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Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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851 FXUS63 KILX 111005 AFDILX Area Forecast Discussion...Updated Aviation National Weather Service Lincoln IL 505 AM CDT Thu Jul 11 2024 .KEY MESSAGES... - Through 9am, patchy fog will result in localized visibilities under 1 mile, which could increase the risk for motor vehicle incidents. - There is a 30-40% chance for showers and thunderstorms this afternoon, posing a lightning risk to those participating in outdoor activities. - There is a 60-80% chance for heat indices over 100 degrees west of I-55 on Sunday, with higher chances area-wide on Monday. This will increase the risk for heat-related illness in vulnerable populations. - Scattered thunderstorms (30% coverage) are expected Saturday, and again late Monday into Wednesday. The timing and location of any severe weather risk remains unclear at this time. && .DISCUSSION... Issued at 130 AM CDT Thu Jul 11 2024 Early this Thursday morning, showers and thunderstorms continue to weaken over east-central Illinois, with some lingering mid level clouds in their wake. These will offset radiational losses through morning, though in breaks between these clouds there will be some patchy fog around after the recent rains; Galesburg`s visibility is already down to 1 1/2 miles and Peoria recently (though briefly) dropped to 5 miles. Mainly driven by the NAMNest and ARW, HREF advertises 20-35% chances for visibilities dipping below 1/2 mile between 5 and 9am, but thereafter radiational warming will result in this boundary layer moisture getting mixed out. Beneath the upper level trough this afternoon, cool advection and widespread diurnal cu will keep us a few degrees below normal for mid July with highs in the low 80s. Near and east of the surface low, meso-a scale uplift and 1000-1500 J/kg SBCAPE will promote a few more pop-up storms this afternoon and evening mainly east of the IL River where we`ve got 30-40% chances in the forecast. Being diurnally driven, those storms will dissolve late evening, simultaneously exiting our area to the east for a period of dry weather tonight through tomorrow night when each CAM is precip-less over central Illinois. Heading into Saturday, the ECMWF and GFS depict a subtle shortwave trough attempting to round the northeast periphery of the ridge building into the region from the west, and suggest storms will develop during the morning across a portion of the ILX CWA amidst overrunning/warm advection. While instability will be on the rise Friday night into Saturday when the NAM suggests 2500+ J/kg of MUCAPE overspread the region as mid level lapse rates steepen to around 7C/km, there`s not a strong signal for shear in the global models with LREF mean only giving us around 20 kt of it between 0m and 500mb. While one shouldn`t expect widespread or significant severe weather Saturday, some locally severe hail seems plausible, though its potential will be limited a bit by warm cloud layer depths around 4km with Donovan heights around 40kft. Given the lack of wind shear, convective outflows are likely to trigger more storms through the afternoon on Saturday which could keep our highs confined to the mid 80s, near NBM`s 25th percentile. Sunday, however, unless the deterministic CMC (unlike the GFS and ECMWF) is correct in its depiction of a morning MCS diving southeast into the area, temps are likely to become warmer as the ridge builds into the region and capping inhibits convective potential. It`ll certainly be a humid day, especially if much of the area gets respectable rainfall totals from Saturday`s storms. LREF mean dewpoint temps reach 76-77 degrees by Sunday afternoon, with around 15% of membership bringing values to 80 or higher. If high temps in fact reach the low to locally mid 90s as forecast by NBM mean (which may be a degree or three high if we have such humid low levels), heat indices would be in the 100-110 degree range during the afternoon - highest west of I-55. While LREF probabilities for heat indices surpassing 105 (local Advisory threshold) are only around 10- 15%, we think the chances might actually be a bit higher, nearer 50- 70% west of I-55 and 25-40% east. Monday continues to look like the hottest day, with forecast highs running about 2 degrees warmer than Sunday (92-95, vs 89-94, degF) and LREF probabilities for dew points of 80 deg or warmer also higher at 20-40%. Based on these forecast temps and dew points, afternoon heat indices would reach the 104-114 degree range area- wide, necessitating a Heat Advisory (70+% chance). However, confidence is not high enough to issue one at this juncture due to potential for convection to offset the heat as the upper level jet remains too close for comfort and each global deterministic model, though varying from run to run in precise timing and location, develops storms at least near our area. Global models and their ensembles exhibit considerable spread in the timing of the ridge breaking down and a trough diving into the region from the northwest, though LREF mean shows 500hPa heights lowering across our area gradually (perhaps due to model timing differences) by midweek. The global models suggest the upper trough`s center will pass to our east, giving the region only a glancing blow with its cool front stalling somewhere near the area. This would suggest uncertainty in three key things: (1) when the area cools down, (2) how much we cool down, and (3) timing of convection. Depending on frontal timing, Monday, Tuesday, and/or Wednesday we`ll need to be on the lookout for severe storms given just how unstable the airmass will be. Given model spread in timing of fropa, LREF mean SBCAPE decreases from 4000+ J/kg Monday to ~3500 Tuesday, to ~1500 Wednesday. Before the cold front arrives, one shouldn`t be surprised if on any given day we have over 5000 J/kg SBCAPE on our raob, with the most warm and humid day currently slated to be Monday. While forward-propagating MCS activity would be favored near and north of the region late Saturday into Monday, the approach of a cool front, an upper trough, and its associated enhanced shear (35-45 kt 0-6km, if you believe the GFS) sometime between late Monday-Wednesday would favor more of an elongated, broken line of storms which could feature a conditional tornado threat. We`ll continue to monitor and reassess with forthcoming model iterations and, of course, our suite of (upstream) observational data. Bumgardner && .AVIATION... (For the 12z TAFs through 12z Friday Morning) Issued at 458 AM CDT Thu Jul 11 2024 Some patchy dense fog has kicked off the morning, with MVFR and occasional IFR visibilities at PIA and BMI where a TEMPO group was added through 14z to account for these fluctuations. Mid morning, dramatic improvement is anticipated as radiational warming burns off the fog, although a brief period of MVFR ceilings is possible (40-50% chance) as diurnal cu develops toward lunchtime. Before long though, cloud bases should rise to between 3000 and 4500 ft as the mixed layer deepens. Scattered showers and possibly an isolated storm are expected mid afternoon through evening area-wide, with the most likely locations being BMI, DEC, and CMI. Chances for lightning were still less than 30%, so a prob30 was not yet issued, although that could be added if confidence increases. Tonight, lack of cloud cover and light winds could once again result in a brief period of fog, for which a group was added to include MVFR visibilities beginning at 09z/4am (Friday) at BMI, PIA, and SPI. Bumgardner && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$