


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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
49
50
481 FXUS63 KLOT 102359 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 659 PM CDT Thu Jul 10 2025 .KEY MESSAGES... - Several waves of showers and storms are possible from this afternoon through Saturday across the area. Some storms may be severe with damaging winds and flash flooding. - Provided low coverage of thunderstorms, Friday may be hot and humid with heat indices near 100 degrees. && .DISCUSSION... Issued at 247 PM CDT Thu Jul 10 2025 Through Friday Night: Several rounds of showers and thunderstorms are expected to impact at least portions of the area late this afternoon through Friday evening. With these storms, the primary concerns will be the threat for strong to potential severe storms, along with very heavy rainfall and possible flash flooding. An MCV continues to be noted in satellite imagery shifting over southeastern MN, with an associated warm air advection wing noted extending southward along the Mississippi into northwestern IL at the time of this writing. To the east, a very warm and moist low- level airmass (low 70s surface dewpoints) continues to destabilize across northern IL early this afternoon, and has recently supported the development of a few isolated showers and storms across southern Livingston and Ford counties in IL. We expect thunderstorm coverage to be on the increase through the remainder of the afternoon, with at least scattered coverage (40-60%) anticipated over northern IL into early this evening as the warm air advection wing near the Mississippi continues to slide eastward into northern IL. Low to mid-level flow is increasing this afternoon (~30 kt) across the Corn Belt along the southern periphery of the southeastern MN MCV and overtop of backed southeasterly near surface flow. This is resulting in ample shear that will be supportive of organized storms late this afternoon into this evening, possibly including some supercells. As such, a low tornado and damaging wind threat exists (level 1 of 5) with these storms into this evening. Outside of the severe threat, a high Theta E airmass (with precipitable water values of 1.75"+) will also be supportive of torrential downpours with these storms. Additional thunderstorms are likely later this evening and tonight as the parent MCV shifts into southern WI. This is likely to promote waves of backbuilding and training east-southeastward storms over the same areas of southern WI and/or northern IL overnight. Given the presence of deep rich moisture (precipitable water values 1.8"+), there is increasing concern that excessive rainfall amounts (2-3"+) could fall across parts of northern IL tonight and potentially result in flash flooding. Flash flooding would particularly be of concern if the heaviest rainfall is concentrated over the Chicago and/or Rockford metro areas. We gave strong consideration to issuing a flood watch for portions of northern IL (north of I-80) given this potential, but with some uncertainty in which areas are most at risk, we opted to hold off at this time to allow the evening shift to get a better handle of observational trends going into the evening. For now, we plan to highlight this threat in our forecast graphics and in an ESF. While some showers and storms could linger into Friday morning, it does look like there will be a several hour break in the activity in the morning and early afternoon. However, additional showers thunderstorms appear likely to redevelop Friday afternoon and evening along the remnant outflow boundary from tonight`s activity. This activity will once again be developing within a well sheared and unstable environment which will support organized and potentially severe storms. Additional instances of heavy rainfall and possible flash flooding will also be of concern with this activity into Friday evening. KJB Saturday through Thursday: Another round of strong to severe thunderstorms (and localized flash flooding) appears quite possible Saturday afternoon, particularly for areas east of I-39. There may be a fairly early end to the storms, with an increasing potential for the footprint of ongoing storms towards/around sunset to be primarily southeast of I-55 and perhaps even farther southeast. There are a few noteworthy mesoscale wildcards/unknowns that will undoubtedly modulate the PM thunderstorm forecast and associated strong to severe t-storm threat. The first of these, as is very common, is the extent of convection lingering into Saturday morning. Deterministic guidance is extremely mixed on this aspect, with a wide range of plausible outcomes. On one end of the spectrum, an extensive robust convective footprint (with associated heavy rain/flash flood/localized severe threats) could plausibly linger through early to mid morning Saturday. The stabilizing cold pool from this may then serve to limit afternoon thunderstorm coverage and intensity, especially with northwestward extent. There`s likely even a plausible scenario in which the afternoon "zone of concern" shifts well southeast of I-55 or so. Then on the other end of the spectrum, if