NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Sep 29, 8:25 AM EST, 4 hours earlier.
Area forecast discussion, Sep 29, 8:25 AM EST, 4 hours before
Area Forecast Discussion
National Weather Service Boston/Norton MA
925 AM EDT Sat Sep 29 2018
.SYNOPSIS...
A dry cold front moves through today as high pressure building
in for the weekend. This brings with it dry and cooler weather.
As the high moves offshore Monday, a warm front spreads east
from the Plains...then lifts north of our area Monday night. The
front will move back and forth across our area through the
week.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
925 AM update...
Stratus and patchy fog across SE MA and portions of CT valley
is gradually eroding, otherwise expect some diurnal cu to
develop given shallow layer of low level moisture in place.
Overall a mostly sunny afternoon. High pres builds across the
Gt Lakes with light W/NW flow, except becoming SW along the
immediate south coast. No change to forecast temps, with highs
upper 60s to lower 70s.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Saturday night will be colder, low-mid 40s, as high pressure
dominates giving us clear skies and an opportunity for efficient
radiational cooling. Flow remains zonal in the mid-levels Sunday
with the surface high shifting over southern New England. It
will be dry with sun and increasing clouds through the day as
the high moves east and winds turn out of the southwest. This
will eventually draw a weak warm front across the region.
Models aren't in great agreement on exact timing but sometime
late Sunday evening/night winds shift out of the southwest
bringing warmer air and more moisture back to the region. Light
winds should allow a seabreeze to set up during the day, with
high temperatures reaching the mid to upper 60s.
&&
.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
*/ Highlights...
- Sweeping frontal boundaries, wet-weather chances, flip-
flopping temperature pattern
*/ Overview...
Tumultuous pattern. Earlier September, N American progressive flow
yielded a N Europe / N Asia traffic jam allowing slow maturation of
storm systems exchanging energy into the Arctic. Meanwhile over the
N Pacific, an anti-cyclonic wave break progressing E, deamplifying,
left a series of cut-off lows that with upstream H5 trof interaction
allowed a NE Pacific rex-block / slowing through the longwave flow
resulting in buckling and pattern amplification. Altogether, with
amplification, a surging Bering Straight H5 ridge, downstream trof
digging, aforementioned cross-polar energy from N Europe / N Asia
pulled in, two points of E hemispheric anomalously warm H85 air are
surging into the Arctic resulting in a S surge of colder air within
the broader trof regime over the W hemisphere / N America.
That being the present synoptic setup, where do we go from here? Per
consensus of deterministic and ensemble forecasts, continued thermal
wind response with warmer air surging into the Arctic should yield
continued H5 trof promotion S into the SW CONUS capturing lingering
energy over the NE Pacific (i.e., aforementioned cut-off lows). The
subsequent downstream H5 ridging as the upstream Pacific pattern
flattens, the N Pacific ridge and very anomalous warm surge cutting
off into the Arctic, could be looking at a mild period over the NE
CONUS for early October.
But a potential fly in the ointment. SW CONUS H5 trof promotion, SW
to NE flow downstream, N Atlantic flow back to being progressive as
Arctic energy becomes bottled up, will be watching cascading energy
thru the week with low centers to our N, trailing frontal boundaries
across our region. Potential wet weather followed by Canadian highs
skirting by, could see just brief periods of warmer weather ahead of
cold frontal boundaries followed by shallow N / NE flow prevailing
as Canadian high pressure follows keeping warmer air to our S.
Significant spread in 2m temperature forecasts beginning October 1st
per ensemble members, more significant throughout the week as likely
forecast models struggle with surface wind outcomes and locality of
airmasses, whether clouds / drizzle with NE flow become an issue.
Will hit the main points and forecast thinking below.
* / Discussion...
Sunday night into Monday...
Focusing warm front, weak isentropic ascent, potential for showery
weather. Stalling W-E across the region, but where? 2m temperature
implications, 29.0z NAM warmest, conceding cooler guidance thinking
an abundance of clouds at least beneath SW capping warmer air. Have
chance PoPs. Nothing of a washout.
Tuesday and Tuesday night...
Warm front lifts seemingly kinked along E waters with SSTs in the
low 60s. SW interior flow, warm sector, warm air advection aloft
limiting mixing to H9 where temperatures flutter around +15C. Low
70s seem certain, add ample sunshine, talking highs around mid 70s.
Rain chances late, overnight, stronger synoptics over N New England
likely will yield showers, low threat of thunderstorms, nothing of a
washout, less heavy rain threat given less lift / forcing.
Wednesday into Wednesday night...
GFS / EC differ in progression of mid-level H5 ridging, influential
surface high pressure shunting the cold front offshore. However both
exhibit a dry inversion beneath which surface NE flow prevails. May
see moisture pool around H9 that with the slightest lift could net
drizzle conditions. May be too generous with temperatures and lesser
cloud cover, cloudy and cooler conditions perhaps prevailing.
Thursday into Thursday night...
If we can open up into the warm sector, could warm up appreciably.
Seemingly katafrontal, question as to whether W overspreading clouds
will impede daytime heating. Confident late sweeping cold front will
have little outcomes with synoptic forcing well removed N/E, flow
along the front mainly parallel. Chance PoPs.
Friday onward...
Potential repeating pattern. High sliding N, onshore cooler flow,
approaching warm front kinked at first, low clouds / drizzle impacts
possible prior to an advecting warm sector before another sweeping
cold front over the weekend.
&&
.AVIATION /13Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Sunday/...Moderate confidence.
Today...
Leftover stratus and patchy fog across SE MA and CT valley will
burn off 14-15z, otherwise VFR. Light NW winds prevailing, but
more SW along S-coastal terminals w
Tonight...
Mainly VFR with light N winds, however can't rule out some
patchy MVFR-IFR to develop again across the interior.
Sunday...
VFR with light winds starting north/northeast, becoming
southwest in the afternoon. E/SE sea breezes likely for
E-coastal terminals around 17z.
KBOS Terminal...Moderate confidence. VFR conditions expected
with west winds becoming northwest in the afternoon.
KBDL Terminal...Moderate confidence. Brief IFR/MVFR conditions
this morning with fog, becoming VFR by early morning.
Outlook /Sunday Night through Wednesday/...Moderate Confidence.
Sunday through Sunday Night: VFR.
Monday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.
Monday Night: Mainly VFR, with areas IFR possible. Slight
chance SHRA.
Tuesday: Mainly VFR, with areas MVFR possible. Windy with local
gusts to 30 kt. Chance SHRA, slight chance TSRA.
Tuesday Night: MVFR/IFR conditions possible. Breezy. Chance
SHRA.
Wednesday: Mainly VFR, with local IFR possible.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Sunday/...
Winds remain lower as the pressure gradient decreases. Wind
speeds should generally remain less than 25 kt, though seas
will take longer to subside across the outer coastal waters.
A Small Craft Advisory remains in effect through Sunday for the
eastern coastal waters for seas of 5 ft.
Outlook /Sunday Night through Wednesday/...Moderate Confidence.
Sunday through Monday Night: Winds less than 25 kt. Areas of
seas approaching 5 ft.
Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Slight chance of rain showers,
slight chance of thunderstorms.
Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance
of rain showers.
Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT Sunday for ANZ250-254.
Small Craft Advisory until 6 PM EDT this evening for ANZ255.
&&
SYNOPSIS...Sipprell/BW
NEAR TERM...KJC/BW
SHORT TERM...BW
LONG TERM...Sipprell
AVIATION...Sipprell/BW
MARINE...Sipprell/BW