Area Forecast Discussion

KEY MESSAGES

- Above normal temperatures are expected Tuesday through Thursday with widespread 70s to low-80s.

- Widespread showers and thunderstorms arrive late Thursday night, and continue into early Friday. There is the potential for redevelopment Friday afternoon into Friday evening.

SHORT TERM

(Through Late Tuesday Afternoon) Issued at 215 AM CDT Mon Apr 20 2026

Surface high pressure was centered over eastern Iowa with another surface high over southwest Arkansas. The northern high has pushed a weak cold front into the area with mid to high clouds stretching from north-central Missouri through the metro area into southern Illinois. As of 07z, surface temperatures ranged from the upper 40s to low-50s with dewpoints largely in the low to mid-30s. Easterly surface winds remain at 5-10 knots with few locations showing lower wind speeds.

Clouds aren't quite thick nor widespread enough to have a broad impact on temperatures. Most locations are expected to fall into the low-40s with mid to upper 30s in the cooler locations of northeast Missouri and west-central Illinois. While patchy frost is possible in the cooler areas, it's a similar theme to yesterday morning. The air remains too dry, the presence of light wind, and now some cloud cover to account for. With the surface ridge departed to the north, winds are not expected to calm. Considering the lack of frost yesterday morning, there's a stronger argument against it than for it.

After a cool start, we'll see temperatures rebound with a subtle, but noticeable warming trend from the southwest. 850mb temperatures rise to near 10C west of the Mississippi River with west-southwest flow rounding the Gulf ridge. Temperatures will largely be similar to Sunday's with exception to central and southeast Missouri. Ridging begins to build west to east from the Plains with warmth having greater influence just north and east of the Ozark Plateau, where temperatures warm into the low to mid-70s.

Maples

LONG TERM

(Tuesday Night through Sunday) Issued at 215 AM CDT Mon Apr 20 2026

As surface high pressure moves west to east across the Upper Midwest and into the Great Lakes late Tuesday, it pushes a weak cold front southward into the mid-Mississippi Valley Tuesday night. RAP/NAM deterministic solutions show the surface boundary stalling near or north of the MO/IA border with a narrow corridor dewpoints in the low-60s pooling along and south of the boundary.

Further south (LSX CWA) of this west-east oriented axis of moisture, air dries rather quickly with dewpoints in the 40s/50s. Mid-level fields show a squeeze play occurring between the boundary and height rises at the northern end of the Gulf ridge. This lines up west to east from northern Missouri into west-central Illinois. West- southwest surface to mid-level flow persists, likely working against the southward progress of the boundary. WAA process already take over at the west end of the front in eastern Kansas and western Missouri. F-gen remains concentrated to the northeast of the CWA with weak convergence riding under a muted upper shortwave. Precipitation that attempts to develop will fall into a deep dry layer that extends from the surface to 800mb. <20% of the LREF members depict measurable precipitation skimming the northeast border of the CWA. Dry conditions are favored with isolated showers at best.

WAA strengthens Wednesday through Thursday as the axis to a mid- level ridge moves overhead from west to east. The warmest air remains over the central Plains, where H8 temperatures are in the mid to upper teens (C). This air is gradually pulled eastward through the 48 hour period, but moderated to the low teens (C) as it approaches the Mississippi River. This is likely to be the warmest stretch of the week with widespread 70s to low-80s.

Long range guidance is coming into better agreement with the late week system. A cutoff, closed upper low traverses the Pacific Northwest and consolidated with a digging trough over Canada. This deepens the surface low over the northern Plains as a cold front extends south of the surface low through the eastern Plains. Strong, southerly pressure gradient flow extends north/south over eastern Kansas, where MUCAPE values approach or slightly exceed 2000 J/kg in the vicinity of a 50-60 kt LLJ. Initial thunderstorm development is favored across the eastern Kansas/northwest Missouri late Thursday. From that point onward, spread begins to increase as result of specifics with convective trends. 35-40 knots of 0-6km shear will support organized multi-cell development that eventually congeals into QLCS line that moves eastward overnight into Friday morning. The ECMWF shows a south-southeast track over southwest Missouri that likely plays off the CAPE gradient over the southwest corner of the state. This doesn't make as much sense when steering flow is out of the southwest. The more likely scenario is a blend between the northern track of the GFS that takes most of the activity through central and northern Missouri.

This disagreement leads to QPF disparity that, in reality, is likely larger than the LREF spreads. They show 0.25"-0.75" between the inner quartile range. Considering PWATS of 1.4-1.5 inches cover the region, convective trends will tap into copious moisture with amounts that will easily exceed IQR within thunderstorms. Thunderstorms will outrace the better CAPE to the west with weakening convection moving into central Missouri around midnight Thursday night. Widespread showers and embedded thunderstorms progress eastward through Friday morning. Much of the area receives another soaking rainfall with ECM/GFS ensemble members loosely clustered around an inch through Friday morning. Several members rise above an inch with the uncertainty in where the complex tracks. This introduces another challenges heading into Friday with additional chances for showers and thunderstorms.

As the upper trough performs a Fujiwara dance with multiple vort lobes over the northern Plains, the front bends more west-east with time. This typically signals a slowing southward progression and ultimately has the boundary bisecting the CWA from northeast to southwest Friday afternoon. Over 90% of the ensemble members show overcast skies holding through Friday afternoon, which limited surface instability. That being said mid-level lapse rates of around 8C line the front over somewhat more modest CAPE values (~1500 J/kg) around 18z Friday. Long range guidance has some development along the boundary with 0-6km shear around 35 knots. Friday will be a day to watch, especially if trends begin to break cloud cover and allow better recovery of surface instability. If so, strong to severe thunderstorms cannot be ruled. This holds true for either round, but will have better support Friday if typical diurnal trends can be realized.

The front slide south Saturday as high pressure clears things out. Yet another surface low ejects out of Colorado into the southern Plain late Saturday, but rain chances likely arrive just after the end of the long range period.

Maples

AVIATION

(For the 12z TAFs through 12z Tuesday Morning) Issued at 604 AM CDT Mon Apr 20 2026

VFR conditions will persist as ridging extends south of a surface high over the Great Lakes. Sustained winds remain under 10 knots at all terminals through late this evening. A few gusts are possible at KUIN, but not until late in the period.

Maples

LSX WATCHES / WARNINGS / ADVISORIES

MO...None. IL...None.