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Area forecast discussion 
National Weather Service Taunton Massachusetts
400 PM EST Sat Feb 28 2015

Synopsis...
a fast moving low pressure system will bring accumulating snow to
the region late Sunday into Sunday night...with the highest amounts
expected south of the Massachusetts Turnpike and away from the
immediate South Coast. High pressure brings dry cold air for the
early week. Low pressure from northern Canada and associated low
forming over the plains Monday will merge...then pass west and
north of New England. This will swing a cold front across the
region on Wednesday. Dry and cold weather returns for the end of
the week.

&&

Near term /until 6 am Sunday morning/...
into this evening...

High pressure pushing east into the region. Temperatures expected to
quickly drop with mostly clear conditions and light winds.

Tonight...

Cold. Conditions will be just right to allow for good radiational
cooling...especially to the east where the impact of increasing middle- to
high-clouds late in the period will be less. Looking at sub-zero
lows for low-lying areas where cold-air drainage effects off the
high terrain are maximized. High-terrain should remain at or above
zero.

&&

Short term /6 am Sunday morning through Sunday night/...
*/ Sunday morning into midday...

Quiet weather. Clouds thickening as winds increase out of the S.
Will see temperatures gradually moderate into the middle- to upper-20s.

*/ Sunday afternoon into early Monday morning...

1) highlights...

- snow Sunday afternoon into Monday morning
- greatest intensity Sunday evening and overnight
- 6+ inch snowfalls expected but exactly where remains uncertain
- annoyingly...forecast guidance continues to wobble

1) overview...

Broad isentropic over-running event of Gulf moisture along the 275-
295k surfaces amplified by a north-stream impulse during the timeframe
possibly invoking an area of low pressure that skirts along the S-
coast subsequently enhancing low-level wind profile. Light-moderate
snowfall is anticipated with locally heavier amounts. The intense
portion of snowfall is expected Sunday evening and overnight ending
early Monday morning.

Model preference given to the European model (ecmwf)/GFS/regional Canadian which will
serve as the consensus of guidance for this discussion. The 28.12z
NAM was considered an outlier with its aggressive-Mode and secondary
low development just offshore.

2) precipitation-type...

Evaluating thermal fields...blend of forecast consensus puts the
-2c h925 isotherm west-east across S-tier of CT/Rhode Island and across southeast Massachusetts with
the 0c 850 mb isotherm just S of Long-Island and Nantucket coastlines.
The consensus of 1000-850 mb critical thicknesses lays right along
the immediate S-coast of Long-Island and Cape Cod.

An all snow-event when considering dynamic-cooling/wetbulbing of the
low-levels ahead of the event. There is perhaps a chance of a brief
mix-over to rain/freezing rain across the cape and islands as S-flow
at the surface is possibly able to nudge above-freezing temperatures
north. Yet as we are in a frozen Tundra...with any rain it would likely
freeze on surfaces which are quite cold. Thus freezing rain prevails
over rain in the forecast.

3) snowfall...

Forecast consensus of a 0.3-0.6 north-S precipitation event with a north-S
snow-to-liquid ratio gradient ranging from 15:1 to 10:1 yields high-
end Snow Advisory criteria with snowfall amounts ranging on average
around 3-6 inches. Areas of 6+ inches are expected...especially from
central-CT east into southeast-Massachusetts /including the upper-cape/. A challenge and
a lot of uncertainty with respect to headlines and decision-making.

Evaluating low-level thermal profiles towards snow-to-liquid ratios
derived via a Cobb-technique...with the greatest lift occurring
just beneath or within the dendritic-growth zone based on a
consensus of BUFKIT GFS profiles...especially in heavier- precipitation
areas...believe the snow will range somewhere between a wet and
fluffy snow. Snow properties from fluffy to wet likely similar to
the north-S gradient as outlined above.

4) headlines...

Hate a wobbling forecast which is typical with such small events.
Uncertainty derives from low-level thermal profiles and precipitation
amounts. Surely expecting snow...but with any shift north-S of colder
air and/or precipitation amounts results in areas shifting from
advisory to warning...or vice versa. Enough to just pull your hair
out. Then there is the timing of the system that has sped up
slightly. A shorter residence time of course would yield lesser
snowfall amounts. Quite a challenging forecast right on the cusp
of headline thresholds.

