Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 211802

Area Forecast Discussion
National Weather Service Taunton MA
Issued by National Weather Service Gray ME
202 PM EDT Wed Mar 21 2018

A coastal storm passing well south of Nantucket will bring snow
mainly late today into early Thursday morning with the greatest
risk of heavy amounts south of the Massachusetts Turnpike.
Upper low pressure lingers over the Maritimes through the
weekend with potential for clouds and scattered rain/snow
showers. Temperatures remain colder than normal through the


2 PM update...

Banding signature becoming organized across LI sound and coastal
CT where heavier precip is developing. Still just light snow or
rain near the south coast of RI and MA with dry conditions to
the north as dry air below 850 mb eroding northern edge of the
precip. Models indicate low level moisture lifting north across
SNE through the afternoon and early evening which will allow
steadier snow to develop across CT, RI and SE MA. Potential for
2-3" accum here toward evening. North of the Mass Pike, mainly
dry conditions will persist through much of the afternoon with
snow developing after 6 pm.

Previous discussion...

Some light snow and rain is moving up along the south coast but
low level dry air is winning the battle further to the north and
precip advancement to the north will be a slow process. Light
precip will be focused near the south coast through early
afternoon with minimal impact given temps above freezing and
light precip resulting in melting on paved surfaces. We do
expect snow to push north toward the Mass Pike mid/late
afternoon while intensity ramps up across portions of CT/RI and
SE MA where accum of 2-3 inches possible by evening.

North of the Pike, it will likely take until late afternoon or
evening near the NH border before snow begins to accumulate so
minimal impact here through the daytime hours.


* Bulk of accumulating snow this evening into early Thu Am
* Abnormally low confidence in snow amounts for a 24 hr forecast
* Greatest risk for heavy snow/iso power outages south of Pike

1) Overview...

A very challenging forecast late this afternoon and particularly
tonight into early Thursday morning. An unusual spread remains
in the model guidance given the event is pretty much inside 24

The main issue is that surface low pressure will be tracking
northeastward towards the Benchmark. However...model QPF fields
vary considerably from the Ukmet/EC which have confine the
threat for heavy snow to far southeast New England.
Meanwhile...the GFS/NAM are much further north and indicate
heavy wet snow right to the NH border. However...this guidance
was also warmer across the far southeast New England coast
especially the Cape/Islands where it indicated the potential for
more rain than snow. The RGEM was the middle ground of the
model guidance and given the tremendous uncertainty is the path
we followed with this forecast. Greatest threat for heavy snow
south of the MA turnpike to near the Cape Cod Canal.

Appears the model guidance is struggling with an initial lead
shortwave and also a lot of dry air in the 800 to 900 mb
layer...north of the MA turnpike. The guidance that erodes that
dry layer quicker indicates heavy snow right to the NH
border...while the other guidance keeps that heavy snow threat
south of the MA turnpike.

The models do indicate some decent mid level banding and
frontogenesis at times...but it seems very transient and mixed
with some dry air. We did notice that the latest HRRR/RAP
struggles to get much QPF north of the MA Pike through mid
evening. Not exactly sure that is correct...but needs to be
taken into consideration.

2) Snow Amounts/Timing:

While some snow will impact the region through the
afternoon...main impact probably not until after 5 or 6 pm this
evening south of the Pike and even later north of that region.
We should see the snow accumulations continue into early
Thursday morning...especially eastern MA where some guidance
hints at mid level banding. As we mentioned snow accumulations
are very difficult...generally went 3 to 5 inches across central
and northeast MA with 5 to 9 inches south of that region but
localized 12 inch amounts are possible. Our lowest snowfall
amounts are in northwest MA where some models show too much dry
air to warrant more than 1 to 3 inches. As for headlines...opted
for winter weather advisories across central/northeast MA with
warnings continuing for most areas south of the Pike. We thought
about downgrading Cape/MVY/BID to an advisory given marginal
thermal profiles...but given uncertainty will defer to the next

3) Wet Snow/Strong Winds Power Outage Potential:

A well mixed northeast low level jet with decent low level lapse
rates will support 40 to 60 mph winds gusts across the coastal
plain. The strongest of those winds will be across the
Cape/Islands and those will occur tonight. These winds alone
may result in isolated power outages. In addition...the greatest
threat for heavy wet snow is south of the MA turnpike where we
will have the potential for isolated power outages.

The snow amounts themselves along with the winds do not look as
strong as what we have seen in the past couple of events. Therefore...
while isolated power outages are certainly possible it does not
look to be as widespread as what we have seen with the past few
events. Nonetheless...if confidence in heavy snow increases
this potential may have to be played up more than current

Later Thursday morning and afternoon...

Some left over snow showers possible later Thursday
morning...but temps will be rising above freezing so any road
issues will quickly improve. In fact...much of the region may
see highs in the lower to middle 40s Thursday afternoon.



* Snow showers/squalls possible late Friday/Saturday
* Below average temps through the period
* Mainly dry early next week with hit or miss showers

Synoptic Overview & Model Preferences...

Overall trends in the guidance suggest a medium to high confidence
level for the extended. Broad longwave trough will remain over the
region through much of the medium range. Several cut-off shortwaves
will eject from the Hudson Bay region into the flow resulting in
wave after wave. This will also result in reinforcing cold air
keeping temperatures below average through the period.

Thursday night into Sunday...

