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000 
FXUS61 KBOX 232101
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
501 PM EDT Mon Oct 23 2017

.SYNOPSIS...
A few showers may develop well ahead of a slowly approaching 
cold front tonight into early Tuesday morning. As the front gets
closer, expect rounds of heavy rain late Tuesday into Wednesday
along with a period of strong to damaging wind gusts possible. 
A few lingering showers are possible Thursday, but the trend 
towards drier, less humid and more seasonable temperatures are 
expected Thursday into Friday. Above normal temperatures may 
return by this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

A stubborn area of very low clouds and dense fog has lingered 
from the mid coast of Maine SW to Cape Ann down to S of Boston 
as seen on GOES-16 prelim, non-operational satellite loop. 
Noting light E-SE winds from about KOWD-KBED-KLWM east to the 
coast as well as into the seacoast of Maine, while the remainder
of the region has generally S winds at 20Z. This is likely due 
to the leftover remains of a backdoor cold front that pushed 
into E Mass Sunday. As the S pressure gradient increases with 
the slow approach of a cold front, expect winds to finally shift
and allow the low clouds and fog to push offshore. Still some 
question when this will happen, but feel it should for a time 
this evening.

With some onshore wind component in place tonight, will still 
see more low clouds and patchy fog, some locally dense, to push 
back along E coastal Mass. Also, with dewpoints in the upper 50s
to lower 60s, will see patchy fog develop away from the coast 
as well as temps approach to dewpoints, mainly after midnight.

Scattered showers will develop early tonight across western 
Mass this evening, which should become more numerous after 
midnight. As an H5 short wave wraps around cutoff low pressure 
over the western Great Lakes, will see the vorticity line push 
slowly E overnight, pushing the front eastward toward western 
New England toward daybreak. Not a lot of instability with the 
leading edge, so expect only showers at this point. QPF amounts 
generally 0.1 inches or less across central and eastern areas, 
possibly up to 0.2 inches across the E slopes of the Berkshires 
as plume of higher PWAT air approaches.

The mild S winds will keep temps well above seasonal normals 
overnight. Temps will bottom out in the mid 50s to lower 60s. 
Could see record tying or setting high minimum temps for 10/24 
at our four long term climate sites. Details will be in the 
climate section.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/...

Expect showers to move slowly across the region during the day 
Tuesday. Very sultry airmass in place with increasing S winds. 
PWAT values up to 1.6 to 1.7 inches will slide up the coast 
ahead of the front. Will also see some heavier downpours develop
with this airmass mainly across western areas during the 
afternoon. Heaviest rainfall looks to hold off until Tue night. 

Good instability also develops across central and western areas
during Tue afternoon. TQ values increase to the upper teens, 
with K indices reaching the lower-mid 30s across central and 
western areas. Have mentioned some thunderstorms in the forecast
as well. Also, with strong low and mid level jet associated 
with the approach surface front and mid level short wave, expect
S winds gusting up to 30-40 mph, highest along the S coast. 

High temps will reach to upper 60s to mid 70s. 

&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
Highlights...

* Period of strong to damaging wind gusts possible Tue into Wed
 
* Heavy rain will result in an urban/street flooding threat Tue-
  Wed along with a risk of isolated flash flooding

* Less humid & more seasonable weather finally return Thu/Fri

Overview and model preferences...
Blocking across the W Atlantic continually sharpens up toward 
Labrador through the long term. This allows for a near constant 
trof reloading pattern over the CONUS, the first of which will 
be associated with the heavy rain/strong winds Tue into Wed, but
after taking on negative tilt and lifting NNE late week, a 
secondary longwave trof develops in the lee of the continental 
divide thanks to reinforcement of arctic influence. Will 
continue with a baseline of ensembles given the highly amplified
pattern, which will take into account most solutions in the 
envelope. 

Details...

Tue Night and Wed...

Met overview...
A cold front becomes gradually parallel to the low-mid lvl flow
as strong upper lvl trof digs and takes on a negative tilt late
Tue night into Wed. The progression of the front across S New 
England is very slow as a result, and is likely to take until 
Wed evening to fully shift offshore. The parent trof has a draw 
of subtropical air with PWATS reaching very close to 2.00 inches
(3 std deviations above normal). As dwpts respond to this 
moisture in the low lvls, soundings become conditionally 
unstable, resulting in 300-500j/kg of CAPE available. Finally, a
strong LLJ, which peaks at about 60 kt at H92 and near 70 kt at
H85 shifts N-S through the area late night into early Wed. This
will lead to a very unsettled, prolonged period of wet wx, 
potentially exceeding 24 hrs.

