Plaincast

What has changed

The most organized severe storms have trended mainly west of our area this evening, but showers and thunderstorms with at least some severe potential will impact the area at times this evening and tonight.

Messages

Showers and thunderstorms will first move into the area between 5 and 8 PM. A few could be severe with damaging winds and heavy rain the main hazards, but the storms with the greatest severity and organization should stay west of the area.

Additional showers and thunderstorms with lower severe potential will accompany a cold front overnight.

A few lingering showers in NE Ohio and NW PA Tuesday and Wednesday, but otherwise drier, much cooler, and less humid through the end of the week

Discussion

KEY MESSAGE 1. The forecast continues to be very challenging and low confidence this evening and tonight in terms of how upstream convection will evolve and impact our region. Two complexes of thunderstorms are currently ongoing. The largest and most organized of the two complexes (MCS's) is currently propagating S to SE across eastern Wisconsin, Lake Michigan, western Lower Michigan, northern Illinois, and NW Indiana. This MCS has produced widespread significant wind damage and multiple tornadoes but will continue to make a hard right turn to the S as it follows the eastern periphery of the thermal ridge, where a very impressive theta e/instability gradient is draped from N to S along a quasi-stationary front. This will allow this MCS to largely miss our area to the west this evening, but the eastern edge of it may clip the I-75 corridor in the roughly 21-00Z (5 to 8 PM) timeframe with gusty winds and heavy rainfall.

The second and smaller of the two MCS's is located over southern Lake Huron and southern Ontario Province. This MCS is being more directly forced by the strong mid/upper shortwave trough that is seen on water vapor imagery digging into the northern Great Lakes this afternoon. As this shortwave dives through the central Great Lakes this evening, the left exit of a 115-120 knot H3 jet streak will generate strong synoptic forcing to sustain this MCS, as well as strengthen a surface low that will move SE into southern Ontario tonight. Despite lower instability across northern Ohio and NW PA (2000 MLCAPE) compared to the extreme instability feeding the larger Lake Michigan MCS, the combination of the jet support and surface low should maintain this Lake Huron convection into our area. Current trajectories time this MCS to reach NE Ohio and NW PA in the 22 to 23Z (6 to 7 PM) timeframe. The aforementioned sufficient MLCAPE combined with 40-50 knots of effective deep layer bulk shear oriented perpendicular to the convection will support at least scattered severe winds, especially as the boundary layer continues to warm late this afternoon, with DCAPE increasing to over 1000 J/Kg. Even in the absence of severe weather, most areas will see some rain and thunder with this activity.

KEY MESSAGE 2. After a lull early tonight, additional showers and thunderstorms are expected ahead of the cold front as the mid/upper jet support moves overhead tied to the shortwave crossing the central Great Lakes. The convection this evening should overturn the atmosphere and effectively use much of the instability, but the aforementioned left exit region dynamics will encourage strong forcing ahead of the cold front to get the additional convection going overnight. This convection should generally not be severe given the lower instability, but gusty winds and locally heavy rainfall will be possible. Maintained high chance of rain overnight until the cold front clears the area.

KEY MESSAGE 3. The mid/upper shortwave will become vertically stacked with the surface low and close off over upstate New York by Tuesday evening. This along with the parent longwave mid/upper trough becoming negatively tilted across the eastern CONUS by Wednesday and Thursday will allow the system to evolve into a coastal low near Long Island. The result for northern Ohio and NW PA will be broad cyclonic flow and associated cold air advection Tuesday through Thursday. A secondary front or surface trough dropping through the region Tuesday afternoon combined with the deepening trough aloft will generate scattered showers, mainly in interior NE Ohio and NW PA, so increased chance of rain Tuesday afternoon. Otherwise, wraparound moisture in the cyclonic flow combined with cold air advection across the warm lakes will lead to periodic lake enhanced showers over far NE Ohio and NW PA through Wednesday before Canadian high pressure finally builds into the central Great Lakes and Ohio Valley late Wednesday and persists through the end of the week, supporting pleasantly cool and dry conditions areawide. Low temperatures will drop into the 50s both Thursday and Friday mornings allowing the air conditioners to get a break before gradual warming over the weekend.