National Weather Service United States Department of Commerce

Forecast commentary -

Anticyclonic flow regime of the last few days now exiting east as height falls associated with the
upstream closed low begin to spread into the area. Temperatures in the low 80s and dewpoints in the
50s this afternoon are characteristic of the resident dry airmass that extends westward into much of
Wisconsin, Indiana, and far eastern Illinois. Areas of shower and thunderstorm activity are ongoing
within the 1.5"+ precipitable water plume that demarcates the airmass boundary which extends from
southeast Minnesota to the lower portions of the Ohio River Valley. Isentropic ascent and increased
convergence along the lead edge of this moisture axis will support either continued or redeveloping
convection over Illinois and Iowa overnight, particularly as upper PV advection increases. Loss of
deeper forcing for ascent as forward progress of the low almost stops altogether by Friday morning
warrants a reduction to chc pops as any band of showers lifting through the forecast area should be
both transient and weakening. Deep layer cyclonic flow and ongoing moist advection will force
dewpoints well into the mid or even upper 60s by late Friday, posing a threat for thunderstorm
development within the weakly sheared, but very moist environment. 12z model progs suggest activity
will be focused along a second wave embedded within the parent low over sw Lower Michigan late
Friday afternoon before spreading east through 00z. Cooler mid-level temps open the door for
moderate destabilization late in the day, but late arrival of forcing and the likelihood of
extensive cloud cover casts doubt on potential. Instead, the more likely outcome is a relatively
quick- moving heavy rain threat late eve/first half of the night with any stronger convection
capable of producing small hail and/or wind gusts to around 40 mph. Shower/tstorm chances will be
pervasive heading into the first half of the weekend. Lack of steering flow on Saturday will favor
some degree of a heavy rain threat depending on degree of destabilization beneath the upper low.

By Sunday, the low pressure system that had been situated over the are will weaken and slide
southward away from the region. Scattered showers/storms will come to an end by late Sunday as high
pressure builds in. For the most part, the area will then remain dry going into next week with only
slight chances for some shower activity. As upper level riding builds in next week, temps will
rebound back into the mid 80s.

. . posted at 344 PM EDT Thu Jul 19 2018