textproduct: Central Illinois

KEY MESSAGES

- A Dense Fog Advisory is in effect along and east of the I-55 corridor through 8am today.

- A Heat Advisory is in effect for locations along and southwest of a Peoria...to Decatur...to Robinson line this afternoon. The advisory expands to include all of central Illinois Monday through Thursday when maximum heat index values exceed 105 degrees.

DISCUSSION

Issued at 330 AM CDT Sun Jun 28 2026

**** Dense Fog Early This Morning ****

Very moist boundary layer conditions exist across central Illinois early this morning as evidenced by 07z/2am surface dewpoints in the upper 60s and lower 70s. While skies have generally remained overcast, there have been enough breaks in the cloud cover to allow temperatures to cool to the dewpoint and areas of fog to develop. CAMs have a good handle on the situation and have been consistently focusing the lowest visbys along/east of the I-55 corridor. With the proper synoptic pattern in place, a clear signal from the models, and area obs beginning to show locally dense fog, opted to issue a Dense Fog Advisory for the eastern half of the KILX CWA through 8am.

**** A Few Thunderstorms Today ****

07z/2am surface analysis shows a nearly stationary frontal boundary extending from northern Missouri to western Kentucky. Scattered convection has formed immediately along the front and will continue to skirt the KILX CWA to the S/SW through sunrise. As upper heights begin to rise in response to a deep trough digging west of the Rockies, the front will gradually get nudged northward today. As a result, a few showers/storms will develop as far north as a Galesburg...to Decatur...to Robinson line later this morning before dissipating before midday. Areal coverage will remain low (20% or less) and any storms that occur are not expected to be severe. Meanwhile further northwest, a cluster of strong to severe thunderstorms aided by a 55-60kt 850mb jet streak over eastern South Dakota are expected to track E/SE across southern Minnesota/northern Iowa this morning. While this activity will eventually outrun the nocturnal jet, ample moist inflow from the south may help maintain the convection in a much weakened state as it crosses into southern Wisconsin/northern Illinois this afternoon. Most CAMs either show the cluster completely dissipating or keep it entirely north of the KILX CWA along/north of I-80. However, both the HRRR and RRFS indicate it may dip a bit further southeastward toward the I-74 corridor. Given current upstream satellite/radar trends and the consistent HRRR/RRFS solution, have included 20-30 PoPs along/north of a Macomb to Bloomington line this afternoon. While the risk for severe weather will be low with the decaying cluster, the latest Day 1 SPC Convective Outlook brushes Knox/Stark/Marshall counties with a Marginal Risk (level 1 of 5) for severe. If the cells manage to drop that far south, think the primary risk will be gusty 40-50mph winds.

**** First Summer Heatwave ****

As has been advertised for the last several days, the first prolonged heatwave of the summer is imminent. As the frontal boundary shifts northward, a hotter airmass will gradually trickle into central Illinois today. The magnitude of the heat will be mitigated by considerable cloud cover across the board this morning and by convective debris along/north of I-74 through the afternoon. As a result, think highs will be held in the 80s across much of the CWA...with readings at or slightly above 90 confined to the W/SW. Decided to leave the current Heat Advisory in place for locations along/southwest of a Peoria...to Decatur...to Robinson line this afternoon, although heat index values will likely remain below advisory criteria of 105 degrees.

Once the front lifts into the Upper Midwest/Great Lakes and upper heights rise in earnest, temperatures will climb into the lower to middle 90s Monday through Thursday. With dewpoints in the middle 70s, heat index values will peak at or above 105 degrees each afternoon. Given increased confidence concerning the strength/position of the upper ridge, have issued a prolonged Heat Advisory for Monday afternoon through Thursday evening.

Models show a series of short-wave troughs gradually flattening and shifting the ridge eastward later in the week. With increased cloud cover and rain chances, temperatures and corresponding heat index readings will cool slightly for the upcoming holiday weekend.

AVIATION /12Z TAFS THROUGH 12Z MONDAY/

Issued at 543 AM CDT Sun Jun 28 2026

Widespread dense fog blankets areas along/east of I-55 early this morning, while visibilities have most recently dropped below 1 mile as far west as KPIA. The dense fog will persist for another couple hours before rapidly dissipating between 13z and 15z. Once the fog lifts, an MVFR overcast will linger through midday before ceilings climb to VFR and/or scatter during the afternoon. An isolated shower/thunderstorm will be possible: however, areal coverage will remain too low to mention at the terminals. Winds will initially be E/NE at around 5kt, then will slowly veer to SE and increase to 10-15kt as the day progresses. Once a warm front lifts into the Upper Midwest, winds will veer to S/SW late tonight into Monday morning.

ILX WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 1 PM this afternoon to 7 PM CDT this evening for ILZ027>029-036-037-040>042-047>054-061-062-066>068- 071>073. Heat Advisory from 11 AM Monday to midnight CDT Thursday night for ILZ027>031-036>038-040>057-061>063-066>068-071>073. Dense Fog Advisory until 8 AM CDT this morning for ILZ038- 042>046-052>057-061>063-066>068-071>073.