textproduct: Central Illinois

KEY MESSAGES

- Unseasonably warm temperatures continue through tomorrow, with highs in the upper 70s to mid 80s this afternoon and a 80-90% chance for highs over 80 south of I-70 tomorrow.

- Gusty (25 to 35 mph) north winds will usher a seasonably cool airmass into the region late Sunday, with lows in the low-mid 30s by Monday morning. Temperatures will then slowly rebound, with a 30-40% chance for highs in the 80s again by Thursday.

- Ahead of the cold front Sunday evening, areas near and south of I-70 have a 20-40% chance for seeing thunderstorms. There is a level 1 of 5 (marginal) risk for damaging winds and large hail with those storms.

- Another chance (30-40%) for showers and storms will accompany a cold front sometime Wednesday night into Friday. Confidence is low in whether a severe risk will materialize.

DISCUSSION

Issued at 116 PM CDT Sat Mar 21 2026

***** UNSEASONABLY WARM THROUGH EARLY SUNDAY *****

Early Saturday afternoon, visible satellite revealed clear skies while temperatures ranged from the upper 70s to low 80s across the Lower IL River Valley to low-mid 70s in east-central IL. RAP mesoanalysis suggested anomalously strong ridging was responsible, with 500mb heights in the lower 570s dam overhead - well-above the 90th percentile (5690) for this time of year. Deterministic and ensemble guidance agree this will continue overnight, with enough of a south-southwesterly PBL wind to hold temps in the upper 50s to low 60s. CAMs generally suggest a cold front will be located near I-80 at daybreak tomorrow, and sink steadily south through the ILX CWA during the day, reaching I-70 sometime between 4-7pm. This will result in a sharp gradient in high temperatures tomorrow, with areas north of I-74 struggling to reach 70 but areas south of I-70 potentially reaching the mid-upper 80s if you believe the RAP/HRRR.

***** 30% CHANCE OF STORMS SOUTHEAST IL SUNDAY EVENING *****

Moisture pooling ahead of the cold front will bring dewpoints at least into the upper 50s, possibly (50-70% chance) lower 60s, in southeast IL - resulting in anywhere from 500-2000 J/kg of MLCAPE. In addition, an EML advected into the region from the Southwest combined with cooling temperatures aloft with the encroaching upper level trough to our north will yield steep (7.5-8.5C/km) 700-500mb lapse rates. With over 50kt deep-layer shear, this parameter space would be favorable for severe weather, especially large hail and strong wind gusts. However, a capping inversion and weak mid-upper level forcing along the front makes it unclear whether storms can develop. At this time, the highest chance for storms (20-40%) will be near and south of I-70 during the 6-11pm timeframe.

***** SHARPLY COOLER LATE SUNDAY INTO MONDAY *****

Surface high pressure will settle into the area quickly behind the front, with those sharp pressure rises favoring a period of gusty (25-35 mph) northerly winds and hence cool advection Sunday evening into Sunday night, when wind chills are forecast to fall into the 20s. This is not particularly uncommon for mid-late March, but it will be quite the change from our weekend of 70s and 80s (which is 20-30 degrees above the 1990-2020 normals).

***** BUILDING WARMTH TUESDAY THROUGH THURSDAY *****

After a seasonably cool day with highs in the low-mid 50s on Monday, warmth will rebuild across the region as upper-level ridging expands eastward from the Plains. Highs in the upper 50s to low 60s Tuesday will give way to widespread 60s Wednesday, followed by 70s to perhaps (30-40% chance) 80s on Thursday. The period was looking dry, but alas the latest National Blend of Models (NBM) includes a 20% chance of showers and storms on Wednesday as deterministic models advertise a weak shortwave disturbance cresting the ridge.

***** CHANCE FOR STORMS, TURNING COOLER LATE WEEK *****

While specifics differ from one model to another, the deterministic global models continue to advertise a cold frontal passage sometime Wednesday night into Friday, offering another opportunity (30-40% chance) for showers and storms area-wide - though the highest chances are once again in southeast IL. The set-up apparent in the deterministic models with bulk shear vectors oriented nearly parallel to the slow moving cold front seems like it would favor training storms and perhaps a corridor of heavy rainfall, though ensemble guidance is not in agreement in whether and where that will occur. Consequently, ensemble probabilities for over an inch of rain in any given location are low: 10-15% from the Low Resolution Ensemble Forecast (LREF) system and 10-20% from NBM. The deterministic GFS and ECMWF suggest 1000+ J/kg MUCAPE and >35 kt 0-6km bulk shear, which would favor a severe weather risk somewhere nearby, though lots can change between now and then. Thus far, confidence has been too low for SPC to introduce a risk area.

The EOF patterns used for WPC's cluster analysis suggest most (50- 55%) of the spread in the LREF is coming from uncertainty in how quickly ridging expands back into the Midwest by next weekend. If a faster solution occurs (40% chance), any late-week cooldown will be short-lived with temperatures rebounding and chances for rain returning over the weekend. With the slower outcome (60% chance), next weekend will stay cool and dry.

Bumgardner

AVIATION

(For the 00z TAFs through 00z Sunday Evening) Issued at 633 PM CDT Sat Mar 21 2026

South-southwest winds will remain in place tonight. A 40-50 kt LLJ develops ahead of an approaching cold front overnight and will bring a period of LLWS through Sunday morning. Winds will veer to the west- southwest by morning, then abruptly shift to the north and become gusty behind the cold front. There is a low chance (<40%) that MVFR ceilings develop with FROPA with an even lower chance (20%) for an isolated shower or storm in eastern IL (KCMI/KDEC) late in the period.

NMA

ILX WATCHES/WARNINGS/ADVISORIES

None.