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Markham, Illinois Weather Forecast Discussion

920
FXUS63 KLOT 202344
AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 644 PM CDT Sat Sep 20 2025

.KEY MESSAGES...

- Scattered thunderstorms expected this afternoon and early evening, a few could be strong with locally heavy rainfall also possible.

- Another round of showers of thunderstorms possible late this evening into the overnight.

- While there will be many dry hours, the threat of periodic showers and thunderstorms will continue through at least the first half of the upcoming work week.

&&

.DISCUSSION... Issued at 205 PM CDT Sat Sep 20 2025

Through early evening:

Morning soundings from DVN and ILX are moist with fairly modest lapse rates. Some clearing across our area this morning is allowing the boundary layer to heat. Modifying the morning soundings, it`d only take sfc temps in the upper 70s with dewpoints in the lower 60s to get MLCAPE around 1000 J/kg with little CINH. Given the expected destabilization and lack of a cap, the question becomes what will force storms.

Water vapor imagery this morning depicts a negatively tilted trough across the mid and upper Mississippi Valley. Well defined vort max is rounding the base of this trough, moving into western Wisconsin late this morning. Diffluence is quite evident over northern IL on water vapor imagery in the base of the trough and on the southern flanks of the vort max.

Given the destabilizing boundary layer and sustained ascent associated with the diffluent and divergent flow aloft, we should see an increase in coverage of showers and thunderstorms the remainder of this afternoon. Greatest coverage of thunderstorms, including a threat of a couple of stronger storms, should be over the eastern half or so of our CWA. Weak shear profiles will likely limit the severe threat, but certainly can`t rule out a couple of stronger to possibly marginally severe storms this afternoon.

Tonight:

Low confidence in evolution of things tonight. Water vapor imagery suggests perhaps a weak MCV over southern IA which could help maintain isolated to scattered coverage of showers and thunderstorms through the evening and into the overnight hours. Extrapolation of this feature would tend to suggest that the associated ascent would largely move east of our CWA by later tonight, though some modest (15-20kt) southerly low level flow could maybe provide enough theta-e advection to maybe keep some convection going in the wake of the wave. The 12z HREF was still pretty bullish with convection overnight, so maintained higher chances pops despite the lower confidence and less of a signal in most other models.

Sunday:

Synoptically, there`s definitely a play for there to be less coverage of showers and thunderstorms and possibly a mostly dry day for some (many?) areas Sunday. Aforementioned negatively tilted trough looks to lift north and away from our area with neutral to even weak height rises locally Sunday. Another vort will dig southeastward into the upper Mississippi Valley Sunday afternoon, but forcing with this trough looks to remain well west and northwest of our area. With a warm and moderately unstable air mass in place, going to maintain some chance pops, highest over the eastern CWA.

- Izzi

Sunday Night through Saturday: Issued at 328 AM CDT Sat Sep 20 2025

Intermittent shower and thunderstorm chances will persist Sunday night through Tuesday as additional disturbances ripple through the west to northwesterly flow aloft. A chaotic upper pattern into the middle and end of next week will likely continue to result in lower-than-typical forecast confidence. Some signal remains in the extended guidance for at least a brief return to drier conditions (especially near/north of I-80) in the wake of a cold front towards midweek, but depending on the influence of another upper low towards the end of the week, this could end up being short-lived. The latest GFS in particular would indicate little/no break in the action with a Fujiwhara dance taking place between several robust disturbances across the central CONUS.

Temperatures this upcoming week will start out solidly above normal (along with fairly humid conditions as well), with highs well into the 70s to near 80F through Tuesday, then trending closer to normal and a bit less humid in the mid to late week.

Carlaw/Castro

- Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/... Issued at 644 PM CDT Sat Sep 20 2025

Main Concern:

- Potential for SHRA and isolated TS overnight into Sunday morning and then scattered TS mid-late Sunday afternoon

Widely scattered showers will be possible starting in the late evening and then shower coverage should increase some overnight into Sunday morning, which may also coincide with isolated embedded thunderstorms. Forecast soundings are rather unfavorable for lightning, so while it certainly can`t be ruled out at the terminals, confidence in occurrence is only 15-20% and as such opted to remove TS mention from the PROB30s. Another round of widely scattered TS is possible over the Chicago metro and points south and southeast mid to late Sunday afternoon. Maintained PROB30 mention for this in the ORD and MDW TAFs and included it in the DPA and GYY TAFs as well.

Winds will generally be light and variable until shifting to southwesterly on Sunday, but speeds will still be predominantly less than 10 kt. Aside from flight category reductions in/near showers and any thunderstorms, expect VFR conditions. The minor exception may be brief MVFR CIGs at RFD during the day on Sunday.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None.

&&

$$

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NWS LOT Office Area Forecast Discussion

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