Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 162200
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
500 PM EST Mon Jan 16 2017

.SYNOPSIS...
Low pressure will move from the Great Lakes to New York state
with secondary low pressure forming near Long Island Tuesday
and passing east of Nantucket by Wednesday. A mixed bag of
wintry weather will impact much of southern New England from
Tuesday afternoon into Wednesday. Could see leftover spotty
showers along the shore late this week. High pressure will build
along the eastern seaboard with dry, milder conditions into next
weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
450 PM update...

A high pressure ridge will remain over New England tonight
with mainly clear skies to start, then increasing mid and high
level cloudiness from southwest to northeast overnight.
Low temperatures should bottom out in the upper teens in northwest
MA, ranging to the mid 20s elsewhere, except lower 30s on the
Islands.

&&

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

Low pressure moving eastward from the Great Lakes to New York
state will spawn a secondary low pressure area near Long Island
Tuesday morning, which will head east toward Nantucket late
Tuesday night. It will cause a wide variety of messy weather
across our region.

All models are coming into good agreement on the timing.
Precipitation is expected to break out in western MA and northern
CT in the afternoon and spread eastward toward evening.
The most difficult part of this forecast is the precipitation
types.

For this forecast package, we have expanded the Winter Weather
Advisory to include central Middlesex and western Essex Counties
in northwest MA. We have upped snowfall totals significantly over
interior and especially northern MA.

Although the typical 1540m value for 850-700 mb partial thickness
will be to our northeast through the bulk of the event implying
warm air aloft, cold air will remain in the low layers of the
atmosphere. The 1000-850 mb partial thickness values remain below
1310m over a large part of MA through Tue. night. In fact, the
ECMWF and GFS show a pronounced lowering of the 1540m line down
to 1535m in northeast MA as heavier precipitation moves in Tuesday
night and evaporational/dynamic cooling takes place. All model
snowfall algorithms and the NCEP WPC forecast have at least 3
inches of snow, with perhaps as much as 6 inches occurring in the
Route 2 corridor of northern MA. Sleet and freezing rain will be
the predominant type of precipitation in northern CT and in
western and central MA south of the MA pike, before likely
changing to all rain. Up to one-tenth inch of ice accretion is
expected, although we cannot rule out local two- tenths amounts,
especially in the slopes of the Berkshires.

In northern RI and interior eastern MA, an inch or two of snow
is possible before turning to rain. All rain is expected in
southern RI and southeast MA.

A lot could change with this situation. A small change in
temperatures, both at the surface and aloft, could result in
big changes of precipitation type and corresponding snowfall
amounts.

A thin coating of ice on untreated roadways can be dangerous.
Please slow down and use caution if driving late Tuesday and
Tuesday night.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
*/ Highlights...

 - Winter Weather Advisories posted for a wintry mix
 - Western and Central MA, as well as N CT
 - Tuesday evening into Wednesday
 - Ice accretion forecast up to two tenths, impacts to travel

*/ Discussion...



Remainder of the week into the weekend...

Individual shortwaves either emerging from the N-branch of Pacific
flow or from the preferred troughing regime over the SW CONUS. Feel
there is poor handling within deterministic model forecasts and thus
preference to ensemble means to smooth out the noise. In the end,
the squeeze of high pressure between a downstream blocking pattern
as inferred from a -NAO trend and preferred low pressure development
to the lee of the Rockies may yield a longer period of dry weather
till we get into next week. Low confidence. Will keep any outcomes
simply chance for now and not get into any particulars. Much of the
focus is on the next 24 to 48 hours where confidence is higher.

&&

.AVIATION /22Z 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 Night/...High Confidence.

2150Z update...

Tonight...VFR.

Tuesday...VFR to start. MVFR developing from W to E in the
afternoon. Wintry mix of SN, IP, FZRA, and RA spreading east
during the afternoon. Conditions then becoming IFR throughout the
region. Snow may be locally heavy reducing visibility in the
higher terrain areas of northern and western MA.

Tuesday night...IFR in SN, IP, FZRA interior, changing to rain and
in RA in RI and southeast MA. Easterly winds gusting to 20-30 kt
near the coast late at night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Wednesday through Saturday/...Moderate confidence.

Northerly winds throughout, gusts up to 30 kts at time especially
across the E coast of MA. Improvement through Thursday with only
ocean effect MVFR ongoing across E MA. Winds diminishing out of
the NW. Low confidence forecast Friday and beyond.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High Confidence.

450 PM update...

Tonight...Have dropped Small Craft Advisories for this evening, as
wind gusts have diminished to below 25 kts. Dry weather and good
visibility.

Tuesday...Winds becoming easterly and increasing to 10 to 15 kt.
A chance of rain late in the afternoon, especially along the RI
coast.

Tuesday night...Low pressure will be passing near or just south of
Nantucket. Small Craft Advisories have been issued for the overnight
hours into Wednesday for the eastern outer waters. Seas will be
increasing to 3 to 6 ft as easterly winds persist...and gusts
increase to 20-30 kt over the eastern outer waters.

Outlook /Wednesday through Saturday/...Moderate confidence.

Area of low pressure develops S offshore deepening N/E towards SE
coastal Canada. Gusts up to 30 kts during this development with
waves potentially building to 10 feet on the E outer waters. Will
see wind and wave activity diminish into Thursday as the storm
continues to slip E and winds turn out of the NW becoming light.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST
     Wednesday for MAZ003>005-010>012-026.
     Winter Weather Advisory from noon Tuesday to 7 AM EST
     Wednesday for MAZ002-008-009.
     Winter Weather Advisory from 4 PM Tuesday to 7 AM EST
     Wednesday for MAZ006.
RI...None.
MARINE...Small Craft Advisory from 1 AM to 5 PM EST Wednesday for
     ANZ250-251-254-255.

&&

$$
SYNOPSIS...EVT/GAF
NEAR TERM...GAF
SHORT TERM...GAF
LONG TERM...EVT
AVIATION...EVT/GAF
MARINE...EVT/GAF



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