Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 170858
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
358 AM EST Tue Jan 17 2017

.SYNOPSIS...

A mixed bag of wintry weather will impact much of S New England
from later today into Wednesday morning. Could see leftover spotty
showers lingering Wednesday. High pressure builds over the region
for Thursday through the weekend. Much of the period looks dry and
seasonably mild. Low pressure developing off the Carolinas Friday
could bring some light rain or snow Saturday, but at this time
most of that should remain offshore. A more substantial weather
system may develop over the Plains this weekend, and has a chance
of bringing precipitation to our area Monday.

&&

.NEAR TERM /UNTIL NOON TODAY/...

Quiet this morning this increasing and thickening cloud decks as E
onshore flow warms temperatures into the upper 30s to low 40s. The
blanket has kept temperatures from dropping considerably overnight.
Lows this morning observed in the 20s for the most part with a few
spot locations in the teens. Low 30s along the immediate S-coast
and over the Islands.

&&

.SHORT TERM /NOON TODAY THROUGH 6 PM WEDNESDAY/...

*/ Highlights...

 - Winter Weather Advisories continue...
 - Threats developing towards the PM commute
 - Wintry mix of snow, sleet, freezing rain
 - Snow accumulations generally 2 to 4 inches across MA
 - Ice accums of a trace to a tenth mostly over the Berkshires

*/ Discussion...

Overview:

An initial 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 of an isentropically ascending warm-moist conveyor belt
over a shallow surface cold dome. Subsequent mixed-bag of precip
beginning during the later-half of Tuesday into Tuesday evening,
sweeping W to E. Transition to an immediate offshore low overnight
ahead of a mid-level positively-tilted trough across regions of
favorable baroclinicity offshore, colder and drier air is drawn S
across New England. Mixed-bag of precipitation transitions to mainly
rain and/or snow as storm exits through Wednesday.

Timing:

Focus on the later half of Tuesday into Tuesday night with height of
the outcomes Tuesday evening. Lingering light showery weather into
Wednesday.

Trends:

With onset will see low-levels saturate towards their respective wet-
bulb, this as warmer air and rising thicknesses encroach from the SW.
Will see onset of rain change over to a mix of snow, sleet and/or
freezing rain, depending on location. As low pressure develops S of
New England overnight into Wednesday will see a combination of cold
air being drawn S from the 1030 high over SE Canada with an E flux
of milder onshore winds. Precipitation types vary. Dependency on the
temperature at a particular location and the thermal profile aloft,
whether there is ice present as well. Will discuss the uncertainty
below.

Drier air working in from the W towards Wednesday and through the
day, low-levels still saturated with a measure of lift, some light
showery weather lingers. Ice in the column in question, but also
will see temperatures warming. Some places, particularly the high
terrain, may still be dealing with a wintry mix of light precip.

Uncertainty:

Thicknesses and low level thermal profiles in doubt with respect to
the depth and magnitude of both intruding warmer air around H8 and
colder air immediately below to the surface. This in addition to
surface 2m temperatures and whether ice is present in the column.
Subsequently there is a whole mixed bag of precipitation outcomes
that make it entirely challenging to get the forecast exactly right.
There are going to be some tight gradients with respect to outcomes,
especially across interior S New England, more specifically Essex
and Middlesex Counties in MA, and likely some angry customers that
we didn`t get the forecast right.

Several concerns and unknowns:

1) Duration of precipitation-type. For example, whether we get more
sleet and/or freezing rain than snow which yielding lower snowfall
totals. Forecast guidance has trended slightly colder limiting the
intrusion of warmer air aloft, more than likely with the stronger
and prolonged presence of low pressure developing S of New England
overnight. Plus greater indications of ice being present within the
column throughout the event, more than likely with mid-level ascent
per follow-up positively tilted H5 trough. Still challenges in the
interpretation of bufkit profiles as to the magnitude and strength
of respective warm intrusion around H8 and colder air immediately
below possibly down to the surface (think Bergeron principles).

Consensus of probabilistics have increased in probability of a 6-
inch snowfall as have deterministic forecast snow algorithms. Am
inclined to believe we`ll see greater snowfall outcomes over N MA
especially over the high terrain before mixing with or transitioning
over to sleet and/or freezing rain. This in agreement with the
consensus of forecast guidance and WPC forecast and probabilities.

