Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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689 FXUS63 KILX 071729 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 1229 PM CDT Sun Jul 7 2024 .KEY MESSAGES... - There is around a 20-30% chance for thunderstorms west of the Illinois River this afternoon and evening, with more numerous showers and storms (40-50% chance) area-wide tomorrow. - Tuesday into Wednesday, a swath of heavy rainfall is expected somewhere across central or southern Illinois, with the highest chance (10-15%) for more than 3 inches of rain south of I-72. There is a level 2 of 4, slight, risk for excessive rainfall south of the I-72 corridor. && .UPDATE... Issued at 1031 AM CDT Sun Jul 7 2024 Central IL sits between weak sfc high pressure over the Appalachians and sfc low pressure over the Canadian Prairie, resulting in modest southerly winds across the area. Regional radar shows scattered showers and isolated thunderstorms present across SE IA and far N MO, generally moving to the northeast. Much of the short-term forecast remains on track, with the latest 12z CAMs depicting a similar evolution as that shown in the existing PoP forecast - which has precip largely confined to areas west of the IL River through the daytime hours today, and gradually spreading as far east as the I-55 corridor tonight. PWAT values are not anomalously high for this time of year, with the 12z soundings from ILX and DVN sampling 1.23" and 1.46", respectively. However, given the relatively slow eastward progression of the corridor where showers/storms are focused, there`s a low chance of locally heavy rainfall, with the latest HREF LPMM showing isolated precip totals as high as 2" through early Mon AM. The probability of exceeding 1" of rain (through 7 AM Monday) is still below 25% at any one location, though. Erwin && .DISCUSSION... Issued at 127 AM CDT Sun Jul 7 2024 Quiet conditions continue this Sunday morning, as surface high pressure dominates the region`s weather. Nighttime microphysics satellite shows mid to high clouds streaming toward central and northern Illinois from the west, and regional MRMS base reflectivity shows some scattered showers beneath those clouds. The 07.00z suite of CAMs generally agree this activity may graze our northern county or two late morning, but by and large the area should stay dry through early-mid afternoon. The aforementioned cloud cover may inhibit radiational warming a little, and hence limit depth of afternoon mixing; nevertheless, middle of the road guidance from a blend of CAMs brings highs to the upper 80s most locations, with a few spots touching 90. HREF mean brings MUCAPE values to between 1000 and 1500 J/kg across areas near and west of I-55 this afternoon, when CAMs depict various convective scenarios unfolding near outflows from upstream convection and a potential MCV. Although shear near an MCV could be locally higher than blended guidance would suggest, the HREF suggests probabilities for more than 30 kt 0-6km bulk shear are generally less than 30% - indicating support for organized and sustained storms is lacking. Given mid level dry air in place, can`t rule out some localized gusty winds and brief heavy rains with collapsing storms as various iterations of the HRRR would suggest; however, the same model may be depicting a more impressive low-level inverted V type sounding than reality if it`s overmixing as it did yesterday when surface minimum afternoon dew points were about 5 degrees warmer than it suggested. Severe chances remain generally lower than 5%. Shear actually increases a smidgen tonight into tomorrow morning, and with an ill-defined mid level shortwave lifting northeast through the area ahead of its parent upper trough (in the Upper Midwest) we`ll have some meso-a/synoptic support for a few lingering showers and possibly a rumble of thunder. Marginal (1000 or so J/kg SBCAPE) instability returns tomorrow for perhaps a few more storms during the afternoon and evening, though coverage is only looking 40- 60% (highest west of I-55) at best with many locations staying dry. Things get a little murkier as we head into the mid week time frame with the approach of Beryl`s remnants which have the potential to bring quite a bit of rain to parts of the area. Ensemble guidance has been trending northward with this feature. For example, the LREF mean PWAT Tuesday evening at arbitrarily-chosen Lawrenceville climbed from 1.73" to 1.80" to 1.95" from the 05.12z to 06.00z to 06.12z iterations; for reference, the ILX raob 90th climatological percentile is 1.73", while its moving average daily max is 2.09", for 00z July 10th. Granted, there`s still a significant amount of ensemble spread in how moist the airmass gets with the approach of post-tropical Beryl, with the 10th to 90th percentile PWAT range spanning 1.1 to 2.5 inches at the same location (KLWV), where the rainiest members have over 4 inches of rain falling while 50% of members have 24 hour precip totals somewhere between a tenth of an inch and 1.5 inches. All three deterministic models now have a notable surface low passing through a portion of central or southeast Illinois late Tuesday into Wednesday, with some corridor of heavy (i.e., 2+ inch) rain totals. And while this would be beneficial if it fell slowly enough for the ground to absorb it, we`re a little concerned the global models are having a bit of a resolution issue, and that in reality there will be a narrow corridor of 4+ inches of rain that may result in some hydrological issues (especially if it overlaps with locations that saw the most of the Sunday-Monday precip), while much of the area just doesn`t get enough rain. Given how dry the area is, thinking is we can take quite a bit of rain so the overall flooding risk is still not particularly high, but it will certainly bear watching. One other noteworthy mention would be potential for gusty gradient winds which at this point aren`t looking strong enough to do any damage, although a small subset of ENS members depict gusts over 45 mph in our southeast Wednesday afternoon, and with the strengthening trends in Beryl`s remnants it wouldn`t be prudent to completely discount it. We`ll continue monitoring model trends for both heavy rainfall and wind potential as the potentially stormy mid-week approaches. The eastern half of the CONUS is slated to remain troughy to close the work week, which would suggest chances for precipitation can`t be safely eradicated from our forecast while temps linger near to perhaps slightly below normal (typical mid-July highs are in the mid- upper 80s). Heading into the upcoming weekend, the trend is for the wicked ridge of the West to expand gradually east, though how quickly that occurs is in question. At some point next weekend into the following week (of July 15th), we`ll find ourselves near the east-northeast periphery of that ridge with the westerlies mainly north of us...though maybe a little too close for comfort given seasonably uncomfortable evapotranspiration-enhanced moisture and attendant instability. Bumgardner && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1228 PM CDT Sun Jul 7 2024 High pressure has drifted east of the terminals allowing a modest S/SSW breeze to overspread central Illinois. Showers and thunderstorms are expected to develop across MO/IA later today and slowly push into portions of central Illinois overnight into Monday morning. Through midday Monday, precip is expected to stay mainly along or west of the I-55 corridor. Outside of temporarily lower conditions in showers or storms, VFR conditions will prevail through the period. Deubelbeiss && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$