
heat release per Soulik and Cimaron, becoming baroclinic, amplified
N Pacific standing wave pattern with H5 Bering Sea trof, NE Pacific
ridge, and subsequent W CONUS trof. The latter, an associated anti-
cyclonic Rossby wave break and attendant PV-streamer influencing
continued organized tropical convection out towards Hawaii. This and
upstream trends as seemingly the main Polar low retrogrades back N,
H5 ridging builds and is maintained over the SE CONUS extending into
New England into the end of the month per SW-NE pump. Ensemble means
signaling anomalous H85 warmth, CPC 6-10 and 8-14 day temperature
outlooks with high confidence noting warmer than average conditions.
Can't rule out dips within aforementioned SW-NE CONUS flow, trailing
cold fronts and chances of showers / thunderstorms, however becoming
diffuse up against the H5 ridge as associated mid-level energy lifts
N/E. It's back to N/W New England scattered wet-weather activity
that falls apart before ever reaching the S/E coast.

Low confidence forecast further out in time, not noting any targets
of opportunity. Overall, looking dry and comfortable Thursday into
Friday, followed by the return of warmer, more humid conditions for
the weekend into next week. Accompanying chances of showers and
thunderstorms give increasing instability. Too early to say with any
certainty as to strong / severe storm threats as well as any flash
flooding potential, however such risks are being considered given
antecedant conditions in addition to past storm trends / outcomes.


Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

2 AM update...

Thru 12z...only moderate confidence given some uncertainty on
northwest extent of MVFR-IFR conditions.

Area of MVFR/IFR cigs over southeast MA will slowly drift north
and west into the KBOS-KPVD corridor later this morning. Could
be some spotty drizzle associated with this low level moisture.
Otherwise VFR elsewhere thru 12z.

After 12z...moderate confidence given some uncertainty how
quickly MVFR-IFR conditions across RI and eastern MA lift along
with areal extent of these clouds.

MVFR-IFR cigs across RI and eastern MA this morning should lift
to VFR-MVFR by afternoon but some uncertainty how quickly this
occurs along with areal coverage. Mainly dry day with the
exception of spotty light drizzel this morning over Cape Cod and
Islands. Farther inland any MVFR cigs this morning will lift to
VFR along with dry conditions.

Tonight...moderate confidence.

VFR-MVFR to start the evening then MVFR-IFR becoming widespread
after midnight with rain and embedded T-storms developing late
and heavy downpours possible toward morning. Low probability of
local LIFR inland in developing fog and rain. Marginal low
level wind shear possible late as warm front approaches.

Wednesday...moderate confidence.

Morning warm front yields widespread IFR with possible LIFR in
fog, showers and embedded T-storm with locally heavy rainfall.
Improving to MVFR-VFR in the afternoon however a second round of
T-storms possible especially inland with strong winds

KBOS Terminal...some uncertainty on duration of MVFR cigs this
morning before lifting to VFR this afternoon.

KBDL Terminal...Moderate to high confidence in TAF.

Outlook /Wednesday Night through Saturday/...



Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Today...Modest NE flow 10-15 kt with a few gusts up to 25 kt.
Spotty drizzle and fog possible this morning but improving by
mid morning.

Tonight...warm front approaches from the south so our winds
become southeast. East winds shift to the southeast 15-20 kt.
A few strong thunderstorms are possible toward daybreak with
even the low risk for a brief waterspout


Warm front lifts thru the waters with winds becoming SSW. Vsby
lowered at times in morning fog and scattered T-storm then
improving before second round of T-storms in the afternoon.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt. Local rough seas.

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

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

Friday through Saturday: Winds less than 25 kt.




NEAR TERM...Nocera
LONG TERM...Sipprell