2) The exact evolution of 2m wet-bulb temperatures, where they will
be at or below freezing during the event. Confident N flow as low
pressure develops and deepens S of New England during the overnight
period. Ageostrophic / isallobaric response drawing colder air S
from high pressure over SE Canada with H925 temperatures around -20C
should maintain a level of colder air across the region, especially
the high terrain. Leaning with high-res WRF / NAM solutions with
forecasting 2m temperatures, keeping sub-freezing conditions in
longer especially as low pressure develops immediately S overnight.
Toughest part to determine is the sharp coastal front boundary
likely over Essex / Middlesex Co MA. Going to be a tight gradient in
temperatures complicating the forecast.

3) What happens into Wednesday. Drier air working in aloft as low
levels remain saturated. This subsequent of the low pressure still
deepening S of New England as it moves out to sea. A strong onshore
surface flow while a mid-level trough sweeps the region invoking
ascent aloft. Light showery weather / drizzle forecast but uncertain
is the presence of ice in the column. Some indications, leaning a
mix of drizzle, freezing drizzle, light snow / flurries that either
taper off or change to liquid through Wednesday as temperatures warm
at the surface. Will add to snow total amounts early, complicated by
not knowing exactly where the aforementioned coastal front over NE
MA will setup. Will keep with high-res forecast guidance 2m temps.

Model Preference:

Holding close to the suite of high-res guidance inclusive of the
NAM, WRF model solutions, and Canadian. Trending less with global
model solutions, however evaluating trends from the UKMET with
respect to 2m and H85 temperatures.

Forecast Outcomes:

Snow...A general 2 to 4 inches with locally higher amounts, mainly
over the high terrain, for areas N of the I-90 corridor extending
N/E somewhere in-between the NE corridor of the I-495 / I-95 belt-
ways (where the aforementioned coastal front is expected to reside).
Lighter amounts for areas S into the N-tier of CT / RI.

Ice..Greatest icing expected over the Berkshires, perhaps even the
Worcester and Tolland Hills at the highest elevations, with ice
accretion amounts ranging from a trace up to a tenth of an inch.

Headlines:

No adjustment to headlines both spatially or temporally. Areas such
as Middlesex and Essex County MA are areas of dispute given their
location with respect to an anticipated coastal front. Definitely
going to see a gradient across the region of outcomes but not
exactly sure where.

Continued confidence in tandem with ensemble probabilistics that the
higher elevations as well as N MA will see the greatest impact with
respect to wintry weather. Areas of greatest concern with regards to
impact are across the high terrain, namely elevations in excess of
500 feet, definitely at or above 1000 feet.

Impacts:

Focusing on the Tuesday PM and Wednesday AM commute. Expected wintry
precipitation will likely result in hazardous travel conditions,
especially on untreated roads. Bridges and overpasses are most
susceptible. Echoing the previous forecaster, it only takes a thin
coating of ice to make conditions dangerous.

Best advice is to remain aware of the oncoming weather, stay tuned
to the latest forecasts, and if you have to travel at any time
Tuesday into Wednesday, plan accordingly and give yourself extra
time as necessary.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Big Picture...

Flat upper flow with embedded shortwaves midweek evolves by late
week into a trough west/ridge east pattern.  But this pattern shows
serial low pressure areas ejecting from the western trough into the
Eastern USA.  The first wave moves to the Plains Wednesday night and
jumps to the East Coast Saturday, then moves off. The second wave
moves ashore from the Pacific by Friday, and sweeps to the Southeast
USA by Monday.

Long range models show similar mass fields through Friday, then some
differences emerge over the weekend.  Even with the differences, the
fields show a general agreement on storm occurrence even at day 7. On
the other hand, the features that may affect us next Sunday-Monday
will not reach land until late this week, so plenty of time for
shifting model solutions.

Contour heights at 500 mb spend the forecast period in the 550s or
higher, rather than the normal for January 540s. Meanwhile 925 mb
heights are near normal during late week and above normal over the
weekend. Expect temps on the mild side of seasonable.

Details...

Wednesday night and Thursday... Moderate confidence.

The surface low and its associated upper reflection will shift off
to the east. Low level flow remains N-NE Wednesday night and then
turns from NW Thursday. Forecast moisture fields show lingering
moisture below 850 mb. Thus a potential for clouds much of the time,
with a clearing trend from the west or southwest Thursday afternoon.
Also lingering sprinkles/flurries Wednesday night, then dry
Thursday. Forecast dew points suggest min temps will mostly be in
the 30s, possibly a couple degrees cooler in the northwest. Temps
aloft Thursday will support max temps in the low to mid 40s.

Friday through Sunday... Moderate confidence.

