471 FXUS63 KLSX 101032 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 532 AM CDT Tue Jun 10 2025 .KEY MESSAGES... - Other than a gradual warming trend through the week, which may bring parts of the area to 90 degrees for the first time this year, our sensible weather remains welcomingly tranquil through at least early Thursday. - Unsettled, showery conditions return late Thursday through the weekend, with confidence still fairly low on timing and exact amounts. That said, there is little pointing to any noteworthy hazards beyond thunderstorms. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 348 AM CDT Tue Jun 10 2025 In the wake of a dry cold frontal passage yesterday, something not often seen in this area in June, weak and nebulous high surface pressure is settled over the bi-state region. Weak but present northwest flow aloft is promoting a dry, seasonably cool airmass that will make for fairly comfortable mid-June weather today. The only fly in the ointment will be some high-level smoke from ongoing Canadian wildfires, but they won`t do much more than make the sky just a tad hazy aloft. Otherwise, temperatures will top out in the upper 70s to low 80s today with comfortable humidity. Winds gradually back this evening and become more southerly as the surface ridge shifts east, drawing warm, moist air poleward. The resultant increased, albeit weak, warm air advection will bolster overnight lows 5-10 degrees warmer than last night`s. The warmup continues into the day on Wednesday, but most deterministic guidance shows a weakening in the warm advection with the approach of a broad mid-level wave during the late morning/early afternoon. That, plus the approach of high cirrus during this same time, throws doubt on whether St. Louis sees its first 90 degree day this year (see the Climate section at the bottom of this discussion). Regardless, it`ll be a more summerlike day across the region Wednesday, kicking off several days of near to above-normal temperatures. Between weakened WAA and nebulous forcing/lift, Wednesday will almost certainly stay dry. MRB && .LONG TERM... (Wednesday Night through Monday) Issued at 348 AM CDT Tue Jun 10 2025 Amidst broader, longwave ridging aloft spanning the CONUS, the aforementioned mid-level closed low meanders into the Mid- Mississippi Valley Thursday morning. The speed, position, and structure of this feature still vary among the ensemble guidance membership, which will impact the coverage and timing of showers and a few thunderstorms late Thursday into Friday. Regardless, this amplification in the flow will further promote increased Gulf moisture advection into the region, with NAEFS guidance showing precipitable water values approaching the 90th climatological percentile. With the increased moisture and trough aloft, diurnally- driven maxima in instability is likely to result in disorganized waves of showers and thunderstorms in the region through the weekend. Fortunately, the subtropical jet relegated to the northern tier of the CONUS on the periphery of the longwave ridge, and winds throughout the column struggle to exceed 20-30kts in all available guidance. As a result, organized strong to severe convection is very unlikely. While I can`t rule out pulse convection resulting in a localized damaging wind threat in this setup, that is a worst-case scenario. What is more likely to occur is pockets of locally-heavy rain, especially in the absence of any appreciable steering flow. Finally, with respect to temperatures, the low aloft and resultant clouds/convection will work to very marginally subdue temperatures. This trend is evident in the NBM maximum temperature distribution, which dips a bit on Friday and through the weekend. That said, most of the ensemble envelope is at least near-normal, so showing that this system`s effect on temperatures is likely to be minimal. There is as much uncertainty surrounding this wave`s exit than there is its arrival, but a preponderance of guidance passes the trough axis through the region by Sunday evening. While this ends the more sporadic showery activity associated with the sluggish upper-level low, the flow appears to turn quasi-zonal and remains somewhat unsettled as a result. The NBM holds onto persistent, low chances for showers and thunderstorms through early next week which matches the large-scale setup. A few deterministic models do show more amplified shortwaves glancing the northern fringes of the Lower Midwest, which would promote a threat for stronger convection, but none really stand out as threatening to us locally. MRB && .AVIATION... (For the 12z TAFs through 12z Wednesday Morning) Issued at 531 AM CDT Tue Jun 10 2025 Dry VFR conditions will prevail amidst light, generally westerly winds through the valid TAF period. MRB && .CLIMATE... Issued at 348 AM CDT Tue Jun 10 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. With 90 degrees being highly unlikely today (June 10), 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. && $$ WFO LSX