Issued: 1pm on Wednesday, September 23rd 2020
Technical Forecast Discussion
Short Term (Wednesday 9-23-2020 through Saturday 9-26-2020)
Positive vorticity being generated by tighter height gradients to our south will begin to advect itself north. This is causing troughing in the subtropical jet, bringing about split flow over our region. As the advection of this VORT axis propagates this trough further east-northeastward throughout the day Wednesday, increasing differntial thickness advection over SW Illinois, and SE Missouri will deepen this trough, forming a closed low at 500 mb this afternoon. This closed low will be weak, however. This closed feature will work to tighten height gradients on Wednesday night. This will strengthen the jet winds to our south, forming a very weak jet streak mainly south of the Ohio river. This, along with the remnants of Tropical Storm Beta, will bring about very broad low pressure over the area. Positive differential PVA and *very* weak WAA aloft will generate negative omega to produce thick layers of clouds Wednesday night and Thursday morning. A few scattered showers will be possible Thursday, especially south of US-50, where WAA will be the strongest. This gloomy weather should continue throughout Thursday night and into Friday (although we may see a few breaks in the overcast Thursday night due to some brief aloft CAA). Friday will bring our best chances of rain, as WAA will once again occur in the afternoon, along with positive differential PVA. Friday night into Saturday, the area of positive VORT should exit the forecast area, bringing about NVA. This will work to propagate a ridge to our west into our region during this time. Decreasing differential thickness advection will work to amplify this ridge further north across the state Friday night and Saturday as well. The effects will be noticed at the surface–highs will return to the upper 70s, and lower 80s Friday and Saturday, making it feel more like early September outside. Positive omega from the NVA and CAA aloft will generate sufficient sinking motion to keep skies partly cloudy on Saturday.
Long Term (Sunday 9-27-2020 through Tuesday 9-29-2020)
This is where the weather gets interesting. Very tight height gradients over the northern tier of the country, manifest by a strong subtropical ridge over the southwest states, and a strong polar trough to the north over Canada will drive a very strong jet over that region. This jet will be mainly zonal in nature Saturday night. This feature is driving very strong positive shear vorticity over Alberta, Montana, North Dakota, and Minnesota. The eastern portion of this VORT axis (Minnesota), being more meridional in nature than the rest of the feature, will begin to be advected eastward throughout Saturday night, lowering heights over Minnesota, Wisconsin, upper Michigan, and Ontario. Meanwhile, over our region, ridging will keep us warm at the surface, with highs continuing in the lower 80s and upper 70s for Sunday. Sunday afternoon and night, PVA and WAA aloft will generate negative omega for cloud and precipitation formation. Back to our north and west, strongly decreasing differential thickness advection over North Dakota will work to dig this jet down into the Midwestern states. This will occur throughout the day Sunday. With an already tight height gradient becoming tighter due to this, a jet streak will develop over eastern South Dakota, through Iowa, and northern Illinois. The left exit of this will cause broad cyclogenesis by 8 AM Sunday, along with frontogenesis due to the VORT axis’s low pressure perturbation causing surface confluence along it. This cold front will be located over eastern Illinois, northwestern Indiana, and southern lower Michigan by 8 AM Sunday, although differences in timing between the mid range models exist at this time. FROPA over southeastern Ohio should occur between 2 and 8 PM on Sunday. With a weak thermodynamic profile, severe convection from this cold front should not be expected at this time (although we will likely see rain with a few rumbles of thunder Sunday). After FROPA Sunday, much of the region will be under the influence of a neutrally-tilted trough, with an axis from northern Lake Huron down to western Ohio/eastern Indiana by late Sunday night and early Monday morning. Out west, strong NVA and negative differential thickness advection will amplify the subtropical ridge over the western states. This will further tighten height gradients over the Pacific northwest, creating even stronger jet winds and positive shear vorticity. With localized strong areas of positive vorticity, positive vorticity advection should begin occurring over Alberta and British Columbia by late Sunday night. With the trough over our area, Monday’s highs should return back to the lower 70s, with partly cloudy to mostly cloudy skies due to PVA occurring over our area. Meanwhile, the PVA previously mentioned over Alberta and British Columbia will lower heights over this area, and propagate the lowered heights further east. Strongly positive differential thickness advection will quickly work to dig this feature down into the Midwestern states throughout Monday night into Tuesday, further tightening the height gradient. This will create a very strong jet streak, located from northern Illinois, through southern Iowa, northern Missouri, and Nebraska. At the left exit of this feature (over southwestern Wisconsin), very strong cyclogenesis will occur (with pressures as low as 990 mb possible near the center of this feature). With a *very* strong positive VORT axis, a low pressure perturbation will occur, causing SFC confluence and therefore frontogenesis late Monday night and early Tuesday morning across Illinois, Indiana, and Missouri. The low and associated front will follow its forcing eastward throughout the day Tuesday, with FROPA occurring between 2 and 4 PM Tuesday afternoon. This front will be quite strong. However, with once again weak thermodynamic profiles, severe convection looks unlikely at this time. Highs on Tuesday should only reach the lower 60s, with rain likely. Behind this front, temperatures across southeast Ohio will struggle to get into the 60s, with temperatures across northern Ohio struggling to get into the 50s.