shower/storm coverage is at a minimum through most of Saturday morning, that would increase the likelihood of more widespread afternoon storms with a damaging wind threat for a larger chunk of the CWA (more on this a few paragraphs down). Synoptically speaking on Saturday, mid-level short-waves embedded in a trough axis extending back to northwest Ontario will be accompanied at the surface by a weak surface low and cold front progressing across the region through Saturday evening. The front will encounter a very warm (highs ~84-90F) and moist (exactly how unstable TBD) air mass with dew points well into the 70s. A final piece of the puzzle is the timing of the cold frontal approach and passage, which will also in all likelihood be modulated by the morning unknowns. Operational guidance has overall trended a bit faster with the incoming cold front, which could veer winds earlier, reduce boundary layer convergence, and limit storm coverage, particularly from the I-39 corridor and west. While the ensemble signal for ~60-70% PoPs/coverage is strong enough to maintain those PoPs area-wide in the afternoon, robust convective initiation may favor the eastern or southeastern half of the CWA. Thunderstorm chances into the early evening, likely focused primarily southeast of I-55 (and possibly only over our far southeast CWA) will then quickly come to an end behind the front. Regarding the severe threat, deep layer shear is forecast to be somewhat muted (20-30 kt), which would limit the chance for widespread intense/organized storms. That said, 20+ kt of shear plus steep low-level lapse rates/moderate to high DCAPE and precip loading in downdrafts are ingredients that may yield at least localized corridors of wind damage, especially if small forward propagating segments can develop. Given the overall uncertainty in the setup and storm intensity limiting factor, SPC`s threat level 1 of 5 (marginal risk) in the day 3 outlook update appears quite reasonable. Saturday evening`s cold front should push far enough southeast into Sunday, along with very dry mid-level air and neutral to positive mid-level height tendencies, to result in a mostly (if not entirely) dry and less humid period at least through Tuesday morning. The 12z NAM was regarded to be an unrealistic outlier with additional showers and storms later Saturday night into Sunday. Thunderstorm chances may then again uptick midweek with the next short-wave trough and accompanying cold front. Temps will continue to average solidly above normal with moderately humid to muggy conditions common to mid July, and then possibly cool to below normal by the end of the work week in the wake of the cold front passage. Castro && .AVIATION /00Z TAFS THROUGH 06Z SATURDAY/... Issued at 659 PM CDT Thu Jul 10 2025 Key messages for the 00Z TAFs: - Clusters of strong TSRA expected to affect the region through tonight, with additional TSRA development likely Friday afternoon/evening. - Brief IFR/MVFR vis and MVFR cigs possible in TSRA. - Relatively low confidence in hour by hour wind directions and Thunderstorm timing Friday due to thunderstorm outflow/cold pool effects of preceding storms tonight. Scattered to numerous thunderstorms have developed from eastern IA into northern IL/southern WI since mid-afternoon, in association with an eastard-moving convectively augmented mid- level disturbance and a warm frontal boundary which drapes from near DBQ to near IKK/LAF. Clusters of storms are expected to persist across the forecast area this evening and into the overnight hours as the mid-level disturbance tracks slowly east, and TSRA outflow enhances the frontal zone. Confidence decreases in thunderstorm coverage after midnight as current eastern IA/IL storms work east of the area, though redevelopment is possible overnight as a west-southwest low level jet continues to impinge on the outflow enhanced front. Storms this evening/overnight will likely leave a residual cold pool (outflow-cooled air) across the area by Friday morning, with a diurnal minima in TS coverage/intensity expected. Additional thunderstorm development is then likely Friday afternoon into Friday evening ahead of another approaching mid-level disturbance during diurnal peak heating and destabilization. All trends from early Friday morning through Friday evening are predicated on locations of outflow boundaries and residual stable cold-pools, thus confidence in details diminishes markedly with time. Surface winds across the terminals have generally been east to southeast to the north of the warm front this afternoon, but are becoming convectively disturbed with current storms. Outside of convectively-disturbed TS outflows, synoptic directions should continue to favor a southeasterly wind direction overnight, becoming more south- southwest Friday. Like TSRA timing however, confidence in details of wind directions is also relatively low Friday. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Flood Watch through Friday morning for ILZ003-ILZ004-ILZ005- ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103- ILZ104-ILZ105-ILZ106. IN...None. LM...None. && $$ Visit us at weather.gov/chicago