Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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517 FXUS63 KILX 050140 AFDILX Area Forecast Discussion...Updated National Weather Service Lincoln IL 840 PM CDT Thu Jul 4 2024 .KEY MESSAGES... - Dry conditions will prevail for July 4th activities this evening. - The best chance for thunderstorms (40-50% chance) will focus along and south of a Paris to Shelbyville line late tonight. && .UPDATE... Issued at 840 PM CDT Thu Jul 4 2024 01z/8pm surface analysis shows a cold front extending from far western Wisconsin to Oklahoma. A broken line of convection has developed along the northern part of the boundary from Wisconsin into central Iowa. These storms are being maintained by a modestly unstable airmass with MLCAPEs of 1000-1500J/kg and strong upper support from a vigorous short-wave trough over Minnesota/Iowa. Meanwhile further south along the front, robust clusters of thunderstorms have developed from southwest Missouri into Oklahoma thanks to a much more unstable environment with MLCAPEs of 2500-3500J/kg. In between the two areas of convection, the boundary is currently inactive immediately west of central Illinois. Cannot rule out a stray shower as the front pushes eastward overnight, but think most locations north of the I-70 corridor will remain dry. Further south, some of the storm clusters currently ongoing across southwest Missouri may lift northeastward ahead of the cold front and spill into the SE CWA toward dawn Friday. Have therefore maintained 40-50 PoPs along/south of a Paris to Shelbyville line late tonight. Overnight lows will drop into the middle 60s behind the front across the Illinois River Valley, but will remain in the lower 70s along/south of I-70. Barnes && .DISCUSSION... Issued at 230 PM CDT Thu Jul 4 2024 This afternoon, a shortwave trough is noted lifting from central Illinois into northeastern Illinois with scattered showers occurring, mainly north of a Macomb to Paris line. Latest RAP suggests very weak instability is in place, less than 200 J/kg but forecast soundings show the instability does poke up above -10C at times and an isolated storm could not be ruled out. As upper wave continues to move northeast, subtle shortwave ridging will move in behind and should keep conditions dry late this afternoon through much of the evening. Late this evening and overnight, deep/closed H5 low is progged to dig across the Upper Midwest while attendant surface cold front sweeps across central Illinois overnight into early Friday morning. Showers and a few storms will accompany these features, spreading back into the Illinois River Valley likely around or just after midnight tonight, then across the remainder of central Illinois overnight. A narrow plume of weak instability (MUCAPE around 300 J/kg) will support a few storms, but the severe threat appears low given the unfavorable diurnal timing and weak instability. Precip amounts will be light overnight across central Illinois. NBM 75th percentile generally runs less than a tenth of an inch north of I- 70, with up to around a quarter inch south. Behind the departing cold front, slightly drier (low to mid 60) dew points will begin to advect across central Illinois along with slightly cooler temps. Expect highs in the upper 70s to low 80s across the area Friday afternoon. Despite the drier air, steep low level lapse rates and lingering low level cyclonic flow should drive some cold-air stratocumulus development Friday afternoon. Forecast soundings suggest instability may be several thousand feet deep allowing a few light showers or sprinkles to develop, especially near and north of I-74. Otherwise, dry weather is expected to prevail Friday and continue into Saturday as high pressure spreads from the central Great Plains Friday across the mid Mississippi Valley Saturday. Heat and humidity begin to work back across central Illinois Sunday as the ridge axis shifts to our east allowing return flow to overspread the area. Precip chances will return early next week as a deep upper trough slowly moves east across the region. Deubelbeiss && .AVIATION... (For the 00z TAFs through 00z Friday Evening) Issued at 640 PM CDT Thu Jul 4 2024 VFR conditions will prevail through the entire 00z TAF period. Winds will initially be E/SE at around 5kt early this evening, then will veer to SW toward midnight as a cold front approaches from the northwest. Once the front passes, winds will become W/NW and gusty on Friday...with forecast soundings and numeric guidance suggesting gusts in the 20-25kt range. FEW-SCT clouds between 3500 and 5000ft will persist through tonight, then will clear out after FROPA. With a pocket of cold air aloft dropping southeastward out of the Northern Plains, steep lapse rates will develop by peak heating Friday. NAM Cu-rule indicates SCT-BKN diurnal clouds at 3500-4000ft during the afternoon. Barnes && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$