Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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340 FXUS63 KLSX 092246 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 546 PM CDT Tue Jul 9 2024 .KEY MESSAGES... - Rain chances will diminish from southwest to northeast through this evening. Additional rainfall amounts of roughly 1-3" are expected mainly over eastern Missouri and southwestern Illinois with locally higher amounts possible. - The threat for strong to severe thunderstorms mainly capable of tornadoes is low across portions of far southwestern Illinois and far southeastern Missouri, but the more favorable conditions for this threat is expected to remain southeast of the CWA. - Relatively calmer weather along with cooler temperatures will persist Wednesday through the early weekend before temperatures warm above normal by the end of the weekend into early next week. && .SHORT TERM... (Through Late Wednesday Night) Issued at 246 PM CDT Tue Jul 9 2024 The remnants of Beryl continue to move through the region, with the center of the system currently over the Missouri Bootheel. North of the center, a widespread shield of light to moderate rainfall is occurring across much of the CWA. Rainfall rates have generally been 0.25-0.5"/hour, with mainly nuisance flooding and ponding on roadways occurring in addition to rises on area creeks and streams. As we get further into the afternoon, there is a low chance (20-30%) that the atmosphere destabilizes enough to produce convective elements through early this evening within the broader rain shield that yield higher rainfall rates and a threat of flash flooding. Therefore, there are no plans to adjust the current Flood Watch. On the note of convection, there remains a threat of strong to severe storms mainly capable of tornadoes over the far eastern- southeastern extent of our CWA. However, given the track of the post- tropical low, climatology and the currently expected parameter space favors that this threat will remain to the east-southeast of the CWA. This evening and tonight, Beryl`s remnants will push east- northeastward, with rain exiting the area through late tonight/early tomorrow morning. As the system begins departing the CWA, guidance consensus is that mid to low-level winds will increase, with some mixing down to the surface. This is expected to produce gusts up to around 35 mph this evening mainly along and east of the Mississippi River. Fog also can`t be ruled out tonight, mainly across portions of central Missouri where clearing may occur prior to sunrise tomorrow morning. Tomorrow, Beryl`s remnants will continue northeastward and merge with an upper-level trough. A shortwave rounding the backside of this trough will move through the Midwest during the afternoon and evening, providing enough forcing to produce widely scattered convection mainly north of I-70. The airmass over this portion of the CWA will be quite unstable, with MUCAPE forecast to reach roughly 2,000 J/kg. However, deep-layer shear will be weak, with values topping out around 20 kts. Given the amount of instability, small hail can`t be ruled out in the strongest cores, but a relatively high melting layer and lack of shear should limit storm strength and hail size. Elmore && .LONG TERM... (Thursday through Next Tuesday) Issued at 246 PM CDT Tue Jul 9 2024 At the beginning of the period, guidance consensus is that the upper-level trough will be gradually moving eastward, placing the Middle Mississippi Valley beneath northwesterly flow. Deterministic guidance shows a subtle shortwave digging equatorward on the backside of this trough through the Midwest. Given the deep northwesterly flow in place over the region, robust moisture return prior to the passage of this shortwave is not expected. Even so, guidance consensus is that there will be just enough to produce a low chance (30%) of rain, with ensembles focusing this rain mainly somewhere along and north of I-70 during the afternoon. For Friday and Saturday, additional rounds of rain are possible as a ridge expands eastward across the CONUS and into the Middle Mississippi Valley, prompting a warm front to move through the region amidst passing weak shortwaves along the periphery of the ridge. However, the signal for rain remains weak among guidance, prompting a low chance (20%) for rain Friday and Saturday afternoon. As the ridge builds in further on Sunday and early next workweek, warm air will advect into the region via southerly to southwesterly low-level flow. In turn, 850 mb temperatures top out in the mid-20s degrees C per ensemble consensus. Climatologically, this favors surface temperatures in the mid to upper 90s assuming deep mixing, and these hot surface temperatures are within the 25th-75th percentile of ensemble guidance. How hot the region stays toward the middle of next week is uncertain, as guidance is split on the southward progression of a trough digging equatorward somewhere over the eastern CONUS. If the trough is able to dig further south, temperatures will cool to some degree compared to earlier in the week, but if the ridge holds out over the region, temperatures will remain as warm into the middle of next week as they are for earlier in the week. Elmore && .AVIATION... (For the 00z TAFs through 00z Wednesday Evening) Issued at 540 PM CDT Tue Jul 9 2024 There are a few items of interest in the 00Z TAF cycle. First, lingering stratiform rain is gradually diminishing this evening, but light rain may intermittently impact local terminals during the first few hours of the period. A few embedded convective cells may also linger into the evening, but are expected to diminish by morning. Heavier bursts of rain are possible within these cells, but confidence is low that they will directly impact local terminals. Some patchy fog may also develop in a few areas overnight and early tomorrow morning, particularly in central MO and possibly UIN where lighter winds are expected. More persistent northerly winds at STL/SUS/CPS will likely limit fog development, but if winds weaken faster than anticipated, fog will be possible here as well. VFR conditions are expected during the day tomorrow, but a scattered to broken field of cumulus clouds is expected. A few showers and weak thunderstorms will also be possible at UIN late in the afternoon. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch until 4 AM CDT Wednesday for Jefferson MO-Lincoln MO- Saint Charles MO-Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO. Flood Watch until 10 PM CDT this evening for Audrain MO-Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO- Reynolds MO-Saint Francois MO-Warren MO-Washington MO. IL...Flood Watch until 4 AM CDT Wednesday for Bond IL-Calhoun IL- Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL- Madison IL-Marion IL-Monroe IL-Montgomery IL-Randolph IL- Saint Clair IL-Washington IL. && $$ WFO LSX