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Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
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899 FXUS63 KILX 301842 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 142 PM CDT Sun Jun 30 2024 .KEY MESSAGES... - There is a 50-60% chance for thunderstorms west of I-55 Tuesday night, and 50-70% chance for much of central and southeast Illinois both Wednesday and Thursday. However, confidence is low in both severe potential and specific timing of storms at any given location. - Seasonably warm and humid conditions will occur outside of thunderstorms. Wednesday looks like the warmest day, with the highest chance (40-60%) for heat indices eclipsing 100 degrees near and south of I-70. && .DISCUSSION... Issued at 142 PM CDT Sun Jun 30 2024 Under the influence of surface high pressure, tranquil and seasonably cool conditions continue across central Illinois this Sunday. As the surface high settles over the Great Lakes tonight, winds will veer to easterly and ease, leading to rapid radiational heat losses in the absence of clouds with forecast lows in the low to mid 50s; in fact, some sites may fall into the upper 40s, according to NBM`s 20-35% probabilities for sub 50 degF Monday morning lows (highest northeast of a Peoria to Effingham line). As heights continue to build into the region tomorrow, large scale subsidence will favor a continuation of dry conditions which, for most of the area, should last into late Tuesday if not Wednesday. Temperatures will return to near normal Tuesday and above normal by Wednesday as southwesterly flow gradually imports to the area a warmer and more humid low-level airmass. Deterministic models and their ensembles, while in excellent agreement in the upper level pattern, have varying solutions for the evolution of the weather maker slated to approach our area late Tuesday into Wednesday - not surprising given differential heating boundaries from upstream convection will serve as triggers for more convection...and global models simply lack the resolution to handle those features well. Not to infer a model cannot be consistently wrong, I would like to point out there`s some consistency, from one iteration to the next, in the deterministic ECMWF`s lightning activity forecast, which would suggest storms upstream across Iowa weaken as they enter our CWA and finally fall apart near the I-55 corridor Tuesday night, while more develop south of I-72 on Wednesday, with renewed storms perhaps making it a tiny bit further north Wednesday night into Thursday. Conceptually, and in accordance with ensemble guidance, this seems like a reasonable scenario. Mean 500mb heights from the LREF decrease a bit over our region early Wednesday when the deterministic models show a cold front pushing into the area, but then they hold steady as the front stalls and potentially lifts slightly northward heading into Thursday. The area of greater than 30% LREF joint probabilities for more than both (1) 30 kt 0m-500mb shear and (2) 1500 J/kg SBCAPE stays west of our area Tuesday night*, extends south of I-72 on Wednesday, and then remains roughly southwest of a Peoria to Terre Haute line on Thursday. Nonetheless, the precise location of the boundary will dictate both daily high temps, humidity, and the location, timing, and strength of thunderstorms. Since this, being a function of multiple waves of convection, is generally not well-resolved by global models, we can`t get too specific during this forecast package. However, we`re likely to have seasonable warmth and humidity in parts of central and southeast Illinois each day, and a conditional threat for severe thunderstorms - confidence in which remains low (5-10%) at any given location. *Deterministic guidance suggests that while sufficient SBCAPE values for convection will be displaced to our west Tuesday/night, MUCAPE will increase as the LLJ noses into our western counties (mainly near and west of the IL River). Eventually, global ensembles would suggest a trough will at least graze our area to the north, although the magnitude and precise timing of any cool down is a subject of considerable model variability still. By mid next weekend, the LREF mean portrays west- northwest flow across the region, with weak troughing across the Great Lakes and a ridge building in the West. Most members of the GEPS and ENS have negative PC1s (principle components) for EOF1, suggesting less troughing over the Great Lakes and less ridging across the Great Basin/West than the mean of the LREF depicts, while the highest concentration of GEFS membership is in cluster 3 which suggests just the opposite. The deterministic forecast takes the middle of the road and has seasonable temps and 10-25% chances for showers/storms all weekend. Bumgardner && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1227 PM CDT Sun Jun 30 2024 The morning cumulus generally along and east of I-57 has gradually lifted into the VFR category while simultaneously thinning. This trend should continue into the afternoon as mixing deepens with surface heating, with VFR conditions slated to continue throughout the remainder of the forecast period. Northeast winds will simultaneously ease and veer to easterly overnight, picking back up to around 10 kt just after sunrise Monday. Bumgardner && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$