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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 171407
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
907 AM EST Sun Feb 17 2019

.SYNOPSIS...
High pressure will maintain dry weather through today. Accumulating 
snow overspreads the region late this evening and overnight from 
west to east and continues into Monday, in association with a low 
pressure tracking south of New England. Drier and colder conditions 
follow Monday night into Wednesday. Another low approaches the 
region which will bring snow Wednesday night, likely changing to a 
wintry mix inland, and over to rain along the coast before ending 
Thursday. Dry conditions should return Friday into Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

905 am update...

Previous forecast is on track. A ridge of high pressure will
result in plenty of sunshine into the afternoon along with 
light winds. High temperatures will recover into the middle to 
upper 30s by mid afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
* Models have trended upward with precip/snowfall tonight into
  Monday.
* Issuing Winter Storm Watches for RI and SE MA and Winter
  Weather Advisories for N CT and Mass Pike corridor into Boston
  and coastal NE MA.

We are becoming more confident that accumulating snow will
affect SNE later tonight into Monday, especially along and south
of Mass Pike where we expect to see highest totals. However,
there are still some conflicting signals in model guidance as to
how much snow will fall.

Overall pattern is not one that favors widespread 6"+ snowfall
totals. Closed mid level low is expected to weaken as it runs
into confluent flow aloft across Northeast, but trend has
continued to bring everything a little farther north. Snow
growth is not ideal as best lift is centered in colder air
around -20C but combination of good overruning and some ocean
enhancement on N/NE flow near coast should compensate and help
generate widespread 2-4" snowfall across SNE, especially along
and south of Mass Pike.

That said, we do have some concern for higher snowfall in RI and
SE MA, especially along Route 44 corridor from Providence to
Taunton and Plymouth, where we could see as 6 or 7" in some 
locations. Models have trended upward in QPF including ECMWF,
UKMET, NAM (ARW/NMM), and even HREF (out to 36 hours). EC
ensemble has decent probability of 0.50" QPF in SE MA as well.

As a result we are issuing a Winter Storm Watch for RI and SE 
MA (including Cape Cod) which is where we have 50% confidence in
exceeding 6" of snow in a few locations. Typically we would go 
right to Winter Weather Advisories but with potential for higher
snowfall totals and lingering uncertainty we would like to 
allow next shift to get a look at 12Z runs to see if trends 
persist, especially since snow does not begin until late 
tonight. Keep in mind it's certainly possible that the Watch 
gets converted to Advisories later today.

Farther north, we are issuing Winter Weather Advisories for
northern CT and along the Mass Pike corridor (including Boston)
to coastal NE MA, where we have most confidence in 2-4" of snow.
It's still uncertain farther north since there may be a tight
gradient between snow/no-snow so we will let next shift consider
expansion of advisories later today. 

As for timing, snow arrives in CT valley 9-11 PM and reaches
eastern MA 11 PM-1 AM. Most of snow will accumulate overnight
which is when we have best overrunning and deep moisture through
column, and even period of decent snow growth in RI and SE MA.
Low passes SE of Nantucket Mon morning and takes most of lift
with it, and with drying at mid levels we should see lighter
snows most of day, especially in eastern MA where N/NE flow will
continue to bring some enhancement. Everything winds down during
afternoon from W to E.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
* Dry but turning colder Monday night into Wednesday
* Another chance of snow Wednesday ngt but likely changing to a 
  wintry mix inland and rain along the coast Thursday
* Dry weather follows Friday and likely lingers into Saturday

Overview...

Large scale flow across the CONUS this period features a high 
amplitude western trough and a downstream southeast/subtropical 
ridge along with confluent flow across New England as northern 
stream remains somewhat amplified. Forecast dilemma this period 
revolves around amplitude of northern stream which will dictate 
position of confluent flow across New England/Maritimes and 
accompanying dry/cold airmass. This will ultimately determine the 
storm track and ptype for Wed night/Thu event. 

Details...

Mon night/Tue/Wed ...

Drying trend develops Mon night as coastal low ejects out to sea in 
response to northern stream short wave moving across New England. 
Good cold air advection (CAA) behind this departing short wave with 
925 mb temps dropping to -10C to -12C across southern New England 
Mon ngt into Wed. This will support temps colder than normal with 
highs only in the upper 20s to lower 30s both Tue and Wed and teens 
at night. Blustery NW winds Mon ngt and Tue will provide cold wind 
chills as well. Winds diminish Wed as 1030+ mb high pres builds over 
the region.  

Wed night/Thursday ...

Next surge of moisture approaches from the southwest Wed ngt. Column 
is initially cold enough to support all snow. However with southeast 
east ridge building up the eastern seaboard storm track shifts 
farther north closer to New England. Also with mid level trough 
tracking well northwest of New England mid level warm air 
overspreads the area Thu. However with northern stream short wave 
slow to exit Newfoundland, cold high pres will linger over the 
maritimes and provide cold air damming across interior southern New 
England for at least at time into early Thu. This will increase the 
odds of shallow cold air lingering across the interior and 
supporting a wintry mix. Closer to the coast snow likely changes to 
rain given a storm track along or near the south coast. Although 
given event is still 96+ hrs out in time we don't want to lock into 
only one weather scenario. 

Friday/Saturday ...

Coastal low and associated mid level trough from Thu moves east of 
New England and is replaced by dry NW flow aloft and seasonable 
temps Fri into Sat. Next system approaches Sat night/Sunday with 
the threat for another round of wintry precip.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

VFR through early tonight other than the potential for brief
periods of marginal MVFR cigs flirting with outer-Cape/ACK. 
N/NW winds become light E/SE this afternoon. Light snow begins 
near BAF/BDL around 04Z, reaches BOS/PVD around 06Z, and HYA/ACK
by 08Z. Conditions quickly lower to MVFR at onset and to 
IFR/LIFR within two hours after that. Snow, or a mix of snow and
sleet near South Coast, continues through daybreak with 
IFR/LIFR conditions before improvement 12Z-18Z from west to 
east, possibly a little later on Cape Cod. 

Accumulations of 2-4 inches are expected across most of southern
New England, most of which will fall prior to 12Z Monday.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SN.

Tuesday through Tuesday Night: VFR. Breezy. 

Wednesday: Mainly VFR, with local IFR possible. Chance SN.

Wednesday Night: Mainly IFR, with areas MVFR possible. Breezy.
SN likely, PL likely, FZRA likely.

Thursday: Mainly VFR, with areas MVFR possible. Breezy. Chance
RA, chance FZRA, chance PL.

&&

.MARINE...
Forecaster Confidence Levels... 

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

Short Term /through Monday/...High confidence.

High pressure builds over region today with diminishing winds
and seas.

Low pressure will track SE of Nantucket Mon morning but is not 
expected to strengthen very much, so NE winds should remain 
below 25 kt. Seas will build on outer southern waters so we 
issued SCA for tonight into Monday. Snow tonight will change to
sleet and rain on waters south of Islands, but should remain as
snow into Monday on remainder of waters with low visibility at
times.

Outlook /Monday Night through Thursday/...

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of
snow. Areas of visibility 1 to 3 nm.

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

Wednesday: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of snow. Areas of visibility 1 to 3 nm.

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Rain likely, chance of snow, sleet likely,
freezing rain likely. Visibility 1 to 3 nm.

Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain, freezing rain likely.

&&

.TIDES/COASTAL FLOODING...
High astronomical tides next week with several 12 ft tides in Boston 
Wed and Thu! Thus won't take much onshore flow to produce minor 
coastal flooding along the eastern MA coastline. Greatest risk comes 
late Wed into Thu as surface wave tracks along or near south coast 
combining with Maritime high pressure to provide increasing onshore 
flow.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 10 PM this evening to 1 PM EST 
     Monday for CTZ002>004.
MA...Winter Storm Watch from late tonight through Monday afternoon 
     for MAZ017>022.
     Winter Weather Advisory from midnight tonight to 1 PM EST 
     Monday for MAZ007-013>016.
     Winter Weather Advisory from 10 PM this evening to 1 PM EST 
     Monday for MAZ009-011-012.
RI...Winter Storm Watch from late tonight through Monday afternoon 
     for RIZ001>007.
MARINE...Small Craft Advisory from 3 AM to 6 PM EST Monday for 
     ANZ254>256.

&&

$$
SYNOPSIS...Nocera/JWD
NEAR TERM...Frank
SHORT TERM...JWD
LONG TERM...Nocera
AVIATION...Frank/Nocera/JWD
MARINE...Nocera/JWD
TIDES/COASTAL FLOODING...Staff