Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Thu Apr 16 2020

Valid 12Z Thu Apr 16 2020 - 12Z Sun Apr 19 2020

...CO Rockies through the central Plains, mid MS Valley, upper
Ohio Valley, Southern Great Lakes, New York/New England...

Days 1-3...

In the CO Rockies, a closed 700 mb circulation causes low level
convergence, pooling of moisture, and ascent
today. Upslope flow is also likely into the eastern slopes of the
Front Range in response to increasing easterly flow behind a cold
front and around the developing low.  The snow tapers the the
circulation departs to the east over the plains and the strength
of the low level convergence wanes. The current model consensus is
for around 6-10 inches in the front range and portions of the
foothills west of Denver north across northern Co.

A warm front will lift across KS/MO/IL and likely stall as the
surface low develops and tracks along this baroclinic gradient. At
the same time, a strengthening low level jet from the S/SW will
transport anomalous moisture into the region, with ascent being
provided through isentropic lift atop this boundary. The
combination of isentropic ascent and intensifying frontogenesis
along this temperature gradient will drive heavy snow in a narrow
band from southeast NE, southern IA to adjacent far northern MO
border, and into northern IL.

There remains spread in the latitudinal placement of this heavy
band, but it appears the heaviest snow will fall in southern Iowa
to adjacent far northern MO, with snowfall rates of 1-2"/hr. WPC
probabilities are high for 6 inches along this axis.  Uncertainty
exists further south as the ECMWF and UKMET show warm advection
increase low level temps above freezing in much of northern MO.
Uncertainty to the north exists in northern NE/IA/IL exists due to
weakening ascent and drier air the further north you go.  WPC
probabilities for 4 inches are low in Chicago, and peak about 30
miles further south in northern IL.

On day 2, Friday through Friday night, the low moves quickly along
the baroclinic gradient across the Southern Great Lakes and into
New England. Moisture should surge northward on , and where the
column is cold enough for snow either through advection or dynamic
cooling, a swath of snow is likely from northern Ohio through the
PA/NY border, and into higher elevations of southern New England.
There remains considerable uncertainty into the exact placement of
the heaviest snow, but WPC probabilities currently indicate a
moderate chance for 4 inches in the higher terrain of northern PA
and southern NY (Catskills), and into the Berkshires of MA.

The event winds down as the system moves off the coast Saturday
morning.

For Days 1-3, the probability of significant icing (0.25 inch or
greater) is less than 10 percent.

Petersen