NWS Forecaster Discussion
BOX
Area Forecast Discussion National Weather Service Boston/Norton MA.
000
FXUS61 KBOX 210352
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
1052 PM EST Wed Feb 20 2019
.SYNOPSIS...
Strong high pressure moves off to the east tonight. Low
pressure from the Western Gulf of Mexico will redevelop along
the East Coast tonight and move across New England early
Thursday. This will bring a period of snow and ice, changing to
rain along the coast. High pressure follows with drier air
Thursday afternoon through Saturday. Another round of a wintry
mix of precipitation is possible Saturday night into Sunday,
followed by dry and seasonable conditions next Monday and
Tuesday.
&&
.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
1052 PM Update...
Steadier wintry precipitation will continue to overspread
southern New England overnight. Have already observed a switch
to freezing rain, then back to snow at KBDL, and other locations
across northern CT. Looking at the 21/00Z KALY and KOKX
soundings, the most likely reason for this is a relatively dry
dendritic snow growth region. Where there is just enough lift
and moisture in this region, snow makes it to the ground.
Otherwise, freezing rain.
KBOX correlation coefficient data placed the melting layer has
pretty much reached the south coast of MA and RI, as well as
moved into northern CT. Still expecting a transition to freezing
rain and rain after midnight. Based on observed soundings, am
slightly less concerned about sleet.Will continue to monitor
trends.
Previous forecast pretty much on track, but have updated to
bring near term conditions current with latest observations.
Previous Discussion...
Broad upper ridge and surface high pressure are moving east of
New England. Shortwave moving out of the Plains will cross the
Great Lakes tonight and Northern New England Thursday. Our prime
concern for tonight is a coastal secondary low developing off
North Carolina. This will move up the coast tonight and pass off
the Southern New England coastline roughly 09Z to 12Z Thu.
Warmer air aloft is also moving up the coast. Temps above
freezing air at 850 mb should reach the Mass Pike around 1 AM
and the NH border around 4 AM. SE winds should bring surface
temps above freezing along the coast and the southeast interior
during the overnight. Weak cold air damming over western and
central sections may hold in surface cold air through the night.
Because of this, we expect a variety of wintry weather during
the night. Snow all areas will mix/change to sleet and freezing
rain during the middle of the night, then coastal and some
southern areas will change to rain between midnight and sunrise.
Saturation in the prime growth zone aloft will be limited to a
couple of hours, so accumulations will be limited. Best
potential for snow is one to two inches. Freezing precipitation
overnight will also bring a layer of ice, but this will be
roughly one-tenth inch or less.
The ice that follows the snow will be the more important threat
in creating slippery spots on untreated surfaces. The Winter
Weather Advisory continues through the night.
&&
.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Thursday...
Coastal low pressure moves off to the east in the morning.
Lingering light rain/drizzle is possible at the start,
especially along Eastern Mass and RI. Drier air moves in later
in the morning and afternoon. and should eventually eat away the
low moisture and clouds.
Clearing skies will allow daytime heating, with temperatures in
the mixed layer favoring max sfc temps in the mid 40s to low
50s. Winds in the mixed layer will be near 30 mph, so expect
gusts in that vicinity during the afternoon.
Thursday night...
High pressure builds over the Great Lakes, setting up a
northwest flow over New England. This should maintain dry
weather as well as a return of the colder air, with overnight
min temps in the 20s and lower 30s.
&&
.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
* Dry and seasonable conditions Friday into Saturday
* Another round of wintry weather early Saturday night,
changing to rain by mid morning Sunday
* Dry and near seasonable conditions return Mon/Tue, with
potential for strong winds Mon into Mon night
* Another weather system may approach Tue night/Wed
Overview...
12Z model suite continues to signal amplified pattern
continuing especially across the western U.S., with H5 long
wave trough around the eastern Rockies into western Plains
states. Downstream nearly zonal flow exiting through the Great
Lakes to New England, while subtropical ridge extends from the
eastern Gulf of Mexico to the southwest Atlc. This will keep an
active, progressive storm track across the N central and NE U.S.
through at least the upcoming weekend.
Models and ensembles signaling that the western H5 trough may
break down during late Fri and the upcoming weekend as a
leftover progressive cutoff low moves in the northern stream
flow. It appears the system will merge with another cutoff
system across N central and eastern Canada. By Sun night or Mon,
looks like a nearly zonal flow sets up across most of the lower
48, though some broad mid level troughing across the northeast
and lowering H5 heights will bring below normal temps by the
middle of next week.
Another system may push eastward in the northern stream flow
around the Tue night/Wed timeframe, but individual model
members showing quite a wide solution spread so a lot of
uncertainty with the timing and track of this potential
system.
Details...
Friday through Saturday...
High pressure builds E out of the Great Lakes Friday, crossing
the region on Saturday. With the general W wind flow in place,
expect temps to reach close to seasonal levels both days.
As the center of the high crosses New England Saturday, may
start to see mid and high clouds increase from S to N during the
afternoon.
Saturday night through Sunday night...
Strengthening low pres, lowering to about 975 to 980 hPa by
Sunday morning, will move across the central Great Lakes into
western Quebec. Frontal system extends to the S and E of this
low, with another low forming on the front. As this frontal low
pushes across the region, will see enhanced SE wind flow,
bringing influx of low level moisture which will combine with
PWAT plume working across the region (on the order of 2-3 SD
above normal as seen on 12Z GEFS forecast). So, have forecasted
QPF amounts from around 0.6 to 1 inch, highest along S coastal
areas by the time it tapers off Sun evening.
