454 FXUS63 KILX 291755 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 1255 PM CDT Mon Jun 29 2026 .KEY MESSAGES... - A Heat Advisory is in effect from today through Thursday as heat index readings peak at 105-110 degrees each afternoon. - Thunderstorm chances will gradually increase Friday into the upcoming holiday weekend. && .DISCUSSION... Issued at 347 AM CDT Mon Jun 29 2026 **** Hot and Humid Weather **** 08z/3am water vapor imagery shows a pronounced upper trough along/west of the Rockies with a corresponding downstream ridge centered from the Gulf Coast northward to the Great Lakes. Thanks to rising upper heights, the persistent frontal boundary that brought several days of showers/storms to southern Illinois and the Ohio River Valley has lifted northward to the I-74 corridor. The front will continue its northward trek into the Upper Midwest/Great Lakes today, placing all of central and southeast Illinois squarely in a hot/humid airmass. Partial cloud cover will do little to mitigate the warming, as forecast soundings and numeric guidance suggest afternoon highs climbing into the lower 90s. Even hotter conditions will be on tap for Tuesday and Wednesday: however, guidance including the MET/MAV/NBM appear to be a few degrees too warm in the upper 90s. As the ridge axis shifts eastward, 500mb heights will drop into the 592-594dm range over central Illinois. In addition, plentiful boundary layer moisture with dewpoints in the middle 70s will temper the heating. As a result, think highs will only rise a couple of degrees into the middle 90s for Tuesday through Thursday. Nonetheless, afternoon heat index values will exceed 105 degrees while overnight readings only drop into the middle to upper 70s. While these conditions are certainly not out of the ordinary for this time of year, this is the first time this summer the region will experience a prolonged period of heat/humidity. As a result, the Heat Advisory will continue through Thursday. Increasing cloud cover and convection chances will likely result in a slight cooling by Friday, although the advisory may eventually need to be extended for locations along/south of the I-70 corridor if trends continue. **** Thunderstorm Chances Return by the End of the Week **** A series of short-wave troughs ejecting northeastward out of the Rockies will gradually flatten the prevailing upper ridge by the end of the week. While the main baroclinic zone will remain focused from Michigan southwestward to Nebraska, a more significant wave may nudge the front a bit southward and bring an increased rain chance to at least the northern half of the KILX CWA on Friday. The 00z Jun 29 GFS/ECMWF/GEM are all in excellent agreement concerning this process, so have therefore introduced 30-40 PoPs across the area. Will need to keep an eye on how far south the front gets nudged, as the latest WPC outlook indicates a marginal risk (5-15% chance) for excessive rainfall as far south as Galesburg and Lacon. Once the Friday short-wave exits, a flat west-to-east zonal flow pattern will prevail across the Midwest as all models show the ridge retrograding westward toward the Desert Southwest. It is difficult to pinpoint/time any subtle short-waves embedded within the flow at that time range, so forecast confidence for the 4th of July remains poor. The current model suite does not show any distinct forcing mechanism other than perhaps lingering outflow boundaries, so rain chances should be low. After that, another stronger short-wave will track across the Plains and dig southeastward into the Ohio River Valley. This will push a cold front through central Illinois on Sunday, followed by a return of more seasonable temperatures in the upper 80s by early next week. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1248 PM CDT Mon Jun 29 2026 VFR conditions will be most common through this TAF cycle as an upper-level ridge settles over the regional terminals. The only potential failure modes are if sky coverage increases more than currently forecast, becoming BKN rather than SCT. With low- level conditions being so humid, ceiling heights will be in the MVFR range during the afternoon. Otherwise, patchy and shallow, near- surface fog could result in brief periods of MVFR visibility across the regional ASOS sensors Tuesday morning. Low-level winds will remain southerly, occasionally gusting to 20 kts this afternoon before diminishing overnight. && .ILX WATCHES/WARNINGS/ADVISORIES... Heat Advisory until midnight CDT Thursday night for ILZ027>031- 036>038-040>057-061>063-066>068-071>073. && $$ KEY MESSAGES...Barnes DISCUSSION...Barnes AVIATION...MJA