Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 161746
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1246 PM EST Mon Jan 16 2017

.SYNOPSIS...
High pressure maintains dry and seasonably cold conditions into
Tuesday morning. Low pressure pushing E out of the Ohio River
Valley will transition S of New England into Wednesday ahead of
which there is the likelihood of snow and ice impacting much of
the interior, specifically late Tuesday into Wednesday morning.
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 PM THIS EVENING/...
1245 PM update...

No changes to the near-term forecast.

Quiet weather this afternoon with sunshine and temperatures in the
mid 30s to lower 40s. Southwest winds have been gusting to 10 to
20 mph along the coast.

All attention is being focused on the complicated forecast for
Tuesday night. Precipitation type will hinge on the exact vertical
profiles, which will be challenging for this situation. Could see
a little more snow than currently forecast along the VT/NH border
but not totally convinced yet.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
High pressure remains in control tonight, even with its core to
our southwest. Not much moisture to work with, so a weak cold
front late tonight should move through without much fanfare.

Forecast gets much more complicated during Tuesday. Only have
moderate confidence in the details at this point. 16/00Z guidance
continues to suggest precipitation arrives during Tuesday
afternoon, particularly across the western half of southern New
England. The main issue there, will be low level humidity. It`s
not expected to be very high to start, and am thinking it will
take a while for the column to saturate, and permit the
precipitation to reach the ground.

However, that will have the consequence of evaporational cooling,
which could lead to a wintry mix. Thinking a rain/snow mix would
be most likely at the onset, before the arrival of warmer air
aloft introduces the possibility of sleet and freezing rain as
well. The Tuesday afternoon commute could get messy, particularly
across the higher terrain of central and western MA. These details
are not set in stone. Will need to monitor over the next 24-30
hours.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
*/ 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...

Tuesday Evening into Wednesday...

An over-running setup ahead of low pressure emerging out of the C
CONUS against a 1030+ high over SE Canada. Focus upon a front-end
thump with low-mid level isentropic lift of a warm-moist conveyor
belt of more continental-origin ascending above a shallow surface
cold dome courtesy of cold air damming. Anticipating an interior
wintry mix consisting of snow to sleet to freezing rain, possibly
changing over to all rain, with precipitation onset roughly around
late Tuesday into Tuesday evening W to E across S New England,
concluding into Wednesday as low pressure transitions S of New
England in regions of better baroclinicity and out to sea. Areas
of greatest concern with regards to ice accretion are across the
high terrain, namely elevations in excess of 500 feet, definitely
at or above 1000 feet.

Some keys to the forecast: 1) Low level thermal profile especially
at the surface, 2) the presence of ice within the column, and 3) the
magnitude of lift / forcing upon available moisture. Focus closely
on a non-GFS consensus and probabilistic guidance for late Tuesday
into Wednesday morning timeframe.

To put it simply, the warm intrusion aloft around H8 as the layer
below down to the surface saturates below-freezing, down to the wet-
bulb, yields a transition of precipitation across the interior as
most likely we`re all aware. Greatest concern is for those areas
of high elevation above 500 ft especially 1000 ft where surface
temperatures are most likely going to be at or below freezing,
maintained by a N ageostrophic / isallobaric flow from regions
with notable snow depth.

Precipitation falling, and likely transitioning from, sleet and/or
freezing rain potentially not going over to plain rain throughout
the entire event across the interior. Some snow is possible at the
very beginning but feel it will be brief for two reasons: 1) the
warm intrusion around H8 is already pushing N/E across the region
into NH at onset of precipitation, and 2) drier air within the mid
levels is not far behind, eroding moisture within the dendritic
growth zone around roughly midnight. Mainly rain for the coastal
plain.

Continued strong signals among the probabilistic guidance. From CIPS
analogs there is an increasing signal of a significant freezing
rain event. EPS/GEFS/SREF signal precipitation onset closer to
Tuesday evening, targeting higher elevations with the greater
likelihood of seeing a wintry mix. While the high-res guidance
targets the high elevations such as the Berkshires, Worcester,
Litchfield and Tolland Hills, SREF plumes exhibit some chaos with
respect to precipitation outcomes at point locations. Thus there
is a level of uncertainty.

But despite the level of uncertainty, Winter Weather Advisories have
been issued for Western and Central MA as well as N CT. There has
been a persistent signal of sleet / freezing rain over the advised
area by a consensus of forecast guidance and probabilistics. Only
need a trace of freezing rain to reach advisory criteria.

Significant ice accretion expected with one to two tenths forecast
in the Berkshires in agreement with the forecast from WPC. Adverse
travel impacts accordingly. Also forecasting up to an inch of snow
and/or sleet accumulation prior to the transition to freezing rain.
Most of these snow / sleet accumulations across the Berkshires and
Worcester Hills. Continued concern of ice accretion for those areas
across the interior that see a change over to rain. Not expected to
warm quickly and given the likelihood that the ground will be near-
freezing there is the possibility of continued ice accretion even
with air temperatures above freezing. Something to keep in mind.


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 /18Z 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/...High Confidence.

1745z update...No changes to the near-term forecast.

VFR. Dry weather and mainly light winds through Tuesday morning.
MVFR developing from W to E Tuesday afternoon. Increasing risk for
a wintry mix of RA, SN, FZRA and IP across the higher terrain of
central and western MA late Tuesday afternoon.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

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

Conditions lowering late Tuesday into Tuesday night to IFR-LIFR.
Threat of SN/IP/FZRA especially across the interior. Ice accretion
forecast along with some potential low visibilities. 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/...High Confidence.

1245 PM update...No changes to the forecast.

This afternoon and tonight...High pressure moves S of the waters
this afternoon. SW winds have developed and with a brief increase
in the pressure gradient this afternoon and evening, seeing some
gusts up to 25 kt. Marginal Small Craft Advisory continues for some
of the eastern coastal waters, especially the outermost waters.
Dry weather and good visibility prevail.

Tuesday...A weak cold front should move through during the
morning, followed by a quick wind shift to the east; ahead of an
approaching warm front. Good visibility should prevail, with only
a chance of rain across the RI coastal waters late in the day.

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

Warm front lifting across the waters ahead of which easterly flow
increases, strongest Tuesday night into Wednesday as an 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 7 PM Tuesday to 7 AM EST
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 4 PM Tuesday to 7 AM EST
     Wednesday for MAZ002-008-009.
     Winter Weather Advisory from 7 PM Tuesday to 7 AM EST
     Wednesday for MAZ003-004-010>012-026.
RI...None.
MARINE...Small Craft Advisory from 4 PM this afternoon to 11 PM EST
     this evening for ANZ231-251.
     Small Craft Advisory from 4 PM this afternoon to 2 AM EST
     Tuesday for ANZ250-254.

&&

$$
SYNOPSIS...Belk/Sipprell
NEAR TERM...Field
SHORT TERM...Belk
LONG TERM...Sipprell
AVIATION...Field/Belk/Sipprell
MARINE...Field/Belk/Sipprell


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