Area Forecast Discussion

KEY MESSAGES

- A frost or hard freeze continues this morning across the area, with a Freeze Warning remaining in effect for eastern Iowa and northwest Illinois, with a Frost Advisory for Scotland and Clark Counties in northeast Missouri

- Warmer conditions are forecast for Monday through Wednesday night, with a low to medium chance (10-40%) of showers and storms Tuesday PM for areas south of Interstate 80

- Widespread showers and storms return Thursday into Friday morning, with the potential of some storms becoming strong

SHORT TERM /THROUGH TONIGHT/

Issued at 248 AM CDT Mon Apr 20 2026

An area of high pressure has settled across the western Great Lakes region early this morning, leading to light and variable winds and clear skies across the area. Enhanced radiational cooling has commenced with these conditions, with temperatures falling to between 30 to 36 degrees over most of the area as of 2 AM/07z this morning. Expect temperatures to continue to fall to a low this morning of upper 20s to the north and lower 30s to the south, leading to a widespread frost or hard freeze. With this said, the Freeze Warning/Frost Advisory remains in effect through 9 AM/14z this morning.

After the chilly start, a very pleasant day is on tap to begin the work week, as the high pressure system slowly translates eastward today. Southerly return flow on the western side of the high should help elevate temperatures to more seasonal values compared to the last few days, warming to the middle 50s over northwest Illinois to the middle 60s towards far southeastern Iowa/west-central Illinois/northeast Missouri. There could be a slight southerly breeze, with gusts to around 20 mph, but otherwise, lighter winds are expected compared to the last few days.

Southerly flow will continue tonight in the wake of the departing high, with overnight lows dipping to the middle/upper 40s to the lower 50s, which is actually around 5-10 degrees above average for this time of the year.

LONG TERM /TUESDAY THROUGH SUNDAY/

Issued at 248 AM CDT Mon Apr 20 2026

Mostly dry conditions will continue Tuesday through Wednesday night as a longwave upper-level ridge develops over the Intermountain West region. The only exception for precipitation during this time is a low to medium chance (10-40%) of showers and thunderstorms for Tuesday PM south of Interstate 80 as a weak baroclinic zone settles across the area. A subtle shortwave trough is progged to also pass through the area, slightly enhancing large-scale forcing for ascent. Instability and kinematics appear to be sufficient for storms to develop, with SBCAPE values around 500 to 1500 J/kg per the 20.00z HREF and various deterministic global models. Mid-level lapse rates actually appear pretty stout, with values between 8 to 9 C/km per several models. At this time, thinking that if any convection develops along the boundary, it should be more isolated in nature due to a lack of favorable boundary layer moistening and a capping inversion in place. However, if convection can get going, I can't rule out perhaps some locally gusty winds and small hail with any storms, given the steep mid-level lapse rates. At this time, we are not outlooked for any severe weather due to the factors mentioned earlier.

Attention then turns to Thursday into Friday, as a broad, upper- level trough develops over the western CONUS on Tuesday and slowly makes its way eastward, becoming more negatively tilted with time. Likely chances (70-90%) of showers and storms will increase Thursday afternoon and evening, and continue into Friday morning. This scenario has been consistent with bringing a surface cold front attendant to the trough, with a pre-frontal shortwave impulse ahead of the boundary. The convective parameter space still shows more modest instability and shear, with MLCAPE values around 500 to 1000 J/kg and deep-layer shear around 20 to 30 knots. The various machine learning severe weather probabilities still show notable values for our region (equivalent to a Slight Risk), likely owing to the stronger large-scale forcing.

As we go into next weekend, the upper-level trough does pass through the area, but a large upper-level low appears to take shape over south-central Canada, placing our region under broad cyclonic flow. This will bring generally cooler conditions, with periodic chances of showers and perhaps a few storms as a couple of mid-level shortwaves revolve around the upper low.

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 625 AM CDT Mon Apr 20 2026

VFR conditions to prevail through the period. High pressure will slide east into the Ohio river valley, with return flow developing this afternoon. A tightening pressure gradient will allow for south winds to increase, but remain under 15kts. An occasional gust over 20kts will also be possible late this afternoon. A strengthening LLJ tonight will bring a risk of LLWS at all terminals and have included mention with this issuance.

HYDROLOGY

Issued at 1229 PM CDT Sun Apr 19 2026

The heaviest rains on Friday afternoon and evening fell in the La Moine, Pecatonica and Rock Rivers and these rivers will see the fastest responses over the next several days. The Rock River at Moline is forecast to approach major flood stage on Tuesday with the crest continuing through Wednesday before falling through the end of the week. Rises continue on all area rivers with longer duration rises on the Mississippi River mainly from Rock Island south.

The forecast remains largely dry through Wednesday night before another storm system moving across the area brings showers and storms to the area Thursday. Additional rainfall may prolong flooding depending on which basins the rain falls.

DVN WATCHES / WARNINGS / ADVISORIES

IA...Freeze Warning until 9 AM CDT this morning for IAZ040>042- 051>054-063>068-076>078-087>089-098-099. IL...Freeze Warning until 9 AM CDT this morning for ILZ001-002-007- 009-015>018-024>026-034-035. MO...Frost Advisory until 9 AM CDT this morning for MOZ009-010.