Area Forecast Discussion
Issued by NWS Northern Indiana
Issued by NWS Northern Indiana
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627 FXUS63 KIWX 200745 AFDIWX Area Forecast Discussion National Weather Service Northern Indiana 345 AM EDT Thu Jun 20 2024 .KEY MESSAGES... * Hot and humid conditions will continue through Saturday. Peak afternoon heat indices will approach 100 degrees in many locations, hazardous to sensitive and vulnerable groups. The heat headlines have been extended through early Saturday evening. * Isolated to scattered thunderstorms are possible this afternoon and again Friday afternoon, mainly north of US Route 30. Showers and thunderstorms become likely Saturday night into early Sunday. && .DISCUSSION... Issued at 340 AM EDT Thu Jun 20 2024 Very little change from previous forecast as stagnant synoptic pattern allows for continuation of dangerous heat and humidity and peak afternoon heat indices around 100 into Saturday. With consistent signals of heat indices in upper 90s to around 100, have extended the current heat headlines through Saturday evening. Upper level longwave ridge will exhibit some slight retrogression over the next few days which will keep heat dome entrenched across most of the Ohio Valley. One difference in today`s weather will be a very weak, shallow sfc trough dropping across southern Lower Michigan/Lake Michigan vicinity with some light northwest winds developing. This weak onshore component will provide some relief from the excessive heat for the Lake Michigan shoreline areas. This weak boundary also should help to focus isolated to scattered afternoon showers and thunderstorms. Have maintained inherited gradient in PoPs from south to north as retrograding upper level ridge will limit instability magnitudes across southern half of the forecast area. Some slightly warmer mid levels today, and perhaps slightly lower low level dew points should result in peak afternoon instability magnitudes generally weaker than yesterday. However, with surface based CAPES on order of 1000 J/kg and steep low level lapse rates/mid level theta-e minimum in place, cannot discount potential of a few storms producing primarily sub-severe gusts of 40+ mph. A few storms may persist into this evening before exhibiting a similar diurnal diminishment as yesterday evening. The best chances of reaching peak afternoon heat indices around 100 today will be in the Excessive Heat Warning area, while elsewhere generally mid to upper 90s heat indices are expected. For Friday, a continued southwest drift of upper level anticyclonic circulation center into southern Indiana/western Kentucky is expected. With a similar thermodynamic environment in place for Friday would expect similar high temperatures in comparison to Thursday. Sfc boundary which washes out across the southern Great Lakes today, will continue to lose its integrity into early Friday. This may lesson coverage of diurnal pop up showers and storms on Friday with low level moisture quality also somewhat in question in terms of overcoming weak mid level warm nose. For Saturday, some slight suppression of upper level ridge continues, but emergence of more substantial upper level short wave forcing from the lee of the Rockies will allow for some better southwest low level return flow during the day Saturday. This should result in a more uniform mid 90s high temperature distribution across the area. Sfc dew points should be held in check in this better mixing environment, which will result in only modest differences of apparent temperatures from air temperatures. Nonetheless, peak afternoon heat indices should top out around 100 across much of the area on Saturday. With confidence medium to high in minimal impacts from clouds/isolated convection, did go ahead and extend the current heat headlines through Saturday afternoon with primary focus on cumulative multi-day effects of heat of this magnitude. Better chances of showers and thunderstorms arrive Saturday night through Sunday as stronger mid/upper level short wave tracks across the western Great Lakes. Still some notable differences in how a few of these upstream waves phase across the Upper Midwest/Great Lakes for this period which will have an impact on sharpness of frontal convergence and associated moisture pooling. This setup does appear to feature limited instability magnitudes though, given weak mid level lapse rates and late timing of this frontal forcing Saturday night into Sunday. Slightly cooler air arrives for Sunday (mid 80s to around 90), which should continue into Monday (mid 80s) before warmer low level air rapidly advects eastward downstream of next northern tier CONUS upper trough. This should result in high temps back into the lower 90s for much of the area by Tuesday. Additional chances of showers and thunderstorms will be in place for Tuesday and Wednesday as decent instability gradient could set up from the Mid MS Valley to the southern Great Lakes, although temporal fine tuning of these chances will be needed over next several forecast cycles. Passage of this associated front should take temps back to more seasonable levels toward the end of this forecast valid period. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 159 AM EDT Thu Jun 20 2024 Showers and storms have nearly all dissipated with the loss of daytime heating. A weak front was drifting south into far northern Indiana and had become more diffuse. The front was draped just south of the Michigan border. Latest model guidance favors the front becoming stationary over the area. Given the diurnal flare to convection with the front in the vicinity later this afternoon and evening, have placed a VCNTY TS at SBN. Otherwise, VFR conditions will prevail. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...Heat Advisory until 8 PM EDT /7 PM CDT/ Saturday for INZ005>008-012>015-020-022-023-103-104-116-203-204-216. Excessive Heat Warning until 8 PM EDT Saturday for INZ009-017- 018-024>027-032>034. Air Quality Alert until midnight EDT /11 PM CDT/ tonight for INZ020-103-203. OH...Excessive Heat Warning until 8 PM EDT Saturday for OHZ001-002- 004-005-015-016-024-025. MI...Heat Advisory until 8 PM EDT Saturday for MIZ078>081-177-277. MARINE...None. && $$ DISCUSSION...Marsili AVIATION...Skipper