Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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068 FXUS63 KILX 151744 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1244 PM CDT Mon Jul 15 2024 .KEY MESSAGES... - Oppressive heat and humid is expected today, with highs in the mid 90s and peak heat indices ranging from 105 to as much as 115 degrees in some places. Cooler air arrives by mid-week, but areas south of I- 70 could see heat indices remain above 100 degrees on Tuesday. - Another round of severe thunderstorms is likely this evening after 7 or 8 PM (level 3 of 5 risk). The primary hazard is damaging winds, but a few tornadoes are also possible. - The thunderstorm chances shift south of I-72 Tuesday evening (50- 70% chance), with some potential for severe thunderstorms (level 1 of 5) or excessive rainfall (level 2 of 4). && .UPDATE... Issued at 1104 AM CDT Mon Jul 15 2024 An oppressively hot and humid day is unfolding across central Illinois as 15z/10am temperatures have already reached the middle to upper 80s...well on their way to afternoon highs in the middle 90s. With dewpoints in the middle 70s with localized pockets in the upper 70s, afternoon heat index values will peak from 105 to 110 degrees. The only mitigating factor to the heat will be southwesterly winds gusting to around 20mph. A vigorous short-wave trough tracking into the Upper Midwest will trigger a cluster of convection across northern Iowa/southern Minnesota late this afternoon. This activity will rapidly grow upscale into an organized MCS...then track E/SE into Illinois this evening. At this time, it appears storms will approach the far NW KILX CWA by around 8pm. Damaging winds in excess of 60mph, isolated tornadoes, and torrential rainfall will be likely with the storms as they race along/north of the I-74 corridor later this evening. The storms will then tend to settle further southward into central Illinois overnight with a diminishing severe weather risk with time. Barnes && .DISCUSSION... Issued at 313 AM CDT Mon Jul 15 2024 The "ring of fire" remains active along the northern periphery of an upper ridge that remains centered over the SW US, with several MCS developing over the last few days - including one ongoing across central IL as of 2am/07z - and another anticipated Monday evening. Regarding the ongoing MCS, it has largely become outflow dominant with cloud temps trending warmer, with the exception of the western flank. New cell generation continues to occur along the surging outflow. Forward-propagation Corfidi vectors from the RAP favor a continued southeastward motion of this MCS, but as it does so it will steadily be moving into a less sheared environment - and thus the expectation is that the severe threat is and will continue to trend lower. Will have to keep an eye out for locally heavy rainfall on the western flank of this MCS over the next few hours, as recent radar imagery shows backbuilding storms tracking across the same parts of Fulton/Tazewell counties. 3-hour flash flood guidance in these areas is a little over 2", and radar estimates are starting to approach 2" is western Tazewell. The latest forecast has PoPs progressing all the way into southeastern IL for this MCS, gradually diminishing with southeastward extent as confidence decreases in its longevity. The MCS will likely result in a cloudier start to the day across the southeastern half of the forecast area. While there`s still some uncertainty, the expectation is that skies transition to mostly clear/partly cloudy by the afternoon. Assuming the cloud cover does in fact clear out, forecast highs should push into the mid 90s, with dewpoints in the mid/upper 70s resulting in oppressive heat. Peak heat index values could climb over 110 degrees, particularly in locations west of I-55, which is approaching excessive heat warning criteria. However, given the uncertainties in the cloud cover forecast opted to stick with the existing heat advisory. Regarding the severe potential this evening (level 3 of 5 risk), the HREF depicts a strongly unstable environment, with mean MUCAPE values approaching 4000 J/kg coincident with 30-35 knots of deep layer shear. Forecast soundings show very high DCAPE values approaching 1400 J/kg. These instability parameters favor strong updrafts/downdrafts capable of producing severe wind gusts. CAMs like the HRRR support this notion, with some runs of the HRRR depicting thunderstorms producing scattered wind gusts over 70 mph. The latest CAMs are actually in fairly good agreement regarding the storm evolution on Monday, with storms forming first over IA during the late afternoon/early evening, then quickly growing upscale and reaching the northwest part of the ILX CWA after 8-9 PM. Once these storms move into the ILX CWA, they`re expected to pose a severe threat for at least several hours, but the exact evolution and potential end time of this expected MCS remains uncertain. Damaging winds are the main hazard, but the potential for line-embedded tornadoes also appears concerning. Guidance has a strong 925mb LLJ developing around the time storms move into the ILX CWA, with 925 mb winds approaching 40-45 knots, which results in 0-1 km shear vectors of almost 30 knots. The pattern shift beings on Tuesday, as a cold front begins to sink into the area. Additional storm development is possible along/ahead of the front, mainly south of I-72 Tues PM (50-70% chance). The environment ahead of the front continues to exhibit ample instability, but weak shear that makes an organized severe threat look less likely compared to Mon night. There is a level 1 of 5 risk of severe weather on Tuesday. Areas ahead of the front will remain hot and humid, and the heat advisory may need to be extended for areas south of I-70 on Tuesday for heat indices over 100 degrees. The other concern with the storms Tues is the potential for locally heavy rainfall/isolated flash flooding, as forecast PWAT values approach 2.3". While we are near the climatological peak of PWAT values in central IL, those values are still quite extreme, well above the 90th percentile and approaching the maximum of the ILX sounding climatology, regardless of time of year. While isolated, some CAMs do depict isolated rainfall totals in excess of 2-3". No significant changes occurred in the extended forecast, as the pattern shifts to below normal temps and dry, save for some lingering precip chances in SE IL near the frontal boundary Wednesday. Erwin && .AVIATION... (For the 18z TAFs through 18z Tuesday Afternoon) Issued at 1244 PM CDT Mon Jul 15 2024 Primary aviation forecasting challenge will be potential convection later this evening into the overnight hours. CAMs suggest a cluster of thunderstorms will develop across northern Iowa late this afternoon, then expand and track E/SE into Illinois this evening. While the exact southern extent of the storms remains somewhat in question, confidence is growing that the I-74 terminals will be impacted by thunderstorms. As a result, have added a TEMPO group for thunder at KPIA between 04z and 07z...then further southeast to KCMI between 07z and 10z. Given uncertainty, have opted not to include thunder at either KSPI or KDEC until trends become better established later today. Once the main storm cluster passes, scattered showers may linger along/south of I-72 well after sunrise. Winds will be SW at 8-14kt, then will begin to veer to the W by Tuesday morning as a cold front approaches from the northwest. Barnes && .ILX WATCHES/WARNINGS/ADVISORIES... Heat Advisory until 8 PM CDT this evening for ILZ027>031-036>038- 040>057-061>063-066>068-071>073. && $$