Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Thu Oct 31 2019

Valid 12Z Thu Oct 31 2019 - 12Z Sun Nov 03 2019


Days 1-2...

...Illinois to the Upper Great Lakes...

Wintry precipitation is likely from IL northeast across Michigan,
with higher amounts of snow in the northern L.P. of MI. This heavy
snow will form beneath the deformation band pivoting slowly across
the region, coincident with 850-600mb frontogenesis.  Snow amounts
near Lake Michigan will be limited by warming from the lake
causing some of the QPF to fall out as liquid, before cold
advection causes the change over to all snow.

Once the transition occurs, bursts of moderate to at times heavy
snow rates are possible where ascent occurs within the dendritic
growth zone. The longest period of snowfall is likely in northern
lower MI, and taper as the low pressure system departs for Ontario
this evening. Probabilities are as high as 30% for 6 inches of
snowfall.

Lighter amounts of freezing drizzle are possible in the dry slots
of northern IL. Freezing drizzle has been observed in west central
IL locations not far from the MO border in the dry slot, which is
progged to move northeast across IL this morning.
This threat ends late today as the storm departs and ascent tapers
and drying aloft occurs.

The probability of significant icing (0.25 inches) is less than 10
percent all 3 days.

Petersen