Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS61 KBOX 190527

Area Forecast Discussion
National Weather Service Taunton MA
127 AM EDT Mon Mar 19 2018

Dry but unseasonably cold weather will continue through tomorrow.
High pressure brings dry weather Tuesday. A coastal storm moves
up from the Mid Atlantic coast Wednesday and then moves off to
the east Thursday. High pressure brings drier weather Friday and
early Saturday.


10pm update...

Yet another weak and progressive shortwave will rotate through N
New England this evening. However, outside of the SCT CU where
Great Lake moisture is available, and a slight increase in CI
mainly SKC conditions should continue. OBS already showing
several locations have decoupled within the last couple of hours
so have leaned most heavily on cold MOS guidance for overnight

Otherwise, the forecast is on track this evening.


Monday into Monday night...

High pressure centered over south-central Canada will continue to
influence our weather into Monday night, with dry conditions and
cool temperatures persisting. Temperatures will be slightly warmer
than today, with highs in the mid to upper 30s. Cannot rule out a
40F but the overall trend is still below normal. A weak shortwave
will pass well east of the region, which will tighten the pressure
gradient. Thus gusty winds near 25 MPH is possible across northern
and eastern MA.

Temperatures overnight will be slightly warmer as WAA begins to
advance into the region ahead of VA/NC developing cyclone. Cloud
cover will also be on the increase during the overnight hours which
will limit any diurnal cooling.


*/ Highlights...

 - Tuesday, keep it cold and dry, N winds, skirting S-coastal snow
 - Wednesday into Thursday, monitoring potential storm
 - Friday into the weekend, keep cool, dry pattern for now
 - Pass on any later potential weather features into next week

*/ Overview and discussion (Tuesday through Thursday)...

Continued evaluation of possible midweek wintry-weather. Suppressing
SE Canadian gyre against W-E Pacific energy train. With an upstream
N-Pacific pattern ridge amplification, downstream H5 trof-ridge-trof
sharpens, preferred NE CONUS H5 troughing, however neighbored W by a
forward leaning H5 ridge / mild SW Pacific-punch while E a wide-open
N Atlantic now with +NAO. While progressive pattern preferred, there
are still wildcards: morphology of 1.) suppressive SE Canadian gyre,
and 2.) upstream central N American H5 ridging. Likely more but both
implicative as to sharpness of E CONUS H5 trof and mid-level feature
potency. Could give longer residency towards low maturation, backed
further N/W to our coast, subsequent bigger impacts, outcomes ...
that is if the H5 ridge upstream highly amplifies while suppression
N/E lessens, the opportunity for N-stream energy to cascade rearward
within the E CONUS H5 trof. Flat and fast versus sharp and slower.

Evaluating isentropes, subsiding S split flow, Canada into E Great
Lakes split towards OH River Valley and NE CONUS initially. Dry.
Beneath mid-level confluence along pressure surfaces, lessening
while pushing E. Can see W-E stacked frontogenesis with height S-N
Tuesday with the first passing low. But moisture present, and lift,
especially in the dendritic growth zone? And to what intensity is
lift, precipitation, especially with consideration of timing (i.e.,
daytime), low-level thermal profiles? Low PoPs, skirting S-coastal
snow for Tuesday, light accumulation if any.

Then late Wednesday into Thursday, the second low, SW-NE fronto-
genetical banding re-emerging. Stronger synoptics, deeper lift and
pull back beneath mid-level features, N/E suppressive wave lesser.
Conveyor belt motions amplified, some concern with a cold, dry belt
wrap around along the isentropes, but expect moisture present along
with lift, especially within the dendritic growth zone, a measure of
trowaling. But to what intensity is lift, precipitation, especially
noting timing (i.e., daytime), low-level thermal profiles? Far too
much spread to get into specific details, and yes, even considering
the latest 18.12z EC. It`s just one model run, prefer consistency
before jumping on it.

Keep in mind, a lot of moving parts, chaotic, so expect adjustments
in later forecasts. Focus Tuesday through Thursday. Preference to
ensemble means / WPC forecasts over operational guidance. A nod to
SREF / CIPS analog probabilistics (though with CIPS, not favoring 60
hr NAM-derived forecast guidance, while the 84 hr GFS-derived looks
reasonable). Recognize SE New England at higher risk with latest 12z
guidance and probabilistics.

Light snow around late Tuesday, if any, along the S-coast. A better
chance of light to moderate snow late Wednesday into Thursday. As to
amounts ... still too early to say. Winds an issue during the latter
period, gusts of 30-40 mph, initial guess. Roughly 10.6 foot Boston
tide 315a on Thursday potentially during strongest winds. Can`t rule
out coastal flood issues for E-coastal MA.

An first-cut 72-hour snowfall grid is published with highest amounts
of 2-4 inches S-coastal New England, Cape and Islands, but take this
as a first-guess with low confidence. Forecasts will change.

Continued advice, stay informed, keep tuned to later forecasts.


Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence.

Through today...
VFR. A few early AM SCT-BKN 040-070. NW winds with a few gusts
15-20 kt.

VFR. Weaker NW winds.

VFR through the day with increasing high and mid clouds. Winds

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...Moderate Confidence.

Tuesday: VFR.

Tuesday Night: VFR. Windy with areas gusts to 30 kt.

Wednesday: Mainly VFR, with local MVFR possible. Strong winds
with local gusts to 40 kt. Slight chance SN.

Wednesday Night: Mainly VFR, with areas MVFR possible. Strong
winds with gusts to 40 kt. Slight chance SN.

Thursday: Mainly VFR, with local MVFR possible. Strong winds
with local gusts to 40 kt. Slight chance SN.

Thursday Night: VFR. Breezy.

Friday: Breezy.


Short Term /through Tuesday/...High confidence.

Progressive northwest flow across the waters will lead to seas at or
near 5 feet through tonight. SCA will linger into tomorrow,
especially for the eastern waters, as winds will pick back up to
around 20-25 kts by the afternoon. Conditions should begin to
subside by the overnight hours.

Very cold airmass in place combined with gusty winds will result in
some freezing spray across the eastern waters. A freezing spray adv
has been issued.

Outlook /Tuesday Night through Friday/...Moderate Confidence.

Tuesday: Winds less than 25 kt.

Tuesday Night: Low risk for gale force winds with gusts up to
35 kt. Seas locally approaching 5 ft. Slight chance of rain,
slight chance of snow.

Wednesday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. Chance of snow, chance of rain.
Local visibility 1 nm or less.

Wednesday Night: Moderate risk for gale force winds with gusts
up to 45 kt. Rough seas up to 18 ft. Snow likely, rain likely.
Local visibility 1 to 3 nm.

Thursday: Low risk for gale force winds with gusts up to 40 kt.
Rough seas up to 18 ft. Chance of snow.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Local rough seas.


MARINE...Small Craft Advisory until 2 AM EDT early this morning for
     Small Craft Advisory until 2 PM EDT this afternoon for ANZ231-
     Freezing Spray Advisory until 2 PM EDT this afternoon for
     Small Craft Advisory until 2 AM EDT Tuesday for ANZ250-254.


LONG TERM...Sipprell
MARINE...Dunten/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.