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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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303 FXUS63 KLSX 040535 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1235 AM CDT Thu Jul 4 2024 .KEY MESSAGES... - Scattered showers and thunderstorms are expected this afternoon. through Thursday morning. A few of the storms could be severe this afternoon and early this evening mainly over southeastern Missouri and southwestern Illinois, with the main threat being damaging wind gusts. - Showers and thunderstorms through tonight will be capable of locally heavy rainfall, leading to a threat of flash flooding. This is particularly the case for locations that received rainfall Wednesday morning. - After a lull in showers and thunderstorms late Thursday morning into Thursday afternoon, another round of showers and thunderstorms is expected Thursday evening. Some thunderstorms will be strong to severe, primarily capable of damaging wind gusts and locally heavy rainfall. - The combination of heat and humidity will allow heat index values to climb to around 105 degrees over parts of southeast Missouri where a Heat Advisory is in effect into early this evening. && .SHORT TERM... (Through Late Thursday Night) Issued at 325 PM CDT Wed Jul 3 2024 A very active stretch of weather is expected over the next 24-36 hours across the CWA. Currently, a cold front has sagged southward into the CWA from the north, and is roughly draped along the I-70 corridor. South of this front is at least a couple of outflow boundaries from earlier convection, with agitated cumulus and a couple of thunderstorms forming along them across portions of southern Missouri. This convection is forming in an environment characterized by MLCAPE of 2,500-3,500 and effective bulk shear of 35-40 kts per recent SPC mesoanalysis. This will lead to a threat of isolated to scattered showers and thunderstorms mainly south of the front as weak upper-level divergence via a shortwave moves over the area, with the better focus for strong to severe storms along and south of the outflow boundaries over far southeastern Missouri and southwestern Illinois. The main threat is damaging wind gusts, though brief instances of hail up to 1" are possible, and this threat will exist through the early evening before instability wanes. The CWA will then see a brief lull in convection later this evening as the first round of convection dies off and/or moves eastward out of the area. The low-level jet will then ramp up and nose into the region, interacting with the front to produce scattered showers and thunderstorms through the overnight hours across much of the CWA. Severe weather is not expected with this overnight round, though convection will be capable of locally heavy rainfall given deep warm cloud depths and precipitable water of at least 2". With all the rain that fell across portions of central Missouri, and training thunderstorms expected this afternoon into tomorrow morning across southeastern Missouri and southwestern Illinois, a Flood Watch has been issued for this portion of the area due to the threat of flash flooding. As the low level jet weakens and veers tomorrow morning, convection will gradually come to an end from west to east, kicking off another lull in rain chances for much of the CWA. Late morning and afternoon showers and thunderstorms can`t be ruled out for portions of the area as a subtle shortwave passes overhead, though confidence in this occurring is low given capping and a lack of instability seen in deterministic model soundings, as well as a lack of a signal for convection in the CAMs. The greatest chance for showers and thunderstorms comes Thursday evening as another cold front surges into the area. Ahead of this front late Thursday afternoon and evening, the 12z HREF mean has approximately 3,000 J/kg of SBCAPE and 30-35 kts of 0-6km bulk shear over portions of western and central Missouri. Given the environment, multicellular convection is expected to form along this front during the late afternoon, eventually congealing into a line or multiple line segments as it moves east-southeast. Currently, this convection is expected to remain mainly along and west of the Mississippi River. The main threats with this convection, especially with any line segments, is damaging wind gusts. Locally heavy rainfall is also expected, and given it will likely track over portions of central Missouri where copious rainfall has occurred recently, there will be a threat of flash flooding. Also of concern given the amount of outdoor events for the holiday is frequent lightning associated with convection during the peak firework display timeframe. Even if thunderstorms are not severe, they will pose a threat of lightning to those who are outdoors. Convection and rain will move out of the area during the early morning hours of Friday as the cold front sweeps through the region. Elmore && .LONG TERM... (Friday through Next Wednesday) Issued at 325 PM CDT Wed Jul 3 2024 Given the plethora of hazards in the short period, only a brief synopsis of the extended period will be given. Guidance consensus is that upper-level troughing will persist over the eastern U.S. through this portion of the period, leading to high confidence in an extended stretch of near normal to below normal temperatures. Rain chances will be confined to shortwaves diving south-southeastward within the northwesterly flow over the Middle Mississippi Valley. There is little consensus in the placement and timing of these shortwaves among guidance, leading to broad-brushed low rain chances both temporally and spatially starting early next week. However, plenty of dry time is expected through this portion of the period. Elmore && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1223 AM CDT Thu Jul 4 2024 Showers and thunderstorms will continue to develop and affect COU/JEF/STL/SUS/CPS terminals through 12-15Z with MVFR/IFR ceilings and visibilities in moderate-heavy rain at times. There will be a shorter period of time between 11-15Z where this will happen at UIN. Then this first batch of rain will move out by 15Z and then the chance for showers and thunderstorms will be low enough to not mention in any of the TAFS between 15-00Z. Then chances will increase again after COU/JEF after 00Z and at STL/SUS/CPS after 03Z. Once again, these storms will have will have the potential to reduce ceilings and visibilities to MVFR or IFR and as well as wind gusts over 35 knot and possible hail. Britt && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Flood Watch through late tonight for Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO- Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO. IL...Flood Watch through late tonight for Monroe IL-Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX