textproduct: Central Illinois

KEY MESSAGES

- Unseasonably warm temperatures continue through Saturday, with highs reaching the upper 70s to low 80s. There is an 80% chance that temperatures will exceed 80 degrees across the lower Illinois River Valley on Saturday and south of I-70 Sunday.

- A conditional risk for severe storms exists Sunday afternoon and evening for southeast Illinois. While storm development is uncertain (30% chance), any storms that form could become severe.

- A sharp pattern change follows Sunday's cold frontal passage through the region. High temperatures will drop back into the 50s by Monday.

DISCUSSION

Issued at 111 PM CDT Fri Mar 20 2026

Unseasonably Warm Pattern (Today through Saturday)...

A persistent, stout upper-level ridge centered over the Desert Southwest remains the dominant feature steering our weather pattern into the weekend. This synoptic setup effectively shunts the polar jet stream well north into the Canadian Rockies, placing central Illinois firmly on the warm side of the jet resulting in unseasonable and near record warmth. Daily high temperatures are forecast to climb into the mid 70s to low 80s through Sunday.

A weak low pressure system digging across the Great Lakes today will drag the tail end of a cold front across the region this afternoon and evening. While this feature is expected to be moisture-starved and not produce any measurable precipitation locally, it will induce a wind shift. Southwest winds will transition to breezy northwest winds in the afternoon and evening hours.

Saturday will see the peak of the warmth, with NBM probabilities for reaching 80 degrees or higher ranging from around 40% along the Peoria to Springfield line up to near 80% across the lower Illinois River Valley.

Medium-Term Pattern Change and Conditional Severe Risk (Sunday)...

A series of subtle shortwave troughs will initiate the gradual breakdown of the upper ridge this weekend. The most impactful of these will be a stronger shortwave digging across the Upper Midwest and Great Lakes on Sunday, which will drive a cold front across central Illinois late in the day. This frontal passage provides our next opportunity for precipitation, though confidence remains low.

Temperatures on Sunday will again be highly anomalous, with 80+ degree probabilities reaching 60-90% south of I-70. Forecast soundings ahead of the front suggest weak to moderate instability (300-1000 J/kg MLCAPE) will develop, coinciding with strong deep- layer shear (40-50kt). The parameter space is highly favorable for severe weather if convection can be initiated. However, a stout capping inversion and a lack of strong upper-level forcing remain the primary limiting factors for widespread convective development. The Storm Prediction Center (SPC) has introduced a Marginal and Slight Risk for severe weather across portions of southeast Illinois on Sunday.

Extended Period (Next Week)...

Behind Sundays cold front, temperatures will briefly return closer to seasonal norms on Monday as high pressure builds southeastward from the Canadian Prairies. Northerly flow will limit Mondays afternoon highs to the 50s.

The pattern shifts again starting Tuesday as the surface ridge axis moves east, allowing the baroclinic zone to lift back north across the region. This will kick off a gradual warming trend through the remainder of the work week. The main jet stream will remain overhead, potentially steering low-amplitude shortwaves across the area and bringing scattered showers. However, model agreement on precipitation is currently poor, which is reflected in the blended NBM guidance keeping PoPs low (generally below 15%) through mid-week. Another stronger cold front is possible late in the week, bringing the next chance for more widespread showers and storms.

Deubelbeiss

AVIATION

(For the 18z TAFs through 18z Saturday Afternoon) Issued at 1202 PM CDT Fri Mar 20 2026

A cold front is moving across central Illinois this afternoon. No precip is expected with the front but SW/W winds ahead of the front will gradually veer to the northwest this afternoon and evening. High pressure translates across the region through the day Saturday causing winds to continue to gradually veer around the dial, eventually setting up out of the SE Saturday morning and shifting back to the S/SW Saturday afternoon. VFR conditions are expected through the period.

Deubelbeiss

ILX WATCHES/WARNINGS/ADVISORIES

None.