Ricky at LOT did a nice layout of the storm situation. Also mentioned the concerns near the lakeshore for Chicago.
For the all important busy Thanksgiving travel period, there are
a few features of interest. The past few model cycles have shown
an increasing signal for warm advection and frontogenesis driven
period of snow (modest accumulations) and associated travel
impacts late Tuesday night into/through the Wednesday morning
commute. In collaboration with neighboring offices, adjusted
PoPs into the (30%) chance range across the area.
Even more notably, a fairly moisture laden southern stream wave
is likely to emerge from the southern Rockies during this period.
Strongly confluent flow over northeastern North America caused
by a deep PV lobe near James Bay will tend to cause more zonal
flow less prone to strong amplification of the southern stream
wave. With that said, one of the key sensitivities in the
forecast is the potential for a northern stream wave crossing
the northern Rockies to phase with the southern stream mid-level
wave. This constructive interaction (or lack thereof) has major
implications on the track, strength, and wintry impacts caused
by the surface low pressure system passing to our south later
Wednesday into Thanksgiving Day.
Given the noteworthy increase in ensemble membership for a
stronger system tracking farther north near the Ohio River
Valley, increased PoPs into the 30-50% range area-wide Wednesday
afternoon into Thanksgiving. The higher PoPs are still indicated
south of I-80 to account for the slightly more suppressed
solutions that comprise a sizeable ensemble membership of their
own. Lower-level thermal progs appear cold enough for a
primarily wet (due to temps generally near/above 32F) snow
event, barring a much stronger solution which would bring the
rain/snow line farther north. For this reason, did hold onto a
chance of rain primarily south of I-80 Wednesday evening/night.
Northeast winds off the mild waters of Lake Michigan may pose
some p-type and/or accumulation efficiency issues along the
lakeshore, including Chicago, if the stronger/wetter outcome
comes to fruition. Nevertheless, the Wednesday afternoon into
Thanksgiving period bears close watching for accumulating snow
and travel impacts in the general region. Regardless of how
Wednesday PM-Thanksgiving Day plays out, another northern stream
trough could bring a chance of light snow/snow showers
Thanksgiving night into early Friday.