Expect a mix of snow, sleet, freezing rain and even rain across
the coastal plain at the onset of the precip Sat evening, then
should gradually change to rain from S-N mainly around or after
midnight. So, could see upwards to an inch of snow across NE and
N central Mass, but more problematic could be some icing across
the east slopes of the Berkshires into the northern Worcester
hills before the changeover.
May see leftover light rain across N and E Mass Sun evening,
which could change to snow across the inland higher terrain
overnight.
Temps will recover to the mid 40s to around 50 as the precip
ends on Sunday. Westerly winds will start to increase, becoming
gusty along the coast as higher terrain as strong low level jet
with excellent low level mixing (with LL lapse rates on order
of 9C/km or greater through H95-H85) crossing the region.
Monday and Tuesday...
As the strong low spins across the Maritimes, excellent low
level mixing continues on Monday, with low level jet up to 50-55
kt! So, could strong gusts could be realized with the continued
strong low level mixing. Could see gusts up to 35-45 kt, highest
across the higher terrain and along the coast through Monday,
slowly diminishing Monday night as the pres gradient finally
relaxes from the Maritime low. Wind headlines may be needed.
Tuesday night and Wednesday...
Rather high uncertainty on approach of another fast moving system
during this timeframe. May see another round of snow move in Tue
night, with possible mix or change to rain across southern
portions of the region during Wed.
&&
.AVIATION /04Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Thursday Night/...High confidence.
Tonight...
Mainly MVFR-IFR late this evening. Warmer air aloft will change
the snow to sleet and freezing rain between 9 PM and Midnight.
Warmer air moving in at the surface across coastal terminals on
southeast winds will change the precip to rain roughly 3 AM to 6
AM.
Thursday...
Coastal storm moves off to the east. Lingering low clouds and
drizzle during the morning, but this should diminish as drier
air moves in on increasing west winds. Becoming VFR by
afternoon.
Thursday night...
VFR. Breezy west to northwest winds.
KBOS Terminal...High confidence in TAF trends. Moderate
confidence in timing precipitation type changes.
KBDL Terminal...High confidence in TAF trends. Moderate
confidence in timing precipitation type changes.
Outlook /Friday through Monday/...
Friday: VFR. Breezy.
Friday Night through Saturday: VFR.
Saturday Night: Mainly MVFR, with local IFR possible. Breezy.
RA, FZRA, chance SN, chance PL.
Sunday: Mainly MVFR, with areas IFR possible. Breezy. RA, FZRA.
Sunday Night: Mainly VFR, with local MVFR possible. Strong
winds with areas of gusts up to 45 kt. Slight chance RA.
Monday: VFR. Strong winds with gusts up to 45 kt.
&&
.MARINE...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Thursday Night/...High confidence.
Tonight...
Light southwest winds turn from the southeast. Wind gusts
increasing to 30 knots after midnight. Seas building to 5-8
feet on the southern and southeast waters after midnight. Snow
changes to sleet and then rain after midnight. Small Craft
Advisory on most waters.
Thursday...
Rain tapers off in the morning. Winds shift out of the west.
Gusts increase to 30 knots by afternoon. Seas linger at 5-8
feet on all exposed waters.
Thursday night...
West-northwest winds gusting to 30 knots. Seas 5-7 feet on the
exposed waters.
Outlook /Friday through Monday/...
Friday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of seas approaching 5 ft.
Friday Night through Saturday: Winds less than 25 kt.
Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft. Rain.
Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Rain.
Sunday Night: Low risk for gale force winds with gusts up to
45 kt. Rough seas up to 12 ft. Slight chance of rain.
Monday: Low risk for storm force winds with gusts up to 45 kt.
Rough seas up to 13 ft.
&&
.TIDES/COASTAL FLOODING...
High astronomical tides are forecast for Thursday, with heights
near 12.5 ft at BOS and close to 6 ft at PVD.
Main concern for Thursday is the mid morning high tide in
Narragansett Bay and Buzzards Bay, and the late morning high
tide along the east coast. Guidance is suggesting a surge up to
1 ft is possible near high tide which may bring storm tides to
near flood stage at PVD and BOS. Expect pockets of minor coastal
flooding, mainly in the usual spots. A Coastal Flood Statement
has been issued.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 7 AM EST Thursday for CTZ002>004.
MA...Winter Weather Advisory until 7 AM EST Thursday for MAZ002>021-
026.
RI...Winter Weather Advisory until 7 AM EST Thursday for RIZ001>007.
MARINE...Small Craft Advisory from 2 AM Thursday to 7 AM EST Friday for
ANZ232>234.
Small Craft Advisory from 2 AM to 7 AM EST Thursday for ANZ231-
251.
Small Craft Advisory until 7 AM EST Friday for ANZ235-237-250-
254>256.
&&
$$
SYNOPSIS...WTB/EVT
NEAR TERM...WTB/Belk/EVT
SHORT TERM...WTB
LONG TERM...EVT
AVIATION...WTB/Belk/EVT
MARINE...WTB/EVT
TIDES/COASTAL FLOODING...