Chicago NWS
What we know... Strong upper level disturbance over the northern
Pacific is progged to move over the Pacific Northwest this evening
and tonight and dig into the southern/central Great Plains early
on Sunday. Surface low will deepen in response lifting from
Missouri early Sunday morning across the Midwest Sunday evening.
Warm advection wing will overspread the region early in the day
Sunday allowing the column to saturate and for precipitation to
eventually reach the surface. P-type details will still have to be
worked out, but precip may start off as rainfall in some areas
before transitioning over to snow for the remainder of the event.
There will likely be a swath of heavy snow left of the surface low
track. Confidence is highest in accumulating snow over far north
and northwest Illinois and diminishes farther south.
Details that still need to be worked out... Surface low track is
the main area of uncertainty. While there has been a pronounced
southward shift from yesterday`s guidance, the latest suite of 12Z
guidance still has important inter-model differences that will
impact the location of heaviest snow. Meso-scale banding will be a
concern within the axis of heaviest snow. Guidance indicates a
corridor of strong f-gen which will help enhance snow totals for
some locations. In addition, lapse rates above 700/600mb remain
steep at times and could help to promote a deep vertical response
to forcing. SREF/GEFS plumes really highlight the uncertainty well
showing a broad range of no snow to over a foot of snowfall in
some members.
Bottom line... A winter storm appears likely somewhere across
eastern Iowa, northern Illinois, and southern Wisconsin, though
details in the placement of the heaviest snow remains low within
50- 100 miles. Rain/Snow line is favored to set up over the CWA,
but again confidence is low within 50-100 miles. Either way,
travel impacts Sunday are likely, and may potentially become
significant for some locations.
