Scientific Forecaster Discussion
fxus61 kbox 210906
Area forecast discussion
National Weather Service Boston/Norton Massachusetts
406 am EST Thu Nov 21 2019
high pressure will settle over southern New England today with
mostly sunny and seasonable weather conditions. Mild and gusty
Friday as a cold front swings through New England. Colder air
moves in behind the front Friday night and Saturday. Low
pressure moving from the southern Appalachians Saturday night
will bring rain to the coast and a wintry mix to rain inland as
it passes near Cape Cod Sunday. High pressure brings dry weather
Monday and Tuesday. A cold front may bring scattered showers
Near term /until 6 PM this evening/...
though some residual overcast lingers towards Cape Cod and
Nantucket, and some leftover wrap-around moisture/stratus in the
Berkshires and the higher terrain in northern Worcester County,
skies early this morning have finally cleared.
Surface weather pattern today features an exiting coastal low
expected to move into the eastern Canadian Maritimes, while an
expansive surface ridge was building across the north-central
Appalachians into central/southern New York. Northerly pressure gradient
responsible for modest north/northwest breezes yesterday into the overnight
will continue to slacken moving through the day today, with winds
tending light to calm as ridging both sfc and aloft builds into
southern New England. Low-level water-vapor imagery shows good
drying in vicinity of the ridge, and the idea for mostly clear skies
remains on track for today. Ridge will tend to shift eastward late
in the day, allowing for a SW surface flow to set up.
Certainly compared to the last couple days where blanket of overcast
predominanted, today will be much nicer weather-wise. Plenty of
sunshine with temperatures warming into the mid-upper 40s, possibly
some isolated 50 degree readings towards the South Coast into the
Hartford Metro area.
Short term /6 PM this evening through 6 PM Friday/...
Mid/upper ridge axis builds into the waters east of Cape Cod by
tonight, and a low-level SW jet 35-40 kts encourages warm and moist
advection to increase in earnest as we move into the evening.
Several guidance sources especially into the eastern Berks/Pioneer
and CT valleys were indicating some light quantitative precipitation forecast in the warm advection
regime tonight. Model forecast soundings however show dry low-levels
with what looks like mid-level clouds, so have opted to keep the
forecast dry until top-down saturation can complete - mainly early
in the day on Friday.
850 mb temperatures warm given the thermal advective pattern into
the +2 to +5c range by 12z Friday. Given this, and light SW winds 5-
10 mph overnight and the increase in clouds, opted for early lows in
the 30s before temperatures show a non-diurnal temperature increase,
with temps by daybreak Friday into the upper 30s to mid 40s areawide.
Partly to mostly cloudy conditions, though with somewhat mild and
breezy southwesterly warm-sector conditions should exist early into
Friday. We will then await a strong upper-level disturbance and
associated strong surface cold front moving in from the eastern
Great Lakes/western New York for the afternoon. A strong shot of colder
air is expected to overspread the area during the second half of
Friday with the front's passage, resulting in sharply falling late-
afternoon temperatures and developing breezy to gusty northwest winds both
over the land and especially out on the waters.
Pre-frontal, opted to raise temperatures into the mid to upper 50s
away from elevation, given some benefit from southwest downsloping
with 850 mb temps in the +5 to +7c range. Could see some spots reach
60 but would take more peeks of sun that look plausible at this
point. Should see some southwest breezes with gusts to 20-25 mph
limited by shallow mixing/progged inversion.
Seems that most model guidance supports progressing the front
into our western counties by around 16-18z, then racing quickly
towards the coast and offshore by around 22-00z. Did note models
were somewhat wetter with forecast qpf, but think limited
moisture to work with and the fast forward speed makes for this
being a low-quantitative precipitation forecast frontal passage (a few hundreths to a tenth at
most). Once the front clears, strong cold advection (850 mb
temps will plunge to values -6 to -9c across the cwa) will
steepen lapse rates considerably and encourage gusty northwest/west-northwest
winds to develop across land areas by the late afternoon, with a
good likelihood of gale conditions across most waters. Should
see sub-Wind Advisory northwest wind gusts 30-35 mph with up to
40 mph possible, with falling temps into the 30s producing some
wind chills down into the 20s by early Friday evening in
northern Worcester County and the Pioneer Valley.
Long term /Friday night through Wednesday/...
* rain near the coast and a wintry mix to rain inland Sat night-sun
* dry and seasonable Mon-Tue
* scattered showers Wed
Active pattern continues over weekend as southern stream trough
rotates through New England, as its associated surface low tracks
near or just east of Cape Cod Sunday. Broad trough then develops
across much of Continental U.S. Next week as a northern stream short wave
rotates through region Wednesday.