The event does take place mainly from Sunday evening into the early
morning hours on Monday during which time there is not much traffic
on the Road. Is this a high impact event that warrants warnings? A
question to struggle with as warnings are necessary when we are
confident in meeting or exceeding 6+ inch criteria. Yet the question
in front of US is where will the higher amounts of snow focus? Have
to make some sort of call...the needs of the many outweigh the needs
of the few.

Advisories posted where there is high confidence of snow but likely
to remain below the 6-inch threshold for warning. Watches continue
and have been extended to the S-coast to cover for the uncertainty
of the area either falling within or in excess of advisory-level
snows. Again...6+ inch amounts are anticipated but exactly where is
uncertain.

5) closing notes...

Some concern for the early Monday morning commute for east/southeast Massachusetts and
Rhode Island. While most of the snow is expected to be immediately offshore
with sunrise Monday...commuters could encounter slick and slippery
Road conditions. Snow could possibly linger over southeast Massachusetts so there is
the potential for reduced visibilities.

&&

Long term /Monday through Saturday/...

Big picture... the broad upper flow shows a ridge in the east and
trough in the west with a west southwest flow from the plains to the
eastern USA coast. Within this flow are both northern and southern
streams at the start of the week. The western trough migrates east
during the midweek at which time the two streams merge to form a 140-
160 knots upper jet feeding northeast into the Great Lakes and St
Lawrence Valley. This jet lingers over or near New England Thursday-
Friday before moving off. The western trough moves across New
England by the end of the week.

Dailies...

Monday... Sunday storm moves off past Nova Scotia while the
supporting upper shortwave and cold pool swing across New England.
The coolest air aloft is in northern New England while leftover
moisture lingers below 800 mb through the day. The GFS is faster
than the European model (ecmwf) in bringing rising pressure to the region but have 1
mb/hour rises here by evening. Put together...expect a clearing
trend but with leftover clouds especially north of the Mass Pike.
Sustained northwest winds should rise to 15-20 kts. Mixing reaches
to at least 850 mb and possibly a little higher...which supports
gusts to at least 30 knots and possibly 35 knots. Temperatures at 850 mb
will be -11c to -14c...mixing this to the surface would support maximum
surface temperatures in the 30s. Min temperatures upstream this morning were in
the single numbers with a few spots below zero. With light pressure
gradient especially in the west...we tended a few degrees cooler than
consensus.

Tuesday-Wednesday... high pressure in control much of Tuesday but
moving offshore during the afternoon. Cross sections show increasing
moisture at middle and high cloud level...so increasing high clouds.
System moving into the Great Lakes spreads precipitation into New York...but
warm advection lift remains mostly to our west through the
afternoon. Some lift forecast at 290k /about 730 mb at the time/ but
none below that. Meanwhile dewpoints in the single numbers progress
into the teens during the day...but enough of a temperature/dewpoint spread
to slow the advance of any precipitation from the clouds by a few hours.

Warm advection lift much of the night supports the idea of precipitation.
Temperatures start cold during the evening but should rise
overnight. The question remains as to how high they will climb. Even
with the surface system tracking up the St Lawrence Valley there is
cold ground and a cold snowpack. This would allow the warmer temperatures
to scream overhead while the immediate surface remains several
degrees colder. We trimmed temperatures back a little from model
values...but low confidence at this time due to the potential that
surface temperatures could remain in the 30s.

Quantitative precipitation forecast forecasts range from 0.50 to 1.00 inches with the higher amounts
tending toward the South Coast. This points to a messy storm with
initial snow accums of several inches followed by a change to
measureable ice accumulation and then a change to rain with
potential for poor drainage flooding in between the snow banks.

The cold front should move through during the afternoon/evening with
a shift to northwest winds. This will change the precipitation back to snow
before tapering off.

