NWS Forecaster Discussion
BOX
Area Forecast Discussion National Weather Service Boston/Norton MA.
000
FXUS61 KBOX 300231
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
1031 PM EDT Tue Sep 29 2020
.SYNOPSIS...
An approaching cold front will bring widespread heavy rain and
strong to possibly damaging winds late tonight into early Wednesday,
with highest rainfall totals across western Massachusetts and
Connecticut. Cooler, less humid and drier air then moves in later
Wednesday into Thursday. Cooler and drier air then moves in
later Wednesday into Thursday. More scattered showers are
possible Friday, followed by a cool dry weekend. Another round
of showers is possible Monday and Tuesday.
&&
.NEAR TERM /UNTIL MIDNIGHT TONIGHT/...
1020 PM Update...
Forecast is largely on track so only made slight changes. A
couple of things:
1)Issued a SPS for locally dense fog along the MA/RI South
Coast through the pre-dawn hours as vsby have come down to 1/2
to 1 mi with dew pt depressions near zero.
2) The concern remains for damaging winds overnight. Bufkit
soundings show up 50 to 60 kt gusts possible towards the pre-
dawn hours for parts of Eastern MA and RI (especially away from
the immediate coast). 850-950mb winds are forecast to be between
65 to 80 kt so if a fine line of convection were to develop,
there would require a short-fused High Wind Warning. Currently
not enough confidence to upgrade the current Wind Advisories but
we will continue to monitor the trends overnight.
PREVIOUS DISCUSSION...
The heavy rain & strong wind threat will wait until after
midnight when the stronger forcing arrives with the cold front.
That being said, a modest southerly low level jet combined with
Pwats of 2 inches will still result in scattered showers this
evening. However, it will not be a complete washout this
evening.
As for temperatures, given the anomalously moist/southerly flow it
will remain quite mild and humid too. Temperatures should only drop
into the upper 60s to the lower 70s this evening.
We think enough wind in the boundary layer will keep fog in most
locations rather patchy/isolated this evening. The exception might
be towards the south coast, Cape and Islands where high dewpoint air
blowing over the ocean may result in more widespread fog developing
with the cooling boundary layer. It is possible that we may need to
consider a short-fused dense fog advisory until the stronger
winds/rain arrives.
&&
.SHORT TERM /MIDNIGHT THROUGH 6 PM WEDNESDAY/...
* Heavy rain/isolated thunder risk overnight into mid morning Wed
* Period of strong to possibly damaging winds between 3 and 11 am Wed
Overnight into mid morning Wednesday...
The main concern will revolve around the potential for a period of
strong to damaging wind gusts and heavy rainfall/isolated
thunderstorm potential overnight into mid morning Wednesday.
A strong cold front will approach from the west overnight as a wave
of low pressure lifts north across western New England; rapidly
intensifies as it lifts into Quebec. We will break things down a bit
more below.
1) Strong to Damaging Wind Potential:
In response to the rapidly intensifying low pressure system a very
strong southerly LLJ will develop. The GEFS is indicated the 850 mb
LLJ of 4-5 standard deviations above normal with even a sliver of 5
to 6 above normal in eastern MA. The 12z NAM seems to have the most
potent LLJ of all the guidance. As it enters eastern MA it indicates
80-90 knots at 850 mb and even 70-80 knots down to 925 mb.
The bigger question is how much of this wind is able to mix down
give the low level inversion. This is always a forecast dilemma for
forecasters in these setups. It ends up very dependent on the
surface temperature and any convective elements. It does appear that
overnight surface temps will be in the lower 70s across much of the
region given strong warm advection. This should allow for a period
of southerly 40 to 50 mph wind gusts across much of the region,
particularly eastern MA/RI under the core of the LLJ. There is a low
risk for a few gusts up to 60 mph if a fine line/convective elements
become involved, but confidence is not high enough to issue any High
Wind Warnings. Therefore, Wind Advisories are posted for much of the
region. The strongest winds are expected roughly between 3 am to 11
am from west to east, lasting 2-4 hours in a given location. Even if
we end up on the lower end of the anticipated gusts, given heavy
rainfall and fully leafed trees there is an increased risk for
isolated power outages.
2) Heavy Rain/Isolated Thunder Threat:
The strong jet dynamics mentioned above will combined with Pwats on
the order of 2 inches. This is 3 to 4 standard deviations above
normal for this time of year. This recipe will result in a band of
heavy rain sweeping across the region overnight into mid morning
Wednesday. The heaviest rain should be across interior MA & CT in
vicinity of the surface low pressure system where the best low level
convergence exists. Rainfall amounts of 1 to 2 inches are
anticipated with localized amounts of 2.50 to 3 inches possible.
Across eastern MA/RI, bands heavy rain will be possible but being
further removed from the low level forcing amounts of 0.50 to 1 inch
are more reasonable. While some nuisance poor drainage street
flooding is possible, do not think amounts will rise to the level of
producing significant flooding. Therefore, no flood watch will be
issued with this package.
Instability is quite marginal, but we may generate a few hundred
J/KG of MUCape. Given the strong forcing with this system though
some embedded thunder is certainly possible. If this does
materialize, the threat for localized stronger wind gusts along with
localized higher rain amounts would increase.
Late Wednesday morning into Wednesday afternoon...
