textproduct: St. Louis

KEY MESSAGES

- Very warm, near daily record high temperatures are expected in many areas again tomorrow, but a sharp cold front will bring more typical temperatures Monday (along with gusty winds late Sunday).

- Elevated Fire Danger for erratic fire behavior is expected Sunday afternoon in parts of the area, mostly across southeast MO and a limited portion of southwest Illinois.

- An isolated thunderstorm or two is possible in southwest Illinois late tomorrow afternoon and evening.

SHORT TERM

(Through Late Sunday Night) Issued at 142 PM CDT Sat Mar 21 2026

While we don't anticipate major weather hazards during the short term period, there are a few items to discuss, including near record warmth and elevated fire weather this afternoon and again tomorrow, followed by a sharp cold front and perhaps a few showers and thunderstorms late in the afternoon and evening.

Before we get to tomorrow, we still have a few more hours of heating left this afternoon, along with the potential for elevated fire weather conditions. As of noon, temperatures have risen to within a degree or two of where they were yesterday, and are on pace to reach the upper 80s to near 90 degrees east of the Mississippi river. This places all three climate locations (St. Louis, Columbia, Quincy) within range of daily record high temperatures again today. Meanwhile, relative humidity values have already dipped into the 30s and 40s early this afternoon, and will likely bottom out between roughly 25 to 35% in most areas, perhaps slightly lower in central/northeast Missouri. While wind speeds have been very light so far, areas of northeast Missouri and west-central Illinois may see wind speeds reach 10-15 mph during the middle of the afternoon, resulting in a brief period of elevated fire weather conditions. While the overlap of breezy winds and low humidity will be brief and limited in coverage, vegetation is currently very dry, which may exacerbate the fire weather threat.

While quiet conditions are expected overnight, a persistent southerly breeze is expected overnight thanks to an approaching low pressure system and cold front from the northwest. This will maintain persistent warm air advection aloft, and will set the stage for yet another very warm afternoon tomorrow ahead of the advancing front. In fact, in areas that remain south of the boundary during the day, compressional warming may even add another few degrees to the afternoon high temperatures, and we may see a few 90s during the warmest part of the day. Not only this, but even though surface dewpoints will be slightly higher than they were yesterday, these very warm temperatures are still likely to result in minimum humidity values reaching near 25 to 35% in the Ozarks. Between that and a persistent 10-15 mph southwest breeze in these areas, along with the previously mentioned dry vegetation, conditions will once again support an elevated fire weather threat. While these conditions are not extreme, we recommend leaving the outdoor burning to land management agencies, as fires may behave erratically.

As the cold front moves through areas south of I-70 late in the afternoon and evening, it will begin to interact with gradually pooling low level moisture and modest instability (1000-1500 J/kg MUCAPE). Model forecast soundings continue to indicate that the warm sector will be characterized by both high LCLS and a very stout layer of warm air aloft, and surface based initiation remains much more likely to our east. However, there is a slightly better chance for a few elevated showers and thunderstorms to develop behind the front as it passes, although chances for this activity are also slightly better across the Ohio River Valley. Still, it won't be out of the question to see an elevated shower or brief thunderstorm considering the forecast instability, steep lapse rates above the warm nose, and strong winds aloft. The most likely hazards would be small hail and lightning, with better chances for severe storms to our east.

Also, winds will become sharply northerly and increase behind the cold front, with non-thunderstorm wind gusts very likely to reach 30 to 40 mph in many areas between late afternoon and late evening. Wind fields within the mixed layer to not currently appear to be strong enough to warrant a Wind Advisory at this time, but this will need to be monitored.

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LONG TERM

(Monday through Next Saturday) Issued at 142 PM CDT Sat Mar 21 2026

Behind Sunday's cold front, an expansive area of surface high pressure will settle into the Mississippi River Valley, bringing with it a much more seasonable airmass and temperatures. While highs are likely to be about 30 degrees cooler than the day prior, this only will bring us down to near seasonal averages. While wind speeds will be a limiting factor, humidity is also forecast to be low enough that we will need to monitor the potential for some localized elevated fire weather conditions as well. Otherwise though, Monday is expected to be a generally comfortable and dry day.

This cooldown will be brief, though, as a broad upper ridge will build again across the western and central CONUS during the mid-week period. Once again, the core of the upper ridge will be located to our west with northwest flow aloft overhead, building heights and southerly low level winds will drive another warmup between Tuesday and Thursday. This will be slow at first, as the northwesterly flow aloft will leave the area exposed to a subtle clipper and weak cold front Wednesday (along with a very modest chance for light rain), but confidence is increasing that by Thursday temperatures will once again climb well into the 80s. In fact, Thursday's setup has a lot of similarities to Sunday's, as a cold front will likely approach the area from the north, with persistent southwesterly low level flow and warm advection ahead of it in the warm sector. Ensemble mean 850 mb temperatures are once again forecast to reach or exceed the 99th percentile, and unless the surface front arrives slightly faster than expected, this will very likely result in another very warm day.

Meanwhile, ensemble probabilities for precipitation are also increasing in latest guidance as this cold front moves through the area Thursday/Friday, albeit with some slight timing differences among members. Ensemble mean PWAT forecasts have increased slightly to near the 90th percentile of climatology, and we also note an increase in instability projections as well. This appears to be driving a modest signal for strong/severe thunderstorms in long range CIPS/CSU output, and this trend will also need to be monitored as we get closer to Thursday. At this time, confidence is low in the quality of moisture and degree of instability that will be realized, in addition to the typical timing concerns with frontal passages at this distant time range.

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CLIMATE

Issued at 242 AM CDT Thu Mar 19 2026

Record Highs at STL/COU/UIN for 3/21-3/22 STL COU UIN 3/21 90 (1907) 92 (1907) 88 (1907) 3/22 88 (1907) 90 (1910) 84 (1907)

CLIMATE

Issued at 242 AM CDT Thu Mar 19 2026

Record Highs at STL/COU/UIN for 3/21-3/22 STL COU UIN 3/21 90 (1907) 92 (1907) 88 (1907) 3/22 88 (1907) 90 (1910) 84 (1907)

LSX WATCHES/WARNINGS/ADVISORIES

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