Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Fri Nov 09 2018

Valid 00Z Sat Nov 10 2018 - 00Z Tue Nov 13 2018

...Great Lakes...

Ongoing snow in the Great Lakes will begin to transition from a
synoptically-driven event to a lake effect event tonight. A
surface low had formed near Lake Erie as of 18Z Friday, and will
develop north into interior Quebec and occlude overnight and into
Saturday. A pronounced surface trough extended to the northwest
across Lower Michigan, and toward the western end of Lake
Superior. Models are in good agreement with the strongest signal
for snow accumulations along this trough during the evening, which
extends vertically into the 925-850mb layers as well with a
similarly pronounced deformation axis.  As the low occludes, the
deformation zone should begin to pivot and race to the southeast
between 06-12Z tonight, which will mark the beginning of some
modest cold-air advection and the transition to a more lake effect
event. After that, there will be about a 6 hour window before
drier air and a surface high begin to diminish the lake effect
snow, arriving around 18Z in the western lakes (Superior,
Michigan) and 00Z Sunday in the eastern lakes (Huron, Erie,
Ontario). Variable wind directions through much of the event may
limit lake effect contributions to the totals, but some stronger
bands will be possible as hi-res models do show several hundred
j/kg of lake-induced CAPE. High (over 70 percent) probabilities of
4+ inches of additional snow after 00Z Saturday stretch from the
Keweenaw Peninsula, the central Upper Peninsula of Michigan, and
ESE toward the Mackinac region where the Lower and Upper
Peninsulas meet. Most of that should fall in the next 12 hours.


...Adirondacks and Interior New England...

To the east of the occluding low in Quebec, strong warm air
advection and northward moisture flux will lead to widespread
precipitation across the Northeast. A secondary low is likely to
form along the coast, and may focus some heavier convective rain
bands in the warmer air mass. This may intercept some of the
low-level moisture before it can reach the higher elevations of
interior New England, and this afternoon's WPC QPF does show a
slight reduction relative to the previous forecast cycle.

The greatest snowfall potential will be constrained to the higher
terrain regions of the Adirondacks and northern Appalachians in
interior New England, as strong low-mid level warm air advection
and a lack of significant antecedent cold air should limit
snowfall at lower elevations. With the significant moisture flux
and a substantial component of the low-mid level flow orthogonal
to the terrain features, there should be a burst of heavy snow
when the precipitation arrives and the temperature profile is
still sufficiently cold. However, after about 6-8 hours of heavier
snow, models show the potential for increased riming and a
possible changeover to sleet. For example, the 12Z NAM shows the
percent of frozen precip becoming reduced below 90 percent,
usually a sign of decreased snow efficiency. Therefore, the
expected heavy snow rates should be balanced by a relatively short
window of impact. Probabilities of exceeding 4 inches of snow are
very high (over 80 percent) from the White Mountains of New
Hampshire into the mountains of northwest Maine; however, the
probabilities of exceeding 8 inches of snow are lower (generally
less than 40 percent) except on the highest elevations above 2500
feet. The WPC snowfall probabilities and forecast were generally
very consistent with the previous forecast as model forecasts were
also very consistent.


...Northern Rockies...

An Alberta Clipper will affect the Northern Rockies from tonight
into Saturday (and also spread some light snow into North Dakota
and northern Minnesota). This will be followed by a second round
of snow on Sunday afternoon and evening due to a secondary digging
wave and an accompanying strong surface high (which will increase
orographic ascent on northward facing slopes). The air mass during
both events should be cold enough to support snow even into the
lower elevations surrounding the mountain ranges in the region,
however the heaviest snow is likely to be tied to the higher
elevations. The mean low-mid level flow through the entire period
should be out of the northwest, so the favored areas for snow will
be on the northern and western slopes.


...Southern Rockies into the Southern Plains...

A sharply digging shortwave should reach the southern Rockies in
Colorado and northern New Mexico by Sunday, and emerge into the
southern Plains on Monday. This is likely to generate some snow in
the Rockies, with the highest probabilities of heavier amounts (8+
inches) focused in the Sangre de Cristo Mountains from southern
Colorado into northern New Mexico. Slightly more uncertainty
exists in the southern Plains, as model temperature profiles do
show some variability. However, there is fairly good model
agreement for at least moderate amounts of QPF (0.25+ inches) from
far northeast and east-central New Mexico, into the Texas
Panhandle, and perhaps into far western Oklahoma as well. The
strength of the digging wave and push of colder air (with 1040+ mb
high shifting down the Plains) should be sufficient to have
all-snow precipitation types in the same area, and thus confidence
is reasonably high in at least several inches of snow across this
broader region.

Some uncertainty exists with the placement of heavier amounts, but
all models do show a frontogenetic circulation extending up into
the dendritic growth layer, so a more focused band of heavier snow
does seem likely. For now, this was placed from east-central New
Mexico into the west-central Texas Panhandle, just south of the
I-40 corridor, based on good agreement from the GFS and ECMWF on
the location of 0.50+ inch QPF. Further east and south, forecast
snowfall amounts were tapered off more quickly, but it should be
noted that the forecast snowfall could increase in these regions.
This would likely either be due to (1) northward model bias, which
occasionally happens with a southward push of a strong high and
accompanying cold air mass in the Plains, and/or (2) changes in
the synoptic evolution of the low pressure system, with increased
banding to the east. The models also consistently showed a
mid-level dry slot wrapping around the southeast flank of the
digging wave on Monday, which could reduce snow efficiency or even
lead to more of a drizzle/rain precipitation type further south
from the Rolling Plains of west-central Texas to the Red River
region. Given the uncertainty on the southern and eastern end of
things, snowfall was kept light for the time being.


The probability for significant ice (0.25 inches or more) is less
than 10 percent outside of several valleys in interior New England.


Lamers