Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
457 AM EST Thu Jan 17 2019

Valid 12Z Thu Jan 17 2019 - 12Z Sun Jan 20 2019

...Western United States...

Much of the expected wintry precipitation in the West will fall in
the Day 1 period as a strong shortwave pushes onshore into
California, and then begins to dig tonight in the Intermountain
West and Central Rockies. The event is ongoing now along the West
Coast, with heavy snow falling in many of the mountain ranges, and
in particular in the Sierra Nevada range. The snow will diminish
in intensity later today and into tonight, but there is still a
high probability (over 70 percent) for over 1 foot of additional
snow after 12Z today in most of the Sierra Nevada range above
5000-6000 feet in elevation.

Into this afternoon and tonight, the higher intensity snow should
begin to shift both north into the Cascades (as the occluded
cyclone moves north over the Pacific), as well as inland (with the
digging trough). The left exit region of a strong jet streak will
favorably align over Utah and Colorado tonight, providing upper
level divergence and a more focused area of ascent. Therefore,
snowfall over 1 foot is possible (over 50 percent chance) through
12Z Friday in many of the ranges of Utah, far western Wyoming, and
western Colorado as well.

As we move into Friday and Saturday, mid-upper level height rises
spread over most of the region, and a ridge will begin to build
in. This should have the effect of suppressing snowfall intensity
in most areas, as well as raising snow levels on Saturday
(limiting the extent of snow to the highest elevations).


...Portions of the Central Plains, Midwest, and Mid South...

The aforementioned wave digging through the West will emerge into
the southern Plains and lead to gradual cyclogenesis on Friday and
Saturday, with a surface low likely to track from Oklahoma into
the Tennessee Valley. There will also be a subtle wave in the
northern stream, moving from the Dakotas toward Lake Michigan.
These waves are expected to eventually phase, but that should
happen to the east of the Mississippi River. Until that happens,
precipitation bands and resulting snowfall may be less focused.
From tonight through Saturday afternoon, there will be a broad
area that is likely (over 70 percent chance) to receive at least 2
inches of snow from SD south to E KS, and east into much of IL.
However, the probabilities of 8 inches of snow in the same area
are generally less than 30 percent. Therefore, a widespread
portion of the Midwest is likely to see several inches of snow.

Closer to the low track over the Mid South, there is a lot of
model variability on how quickly the column cools behind the low,
and then how quickly the mid troposphere dries out. This will make
all the difference between some flurries and several inches of
accumulation. The ECMWF and its ensemble members show the
potential for greater snow in the deformation band on the NW and W
periphery of the surface low, while the GFS and NAM are more
progressive and dry the column faster. Although the probability of
accumulating snow is lower in areas south of the Ohio River, and
southwest into Arkansas and Oklahoma, a favorable coupled jet
structure and deepening low present a reasonably favorable setup
for a band of snow behind the surface low. Nevertheless, the
deformation band, like the low itself, should make steady eastward
progress and thus the favorable forcing is unlikely to be aligned
for a long period of time in any one location in the Mid South.
Ahead of the surface low and closer to the Ohio River, there is a
chance of freezing rain or sleet as well.

The greatest potential for mesoscale banding, and thus higher snow
amounts than most models would be forecasting, would be in two
locations. (1) Along the track of the northern stream wave from SD
into N IL where frontogenesis may be stronger in the dendritic
growth zone, and (2) to the northwest of the surface low track
from far E OK, AR and into W TN and KY. In this second region, if
the column cools fast enough there should be more than enough
strong forcing (coupled jets aligned with well-developed
deformation band) to lead to some heavier snow rates that could
offset warmer ground temperatures and other unfavorable factors to
produce a quick several inches of snow.


...Upper Ohio Valley into the Northeast...

There is a very strong signal for heavy snow from Ohio and
Pennsylvania into the remainder of the Northeast, and this has
been present in models for days now. As the two waves phase, the
warm conveyor belt should broaden and intensify. By 00Z Sunday,
the WAA region should be nearly 600 miles across, from C IN
eastward to NJ with the core of the low-mid level jet (700mb)
increasing to 50-60 knots near the spine of the Appalachians.
Given the positively tilted nature of the mid-upper level trough
and expected steady progression of the cyclone, much of the wintry
precipitation should be driven by the warm air advection. The
aforementioned strength of the warm conveyor belt and available
moisture aloft (700mb mixing ratios approaching 4 g/kg from C OH
into PA and on into S NH) are hallmarks of significant warm
advection snow events. The WPC winter ensemble has a high
probability (over 70 percent) of 8+ inches of snow from northern
Ohio into northern Pennsylvania and southern New York. This is the
most likely area to receive significant snowfall through 12Z
Sunday, and it is also very likely this swath will extend
northeast on Day 4 (beyond the scope of this forecast).

On the southern periphery of the heavy snow area, some freezing
rain and sleet are expected. The probability of 0.25 inches of ice
accumulation is over 30 percent from southern Ohio, to the western
Maryland and southern Pennsylvania. Models may be underestimating
the breadth of the freezing rain and sleet area. With the strong
warm advection aloft, the warm layer may penetrate further north
than models are showing at this time, and a strong surface high
over the Northeast in advance of the precipitation should help
keep low level cold air in place for at least a little while.
Therefore, snow projections could be inflated a bit on the
southern periphery where greater amounts of sleet are possible and
would lower the effective snow ratio for the duration of the event.


The probability of one quarter inch of freezing rain is less than
10 percent on Days 1 and 2.

Lamers