Thursday...diferences in the GFS and European model (ecmwf) in handing the upper jet.
The European model (ecmwf) moves the jet offshore and puts southern New England
into subsidence. The GFS maintains the jet overhead with our area
in the right entrance region. The first scenario ends the precipitation and
cools the air. The second scenario would support lift and generate
waves along the departing cold front...maintaining a chance of
snow over at least southern parts of our area. Our Thursday
forecast tends toward the latter for now.

Friday-Saturday... high pressure builds over the region Thursday
night and Friday with clearing skies and a brief period of cold air.
Temperatures aloft trend up about 10c on Saturday suggesting less chill at
the surface. Another cold front approaches the St Lawrence Valley
and could spread some clouds into our area Saturday
afternoon/evening. But precipitation chances look small.



&&

Aviation /21z Saturday through Thursday/...
forecaster confidence levels...

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

Short term /through Monday morning/...

21z update...

Tonight through Sunday midday...high confidence.

VFR. Lowering and thickening clouds Sunday morning into midday.
Low-end VFR to west-terminals by end of period.

Sunday afternoon into Monday morning...moderate confidence.

Snow overspreads the region W-E. Ceilings/visibilities deteriorate to MVFR-
IFR by early evening...widespread IFR-LIFR Sunday night. Snow to mix
with or change to freezing rain across offshore island terminals overnight.
Precipitation ends west-east toward daybreak Monday....conditions improve.

Kbos taf...moderate confidence. Trends certain but not so much the
actual timing of impacts.

Kbdl taf...moderate confidence. Trends certain but not so much
the actual timing of impacts.

Outlook...Monday through Thursday...

Monday... high confidence. Some IFR/MVFR in snow early morning but
otherwise conditions will improve to VFR during the morning.
Northwest winds will increase with gusts around 30 knots by midday.
VFR and diminishing wind Monday night.

Tuesday...moderate confidence. VFR during the day with increasing
sky cover. MVFR Tuesday evening becoming IFR/LIFR overnight in
developing snow and freezing rain.

Wednesday...moderate confidence. IFR/LIFR in rain and freezing rain.

Thursday...low confidence. VFR north of a hfd-bos line. MVFR ceilings
and areas of MVFR/IFR visibilities in snow.



&&

Marine...
forecaster confidence levels...

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

Short term /through Monday morning/...high confidence.

High pressure nearby will keep winds/seas well below Small Craft
Advisory thresholds into Sunday morning. A weak wave of low
pressure moves across the waters late Sunday into Sunday night...
but weak pressure gradient should keep winds/seas below small
craft thresholds.

Outlook...Monday through Thursday..moderate confidence.

Monday...west to northwest winds increase with frequent gusts to
gale force especially away from the nearshore. This could generate
some light freezing spray especially early Monday night. Seas 5
to 10 feet. Winds and seas diminish Monday night. A Gale Warning
may be needed.

Tuesday... lingering 5 foot seas on the outer waters but diminishing
during the morning. Winds and seas increase again Tuesday night
after midnight ahead of a Great Lakes cold front. South winds will
approach 25 knots with seas building to 5 to 8 feet. Visibilities will
be reduced in snow changing to rain.

Wednesday...southwest winds with gusts 25-30 knots. Seas building
to 6 to 12 feet. Reduced visibilities in rain. A Small Craft Advisory may
be needed.

Thursday...northwest winds will gust to 25 knots...and seas 5 to 8
feet. Both should diminish toward evening. A Small Craft Advisory
may be needed.

&&

Box watches/warnings/advisories...
CT...Winter Storm Watch from Sunday afternoon through Monday
morning for ctz002>004.
Massachusetts...Winter Weather Advisory from noon Sunday to 7 am EST Monday
for maz002>008-010-014-023-024-026.
Winter Storm Watch from Sunday afternoon through Monday
morning for maz009-011>013-015>022.
Rhode Island...Winter Weather Advisory from noon Sunday to 7 am EST Monday
for riz008.
Winter Storm Watch from Sunday afternoon through Monday
morning for riz001>007.
Marine...none.

&&

$$
Synopsis...wtb/sipprell
near term...sipprell
short term...sipprell
long term...wtb
aviation...wtb/sipprell
marine...wtb/sipprell

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