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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1248 am EDT sun Aug 18 2019

Synopsis...
very warm and humid conditions Sunday with an increasing risk
for some showers and thunderstorms. These conditions continue
through midweek, when a cold front will sweep through. Drier and
not as warm late next week.

&&

Near term /until 6 am this morning/...
1 am update:

Showers/storms have subsided but still enough MUCAPE to allow
for leftover showers in parts of CT and NE MA, which should
continue to diminish overnight.

Plenty of low level moisture combined with light winds has
allowed areas of dense fog to develop, mainly in parts of south
coastal Rhode Island and around the islands. May need to issue a dense fog
advisory with next update around 4 am. So far webcams show it's
not too widespread but will need to keep an eye on that.

&&

Short term /6 am this morning through 6 PM Monday/...
Sunday...
warm sector airmass across southern New England with boundary layer winds
turning SW. This will result in low clouds and patchy fog
burning off and giving way to at least partial sunshine. 850 mb
temps 17-18c which normally supports temps into the lower 90s,
but mixing depth may fall short of 850 mb so will go a bit
cooler with highs mid/upper 80s with a few locations in CT
valley possibly reaching 90. Cooler along the coast where sea
breezes develop. Quite humid airmass with dewpoints into the
lower 70s.

Moderate instability develops in the afternoon with CAPES
1500-2000 j/kg. While synoptic forcing is limited, enough
instability to support scattered showers/T-storms in the
afternoon, mainly interior where best instability is located.

Sunday night...
expect scattered showers/T-storms to linger into the evening as
decent instability persists within a well defined Theta-E ridge.
Then convection diminishing overnight. Another warm and humid
night with lows upper 60s to lower 70s. Patchy low clouds and
fog may redevelop.

&&

Long term /Monday night through Saturday/...
guidance shows ridging over the southern USA, shifting to the SW US
during Wed-Fri as a Hudson Bay trough deepens and extends southward
into the eastern U.S. Southern New England remains under the influence of decent mid-
level flow, steering a series of shortwaves in our direction thru
mid-week. Exact timing is uncertain on the shortwaves passing thru,
but with increased moisture, each will have the ability to produce
some showers and possibly some thunderstorms to our area.

A cold front pushes south towards/into our area Monday
night/Tuesday. However there is model uncertainty on whether or not
the front pushes south of sne, at least for a period of time, or if
it stalls over our area. If the 12z NAM solution pans out, we could
get a brief reprieve on shower potential during Tuesday. However
because of the uncertainty, we will have a continued risk for
showers and thunderstorms during this timeframe. The front should
move back north of our area as a warm front during Tuesday
night/Wednesday, which will keep or bring back an unstable airmass
over our area.

Then as the upper trough deepens and pushes short wave energy thru
the eastern Great Lakes region, a cold front will pass thru our
area. The cold frontal passage should pass thru our area during Thu.
The approach and passage of this front will likely bring our
greatest chance for showers and thunderstorms for the week.

High pressure builds in from the west for Fri-Sat. The base of the
upper trough may lag behind, which could keep a cold pool over New
England on Friday. With much drier air in place, model quantitative precipitation forecast is
lacking, and for now will continue with this dry forecast. By
Saturday the upper trough should be to our east, with dry conditions
prevailing.

Until the cold front pushes east of our region on Thu, it will be
very warm and humid. Dewpoints will remain in the mid 60s to low
70s, with relief from the humidity Thu-Fri. With temperatures rising
to around 90 degrees early next week, we may need heat advisories
for a time.

&&

Aviation /06z Sunday through Thursday/...
forecaster confidence levels...

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

Short term /through Monday/...moderate confidence.

Widespread IFR/LIFR through sunrise in low clouds and fog.
Conditions improve to VFR 12z-15z except closer to 18z on Cape
Cod and islands. Scattered showers and thunderstorms develop
during afternoon, mainly along and northwest of a line from bdl-orh-
lwm.

Return to IFR/LIFR in low clouds and fog tonight, especially in
Rhode Island and east MA, as showers/storms subside. Repeat for Monday with
improvement to VFR by 15z-18z then another round of scattered
showers and thunderstorms.

Kbos terminal...moderate confidence in taf. Conditions should
lower to LIFR for a few hours before sunrise.

Kbdl terminal...moderate confidence in taf. Timing of IFR
conditions may be too soon.

Outlook /Monday night through Thursday/...moderate confidence.

Monday night: VFR. Slight chance shra, isolated thunderstorms and rain.

Tuesday: VFR. Chance shra, slight chance thunderstorms and rain.

Tuesday night: mainly VFR, with local IFR possible. Chance
shra, slight chance tsra, patchy br.

Wednesday: VFR. Chance shra, slight chance thunderstorms and rain.

Wednesday night: mainly VFR, with local IFR possible. Chance
shra, slight chance tsra, patchy br.

Thursday: mainly VFR, with local MVFR possible. Chance shra,
slight chance thunderstorms and rain.

&&

Marine...
forecaster confidence levels...

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

Short term /through Monday/...high confidence.

Primary marine concern will be areas of fog and poor vsbys
tonight into Sun morning and and possibly another round Sun
night. Generally light winds and seas through the period.

Outlook /Monday night through Thursday/...moderate confidence.

Monday night: winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers, isolated thunderstorms.

Tuesday: winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers, isolated thunderstorms.

Tuesday night: winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, slight chance of thunderstorms.

Wednesday through Wednesday night: winds less than 25 kt. Areas
of seas approaching 5 ft. Chance of rain showers, slight chance
of thunderstorms.

Thursday: winds less than 25 kt. Local rough seas. Chance of
rain showers, slight chance of thunderstorms.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...kjc/nmb

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