Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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517 FXUS63 KILX 201754 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1254 PM CDT Mon May 20 2024 .KEY MESSAGES... - Heat indices will return to the low 90s tomorrow, posing a risk for heat-related illness in vulnerable populations. - Periods of thunderstorms will continue through Tuesday night north of I-70 and Wednesday further south. Any of these storms could turn severe, but the greatest risk (level 3 of 5, "enhanced") will be near and west of the IL River late Tuesday evening. && .UPDATE... Issued at 1007 AM CDT Mon May 20 2024 Late this morning, a pair of upper level disturbances are moving into portions of the state. An MCV emanating from severe storms in KS yesterday is currently lifting into far NW Illinois while further south, a shortwave trough is lifting across the St Louis area. Either of these features could trigger isolated to widely scattered showers and storms late this morning into the afternoon. Deep layer shear today is sub-marginal for an organized severe weather threat, topping out 20-25kt, but moderate instability with MLCAPE values of 1500-2000 J/kg could support some stronger updrafts with a pulse severe threat and locally heavy rainfall. HREF LPMM indicates a couple small bullseyes of 2-4 inches of rain could occur with any stronger storms, in part owing to slow storm motions around 20 mph and PWats increasing to around 1.5 inches today. Temps are on track to top out in the mid to upper 80s this afternoon. Deubelbeiss && .DISCUSSION... Issued at 132 AM CDT Mon May 20 2024 Latest radar shows an absence of appreciable reflectivities across central Illinois, but also a stretched-out and weakening MCS whose MCV is centered near Kansas City. The past several iterations of the HRRR suggest renewed showers and storms associated with the southern portion of that MCS could erupt 4-8am this morning mainly north of I- 72, but disagree on eastward extent of thunderstorm activity and fortunately all suggest storms stay sub-severe (though isolated wind gusts over 40mph could occur). The CAMs suggest the remnant MCV will lift northeastward across eastern Iowa this morning, leading to regeneration of convection to our north mid to late morning and across our area along its trailing boundary after around 1-2pm. It`s uncertain how much sunshine we`ll get late morning into early-mid afternoon for destabilization as HREF shows abundant mid to high "convective debris" clouds, and soundings generally appear less unstable than yesterday with a weak capping inversion in place. However, forcing for ascent and increased shear along the aforementioned boundary - wherever it tracks - could prove sufficient to initiate and sustain storms, which, given dry mid levels and modest (7-7.5C/km) 700-500mb lapse rates, could generate hail and strong wind gusts (especially if a linear storm mode evolves along the boundary). Virtually all of the CAMs show something across our area during the 3 to 7 pm timeframe, so we`ll be keeping a close eye on things this afternoon and evening. Tuesday`s forecast is quite the challenge, as this afternoon/evening activity will (1) modify the environment into which the low is moving, affecting its development for better or worse...and (2) have an especially large influence over the location of the warm front which could be as far north as Milwaukee or south of I-80 during the day. If it ends up being on southern end of the ensemble envelope, we could be dealing with severe weather as early as Tuesday morning across our northwest; in fact, the 00z HRRR suggests several rounds of convection could ripple along the warm front from northeast Missouri to northeast Illinois (clipping our far northwest CWA) throughout the entire day Tuesday, with each storm carrying a risk for large hail and afternoon/evening storms posing a threat for tornadoes and intense straight-line winds as well. However, if one of the model solutions featuring a farther-north warm front set up materializes, we could go the entire day without seeing any precip. Forecast soundings suggest a loaded gun-type thermodynamic profile suggestive of a strong capping inversion which should prevent convective development outside of some rather strong forcing mechanism, until the upper low starts to draw near during the mid to late evening leading to steepening mid level lapse rates (over 8C/km, if you believe the NAM) and increasing shear (45-55 kt west of the IL River by 7pm). The 00z deterministic ECMWF suggests a broken line of storms reaches Schuyler County around 6-7pm, I-55 around 9-10pm, and I-57 around 11pm, and given the LLJ stretching hodographs for 150-250 m^2/s^2 0- 1km helicity, concern exists that these storms could pack potential for more than just straight-line winds even if the storms wind up being linear. However, if a large enough component of 0-3km shear (30-40 kt) is perpendicular to the cold front, storm mode might be more supercellular (or hybrid) than QLCS...and if that happens we`ll definitely be concerned about all severe hazards, especially west of the IL River. Wednesday, there`s some uncertainty in how far south the cold front will make it before stalling out, and given another mid level wave lifting northeast along and south of I-70 we could wind up with another round of severe storms there during the afternoon and evening. We`ll be watching model trends with the placement of this wave and the frontal boundary to get a handle on Wednesday`s storm potential as well as the northward extent of PoPs. Northwest of the cold front it`ll be near to slightly below normal with highs in the low to mid 70s, while south of it the airmass will remain quite warm and sticky. Thursday into the weekend, predictability decreases as models differ in their depiction of a parade of MCSs riding along a quasistationary boundary draped roughly from the Central Plains to Lower Ohio or Tennessee Valley, bringing potential for heavy rain, strong wind gusts, and isolated hail to a large portion of the country. Since we`ll be near or just north of that boundary, and that boundary will move under the influence of MCS outflows, both precipitation chances and temperatures are impossible to pin down at this juncture. Bumgardner && .AVIATION... (For the 18z TAFs through 18z Tuesday Afternoon) Issued at 1255 PM CDT Mon May 20 2024 VFR conditions are expected to continue through most of the 24 hour period through 18Z/1 pm Tue across the central IL airports. However an isolated thunderstorm could (20% chance) impact the PIA early this afternoon and along I-74 terminals early this evening from 23Z-02Z. Breezy SW winds 13-17 kts with gusts around 25 kts through mid afternoon, will veer south and subside to around 10 kts or less around 00z/7pm. South winds increase to breezy levels again after 15Z Tue with speeds near 15 kts and gusts around 25 kts. 07 && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$