The bulk of the heavy rain will be over by mid morning Wednesday,
but a few left over showers may linger through lunchtime. Otherwise,
the main story will be drier/cooler air working in from the
southwest. While temps will fall into the 60s in most locations by
mid morning behind the front, enough sunshine/heating should allow
them to recover into the 70 to 75 degree range in most lower
elevations during the afternoon. It will remain breezy though and
with dewpoints falling into the 50s the airmass will feel noticeably
cooler/drier than the last few days.
&&
.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
Big Picture...
Broad upper trough sits over the Northeast USA and a tall ridge over
the Western USA through the weekend. At least two shortwaves will
move through the flow around the Northeast trough. Current timing
would bring one shortwave through New England late Friday, the
second on early Monday. The upper flow then becomes more zonal for
the early week.
Contours start near normal values, then cool below normal through
the weekend, then slowly moderate early next week. Expect seasonably
cool temperatures for most of the forecast period.
Model mass fields show a similar overall pattern through Sunday,
then diverge early next week. Forecast confidence is moderate
overall, but lowering for Monday-Tuesday.
Details...
Wednesday night through Friday...
Upper trough remains centered over the Great Lakes with a cooler
southwest flow over Srn New England. Wednesday night and Thursday
look dry, but lingering moisture at 850-mb suggesting diurnal clouds
Thursday. Mixing reaches to 850 mb, and taps temps of 6-8C at that
level...so max sfc temps should be in he upper 60s and lower 70s.
Friday upper trough pushes east, and pushes a cold front into New
England. The supporting upper jet stays to our west and slides up
into Canada. The right entrance region of the jet moves over Srn New
England Friday, providing lift and possibly generating a wave along
the front. This should be enough to generate showers Friday, but PW
values of 1 to 1.25 inches don`t suggest any gullywashers. Temps
aloft 2-6C suggest highs mainly in the 60s, possibly around 70 in
parts of RI/SE Mass. Temps cool into the 40s to around 50 Friday
night.
Saturday-Sunday...
High pressure moves over the region. Expect dry weather. Temps aloft
0-4C supports max sfc temps in the low to mid 60s. Expect a frosty
night Saturday night, with radiational cooling and dew points in the
mid 30s to mid 40s...so min sfc temps will range from the mid to
upper 30s inland to the mid 40s along the coast.
Monday-Tuesday...
Monday shortwave brings rain/showers to Srn New England Monday, but
details vary between the GFS and ECMWF. Both develop a surface low
associated with the upper shortwave. But the GFS takes this surface
low well south of New England while the ECMWF brings the low up
across Srn New England. Will forecast a chance of showers Monday,
with showers ending on Tuesday.
&&
.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Tonight into mid morning Wednesday...Moderate confidence.
Scattered showers continue over CT and Wrn MA early tonight
with conditions deteriorating to MVFR-IFR levels. LIFR
conditions are possible near the south coast along with fog. The
heavy rain and isolated thunder threat will occur between 4z
and 16z from west to east. Southerly wind gusts of 35 to 45
knots anticipated during this time as well with the strongest of
the winds most likely across eastern MA RI. LLWS will be a
concern too.
Late Wednesday morning into Wednesday afternoon. High confidence.
Conditions should improve to VFR in most locations by late
morning/mid afternoon from west to east. SW wind gusts up to 25
knots will remain possible.
KBOS Terminal...Moderate confidence in TAFs.
KBDL Terminal...Moderate confidence in TAFs.
Outlook /Wednesday Night through Sunday/...
Wednesday Night: Mainly VFR, with local MVFR possible. Breezy.
Thursday: VFR. Breezy.
Thursday Night: Mainly VFR, with local MVFR possible. Slight
chance RA.
Friday: Mainly MVFR, with areas VFR possible. Chance RA.
Friday Night: Mainly VFR, with local MVFR possible. Chance RA.
Saturday through Saturday Night: VFR.
Sunday: Mainly VFR, with local MVFR possible.
&&
.MARINE...
* Gale Warnings overnight into Wednesday morning
A rapidly intensifying low pressure system passing well west of the
waters will allow a very strong southerly LLJ to develop ahead of an
approaching cold front. The strongest of the winds look to occur
overnight into Wednesday morning. Despite the inversion, expect a
period of southerly wind gusts of 30 to 40 knots during this time.
Seas should build to between 7 and 11 feet across our open waters.
Gale Warnings have been hoisted for all waters.
Winds will diminish behind the front during the afternoon, but still
expect SW wind gusts around 25 knots.
Outlook /Wednesday Night through Sunday/...
Wednesday Night through Thursday: Winds less than 25 kt. Rough
seas up to 8 ft.
Thursday Night: Winds less than 25 kt. Areas of rough seas.
Friday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain.
Friday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain.
Saturday through Sunday: Winds less than 25 kt. Areas of seas
approaching 5 ft.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory until 1 PM EDT Wednesday for CTZ002>004.
MA...Wind Advisory until 1 PM EDT Wednesday for MAZ004>007-011>024-
026.
RI...Wind Advisory until 1 PM EDT Wednesday for RIZ001>008.
MARINE...Gale Warning until 2 PM EDT Wednesday for ANZ231-232-250-
254>256.
Gale Warning until noon EDT Wednesday for ANZ233>235-237-251.
Gale Warning from 2 AM to noon EDT Wednesday for ANZ230-236.
&&
$$
SYNOPSIS...WTB/Frank/Gaucher
NEAR TERM...WTB/Frank/Chai
SHORT TERM...Frank/Gaucher
LONG TERM...WTB
AVIATION...WTB/Frank/Gaucher
MARINE...WTB/Frank/Gaucher