Area Forecast Discussion
Issued by NWS Northern Indiana
Issued by NWS Northern Indiana
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848 FXUS63 KIWX 240723 AFDIWX Area Forecast Discussion National Weather Service Northern Indiana 323 AM EDT Tue Sep 24 2024 .KEY MESSAGES... - Numerous showers and a chance of thunderstorms today. - There is a marginal risk of severe thunderstorms for a small portion of the forecast area. A tornado is possible. - Additional rain is expected late this week. && .DISCUSSION... Issued at 321 AM EDT Tue Sep 24 2024 As a surface low moves through Indiana, severe thunderstorms capable of producing a tornado are possible this afternoon. The area of greatest interest is from roughly Wabash, to Fort Wayne, to Ottawa; South to Portland and west to Marion. Forecast soundings, including ensembles, depict a weak cap this morning that quickly mixes out prior to noon. Outstanding directional shear in the low-levels exists in the vicinity of the low`s triple point, paired with low LCLs near 500m. Bulk shear maxes out near 25-30 knots paired with modest instability to start, but perhaps ballooning toward 1500 j/kg. The potential failure mode here could be one of two things: 1) lapse rates today are overall rather weak at less than 7C/km (thus, instability is marginal) and 2) the best directional wind shear could occur prior to this afternoon`s destabilization, thus the tornado risk would be notably less with subsequent thunderstorms. This is somewhat depicted in marginal helicity tracks within the CAMs guidance. In lieu of the tornado risk, a few forecast soundings depict low-level lapse rates approaching 7.5 C/km as afternoon temperatures warm into the mid-70s, which presents a non-zero risk of damaging wind gusts and perhaps more likely, small hail. Overall confidence in severe weather occurring is medium. Elsewhere, a nice steady rain (drizzle) is falling over a large portion of the forecast area this morning. This is ahead of a broad 40-knot 500-mb jet over the Tennessee and Ohio river valleys. As the trough aloft deepens this morning, this upper-level wind field will increase, enhancing lift and promoting additional shower development. As destabilization occurs within this dynamic environment the chance of thunderstorms increases through the afternoon (see preceding paragraph). While the synoptic rain shield departs early Wednesday night, lake enhanced rain showers are anticipated tonight amid modest delta-Ts. Low-level wind quickly veers prior to daybreak Thursday putting and end to any lake effect. Patchy fog is anticipated overnight as well. For the end of the week, all eyes will be on Tropical Cyclone 9 as it impacts the Gulf Coast, drifting up over the southern Appalachians. Guidance continues to advertise this cyclone wrapping around an upper-level low over the Southern Mississippi River valley. Closer to home, lingering high pressure just northeast of the Great Lakes delays the overall shower chance here until the weekend when a strong ridge over the Four Corners steers this low to the Midwest. The in-house blend significantly increased POPs Thursday and Friday. This was out of line with the latest model runs and seems to ignore the aformentioned Great Lakes high pressure. Therefore, I leaned closer to the existing (i.e.previous) forecast for that time period. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 123 AM EDT Tue Sep 24 2024 Surface low pressure will move into the area today with multiple round of showers expected. Ceilings will steadily lower through the early morning as this system approaches. IFR ceilings already at KSBN with some brief LIFR possible around 12Z. KFWA will take a bit more time to drop given more dry air entrainment but IFR is likely here as well by around 12Z. Some improvement expected with diurnal mixing especially at KFWA where low track may favor much better mix out. IFR returns behind the low Tuesday night though. There is some chance of thunder (especially at KFWA) but confidence in timing and coverage remains very low at this point. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...None. OH...None. MI...None. MARINE...None. && $$ DISCUSSION...Brown AVIATION...AGD