textproduct: St. Louis

KEY MESSAGES

- Severe thunderstorms end this evening as a cold front pushes drier air into the region, at least briefly.

- The front stalls to our south and wobbles back and forth over the next few days, bringing multiple rounds of showers and thunderstorms Thursday through Saturday night. The heaviest rain will be on Friday. Localized flash flooding and more widespread river flooding is possible.

SHORT TERM

(Through Late Thursday Night) Issued at 249 PM CDT Wed Apr 2 2025

In the very near term, we remain in the warm sector ahead of an approaching front. The moist, unstable air ahead of a prefrontal line of convection has led to increasing instability in an environment of strong speed and some directional shear. We expect thunderstorms to continue developing ahead of the front through the mid to late afternoon before the passage of the front cuts off the moisture and ends the threat. Storms in this environment will be capable of producing tornadoes as well as large hail and damaging winds. Once the front moves through we'll see winds turn westerly and dewpoints falling into the 40s. Temperatures tonight drop into the 40s in western and northern areas where the impact of the front will be felt first. Tomorrow will be cooler for most, in the 50s and 60s. Despite the brief drying behind the front, clouds and rain chances increase as the day goes on Thursday.

In the bigger picture, the surface front stalls to our south near the Memphis area while an upper trough reorganizes over the southwest US. Broad surface low pressure near the Rio Grande ensures a continual southerly flow from the Gulf up into and over the frontal boundary to our south. Most of the heavier activity on Thursday stays to our south, but some may creep into our area from the west during the afternoon and evening. This is more associated with a passing shortwave trough rather than the front itself. This far north of the front we won't have much instability to work with, and if we do it will be elevated. So we're not looking at any significant severe weather threat with this. But this will be the opening round of what will be multiple days of showers and thunderstorms which will produce swaths of heavy rainfall. The flood threat on Thursday is pretty low, but this initial round of rain will wet soils for the upcoming days of rain.

Kimble

LONG TERM

(Friday through Next Wednesday) Issued at 249 PM CDT Wed Apr 2 2025

While the initial shortwave exits to the east Thursday night, the next rounds the trough and moves in from the west on Friday. This will send another surge of moisture into and over the front, shifting the front northward as well. This round of rain centers on Friday afternoon into Friday night and is the most likely time period for our area to see heavy rainfall. After multiple days of flow off the Gulf, moisture levels will be maximizing with surface dewpoints south of the front rising into the low 70s and PWAT rising to almost 2 inches, extreme for this time of year. This suggests rainfall rates may be extreme especially in areas of deep convection. Combine this with a stalled or slow moving boundary which serves as a focus for multiple rounds of storms to develop and we see a very real threat for widespread flooding and localized flash flooding. This threat would locally magnify if the boundary stalls and multiple rounds move over the same area. However at the moment the boundary does look to wobble around, more like a water hose being flung around in the backyard. The boundary lifts north then back south with the passage of the broader wave. This wobbling would reduce the cumulative totals somewhat, but there remains fairly high confidence in a prolonged period of heavy rain. This is especially true on a larger river basin scale where even if the heaviest rainfall axis shifts around over time, much of this water will end up in the same rivers in the end. The primary river basins to be impacted are the Meramec, Kaskaskia, and Black Rivers. Additional flooding on tributary creeks and streams is likely as well. However, the larger rivers like the Missouri and Mississippi (upstream of St Louis) are less likely to see flood impacts.

Yet another wave moves along the wobbling boundary on Saturday initiating one final round of heavy rain along and north of the boundary Saturday into Saturday night. However, the boundary is expected to shift southward for this round, sending the focus for heavy rain just to our south over southeast MO into the Ohio River Valley. If the front doesn't shift as far south then we could see a second round of heavy rainfall in parts of our area which would lead to even more significant impacts. The overall trend in the QPF forecast has come down somewhat from prior forecasts primarily due to the shift southward in this final round of heavy rain. However, we do still have high confidence in the overall environment favoring heavy rainfall with the most widespread impacts in our area on Friday into Friday night.

Regarding the severe weather threat through this period, we remain largely to the north of the surface front. This means convection will be elevated in nature, limiting the threat to primarily hail if we can get enough instability to develop. At this time, there's considerable uncertainty on whether we get this necessary instability, but if we do it would be most likely on Friday when the overall moist air mass gets shoved furthest northward. The SPC outlook for Friday does include a Slight Risk for parts of our area due to this threat for some large hail.

Saturday night's trough is the final trough to affect us in this flow regime. As it shifts the front further southeast it also brings in colder air late Saturday night into Sunday morning. This will also be as the last of the precipitation is ending and there remains a very low chance that this precipitation ends as some snowflakes across central and northern Missouri. About 10 to 20 percent of the 00Z ensemble guidance indicates this switch to light snow, but even if this occurs we do not expect any accumulations considering marginal surface temperatures and even warmer ground temperatures.

Cooler weather lasts for a day or two as another deeper trough dropping into the Great Lakes ushers in a reinforcing shot of cooler air. We're actually looking at a pretty widespread potential for a freeze across the entire forecast area Monday night. This will be more likely if surface high pressure is able to settle into the region during the overnight hours leading to ideal cooling conditions. However, it's possible we see freezing temperatures arrive via cold advection alone. The latest NBM indicates probability of freezing temperatures ranging from 60 percent in our south to as high as 80 percent in our north.

Kimble

AVIATION

(For the 06z TAFs through 06z Thursday Night) Issued at 1031 PM CDT Wed Apr 2 2025

Dry and VFR conditions will continue overnight ahead of the next system that will bring widespread rain and MVFR ceilings to the mid- Missouri and St. Louis metro terminals. Rain will push back into the region mid to late morning on Thursday and largely continue through the remainder of the period. A thunderstorm or two is possible across the area during this period, but energy for thunderstorms will be very limited to confidence in where and when a thunderstorm develops remains low.

MVFR ceilings will develop during the late afternoon to early evening Thursday, pushing up from the southwest. How low ceilings get remains uncertain at this point with some guidance hinting at prolonged IFR conditions. I'm not confident enough in this just yet to include an IFR group in the TAFs.

Winds will remain less than 10 kts through the period, with a variable wind direction during the day before becoming northeasterly around 00Z Friday.

Delia

LSX WATCHES/WARNINGS/ADVISORIES

MO...Flood Watch from 7 AM CDT Thursday through late Saturday night for Crawford MO-Franklin MO-Iron MO-Jefferson MO-Madison MO- Reynolds MO-Saint Francois MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-Washington MO.

IL...Flood Watch from 7 AM CDT Thursday through late Saturday night for Bond IL-Clinton IL-Fayette IL-Madison IL-Marion IL- Monroe IL-Randolph IL-Saint Clair IL-Washington IL.