Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KBOX 240241
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
941 PM EST Thu Nov 23 2017

.SYNOPSIS...
Quiet weather through Friday with dry and cool conditions.
Milder air returns Saturday ahead of a cold front. That cold
front swings through Southern New England late Saturday and
early Saturday night. Blustery and cooler weather follows
Sunday. Conditions trend toward dry and milder again Tuesday
and Wednesday, and then cool again Thursday behind another cold
front.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
940 PM update...

Satellite data continues to indicate some high clouds moving
across New England. Thus far, these clouds have not prevented
temperatures from falling quickly this evening, especially
under an area of clouds across RI and eastern MA. Clouds just
moving into western MA appears to be slightly lower in height
and thicker. These clouds should impact temperatures as they
pass overhead.

No precipitation upstream. So the main adjustment was to tweak
temperatures based on observed and expected trends. 24/01Z LAMP
guidance looked to have the best handle, so used that as the
basis for this forecast update.

Previous discussion...

Scattered to broken mid to high cloud decks filtering across
the region could put a blanket on Thanksgiving festivities
keeping temperatures slightly warmer. Positively- tilted open
wave and attendant weak vortmax sweeps through the region early
across the N-stream, a flattening feature, as energy sweeps up
from the SE. Mid to high clouds sweeping through however into a
region of drier air and subsidence, can already see some erosion
in the latest GOES-16 satellite. Perhaps some blanket /
insolation impacts so did not go with coldest of MOS guidance.
Towards the lower end with forecast light winds, looking at lows
down into the 20s, warmer along the shore.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Nice day on tap. With daytime mixing under mostly clear conditions,
the mid to high clouds eroding out, winds turn S/SW. Warm air
advection proceeding aloft, mixing is limited to around H95.
Looking at highs in the upper 40s to low 50s, edging towards the
warmer end of guidance.

With both N and S stream energy still neighboring our region,
dry subsidence inversion prevails. As we go towards evening, the
cap well in place, mixing up to just beneath, and the boundary
layer cools, likely to see the moisture pooling condense and
subsequently could see the development of low-level stratus
evening and overnight especially over SE New England. Lows down
into the 30s most places, but warmer SE around the low 40s. With
daytime mixing, dewpoints should be low enough that 5 to 10
degree dewpoint depressions prevail. So despite the overnight
N/W looking favorable for radiational cooling, do not expect fog
to be an issue.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Big Picture...

Longwave scale shows a broad trough along the USA East Coast and
West Atlantic, a second trough over the Eastern Pacific, and a ridge
over the the Plains. The Eastern trough lingers much of the week
while the overall flow trends zonal.

Model consensus shows northern stream shortwaves crossing New
England Saturday night/Sunday and again Wednesday. The models also
show a southern stream low crossing the Southeast USA Thursday
trailing the Wednesday northern shortwave.

The mass and thermal fields are similar through Mon night/Tuesday,
but then diverge by showing different timing of the northern and
southern stream features. The feature that becomes the northern and
southern shortwaves next week is currently over the Aleutians and
diving south over the Pacific Ocean...and it reaches the USA West
Coast by Monday. Expect changeable solutions regarding this feature
until that time.

Contour heights and thermal fields are a little above normal
Saturday, trend colder than normal Sunday-Monday, then trend above
normal midweek next.

Details...

Saturday...

Low pressure passes well offshore. General agreement that the
southern shortwave remains unphased with the northern trough.
Meanwhile, the northern trough doesn`t dig much until the offshore
low is moving past us. So expect the precipitation to remain over
the ocean.

Cold front approaches during the afternoon and passes through
Southern New England during the evening/early night. Moisture is
limited below 700 mb much of the time, but runs deep briefly during
the evening. Precipitable water values climb to about 0.85 inches
during the evening, which is above normal for late November. We will
show low-end chance pops during the late afternoon/evening.

Temps in the mixed layer are forecast at 850-mb equiv of -1C to -4C,
so expect max sfc temps in the 50s.  Behind the front, the airmass
cools and dries slowly, so min temps at night should only fall to
the 30s and low 40s.

Sunday-Monday...

Upper trough digs over New England Sunday. Cold pool will be in
place with 500-mb temps at -28C working to destabilize the airmass.
Moisture lingers below 800 mb during this time.  Expect quite a few
clouds to pop during the day, but moisture is too limited for
showers. Mixed layer temps will support max sfc temps in the upper
30s and 40s.

The trough moves off on Monday and high pressure builds surface and
aloft. Expect a fair day with max sfc temps in the 40s.

Tuesday through Thursday...

Another cold front crosses us midweek, driven by the northern stream
shortwave. As noted above, there are timing uncertainties among the
models, so we used a blend. This favors a Wednesday passage at this
time, but the uncertainties noted above may change that exact
timing. This will need to be monitored.  The blend of model data
supports slight chance pops Wednesday with the cold fropa and mainly
dry Thursday.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...High confidence.

VFR. W/NW winds diminish overnight, turning S/SW into Friday.
BKN-OVC MVFR CIGs possible for SE New England into Friday night.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: Mainly VFR, with local MVFR
possible. Breezy. Slight chance SHRA.

Sunday: VFR. Windy with local gusts to 30 kt.

Sunday Night through Tuesday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Small Craft Advisories adjusted to account for seas. Should
conclude toward daybreak Friday morning. High pressure mainly
in control. Good boating weather as swells diminish.

Outlook /Saturday through Tuesday/...

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

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of seas approaching 5 ft.

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

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas.

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 7 AM EST Friday
     for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Belk/Sipprell
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell
MARINE...WTB/Belk/Sipprell


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.