


Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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958 FXUS63 KLSX 300354 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1054 PM CDT Sun Jun 29 2025 .KEY MESSAGES... - Periods of showers and thunderstorms are expected through Monday. - A cold front Monday brings less humid conditions for the middle of the week with pleasant temperatures. Temperatures warm up again later in the week. && .SHORT TERM... (Through Late Monday Night) Issued at 246 PM CDT Sun Jun 29 2025 Multiple clusters of showers and thunderstorms have formed and moved slowly across our general region today, all beneath a subtle shortwave trough aloft. CAMs have been hit or miss with these pockets of thunderstorms today, leading to relatively low confidence in their output going forward over he next 24-30 hours. That makes for a rather difficult forecast when it comes to timing and location of thunderstorm chances. While there is considerable uncertainty on the local level, at a bigger scale we see we remain in a humid and generally unstable air mass beneath a subtle shortwave trough aloft. In that environment we can expect occasional rounds of thunderstorms. Wind shear remains quite weak so the severe weather threat is low and focused to our west in the next several hours where there has actually been some sun today to generate better instability. Guidance is in fairly good agreement that the next round of thunderstorms gets generated across western to central Missouri overnight. Considerable timing and location variance exists among the CAMs on this, but the general idea has good support. Despite some agreement here, confidence remains somewhat lower as most guidance has not handled this afternoon`s storm clusters very well which may play a role in what happens tonight. The greatest thunderstorm chances will be in the western part of the forecast area tonight. A stronger trough is moving out of Montana and into the Northern Plains this afternoon, pushing southeast through the Midwest tomorrow. This brings with it a cold front which ushers in a change in air mass behind it. As the front and associated trough move through our region on Monday, an additional round of showers and storms may develop in the vicinity of this front. The uncertainty on this round centers on two things: the availability of instability and the timing of the front. On the first point, tonight`s round of storms and any lingering showers and clouds will be a limiting factor on the development of more robust instability ahead of the front. With a little lower confidence on tonight`s storms, that in turn gives us a little better confidence that there will be enough sun to generate instability ahead of the front on Monday. On the second point, guidance still varies by several hours on how fast the front moves through and thus where it will be during peak heating. If the front is a bit slower, then convection may begin further north, though it remains possible that the front will be south of the forecast area by the time it is able to take advantage of peak instability. With the arrival of the trough aloft we will have a bit better wind shear available on Monday. So if storms develop there`s a better chance that they organize into clusters with the potential for damaging winds or localized downbursts. This is particularly true if strong instability is able to develop. While this threat exists, the considerable uncertainty regarding Monday`s convection limits confidence and we are currently not messaging this threat. Kimble && .LONG TERM... (Tuesday through Next Sunday) Issued at 246 PM CDT Sun Jun 29 2025 The air mass behind Monday`s front is cooler and much less humid, with its origins in the Canadian Rockies. With drier air and more sunshine, though, the sun will act effectively on this air mass and warm temperatures up into the mid to upper 80s. With dewpoints as much as 10 degrees lower than prior days, this will feel notably less humid even if daytime temperatures are similar. Nighttime lows will fall back into the 60s for the first time in a while due to those lower dewpoints. This mild and less humid air mass lasts through at least Wednesday when surface high pressure moves across the region. Long range guidance is in good agreement that ridging spills out of the western US and into the Plains later this week while a broader trough develops over the Northeast. This will tend to keep things dry within a northwesterly upper flow over our region, though we`ll see temperatures gradually warming as the influence of the ridge becomes more dominant. Low level flow turning southerly will also advect more humidity back into the region just in time for Independence Day. The combination of gradually increasing heat and humidity will cause heat index values to rise through the 90s to lower 100s, closing in on more dangerous levels. Rain chances continue to tick downward as confidence has increased in the subsident northwest flow being present to suppress convection. Kimble && .AVIATION... (For the 06z TAFs through 06z Monday Night) Issued at 1039 PM CDT Sun Jun 29 2025 Expect some potential for fog to develop late tonight into early Monday at UIN/COU/JEF, so have added a TEMPO group between 09-13Z for MVFR visibilities. The potential for thunderstorms late tonight into early Monday morning has decreased, so have removed the PROB30 group at COU/JEF between 06-11Z. There does remain a chance for thunderstorms in the afternoon into the early afternoon with a cold front moving through the area from the from the northwest, so I have PROB30 groups going all of the terminals from 21-02Z. Any of the heavier showers and thunderstorms will be capable of producing MVFR/possible IFR ceilings and visibilities. Winds are expected to stay below 10 knots outside of thunderstorms. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX