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Area forecast discussion 
National Weather Service Taunton Massachusetts
1223 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. Another storm is likely Tuesday night into
Wednesday...with snow and mixed precipitation likely changing to
rain. Cold and dry conditions return for the end of next week.

&&

Near term /through tonight/...

1230 PM update...

No major changes. Only modifications to temperatures and
dewpoints to bring them more in-line with trends.

Previous discussion...

High pressure overhead will allow for sunny skies and light winds.
Temperaturess should recover into the middle to upper 20s in most
locations...but that is still around 15 degrees below normal for
the last day of February. However...the increasing late February
sun angle and light winds will make it somewhat more bearable than
our recent cold waves.

Tonight...

High pressure overhead will allow for another excellent night of
radiational cooling. The normally coldest outlying locations will
drop to between 5 and 15 below zero...while mainly single digits
can be expected for the rest of the region. The one fly in the
ointment is we are expecting some middle/high clouds to increase from
the west after midnight. If they come in a bit earlier then
expected our temperature forecast might be too cold.

&&

Short term /Sunday through Sunday night/...

*** accumulating snow late Sunday and especially Sunday night with
the highest amounts expected south of the Massachusetts Turnpike and away
from the immediate South Coast ***

A digging shortwave will drop towards the Great Lakes on Sunday.
This will allow for an overrunning setup across southern New
England. A nice frontogenesis signature with a developing low
level jet should result in a good slug of precipitation Sunday
evening into Sunday night. Most models are showing a weak
secondary low pressure developing near the South Coast. This will
help to back and strengthen the low level jet along with some
surface convergence resulting in the risk for some high
precipitation totals south of the Massachusetts Turnpike which will explain
more below.

1} timing and ptype:

Dry weather should prevail Sunday morning. Middle level warm
advection should allow light snow to develop during the afternoon
across the interior...reaching the coastal plain by late
afternoon/early evening. The snow will initially be light...but
expect the snow to pickup in intensity Sunday evening and
overnight as the low level jet/forcing increase. Model soundings
show a period of decent Omega in the prime snow growth region.
System is fairly progressive...so the snow should be over early
Monday morning and before rush hour starts.

Ptype will probably be all snow for a lot of the region. It becomes
more uncertain across southern Rhode Island and far southeast Massachusetts. The NAM
model is an outlier and most aggressive in bringing a surge of
warm middle level air northward...which would change ptype to rain
for just about all of Rhode Island and southeast Massachusetts Sun night.
However...most of the other guidance keeps ptype snow except for
areas near the South Coast/cape/islands. So for now will keep the
change over line just south of the Taunton to Providence
corridor...but this may need to be adjusted based on latest model
data.

2)snow accumulations and headlines:

This looks to be a high end Winter Weather Advisory or low end
warning for much of the region. We are generally expecting
snowfall amounts of between 3 and 7 inches...away from the South
Coast where ptype issues will be more problematic. The NAM is
heavier on quantitative precipitation forecast and we opted to go with a GFS/European model (ecmwf) blend. Even
these models still have between 0.40 to 0.60 inches of precipitation from
near the Pike to the South Coast. This seems to be in response to
a weak wave developing near the South Coast...allowing for a more
backed low level jet and better surface convergence. With pretty
good Omega in the snow growth region...snowfall ratios may be
better than 10 to 1 ratios.

With that said...felt it is Worth a Winter Storm Watch from near the
Pike to areas inland from the South Coast...where ptype will
probably end up as mainly snow. This is for the potential of 4 to 7
inches of snow. We are fairly confidence in widespread 3 to 4
inch amounts...but confidence is lower on whether or not we see
the 6 to 7 inch amounts. However...we only need 50 percent
confidence in those higher totals occurring to issue the watch and
it gives the next shift some more options.

3) forecast uncertainty:

How much snow we actually receive will have a lot to do with how
quickly the secondary wave of low pressure develops and how far
north the rain/snow line gets. The NAM model is strongest with
the development of this wave and hence has the highest quantitative precipitation forecast...but
does introduce more ptype issues. Meanwhile...the ggem is the
weakest with this development and barely has advisory level snow
for the region. Currently we are leaning towards a GFS/European model (ecmwf)
compromise...resulting in 3 to 7 inches of snow with the highest
amounts south of Pike and away from the South Coast.

&&

Long term /Monday through Friday/...

Highlights...

* conditions improve Monday
* another storm possible Tuesday night into Wednesday...with snow or
mixed precipitation likely changing to rain
* cold and dry toward the end of the week

Overview and model preferences...

One thing is for certain...with little change in the nao/ao and
the pna expected to rebound from negative values back toward a
more neutral solution...the cold air and longwave trough look to
remain through the long term forecast. The only caveat will be in
the low levels on Wednesday...when a robust SW low level jet will allow low-middle level
temperatures to warm closer to above climatological normals. However...
the cold air initially in place will lead to p-type issues for the
middle week system. European model (ecmwf) deterministic remains the most robust of
the operational models...with higher amplification in the flow
pattern than the GFS and UKMET. It starts much colder...but ends
much warmer. However...do note that some of the gefs and ecens
ensembles agree with the European model (ecmwf). Therefore...will be using the
ensemble blend as a baseline as this takes into account both the
more progressive /and ultimately less robust with the thermal
differences/ and the potential for a more dynamic system. Will use
this blend through the remainder of the forecast as well.

Details...

Monday...

Morning snows come to an end early across east Massachusetts...and flurries
dissipate through the Berkshires and Litchfield Hills as dry northwest
flow moves in. Blustery conditions expected...and even through
highs will still be making it into the low-upper 30s...northwest winds
with gusts 25-35 miles per hour at times will yield wind chills struggling
into the low 20s.

Monday night into Tuesday...

High pressure will move across the region. Dry conditions dominate
although with temperatures remaining below normal.

Tuesday night into Thursday...

Model consensus is now generally in agreement that both the northern
and southern streams phase in some fashion across the central Continental U.S.
With the formation of low pressure which is expected to slide across
southern Canada. Warm front will approach...but given deep snowpack the
low level front may actual stall near the S coast as the low moves
over as the upper warm front slides into central New England. This
presents a good overrunning situation with strong 50-60 knots h92 low level jet
out of the SW. Given that this system is associated with the southern
stream...it also carries with it a fair amount of moisture as
precipitable waters look to reach 2-3 Standard deviations above normal

So all in all..plenty of ingredients for heavy precipitation across the
area. The question will be type...all guidance begin cold enough
for snow at onset Tuesday night into early Wednesday. European model (ecmwf) would suggest
much more and better lift in the snow growth region...so there is
that to consider. In any case...will need to at least watch for
potential warning level snowfall before the warm air begins to
build in from S to north through the day on Wednesday. Even though the European model (ecmwf)
starts the coldest...it ends as the warmest solution...bringing
the h92 +8c line all the way into central Massachusetts. So we do expect a
gradual shift to a wintry mix including sleet and freezing rain
for a time given the deep snowpack across the region. Some areas
of western Massachusetts...and the hills of northern CT may actually stay as a mix of
sleet freezing rain and even maybe some snow given that the surface
warm front may stall. Will have to watch for icy conditions with
as much as a quarter inch of ice not out of the question where it
doesn/T switch to rain with melting /best chance of rain/melt is
in Rhode Island/CT and portions of mass east of I-495 and S of the Mass Pike/.
Heavy rain a threat too given upper level instability and robust
low level jet...so where melting and heavy rain are observed...poor drainage
flooding is a possibility where snow clogs area drains.

This heavy mixed precipitation event could linger into Thursday...the more
amplified European model (ecmwf) and several ecens/gefs members suggest that the
leftover cold front stalls with yet another weak wave of low pressure
moving through Thursday. Other solutions are more progressive and have
less quantitative precipitation forecast /snow potentially/ on the back end. However...by Thursday
afternoon/evening...conditions should be improving /although with
rapid temperature drop/.

Late next week...

Even though there is uncertainty with how amplified the pattern
is by late week. It appears a period of dry but cold weather is likely
as Arctic air once again shifts in behind the exiting middle week
system.

&&

Aviation /17z Saturday through Wednesday/...

Forecaster confidence levels...

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

Short term /today through Sunday night/...

18z update...

Today through Sunday morning...high confidence.

VFR. Increasing middle-level clouds late.

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 may
mix with or change to rain for a time across portions of S-coastal
Rhode Island and Massachusetts...including the cape and islands. Precipitation should rapidly
end from west-east toward daybreak Monday....improving conditions.

Kbos taf...high confidence. VFR into Sunday morning.

Kbdl taf...high confidence. VFR into Sunday morning.

Outlook...Monday through Wednesday...

Monday...high confidence.

Conditions improve early from IFR/MVFR in snows and low ceilings...
with VFR expected everywhere by afternoon. Northwest winds in the
afternoon with gusts 25-30 knots.

Monday night into Tuesday...high confidence.

Mainly VFR. Gradually lowering ceilings on Tuesday.

Tuesday night into Wednesday...moderate confidence.

Mixed wintry precipitation likely starts as all snow...transitions to a
mix of sleet and freezing rain...then to all rain during the day
on Wednesday. Some snow and ice accumulations are likely with IFR or
potentially even LIFR conditions at times.

&&

Marine...

Forecaster confidence levels...

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

Short term /through Saturday night/...high confidence.

1230 PM update...no major changes.

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 sun into Sun night...but
weak pressure gradient should keep winds/seas below small craft
thresholds.

Outlook...Sunday through Wednesday...

Monday into Monday night...high confidence.

SW winds shift to the northwest during the daylight hours as a cold
front sweeps across the waters. Once winds shift northwest in the
afternoon expect gale force gusts 35-40kt. Colder air Tuesday night
could lead to light freezing spray.

Tuesday...moderate confidence.

Winds gradually diminish out of the west-northwest but seas are likely to
remain elevated 5-8ft on the ocean waters. Small craft advisories
will likely be needed.

Tuesday night into Wednesday...moderate confidence.

Snow changes to rain Tuesday night with low visibilities over the waters.
Winds out of the S mainly around 25-30 knots at most. Seas build
12-15 feet by daytime on Wednesday on southeast ocean waters. Small craft
advisories will likely be needed.

&&

Box watches/warnings/advisories...

CT...Winter Storm Watch from Sunday afternoon through late Sunday
night for ctz002>004.
Massachusetts...Winter Storm Watch from Sunday afternoon through late Sunday
night for maz009-011>013-015>019.
Rhode Island...Winter Storm Watch from Sunday afternoon through late Sunday
night for riz001>004.
Marine...none.

&&

$$

Synopsis...Frank/doody
near term...Frank/doody/sipprell
short term...Frank
long term...doody
aviation...Frank/doody/sipprell
marine...Frank/doody

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