Plaincast

What has changed

Decreased confidence in severe convection tonight with smoke becoming too thick to allow for convection. Will need to be on the lookout for isolated instances of very dense fog come morning in river valleys given smoke concentrations.

Messages

Smoke, and visibility obstructions persist through tomorrow.

More widespread storm chances, along with severe and flooding risks, return on Saturday.

Discussion

KEY MESSAGE 1. With surface and deep layer column flow orienting more northwest to southeast on Thursday and especially Friday, ground-level wildfire smoke from fires originating in southern Ontario and northern Minnesota will arrive to our area. Upstream observations show the worst of the visibility dipping down to as low as half a mile with general prevailing conditions between 1-2 miles. In addition, air quality is greatly degraded. The thickest ground-level concentration will arrive late Thursday into Friday. In coordination with OH, PA, and WV Departments of Environmental Protection, Air Quality Alerts are in place for Thursday and Friday. Refer to your state's department for more information.

One thing to note, with low temperatures forecast within a few degrees of dew point temperatures, the potential for localized "super fog" with very low visibilities (<1/4 mi) will need to be monitored in river valleys Friday morning with plenty of smoke- related CCN for water drops to condense onto. Should this occur, a dense fog advisory may be needed tonight, even if visibility interruptions are on a localized basis.

KEY MESSAGE 2. Friday into Saturday the ridge over the Plains will break down and the ridge axis will retreat westward over the Rockies. As this occurs, a shortwave trough will quickly move through the larger trough sitting over eastern Canada. Ensemble members are in agreement that the axis of this shortwave will cross from the the Great Lakes from NNW to SSE during the first half of the day Saturday. On the eastern side of the trough axis, an upper-level west-to-east oriented jet streak will intensify. Forcing from both the approaching shortwave axis and the right entrance region of the upper-level jet streak appear to be present in the region Saturday afternoon, along with favorable environmental parameters for convective hazards.

Instability in the region remains contingent on potential convective development over the lower Midwest on Friday. Current CAMs suggest convection will develop over northern IL, IN, and OH along a boundary of differential heating influenced by the extent of wildfire smoke. The uncertainty in the factors surrounding this convection has led to disagreement between ensemble members on the impact to the convective environment on Saturday. The deciding factor will be the persistence of cloud cover from Fridays convection into the day Saturday.

The 25-75th the blend-of-models forecast percentile spread of surface storm energy is in excess of 1000 J/kg at 18z Saturday, indicating that some members prefer persistent cloud cover while others expect early clearing. In the scenario where persistent cloud cover prevents maximized heating, the existing thermodynamic environment will still favor convective development, meaning all severe hazards cannot be ruled out in any scenario. If clearing occurs in the morning, destabilization will occur earlier and result in a more potent severe threat. Regardless of the level of clearing, favorable thermodynamics in the same region as upper-level forcing will likely result in convective development.

While all hazards are on the table, wind and flash flooding look to be the primary threats. Current model soundings for Saturday show plentiful deep layer moisture and a favorable warm rain layer through 400mb, meaning hail development is less favorable. PW values near 2 across most models indicate high confidence in heavy rainfall production in sustained updrafts. Elevated dew points resulting in low LCLs pose a non-zero tornado threat, but low-level shear is relatively weak which could be a limiting factor.