Wind/severe threat...
LLJ peaks late Tue night into early Wed, with H92 speed values 
around 60 kt even on the typically more conservative ECMWF/EPS 
members. Combination of precip drag and some BL mixing thanks to
conditionally unstable lapse rates will likely lead to some of 
this momentum reaching the ground. Widespread 35-45 mph wind 
gusts are likely, even into interior New England. However, 50-60
mph are also possible, especially across SE MA/RI where the 
core of the LLJ peaks. With some instability, thunderstorms, 
and/or even a fine line of convection will more efficiently mix 
these winds down, potentially leading to localized higher impact
damage. While this is possible anywhere in New England, highest
winds will still be featured across the SE. Wind advisories 
will be hoisted, and given fully leaved trees, will highlight 
all of S New England for this risk, but still monitor for 
localized higher impacts in thunderstorms/fine convective line. 
This coincides with the SWODY2 upgrade to a slight risk, given 
localized gusts could produce more damage in convection. Peak 
will be overnight into Wed morning. One final note, cannot rule 
out a brief spinup given the high shear (helicity 300+) and low 
LCLs within the moist airmass. This is a very low/localized 
risk.

Heavy rain...
High PWATs and the influence of convective potential could 
yield pockets of heavy rain with rainfall rates exceeding 1in/hr
at times. The combination of the slow movement of the front, 
and strong S LLJ suggests a risk for training heavier cells, 
which could lead to prolonged periods of high-rate rainfall. A 
slight E component to the low lvl flow could lead to enhanced 
upslope in E slopes of the Berkshires and Litchfield hills. Even
though dry, with FF guidance nearly 3-4 inches/hr, the long 
timeframe could exceed 3 or 6 hr FF guidance. Flashier streams 
will see a risk for flooding. Therefore, will be hoisting a FF 
Watch with this update mainly for W portions of the forecast 
area, where the enhancement is most apparent. Further E, 
training/heavy rain is still an issue, but the risk would be 
more in the form of urban/poor drainage flash flooding, 
exacerbated if stripped leaves clog drains. We may need to 
expand the FF watch for this risk in later updates. Overall 
widespread 1.5-2.5 inches are expected with localized 3-4 inches
certainly possible. 

Thu...
Dry air moves in through the column aloft first, trapping some 
lower lvl moisture into the day on Thu. The combination of this,
some cold advection and strong cyclonic curvature aloft suggest
a risk for lingering clouds and SHRA through much of the day, 
but not nearly as widespread as the previous 24-48 hours. H92 
temps drop to about +6C on average, so expecting seasonable 
temperatures with highs mainly in the upper 50s and low 60s 
while weak flow overnight and gradual clearing should allow mins
to drop into the 40s.

Fri and Sat...
Dry period as weak mid lvl ridging forms between the exiting 
cutoff/trof to the NE and deepening longwave trof shifting E 
into the prairies. H92 temps hover around +9C to +10C each day, 
suggesting highs will be seasonably warm, mainly in the mid 
60s, although overnight lows will be seasonable thanks to some 
radiational cooling especially Fri night. Dwpts in the low-mid 
40s during the afternoon suggest these will be near the 
overnight lows.

Sun into Mon...
With a very amplified pattern timing/mesoscale features are not
resolved well this far out, but given a similar setup with a 
gradually tilting trof with some subtropical moisture associated
with it, will have to watch for another round of low pres with 
widespread rainfall. Will favor the slower timing of ensembles 
given the meridional nature of the pattern. At the very least, 
another unsettled wet period is possible early next week.

&&

.AVIATION /21Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/....

Through 00Z...Moderate confidence. 
Lingering IFR-LIFR conditions across portions of E coastal Mass
through 00Z. Elsewhere, areas of MVFR CIGS/VFR VSBYS across the
remainder of the region. May see a few isolated showers across 
the E slopes of the Berkshires by 22Z-23Z. S-SE winds increase 
to around 10 kt. 

Tonight...Moderate confidence. 
Conditions lower to MVFR-IFR across N central and western areas
this evening in scattered showers and patchy fog. VFR 
conditions to start across S central and eastern areas, but CIGS
lower to MVFR to local IFR after midnight as showers slowly 
move E. Patchy fog will lower VSBYS to MVFR to local IFR as 
well. S winds gusting to 20-25 kt along S coastal areas after 
midnight.

