


Area Forecast Discussion
Issued by NWS Central Illinois
Issued by NWS Central Illinois
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
738 FXUS63 KILX 132343 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 643 PM CDT Sun Jul 13 2025 .KEY MESSAGES... - Rain Showers will taper off this evening. - Areas of fog will develop overnight, with patchy dense fog possible. - Additional chances for strong thunderstorms and flooding return by the middle of the week (Wednesday - Thursday) and again by next weekend (Saturday). && .DISCUSSION...(through next Saturday night) Issued at 1250 PM CDT Sun Jul 13 2025 A weak front lies across central Illinois, near and south of the I-72 corridor, as shown by early afternoon surface analysis. Scattered showers have developed due to warm-air advection and isentropic lift, driven by a very moist boundary layer (with PWATS exceeding the 90th percentile for mid-July climatology) and weak low-level southwest flow impinging on the surface front. Scattered thunderstorms are expected to develop this afternoon as instability increases south of the boundary. This will be accompanied by an increase in forcing for ascent and deep-layer shear, driven by a mid-level shortwave trough lifting into southern Illinois. Modest CAPE/Shear profiles are expected this afternoon south of the front with the latest RAP guidance supporting MLCAPE values between 1000-1850 J/kg and deep-layer shear between 20-30 kts. Though poor mid-level lapse rates (<6.5 C/km) may hinder updraft development, the presence of steep low-level lapse rates (>8 C/km) near and south of I-70 suggests a risk of damaging thunderstorm winds wherever the stronger updrafts do form. The risk of hail is minimal due to limited instability and shear in the hail growth zone (-10 to -30 C). Similarly, the tornado threat is low, primarily attributed to a relatively straight hodograph and weak streamwise vorticity within the effective inflow layer. This is best illustrated by poor 0-1km SRH values (< 50 m2/s2). Still, any squirrelly storm-scale interactions that could potentially boost tornado potential will need to be monitored in addition to the potential for landspouts that could anchor along the synoptic boundary. The more widespread impact today will be heavy rainfall and localized urban flooding. The latest HREF LPMM QPF guidance supports pockets of 1-3 of rainfall through about 9pm this evening. This seems reasonable given our moisture-laden airmass and the fact that the mean wind and shear vectors are parallel to the slow-moving front. Monday is looking like a dry day for most as the mid-level shortwave departs east of our area and the sfc front washes out somewhere near the HWY 50 corridor. The pattern becomes hard to whip by the middle of the week, with the next chance for rain returning as early as Tuesday. By then, another subtle shortwave impulse appears to lift northward across the Missouri Ozarks, helping to resharpen the washed-out sfc front and nudge it northward back into central Illinois. Relatively weak deep-layer shear but moderate buoyancy near and south of the front could support scattered thunderstorms both Tuesday and Wednesday afternoon. Shower and thunderstorm coverage may then increase Wednesday and Thursday as a pair of mid-level shortwaves lift across the Mid- Mississippi Valley, which will again push a synoptic front through our region. A quick assessment of deep-layer shear and PWATs during this timeframe indicates a potential return of both strong thunderstorms and localized flooding. Chances for rain lower by Friday as the synoptic front exits our area and is replaced by surface high pressure. Areas of dense fog could become prevalent Friday night into Saturday morning across portions of central and southeast Illinois. Rain chances then increase Saturday afternoon and evening as ensemble guidance hints at another shortwave trough moving through quasi-zonal, mid-level flow, ultimately helping push a frontal system into central Illinois. MJA && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 643 PM CDT Sun Jul 13 2025 Showers will depart the central IL terminals to the east early this evening, followed by clearing skies. Until then, a couple hours if IFR to MVFR cigs are possible from KBMI-KDEC eastward early this evening. Overnight, high pressure shifting over the area after recent rainfall will bring favorable conditions for fog development, and have included IFR vsby at all terminals by 09Z, although locally worse conditions are possible. Fog should diminish during the morning, followed by scattered cloud cover around 5000 feet AGL for the afternoon. Winds light and variable, becoming S-SE 3-6 kts by 18Z. DJO && .ILX WATCHES/WARNINGS/ADVISORIES... None. && $$