High pressure builds over the region surface and aloft and should
provide dry weather most of the time. Upper trough, supported by the
left exit region of a southern stream jet, slips through the ridge
Friday night and passes south of New England. The left exit region
is close enough to us to leave open an overnight chance of
precipitation, mainly rain, in areas south of the Mass Pike. This
should quickly move out to sea. The high will then reassert itself
with partly cloudy skies the rest of the weekend. Temps in the mixed
layer remain similar Friday and Sunday, and a little milder on the
cloudier Saturday. Continue to expect highs in the upper 30s to mid
40s, lows in the upper 20s to mid 30s.

Monday... Low confidence.

The main long term models generally agree on the next weather system
coming onshore from the Pacific Ocean about Friday and generating a
closed low over the Southern Plains over the weekend, then trending
northeast toward New England. Currently, all suggest precipitation
moving into our area on Monday, and all hint at cold air lingering
in the interior.

The agreement and the potential weather are impressive, but the
system is still feet-wet over the Pacific and with 7 days for the
pattern to evolve.  We will indicate chance pops for Monday and
watch developments.

&&

.AVIATION /08Z TUESDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

Short Term /through Wednesday/...Moderate Confidence.

9z update...

Today...

VFR much of today. Lowering towards IFR-LIFR by late afternoon
into evening. Precipitation spreading across the terminals W to E
either as RA and/or a wintry mix of SN/IP/FZRA. Low visibility
with SN. Increasing E onshore flow over much of S New England with
a more N flow in the CT River Valley. Gusts up to 20 kts along the
E shore of MA.

Tonight...

IFR-LIFR over much of the region. Either RA or a continued wintry
mix of SN/IP/FZRA potentially changing to RA. Increasing E onshore
flow becoming N towards morning. N flow prevailing in the CT River
Valley. Gusts up to 30 kts for the E MA shore.

Wednesday...

Precipitation diminishing but IFR-LIFR conditions remain. Threats
of either DZ or a wintry mix of FZDZ/SN. N winds persist with gusts
up to 30 kts across the E/SE shore of MA.

Specific Terminals...

KBOS TAF...With the increasing E onshore flow, believe the bulk of
precipitation for the terminal will be in the form of RA. Gusts up
to 30 kts tonight into Wednesday. Lowering down to LIFR with cigs
around 500 feet.

KBDL TAF...Will see a N flow prevail through the CT River Valley.
Initial onset of RA transitioning to a wintry mix of SN/IP/FZRA.
Late today into tonight. Light snow / ice accumulations are
forecast with snow on the order of several tenths and icing
roughly a trace.

Outlook /Wednesday night through Saturday/...

Wednesday night-Thursday... Moderate confidence.

MVFR-IFR cigs Wednesday night and early Thursday. Brief MVFR vsbys
in drizzle possible near the eastern MA coast. Conditions improve to
VFR during Thursday.

Friday-Saturday... Moderate confidence.

VFR. An area of MVFR cigs/vsbys in light rain may move up into
CT/RI/SE Mass Friday night, then shift south offshore Saturday.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Wednesday/...Moderate Confidence.

Increasing E winds beginning late today and continuing overnight
into Wednesday as an an area of low pressure develops and deepens S
of New England pushing E out to sea late. Gusts up to gale force
are possible, especially for the SE outer waters. Waves building
into Wednesday up to 6 to 9 feet especially across the E/SE waters.
Rain expected with the developing low pressure. Could see brief
reductions in visibility along with mist. Small Craft Advisories
are in effect.

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

Wednesday night...

Northeast winds with frequent gusts to 25 knots early in the night.
Winds then diminish overnight. Seas 5 to 10 feet on the exposed
waters will start to subside toward morning. Small Craft Advisories
will continue for most waters.

Thursday...

Seas subside to less than 5 feet by Thursday night. NW winds will be
15 knots or less. Small Craft Advisory may be needed on the outer
waters.

Friday-Saturday...

Northeast winds 15 knots or less.  Seas 4 feet or less. An area of
rain may move across the southern waters Friday night with brief
reduction of visibility.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...Winter Weather Advisory from 1 PM this afternoon to 7 AM EST
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 1 PM this afternoon to 7 AM EST
     Wednesday for MAZ003>006-010>012-026.
     Winter Weather Advisory from 10 AM this morning to 7 AM EST
     Wednesday for MAZ002-008-009.
RI...None.
MARINE...Small Craft Advisory from 11 PM this evening to 7 PM EST
     Wednesday for ANZ230>237-250-251-254>256.

&&

$$

SYNOPSIS...WTB/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell



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