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Area forecast discussion 
National Weather Service Taunton Massachusetts
1015 am EST sun Mar 1 2015

Synopsis...
a fast moving low pressure system will bring accumulating snow
later today and especially tonight with the highest amounts
expected south of the Massachusetts Turnpike.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 3 PM this afternoon/...

10 am update...

Forecast continues to be mainly on track...middle deck and lower clouds
working their way across the area with precipitaiton gradually
working its way through the dry layer below. Kbox radar imagery as of
15z shows precipitation returns as low as 5-8kft. 15z metars indicate light
snow reaching the surface in the Danbury CT area. Continues to
look like 15z-18z for first snowflakes to fall across western
zones...18z-20z easternmost zones. Minor changes made to hourly
temperatures to reflect latest observation...forecast highs look on track.

Early morning discussion...

Middle level cloudiness will stream into southern New England and
thicken during the morning hours in advance of approaching low
pressure. Southwest flow aloft overruning the cold dome at the
surface may allow for some light snow/flurries to break
out across the interior late this morning/early afternoon and then
find their way to the coast by middle to late afternoon. The lift
through middle afternoon is fairly weak...so the areas of snow should
be rather light. Any accumulations that occur will probably be
a dusting to less than 1 inch through that that time. High temperatures
will mainly be in the upper 20s to around 30.

&&

Short term /3 PM this afternoon through 6 am Monday/...
*** bulk of the accumulating snow occurs between 3 PM and 1 am with
the heaviest amounts south of the Massachusetts Turnpike ***

1) overview and timing:

The bulk of the accumulating snow will occur between 3 PM and
1 am across the region. The snow should taper off to flurries
toward daybreak Monday. The models do agree that the better forcing
will occur to the south of the Massachusetts Turnpike. This a result
of secondary low pressure near the South Coast...allowing for
some surface convergence as well as the intensification of the low
level jet.

The biggest issue is the models are not in agreement in how
intense the forcing will be across the region this evening. The
rgem/NAM are the most robust with quantitative precipitation forecast south of the Massachusetts Turnpike. The
NAM shows a period of fairly strong lift of 15 to 20 microbars per
second within the best snowgrowth region this evening. This would
translate to a period of 1+ inch per hour snowfall rates near and
south of the Massachusetts Pike. On the other hand...the GFS also has fairly
strong lift...but keeps it southeast of our region out over the
ocean. The secondary low pressure system on the GFS is weaker and
further southeast...resulting in the low level jet being displaced
further east. The European model (ecmwf)/UKMET solutions seem to be The Middle
Ground which we will lean towards.

2) winter weather headlines and snow accums:

With that said...the other dilemma is what to do with the current
headlines. This does not appear to be the type of setup where we
are going to receive widespread warning snowfall amounts. The
window for the very strong lift/forcing is short-lived and really
would like to see the low level jet a bit more backed to transport
deeper moisture northward. Therefore...feel that for most of the
region Winter Weather Advisory/S for 3 to 6 inches of snow will
cover it. However...given there is nice Omega in the snow growth
region...we might be looking at some decent ratios. So we still can
not rule out the low risk of reaching warning amounts and later
shifts will have to monitor closely...especially across northern CT
and northwest Rhode Island.

We did upgrade a small area from central and southern Rhode Island
into southeast Massachusetts including the South Coast to a Winter
Storm Warning. The nose of the low level jet looks to transport
some deeper moisture into this region. There also could be some
enhancement along the land/sea interface and a weak surface
boundary so went with 4 to 8 inches in this region.

3) ptype and impacts:

The models have trended colder over the past 24 hours.
Therefore...expect this be pretty much a snow event. It does become
a very close call near the South Coast...but were leaning towards it
staying mainly wet snow. If precipitation lightens up that may
increase this risk we see a brief change over to rain.
Meanwhile...we went with a high end Winter Weather Advisory for the
cape because they may change to rain for a time...especially east of
Hyannis. If the change over does not occur...we may need to upgrade
them to a Winter Storm Warning especially near the Cape Cod canal
given expected quantitative precipitation forecast.

The one thing will also have to watch is areas near the South Coast
and Cape Cod. Looks to be a wet snow down that way and on going
roof issues may be exacerbated...especially if any areas that are
able to exceed 6 inches of wet snow.

&&

Long term /Monday through Saturday/...
highlights...

* blustery conditions on Monday with dry weather continuing into
Tuesday.
* A storm system brings mixed precipitation Tuesday into Wednesday.
* Conditions improve Thursday but becoming cold again.

Overview and model preferences...
a split flow merger will be the defining factor through the long
term forecast period. In the middle term...the streams merge thanks
to phasing of northern and southern stream shortwaves upstream as merging
strong jet develops across the NE Continental U.S.. these features will allow
low pressure to gradually deepen and slide through southern Ontario and
Quebec...creating an overrunning event as the system attempts to
occlude. This is the mixed precipitation event we have been discussing
for some time now. Now that NAM has captured it...it appears
colder than most of the deterministic guidance...while the European model (ecmwf)
starts cold and shifts to the warmer side of the envelope due to
the fact that it is more amplified than other models. The
GFS...ecens/gefs means are actually a good middle ground here.
This blend will be used through the remainder of the long term as
well to take into account the various solutions.

Details...

Monday into Tuesday...
remnant snowfalls will be gradually coming to an end through middle
morning Monday...although flurries may linger a bit longer in the
Berkshires. Strong high pressure will then slide across the region
through Tuesday...with mainly dry weather. Temperatures look to remain below
normal given little change in the airmass from previous days.

Tuesday night into Wednesday...
overrunning event expected as warm front will attempt to push north
across the region but is likely to struggle near the surface due to
entrenched cold air and a deep snowpack. However...50-60 knots low level jet
with strong southerly component will allow upper level warming ton
continue unimpeded. In spite of the strong jet at only 3-5kft
above the surface...snowpack will make mixing difficult so not
expecting significant wind issues at this time.

The column starts cold enough for snow at onset Tuesday evening...and
looks like as much as 0.40-0.60 of the total event quantitative precipitation forecast falls as
snow initially everywhere...with gradual changeover during the
early morning hours Wednesday morning from S-N. Therefore...could see at
least advisory if not low-end warning level snowfalls through the
overnight hours before a transition to sleet...then freezing rain.
Freezing rain could lock in for some time and given that h93 temperatures
/even on the coldest guidance/ an average of +4c during the 08z-
16z timeframe...but surface temperatures may hold near freezing for some time
given the deep snowpack across the area. However...straight rain
for a time from about middle morning into the afternoon is likely
until the precipitation comes to an end. Depending on how quickly 2m
temperatures rebound...as much as a quarter inch of ice can/T be ruled
out. Agree with previous forecaster that leaning a bit colder than
MOS 2m temperatures is probably the way to go.

Modest upper level instability as warming occurs in the middle levels
could also mean a period of heavy precipitation...both snow and rain.
With the rain...comes a risk for poor drainage flooding across the
region as snow melts and drains become clogged. Precipitation comes to an
end across most of the region by afternoon Wednesday...but may linger a
bit near the S coast...more on that below.

Wednesday night into Thursday...
confidence in the forecast continues to diminish here partially
dependent on how fast the upper level jet exits to the east and how
amplified the pattern remains in the wake of the low pressure. The
more amplified guidance...led by the European model (ecmwf) keeps the cold front
just offshore and within the equatorward entrance region of the
upper level jet. This suggests S coastal areas could continue to see
precipitation...possibly even changing back to snow for a time well into
the day on Thursday. The more progressive guidance...led by the GFS
would move both of these features offshore such that Thursday is
mainly dry and becoming quite cold as another Arctic airmass
settles in. For now...will continue to lean on the more
progressive GFS but with the ecens/gefs slower members blended in.
The final answer will likely be somewhere in between the two
solutions. In any case...much colder airmass expected to move in.

Thursday night into Friday night...
cold airmass across the region associated with yet another 1035+
hpa high pressure. Will likely see highs only making the teens and 20s
with lows back in the single digits above and below zero.

Next weekend...
some uncertainty here as to whether yet another wave will be
effect the region. Will lean mainly dry for now as the airmass in
place is quite dry and the wave itself is quite weak and shearing
out.

&&

Aviation /15z Sunday through Thursday/...
forecaster confidence levels...

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

Short term /today and tonight/...

15z update..
through midday today...high confidence. Morning tafs look on
track...no major changes made. Flurries should begin late this
morning baf/bdl then spreading W-E. MVFR conditions possible at
bdl/baf by 17z...to the east expect VFR through 17z.

