textproduct: St. Louis

KEY MESSAGES

- A Flash Flood Watch will be in effect from late tonight into early Monday for the threat of heavy rainfall potentially leading to flash flooding.

- Some storms could be strong to severe late tonight into late Sunday evening, bringing the threat of tornadoes and damaging wind gusts.

- Below-average to seasonably cool conditions are anticipated next week.

SHORT TERM

(Through Sunday Night) Issued at 220 AM CDT Sat Jun 20 2026

Regional radar imagery reveals a few isolated pockets of showers that are percolating southeastward and are being aided by weak low- level frontogenesis associated with a weakening boundary. Some pockets of heavier showers featuring intermittent lightning cannot be ruled out as pockets of MUCAPE near 250 J/kg is forecast by guidance to linger throughout the early morning hours, but the best chance of this is across western MO. High-resolution guidance is in good consensus that this activity will continue through 10z mostly across central/northeast MO and quickly fade into the mid-morning as the previously mentioned band of frontogenesis fizzles out. The remainder of today is expected to be dry with seasonably cool temperatures and light surface winds continuing into tonight.

Beginning tonight, our attention turns to a developing lee-side surface cyclone that will initiate convection across KS/NE, with quick upscale growth into an MCS/complex with eastward extent. The commencement of the features above are in response to a shortwave trough that will be traversing eastward into the Great Plains within quasi-zonal flow in the mid/upper-levels. The MCS is forecast by CAMs to be nearing central/north-central MO shortly after midnight as it propagates along the Corfidi upshear vectors and/or favors to track along the eastward extending warm front. As it enters into the area (northeast/central MO), the greatest severe threat will be damaging winds associated largely to any bowing/surging segments that are still present. Additionally, locally heavy rainfall is possible as anomalous PWATs near 2" (~99th percentile of climatology) and a strengthening LLJ leading to increased moisture transport will all be in place. If training or back-building along the southern flank becomes favored across the area, this will enhance the flash flooding threat as thunderstorms will be efficient at rainfall production with >12,000ft warm cloud layers favoring warm rain processes. A widespread 1-3"+ is forecast from this MCS with locations that experience training or multiple rounds of thunderstorms may easily reach >3" of rainfall from early Sunday morning into Sunday evening.

What remains quite uncertain is the evolution of this MCS (intensity, speed, and track), as is typical with these systems. Lingering cloud cover/thunderstorms associated with the remnant MCS throughout the day would decrease instability and thus lower the afternoon/evening severe threat, while less of the above would favor the opposite. Another feature previously mentioned will be a surface cyclone that some guidance resolves will traverse across the area on Sunday. If a surface cyclone does indeed cross the area at an optimal time (afternoon/evening), that would locally enhance the low- level shear, ultimately leading to a greater threat for tornadoes on Sunday, especially near the warm front/surface cyclone. Otherwise, a cold front is progged to be entering western MO Sunday evening which should work to initiate additional convection, particularly near any remnant outflow boundaries left behind from the earlier MCS. With sufficient effective bulk shear (>30kts), organization of thunderstorms is expected with a linear mode favored as the shear vector is forecast to be nearly parallel to the boundary. Analysis of the LREF IQR spread for MUCAPE highlights the uncertainty as the 25th-75th spread is around 1,000-1,500 J/kg across the area. Regardless, with potential overlap of shear and instability, strong to severe thunderstorms will be possible Sunday afternoon/evening with the threats being damaging winds, tornadoes, and large hail. Also, with the mean flow becoming increasingly parallel to the forecast boundary, training of thunderstorms leading to locally heavy rainfall will also be a threat into Sunday night.

As a result, a Flash Flood Watch has been issued for the entire area with a range of start/end times in a window from late tonight into Monday morning.

Peine

LONG TERM

(Monday Morning through Friday) Issued at 220 AM CDT Sat Jun 20 2026

By Monday, the main shortwave trough axis will be crossing over the area which is expected to bring an end to the widespread showers and thunderstorms from west to east Sunday night into Monday morning. A few subtle pieces of mid-level energy are progged by long-range guidance within the wake of the shortwave, which may lead to some lingering shower activity across the area early on Monday. Also, as the surface cyclone lifts to the northeast, it will drag the cold front through the region leading to ample low-level CAA on Monday. As a result, Monday will feature high temperatures around 10 degrees below average.

Northwesterly flow aloft will take hold over the region Tuesday into the end of the week, resulting in a stretch of largely dry conditions along with seasonably cool temperatures. Generally, highs in the upper 70s/lower 80s and lows in the low/mid 60s look to prevail, which is about 5-10 degrees below climatological normals. This is supported well by the LREF, which has narrow 3-5 degree temperature IQR spread through the end of the week.

Peine

AVIATION

(For the 18z TAFs through 18z Sunday Afternoon) Issued at 1215 PM CDT Sat Jun 20 2026

Dry/VFR conditions are forecast into the early overnight hours along with light/variable winds. The threat for showers and thunderstorms increases after 0900 UTC Sunday, mainly for KUIN. Farther south, there is more uncertainty so kept the remaining terminals with a PROB30 group. Any site that does have a direct impact from a thunderstorm may see a brief period of low visibilities (IFR) and/or some gusty winds.

Gosselin

LSX WATCHES/WARNINGS/ADVISORIES

MO...Flood Watch from Sunday morning through late Sunday night for Franklin MO-Saint Charles MO-Saint Louis City MO-Saint Louis MO.

Flood Watch from Sunday morning through Monday morning for Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO- Saint Francois MO-Sainte Genevieve MO-Washington MO.

Flood Watch from late tonight through Sunday evening for Audrain MO-Boone MO-Callaway MO-Cole MO-Gasconade MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-Montgomery MO- Osage MO-Pike MO-Ralls MO-Shelby MO-Warren MO.

IL...Flood Watch from Sunday morning through late Sunday night for Bond IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-Madison IL-Montgomery IL.

Flood Watch from Sunday morning through Monday morning for Clinton IL-Marion IL-Monroe IL-Randolph IL-Saint Clair IL- Washington IL.

Flood Watch from late tonight through Sunday evening for Adams IL-Brown IL-Calhoun IL-Pike IL.