Tuesday...High confidence in trends. Moderate confidence in 
timing. 
Expect mainly MVFR-IFR conditions in low CIGS and areas 
of fog. Expect areas of -SHRA/SHRA to push slowly into the 
region during the day. Rain heavy at times late in the day 
across W areas. Occasional TSRA at times Tue afternoon. S-SE 
winds gusting up to 25-30 kt, possibly to 40 kt in any 
thunderstorms. Highest gusts along S coast, Cape Cod and the 
islands. Areas of LLWS likely, mainly after 15Z.

KBOS Terminal...Moderate confidence in TAF. 
Mainly IFR-LIFR CIGS and IFR VSBYS lingering through at least 
04Z, possibly later. E winds up to 10 kt should shift to SE-S 
overnight. Remaining IFR during at least early portion of the 
morning push.

KBDL Terminal...Moderate confidence in TAF. 
VFR through 00Z, then CIGS lowering to MVFR. May see MVFR-IFR 
VSBYS in scattered showers by around 10Z or so. S winds gusting 
up to 20-25 kt by around midday Tuesday. 

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Mainly IFR, with local MVFR possible. Strong 
winds with gusts to 40 kt. RA, chance TSRA, patchy BR. 

Wednesday: Mainly VFR late, with local IFR possible. Windy with
gusts to 35 kt. RA, patchy BR. 

Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. 
Chance SHRA, chance RA, patchy BR. 

Thursday: VFR. Breezy. Chance SHRA. 

Thursday Night through Friday: VFR. 

Friday Night: VFR. Patchy BR.

Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Have converted Gale Watches to Warnings with this package for 
all waters. 

Expect increasing S-SE winds tonight, gusting up to 20-25 kt 
after midnight. Seas build up to 5-6 ft. Winds and seas highest 
on the southern waters. Reduced visibility in areas of fog. 
Scattered showers across the southern waters late tonight.

On Tuesday, S winds continue to increase, with gusts up to 
30-35 kt. Seas continue to build, up to 6-9 ft. Visibility 
restrictions develop in areas of locally heavy rain and patchy 
fog. Scattered thunderstorms possible. Gale force wind gusts 
likely Tue afternoon across most of the waters. 

Outlook /Tuesday Night through Saturday/...

Tuesday Night: Gale force winds with gusts up to 45 kt. Rough 
seas up to 11 ft. Rain, chance of thunderstorms, patchy fog. 
Local visibility 1 to 3 nm. 

Wednesday: Gale force winds with gusts up to 40 kt early. Rough
seas up to 12 ft. Rain, chance of thunderstorms, patchy fog. 
Local visibility 1 nm or less. 

Wednesday Night: Winds less than 25 kt. Rough seas up to 11 ft.
Rain showers likely, rain, patchy fog. 

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers. 

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

Friday: Winds less than 25 kt. Seas up to 5 ft. 

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

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

&&

CLIMATE...
Overnight lows tonight will likely remain mild with S-SE winds
in place. May tie or set record high minimum temperatures for
October 24.  Here are the records...

October 24     Record High Min
Boston         60/1900 and 2001
Windsor Locks  61/1959
Providence     61/1959 and 2001
Worcester      59/1959

&&



.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for 
     CTZ002>004.
     Flash Flood Watch from Tuesday evening through Wednesday 
     morning for CTZ002.
MA...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for 
     MAZ002>024-026.
     Flash Flood Watch from Tuesday evening through Wednesday 
     morning for MAZ002-003-008>011.
RI...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for 
     RIZ001>008.
MARINE...Gale Warning from noon Tuesday to 11 AM EDT Wednesday for 
     ANZ231>234.
     Gale Warning from 8 PM Tuesday to 9 AM EDT Wednesday for 
     ANZ230-251.
     Gale Warning from 2 PM Tuesday to 8 AM EDT Wednesday for 
     ANZ236.
     Gale Warning from 11 AM Tuesday to 8 AM EDT Wednesday for 
     ANZ235-237.
     Gale Warning from noon Tuesday to 2 PM EDT Wednesday for 
     ANZ250-254.
     Gale Warning from 11 AM Tuesday to 11 AM EDT Wednesday for 
     ANZ255-256.

&&

$$
SYNOPSIS...Doody/EVT

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