This afternoon and tonight...moderate confidence. MVFR to pockets
of IFR conditions develop from west to east this afternoon in
areas of light snow. However...the bulk of the moderate to even
briefly heavy snow will occur this evening when widespread IFR to
LIFR conditions occur. The lower conditions most likely south of
the Massachusetts Turnpike. Ptype pretty much all snow...except for the very
immediate South Coast/cape where snow may mix with or change to
rain for a time. Whether or not that happens is uncertain...but we
do think that at least the islands will change to rain.

Bulk of snow ends by 8 or 9z tonight and much of the the region
should already have VFR conditions by 12z Monday.

Kbos taf...confident in overall trends but exact timing is more
uncertain. While some light snow is possible this
afternoon...bulk of accumulating snow with widespread IFR to LIFR
conditions expected this evening.

Kbdl taf...confident in overall trends but exact timing is more
uncertain. Some light snow is possible late this
morning...likely for this afternoon. Bulk of accumulating snow
with widespread IFR to LIFR conditions expected after 3 or 4 PM.

Outlook...Monday through Thursday...

Monday...high confidence.
IFR/MVFR in remnant snows end early...with transition to VFR
everywhere by afternoon. West-northwest wind gusts up to around 30 knots at
times during the afternoon and evening.

Monday night into Tuesday...high confidence.
Mainly VFR. Winds diminish Monday night.

Tuesday night into Thursday...moderate confidence.
A messy period. Snow begins everywhere late Tuesday into Wednesday night
with moderate accums possible...then expect gradually transition
to sleet and freezing rain during the early morning hours
Wednesday...then to all rain during the day on Wednesday. Some ice accumulate
possible as well. The rain diminishes late Wednesday but may linger
along the S coast and even change to snow into the day on Thursday.
Expect periods of IFR/LIFR conditions throughout the period with
some improvement from north-S on Thursday. Low level wind shear likely especially Tuesday night
and early Wednesday.

&&

Marine...
forecaster confidence levels...

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

Short term /today and tonight/...high confidence.

A weak wave of low pressure moves across the waters later
today and tonight. Weak pressure gradient should keep
winds/seas below small craft thresholds today and tonight. The
system really intensifies as it moves into the Canadian Maritimes
near daybreak Monday...so westerly wind gusts may reach 20 to 25
knots near 12z Monday. Otherwise...rain and snow late today and
especially tonight will reduce visibilities at times for mariners.

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

Monday into Monday night...high confidence.
Winds begin out of the SW then shift to west-northwest in the afternoon.
Wind gusts reach 35-40 knots during the afternoon and overnight so
will be issuing a gale watch. Seas build 10-12 feet on the ocean
waters. Some light freezing spray possible Monday evening into Tuesday
morning.

Tuesday...high confidence.
Winds and seas start at or around Small Craft Advisory thresholds
but then drop during the day such that a period of quiet boating
weather is likely Tuesday afternoon and evening.

Tuesday night into Thursday...moderate confidence.
Low pressure passes well inland...with winds out of the S gradually
shifting to the west late Wednesday into Thursday. Periods of wind gusts 25-30
knots expected although gusts may approach gale force Wednesday evening.
Seas build through Wednesday to around 7-10ft on the ocean waters then
diminish Thursday. At the very least small craft advisories are likely
during this period. Periods of snow changing to rain early Wednesday
with some low visibilities in rain and fog on Wednesday.

&&

Box watches/warnings/advisories...
CT...Winter Weather Advisory until 7 am EST Monday for ctz002>004.
Massachusetts...Winter Weather Advisory until 7 am EST Monday for maz002>016-
022>024-026.
Winter Storm Warning from 3 PM this afternoon to 7 am EST
Monday for maz017>021.
Rhode Island...Winter Weather Advisory until 7 am EST Monday for riz001-008.
Winter Storm Warning from 3 PM this afternoon to 7 am EST
Monday for riz002>007.
Marine...gale watch from Monday morning through late Monday night for
anz231>235-237-250-251-254>256.

&&

$$
Synopsis...Frank/doody
near term...Frank/doody/nmb
short term...Frank
long term...doody
aviation...Frank/doody/nmb
marine...Frank/doody

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