Main concern is potential for some wintry weather Saturday night
into Sunday as low pressure approaches from the southern
Appalachians. Overall pattern looks rather warm with a lack of a
high to the north and with onshore flow, but with hints of cold air
damming inland, temperature profiles suggest rain near the coast and
more in way of light icing across interior (mainly western/central
Massachusetts and perhaps parts of northern ct) later Saturday night into
Sunday morning, before milder air wins out and causes a change to
rain. However, this is based upon the low tracking near Cape Cod,
which is in the middle of the envelope of individual member
solutions, ranging from as far inland as Rhode Island/eastern Massachusetts (warm) to off
Nantucket (cold). Since there is uncertainty this far out, we are
also including mention of light snow/sleet since we are unsure of
how deep the cold air will be initially. Right now any snow
accumulations look to be an inch at best per ensemble probabilities.
Bottom line is there is the potential for light icing, or even a
light snow accumulation across interior southern New England Saturday night into
Sunday morning, which may affect weekend travel.
Beyond the weekend, high pressure will bring dry and seasonable
weather Monday and Tuesday. An approaching cold front will bring a
milder day Wednesday, along with scattered showers. The early
outlook for thanksgiving: dry and seasonable with gusty winds behind
One other note: we are coming into a period of high astronomical
tides Sunday and Monday. Depending on the wind direction and timing
of highest surge, a worst case scenario would bring some minor
coastal flooding with the Sunday high tide on either coastline (5.7
ft MLLW at Providence at 541 am, 11.2 ft MLLW at Boston at 845 am,
and 3.9 ft MLLW at Nantucket at 946 am). Ensemble guidance from
Stevens Institute shows a potential surge of 1.0 to 1.5 ft. Tides
are a little higher Monday morning, but by then we should be dealing
with offshore (west) flow which should limit the threat.
Aviation /09z Thursday through Monday/...
forecaster confidence levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short term /through Friday/...high confidence.
09z taf update:
Leftover MVFR ceilings should become VFR over the next couple
hours; VFR then for all terminals under building high
pressure. Northwest winds 4-10 kts (strongest Cape Cod
terminals) will decrease to light west, then become light
southwest by early evening.
VFR, though will see increasing/thickening cloudiness. Southwest
winds 6-8 kts, though increasing swly low-level jet of 35-40 kts
should produce pockets of low-level wind shear/turbulence late
tonight into early Friday.
Continued lowering to ceilings, but these should remain at VFR
levels ahead of a cold front. Southwest winds 8-12 kts with
gusts to 20-23 kts ahead of the cold front; a brief period of
unrestricted showers will accompany the front before a gusty
wind shift to northwest/west-northwest late in the day. West-northwest/northwest gusts 25-35 kts
mainly after 21z.
Kbos taf...high confidence in taf.
Kbdl taf...high confidence in taf.
Outlook /Friday night through Monday/...
Friday night: VFR. Strong winds with local gusts up to 40 kt.
Saturday: VFR. Windy with local gusts up to 30 kt.
Saturday night: mainly VFR, with local IFR possible. Chance ra,
chance fzra, chance snow.
Sunday: MVFR/IFR conditions possible. Breezy. Rain likely, freezing rain
Sunday night: mainly VFR, with local IFR possible. Breezy.
Slight chance rain showers.
Monday: VFR. Breezy.
short term /through Friday/...moderate confidence.
Small craft advisories remain in effect for most offshore waters
and Cape Cod Bay today, mostly for seas. Seas will tend to
slowly subside through tonight with easing winds. The bigger
concern for mariners is the cold frontal passage late Friday,
expected to bring a period of gale-force west-northwest winds across the
the coastal and outer waters, with the possibility of low-end
gales into Boston Harbor and narrangansett Bay. Have therefore
hoisted gale watches from late Friday afternoon through early
Today: northwest winds 10-15 kts with gusts 20-25 kts early to decrease
to 5-10 kts by afternoon. Seas 4-7 ft outer offshore waters,
though will tend to lower as the day progresses. Good
Tonight: winds become west-southwest and increase to 10-15 kts with gusts
to 20 kts by daybreak Friday. Seas mainly 2-4 ft, though with
leftover 5 footers early. Good visibility.
Friday: west-southwest winds continue to increase to low-end Small Craft Advisory levels
(mainly 15-20 kts with gusts to 30 kts) ahead of a cold front
expected to cross the waters late Friday. Wind shift to northwest with
20-25 kt winds, gusts 35-40 kts, tending on the lower end of
that range in the bays and harbors. Seas build to 5-9 ft,
highest southern and southeastern waters. Light showers could
bring visibilities down to 4-6 miles late Friday afternoon with
Outlook /Friday night through Monday/...
Friday night: low risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft.
Saturday: low risk for Small Craft Advisory winds with local
gusts up to 30 kt. Local rough seas.
Saturday night: winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain.
Sunday: winds less than 25 kt. Local rough seas. Rain likely.
Sunday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Slight chance of rain
Monday: winds less than 25 kt. Local rough seas.
Marine...gale watch from Friday afternoon through late Friday night for
Small Craft Advisory until noon EST today for anz231-255-256.
Gale watch from Friday afternoon through late Friday night for
Small Craft Advisory until 7 am EST Friday for anz250-254.
Small Craft Advisory until 6 PM EST this evening for anz251.