Area Forecast Discussion

KEY MESSAGES

- Oppressively humid through Monday evening (cannot get much more humid than this), with very little relief tonight, followed by Monday afternoon heat indices in many locations peaking 105-115.

- Scattered thunderstorms in the region through Monday, some potentially organized especially late tonight and Monday morning in northwest Illinois. These could be severe with damaging winds (level 2 of 5 risk) if well-organized.

- Some midweek relief behind a cold front Monday night, although smoke from Northwest U.S. and Canadian fires may spread over the area that could continue through Tuesday.

SHORT TERM /THROUGH MONDAY/

Issued at 250 PM CDT Sun Jul 26 2026

Dew points are at the true upper echelon over the area this afternoon, with readings in the CWA between 78F and 82F at almost all ASOS and AWOS locations, with exceptions being a few AWOSs indicating 83F to 86F(!). Coupled with temperatures in the upper 80s to mid 90s, this is providing heat index readings of 100 to 115 for most locations. Such high dew points will offer very little reprieve in cooling tonight, with lows in the upper 70s apart from any convective influences. These lows will be near record daily warm minimum temperatures.

For thunderstorm chances, through this evening there are a few features we are watching that could provide an impetus. One is the warm front near the Mississippi River, which separates temperatures several degrees above 90F from upper 80s. Gradual warm air advection (WAA) atop this has led to some better- defined cumulus in parts of northwest and north central Illinois, and cannot rule out a couple late afternoon storms there. There's also a remnant MCV over the southern CWA creeping southeast. This will likely will spark storms but these may be just beyond our CWA by the time they do (after 5 PM). Finally, there is a short wave and mid-level speed max over far northwest Iowa that will move over the area during this evening. That too may be enough to pop isolated storms. But as is common in this air mass that is highly moist (PWAT 2 inches) and conditionally unstable (MUCAPE of 6,400 J/kg on our 18Z sounding), it is often challenging to forecast areas and timing without some more observational trends. The CAMs naturally struggle too. So just have largely broad 20% chance PoPs to account for these.

For overnight into Monday morning, attention turns to the northeastern side of the heat dome, from Wisconsin southward into Illinois. This area will be under a seasonably strong belt of upper level winds, and at the gradient of strong instability. Well-defined mid level impulses are seen across the northern half of Minnesota this afternoon, and these should be the triggers for mature convection in Wisconsin this evening. More than sufficient deep layer shear (>45 kt) and mid-level lapse rates around 9 C/km should support upscale growth, and steering vectors indicate that to be to the south-southeast. Multiple clusters are favored, possibly even one well-defined mature MCS, that most high-resolution models track from Wisconsin into northern Illinois late tonight/early Monday morning. There is spread in the longitude these storm clusters will take, and accounting for larger than normal uncertainty given the pattern, a reasonable western edge of the storm swath is a little west of the Mississippi River. So all that said have inched up PoPs some, especially in northwest Illinois, late tonight and Monday morning, but the severe risk is at least part conditional on storms being semi- or fully organized into clusters or an MCS. The higher end scenario would be a quickly-propagating mature MCS with a severe wind threat, potentially significant. The more likely scenario is scattered to clusters of storms that would have a scattered severe threat, especially in our far eastern counties.

Outflows from this convection, even if the convection itself is not in our area, are likely to track enough southwest into our area in the morning. These could fester storms especially with a western solution back to even the Mississippi River. This outflow(s) along with any convective clouds lingering through mid-morning is why we felt going with a Heat Advisory for Monday in most of far northwest and north central Illinois was the wiser route. Nonetheless, still oppressively humid, whether highs are upper 80s or mid 90s, and thats the main message for Monday. The 925 mb temperatures (25-30C) would support mid to upper 90s area-wide at this time of year, so those locations that do tap into more full sun, such as parts if not most of eastern/southeastern IA and northeast MO, are favored to see their highest heat index readings of the year (110-115F). Heat safety is a must on Monday.

During the afternoon Monday, most locations look to be markedly capped on forecast soundings, with the exception possibly being the far east due to lingering boundaries and slightly cooler 700-850 mb air. All in all the chances for storms Monday afternoon look low.

LONG TERM /MONDAY NIGHT THROUGH SUNDAY/

Issued at 250 PM CDT Sun Jul 26 2026

A cold front will progress southward on Monday night, with some uncertainty how quickly it moves through the area. Because of that and the very humid and hot conditions during the daytime Monday, have the headlines going all the way through 10 PM. The frontal convergence may be enough to overcome any weaknesses in the cap, which again could be northwest Illinois but still an overall low signal for convection in our CWA.

One note for behind the front is the potential for smoke in the air, possibly even reduced air quality at the surface. GOES Geocolor satellite this afternoon indicated widespread smoke over the northern CONUS Rockies into the western Dakotas. RAP and HRRR extended smoke forecasts indicate this being steered southward behind the front over our area. This is still too far out to tell if it would mix to near-surface, but at least some smoky sky Monday night into early Tuesday looks probable.

After quiet and near to slightly warmer than seasonable temperatures Wednesday and into the day Thursday, a slow moving and fairly well-defined short wave is indicated by guidance to pass over the area late Thursday night into Friday. This looks like the next chance of rain.

AVIATION /00Z TAFS THROUGH 00Z TUESDAY/

Issued at 551 PM CDT Sun Jul 26 2026

A complex of showers/storms is working into our northwest now, which may bring impacts to some locations over the next few hours (until about 03z). Locations with the best chance of seeing impacts will be at CID and MLI, but DBQ cannot be completely ruled out. Although, confidence is low on impacts at this time. Opted to utilize PROB30 groups to message the potential. Otherwise, much of the area will remain quiet through much of the night, with another complex of storms developing north of the area after 09z tonight. Best chances for impacts will be at DBQ and MLI for this complex of storms, moving in from north to south (roughly from 10-15z), but confidence is low. Opted to use PROB30s for this as well due to uncertainty on coverage. With both rounds of storms, brief cig/vis reductions can be expected. Areas of haze/mist can be seen tonight throughout due to higher humidity and light winds.

Winds will remain light and southerly through tonight, increasing out of the southwest after 12z Monday throughout. Higher gusts can be anticipated briefly with any thunderstorm.

DVN WATCHES / WARNINGS / ADVISORIES

IA...Extreme Heat Warning until 10 PM CDT Monday for IAZ040>042- 051>054-063>068-076>078-087>089-098-099. IL...Extreme Heat Warning until 10 PM CDT Monday for ILZ015-016- 024>026-034-035. Heat Advisory until 10 PM CDT Monday for ILZ001-002-007-009- 017-018. MO...Extreme Heat Warning until 10 PM CDT Monday for MOZ009-010.