Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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622 FXUS63 KLSX 252341 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 641 PM CDT Wed Sep 25 2024 .KEY MESSAGES... - Generally dry conditions are expected through Thursday evening/night. - Rain associated with tropical moisture from Tropical Cyclone Helene is likely to move into the region Friday morning, continuing through Saturday morning. - An axis of 1-3" of rain is expected. Recent model trends have shifted this axis further south southeast of the STL metro. However, uncertainty remains in the exact placement of this heavy rain axis. && .SHORT TERM... (Through Late Thursday Night) Issued at 145 PM CDT Wed Sep 25 2024 The cut off low continues to spin over southern Illinois/SE Missouri this afternoon, with the vast majority of our region on the subsidence side of the cut off low and associated trough. That being said, a scattered cumulus field has developed this afternoon, with a few isolated showers ongoing south of the STL Metro toward the St. Francois Mountains. Showers (and the scattered Cu field) should dissipate by late afternoon/early evening with the loss of daytime heating. Generally clear skies tonight paired with light winds should allow for decent radiational cooling, with overnight/morning lows ranging from the low to mid 50s (upper 50s over the STL urban heat island). Fog is not anticipated to be as widespread tomorrow morning, but patchy fog will be possible once again in the river valleys. Tomorrow, the aforementioned cut off low slowly slides a bit further south toward the Missouri Bootheel as mid level ridging noses its way into the Upper Midwest. Meanwhile, Hurricane Helene will move northward across the eastern Gulf of Mexico, making landfall across the panhandle/big bend region of Florida around 7 pm tomorrow. The result for our region will be high temperatures generally in the upper 70s under partly cloudy skies for the daytime hours, with increasing high level clouds overspreading the region from the east/southeast tomorrow evening/night as a plume of tropical moisture wraps around the cut off low and nudges its way into our area. Some drizzle/light showers may be possible as early as late Thursday evening/night across our far southeastern areas, but otherwise rain should off until after midnight Friday morning. BMW && .LONG TERM... (Friday through Next Wednesday) Issued at 145 PM CDT Wed Sep 25 2024 The cut off low continues its slow meander southward early Friday morning, where it will begin to interact with Helene and likely slowly absorb it into one primary center of low pressure. Increased PWATs, potentially as high as 2", and associated showers, is progged to rotate around the tropical low and into portions of our CWA, especially areas south/southeast of STL, during the daytime hours of Friday. There is still a decent amount of uncertainty in the exact evolution of these features and just how far into our CWA the axis of heavier rainfall may progress. The 12z suite of models has continued with a slight southeastward trend for the axis of heaviest rainfall, with a fairly large rainfall accumulation gradient likely setting up across our region. The most recent WPC guidance suggests up to 3" of rain is possible from Friday morning through Saturday morning across the far south southeastern reaches of our CWA (Ellington to Fredericktown to Ste. Genevieve), with only a half inch or so across the STL metro. NBM probabilistic guidance shows roughly a 25% chance for rainfall exceeding an inch, and around a 10% chance for rainfall exceeding 2" at STL (Lambert). For the Ste. Genevieve area, NBM probabilities for rainfall exceeding 1", 2" and 3" are 55%, 33%, and 17%, respectively. High temperatures for Friday into Saturday should remain generally into the mid 70s, with breezy northerly winds (especially on Friday afternoon). Sunday into early next week, the upper-level cutoff low is progged to shift eastward into the Ohio Valley, with mostly diurnal isolated to scattered showers lingering as larger scale forcing exits. Model guidance continues to suggest an upper level trough moving across the Northern Plains into the Great Lakes region early next week, bringing a cold front through the region from northwest to southeast on Tuesday, bringing a much cooler and drier air mass to the region in its wake. BMW && .AVIATION... (For the 00z TAFs through 00z Thursday Evening) Issued at 640 PM CDT Wed Sep 25 2024 Winds will become light and variable overnight with surface high pressure over the region. These light winds along with largely clear skies will create radiational cooling conditions and favor the development of fog at KSUS. I cannot rule out the development of fog at KJEF and KCPS but confidence is too low to include in TAF. Otherwise, expect VFR flight conditions with northerly winds increasing after sunrise and diurnal cumulus on Thursday. MMG/Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX