textproduct: St. Louis

KEY MESSAGES

- Warm, pleasant weather continues through Thursday with the St. Louis and Columbia metro areas potentially reaching 90 degrees for the first time this year.

- Increased humidity marks the return of showers and a few weak thunderstorms late Thursday that persist through early next week.

SHORT TERM

(Through Late Thursday Afternoon) Issued at 336 AM CDT Wed Jun 11 2025

On the heels of what can only be described as a stellar day in the Mid-Mississippi Valley, a weak surface high centered in the Southeast U.S. is drifting east, establishing warmer southerly return flow through the region. Aloft, broad longwave ridging is becoming more established across the CONUS, further supporting modest warming. Despite increased cloud cover from high cirrus streaming north, expect temperatures as much as 10 degrees warmer today than yesterday's. Exactly how warm we get will hinge on how much cirrus can obscure the insolation, along with how strong the warm air advection will be. Among the available guidance, there is general agreement that the WAA will weaken through the morning as boundary-layer winds slacken. CAMs also depict persistent cirrus today to varying degrees of coverage. Both of these factors led to today's temperature forecast to lie near the "cooler" end of the fairly tight envelope of solutions. That said, if one of the factors above is underrepresented, St. Louis and Columbia may see their first 90 degree day this year. With more typical summerlike humidity lagging behind the warmup, today will be another winner for those with outdoor plans.

The combination of weak WAA and increased cloud cover will work to limit radiational cooling tonight, resulting in overnight lows 5-10 degrees warmer than last night. Humidity will continue a gradual uptick overnight into Thursday with deeper southerly flow in advance of a slow-moving mid-level wave drawing moisture poleward. Temperatures on Thursday will be generally similar to today's, though the warmest conditions will likely be along and east of the Mississippi River where cloud cover won't be as robust. If the sites that don't break their streak without reaching 90 degrees fail to do so today, at least St. Louis will have another go at it on Thursday. There is some modest but consequential differences in the timing of the mid-level wave and the precipitation it eventually brings. While most depictions show showers and a few thunderstorms holding off in central Missouri until after sunset, there are some that bring the convection in earlier. That said, most of the day will remain dry across the region and any convection that manages to form will do so in a fairly unimpressive thermodynamic environment for heavy rain or stronger updrafts. At worst, a few garden-variety thunderstorms would occur.

MRB

LONG TERM

(Thursday Night through Tuesday) Issued at 336 AM CDT Wed Jun 11 2025

The large-scale precipitation evolution this weekend is fairly consistent with previous forecasts: more anomalous precipitable water values overspread the region Friday as the flow aloft amplifies. The deep, saturated warm cloud layers depicted in deterministic model soundings would indicate a threat for locally- heavy rain where convection develops. However, in the absence of any stronger low-level jet dynamics (or any kinematics/forcing of note through the column), the threat for an organized heavy rainfall threat is very low (10% or less) on Friday. The area best situated for more widespread showers and thunderstorms (and where our highest PoPs exist) is southeast Missouri and southwest Illinois, where the overlap of anomalous PWATs and a weak low- level jet exists. While the LLJ is subjectively meager, it will support integrated water vapor transport near the 90th climatological percentile in the aforementioned area. LPMM output from high-resolution guidance suggests that a reasonable worst- case from these downpours would be up to 1.00 inches, which would not pose much of a threat.

The mid-level wave staggers east through Friday into Friday night and slows further on Saturday. While this does shunt the weak diffluence and low-level jet east, and along with it the threat for localized downpours, the threat for scattered showers and thunderstorms persists under the closed upper-level low. Instability will remain sufficient for thunderstorms, as most guidance paints 1500-2000J/kg of MUCAPE in the region. Deep-layer shear will remain unsurprisingly anemic with the subtropical jet far to our north and maximum wind speeds through the column of 30kts, which will keep our threat for stronger thunderstorms very low (less than 10%). There is now growing consensus that the trough axis will clear most of the bi- state area by late Saturday night, which would bring a lull in activity into Sunday morning. While it may not be completely dry to end the weekend, showers will likely be fewer, further between, and less impactful overall. When all is said and done, it's likely (70% chance) that the highest rainfall totals in the region across southeast Missouri and southwest Illinois will be below 1.00 inches. Parts of northeast/north- central Missouri may see little to no rain, depending on how the wave evolves.

The pattern through the middle of next week is plagued with weak, transient waves amidst quasi-zonal flow aloft. Closer to the surface, persistent southerly flow will bolster seasonable warmth and humidity. The combination of the unsettled pattern aloft coincident with a diurrnally-unstable boundary layer will promote continued shower and thunderstorm chances centered around the afternoon each day. While the exact structure of these waves will need to be monitored for an uptick in shear, which would threaten stronger thunderstorms as a result, nothing appears particularly concerning from that perspective.

MRB

AVIATION

(For the 12z TAFs through 12z Thursday Morning) Issued at 525 AM CDT Wed Jun 11 2025

MVFR fog currently threatens the low-lying terminals in the St. Louis metropolitan area, but that will end in the next few hours when dry and VFR conditions will prevail through Thursday morning. There is another chance for fog in the Missouri River valley tomorrow morning, but it will depend greatly on cloud cover and temperatures today and tonight. While not currently in the TAFs, this potential will be monitored.

MRB

CLIMATE

Issued at 525 AM CDT Wed Jun 11 2025

St Louis has still not seen 90 degrees yet in 2025. The average first 90 degree reading is May 20. The most recent late 90-degree start was June 10, 2021, and this is the latest first 90 degree day since at least 1995 (June 19). The latest date of first 90 degrees on record was July 4 in both 1961 and 1912.

Columbia has also not yet seen a 90 degree reading in 2025, however the average first date is much later (May 31). The latest date of first 90 degrees on record was July 14, 1904.

Quincy reached 90 degrees on May 15 of this year, about 3 weeks earlier than their average first 90 degrees (June 2).

Kimble/MRB

LSX WATCHES/WARNINGS/ADVISORIES

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