Amplified cyclone which will impact our region Wednesday into
Thursday will begin to pull out towards the Maritimes by
Thursday evening. Lots of dry air will filter back into the
region thanks to northwest flow.

Another digging shortwave will get swept up in the broad longwave
trough and push into the region Friday into Saturday developing a
cold pool aloft. Low level moisture appears to saturate per the NAM
and GFS with some guidance showing lapse rates near 6.5+ C/km. This
could be enough to help trigger snow showers or snow squalls. In
fact the BTV WRF model does show SNSQ parameters above one
across western MA and CT. Best timing appears to be Friday
evening, but axis of greatest potential could still shift.

Saturday and beyond...

Still chilly for the weekend as CAA takes hold of the region.
Another cut-off 500 mb low will strengthen over the region. This low
could bring in some showery weather across the area from off the
ocean. Still a lot of details to work out as the 12z EC tries to
bring offshore low closer to the coast, with the GFS being more
progressive and trending towards dry weather by mid-week. Will wait
until after this upcoming storm to begin to dive into this potential


Forecaster Confidence Levels...

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

Short Term /through Thursday/...

Through 00z...MVFR conditions south of the Pike will deteriorate
to IFR as steadier snow develops, especially RI and SE MA.
Otherwise, VFR will gradually lower to MVFR to the north
reaching far northern MA toward 00z. NE gusts to 25-35 kt with
up to 40 kt Cape/Islands.

Tonight...Moderate confidence. MVFR-IFR conditions in mainly
snow with some LIFR thresholds for a time especially eastern New
Eng. Ptype mainly snow but mixing possible near the Cape Cod
Canal and mostly rain outer Cape/ACK. N gusts to 25-35 kt with
up to 45 kt Cape/Islands.

Thursday...Moderate confidence. Lingering snow especially across
eastern MA may result in some IFR conditions Thu morning.
Otherwise...mainly MVFR conditions, improving to VFR in western
New Eng. N gusts to 40 kt Cape/Islands in the morning,
diminishing in the afternoon.

KBOS Terminal...Moderate confidence in TAF. A period of IFR
expected tonight with accumulating snow.

KBDL Terminal...Moderate confidence in TAF. A period of IFR
expected late afternoon into tonight with accumulating snow.

Outlook /Thursday Night through Sunday/...Moderate Confidence.

Thursday Night: Mainly VFR, with local MVFR possible. Breezy.

Friday: Mainly VFR, with local MVFR possible. Slight chance
SHRA, slight chance SHSN.

Friday Night: Mainly VFR, with local MVFR possible. Breezy.

Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.

Saturday Night: Mainly VFR, with areas MVFR possible. Breezy.
Slight chance SHSN.

Sunday: Mainly VFR, with areas MVFR possible. Windy with local
gusts to 30 kt. Slight chance SHRA, slight chance SHSN.


Short Term /through Thursday/...

Today through Thursday...Moderate to high confidence.

Storm warnings all waters late today and especially
tonight...except for strong Gales across Boston Harbor and Narr
Bay. Northeast wind gusts of 40 to 50 knots expected later today
and especially tonight with good transfer over the ocean. This
should yield 20 plus foot seas across our eastern waters by
daybreak Thursday. Winds should diminish to gales later Thursday
morning and afternoon...but high seas will continue across the

Outlook /Thursday Night through Sunday/...Moderate Confidence.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.

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

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas. Slight chance of rain
showers, slight chance of snow showers.

Sunday: Strong winds with gusts up to 30 kt. Rough seas up to
11 ft. Slight chance of rain showers, slight chance of snow


Main concern continues to be the early Thursday morning high
tide along the eastern MA coast and Nantucket. This looks to be
mainly a widespread minor with perhaps pockets of low end
moderate coastal flooding south of Boston. Overall...a 2 to 2.5
storm surge expected with perhaps a bit higher south of Boston.
We issued a coastal flood advisory for Boston points
north...while we held the current coastal flood watch south of
Boston. Since its a late 2nd period event and minor to low end
moderate coastal flood event...felt we could let the next shift
decide. Some beach erosion likely across the ocean exposed
portions of the outer-Cape and Nantucket.


CT...Winter Storm Warning until 8 AM EDT Thursday for CTZ002>004.
MA...Winter Storm Warning until 8 AM EDT Thursday for MAZ012>023.
     Coastal Flood Watch from midnight EDT tonight through late
     tonight for MAZ019-022>024.
     Wind Advisory until 8 AM EDT Thursday for MAZ007-014>016-
     Coastal Flood Advisory from midnight tonight to 6 AM EDT
     Thursday for MAZ007-015-016.
     Winter Weather Advisory until 8 AM EDT Thursday for MAZ009-011.
     High Wind Warning until noon EDT Thursday for MAZ022>024.
     Winter Weather Advisory from 5 PM this afternoon to 8 AM EDT
     Thursday for MAZ004>007-026.
RI...High Wind Warning until 8 AM EDT Thursday for RIZ008.
     Winter Storm Warning until 8 AM EDT Thursday for RIZ001>008.
     Wind Advisory until 8 AM EDT Thursday for RIZ005>007.
MARINE...Storm Warning until 9 AM EDT Thursday for ANZ231>235-237-
     Gale Warning until 8 AM EDT Thursday for ANZ230-236.
     Storm Warning from 7 PM this evening to 9 AM EDT Thursday for


LONG TERM...Dunten
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