Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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107 FXUS63 KLSX 121732 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1232 PM CDT Fri Jul 12 2024 .KEY MESSAGES... - Isolated to scattered thunderstorms, featuring gusty winds to 40mph and heavy downpours, will continue to threaten portions of the area through early Sunday morning. Again, plenty of dry time is forecast in between convection. - Abnormal heat will impact the region Sunday through Tuesday, with Monday being the hottest and most humid day when heat index values exceed 100 degrees across most of eastern Missouri and western Illinois. - Multiple rain chances exist Wednesday through Friday along a slow-moving front, which will bring temperatures back to more seasonable (if not below-normal) values. && .SHORT TERM... (Through Late Saturday Afternoon) Issued at 321 AM CDT Fri Jul 12 2024 A west-east oriented stationary front is currently draped across the northern fringe of the Ozark Plateau into south-central Illinois. On the nose of a low-level jet and amidst a mid-level shortwave impulse, strong convection blossomed late yesterday night in southeast Kansas and southwest Missouri. As it moves east, however, the mid-level support weakens and most convection struggled to reach the forecast area. The stronger thunderstorms have grown upscale into a north-south oriented line just west of Joplin, Missouri. This feature, or at least the outflow boundary it generates, will act with the stalled front and weakening (but still present) mid-level forcing to promote continued isolated to scattered thunderstorms along and south of I-70 today. With MLCAPE values still more than sufficient for deep convection, gusty winds to 40mph and heavy downpours are the main threat with these thunderstorms. The continued lack of any noteworthy shear will keep nearly all convection beneath severe strength. Otherwise, temperatures today will remain near mid-July normals (upper 80s to low 90s). The strong mid-level ridge centered over the western CONUS is still forecast to expand east gradually overnight into Saturday morning, however the returning nocturnal low-level jet will allow for additional convection to blossom across western Missouri and drift through the forecast area through the day on Saturday. That said, once the LLJ weakens, height rises aloft would make it more difficult for convection to persist into the day. That said, with surface-based instability still in the 1500-2500J/kg range, at least isolated thunderstorms can be expected Saturday with a weak warm front lifting north through the region. In the wake of the warm front, temperatures climb into the 90s areawide as we continue a slow march to abnormal warmth by early next week. MRB && .LONG TERM... (Saturday Night through Thursday) Issued at 321 AM CDT Fri Jul 12 2024 The weak warm front lifts north of the region by Saturday evening as mid-level ridging expands further into the Mississippi Valley. At the same time, a shortwave tracks along the ridge`s periphery and sparks convection in the northern Plains into the Great Lakes region. With the nose of the 850mb low-level jet uniformly forecast to be closer to northeast Iowa, the bi-state area looks too far removed from favorable forcing and instability to pose a threat for strong to severe thunderstorms overnight Saturday into Sunday morning. Remnant convection, possibly on the outflow of the nocturnal MCS to our north, is possible (roughly 20% chance) Sunday morning into the early afternoon, but it will be fighting a very strong capping inversion and mid-level subsidence. By this point, attention turns to the strengthening 850mb thermal ridge and the increasing heat index values. Little has changed from previous forecasts regarding the large-scale pattern: the eastern periphery of the mid-level ridge expands east over us and increased southwest flow from 850mb to the surface takes hold. Temperatures are almost certainly (80-90% chance) to climb into the mid/upper 90s by Sunday for all except the higher elevations of the Ozarks. While southwest winds can act to deepen mixing and lower humidity more than a due south wind, the recent stretch of above-normal rainfall is keeping the topmost layer of the ground relatively moist. Amidst these warmer temperatures, increased evapotranspiration and resident low-level atmospheric moisture should counteract any impacts from the Ozarks. That said, except for directly in river valleys or in the middle of the local metropolitan areas, heat index values will top off in the 100-105 range Sunday. Monday looks to be the hottest day of the forecast period, with temperatures approaching 100 degrees in the major cities and mid/upper 90s for most of the remaining region. With similar humidity to Sunday`s values, and 850mb temperatures/surface flow pattern matching the patterns that historically lead to 105 degree heat index values, the best potential for dangerous heat and humidity exists Monday. Meanwhile, the ridge aloft begins to erode from the north as longwave troughing from Canada and shortwaves tracking along the cyclonic flow push a front gradually south. Tuesday still looks abnormally warm, but there is growing potential that either cloud cover or cooler air from the front itself would begin to impact the region. The forecast highs (and maximum heat index values) have cooled a few degrees compared to previous iterations in response to this scenario becoming more prominent in the ensemble guidance. While some uncertainty in the longwave trough`s position and amplitude exists, the front decidedly arrives in our north by Wednesday. The combination of marginally-cooler air and increased cloud cover/precipitation extent will bring temperatures back to the mid/upper 80s Wednesday, and cooler still into the end of the work week. There are no indications that it will rain continuously from Wednesday onward, but exactly when and where it rains will depend on shortwaves tracking along the stalled boundary and base of the longwave trough (all of which is not known at this point). Each disturbance pushes the front a bit further south, with a gradual drying trend from Wednesday to Friday from north to south. MRB && .AVIATION... (For the 18z TAFs through 18z Saturday Afternoon) Issued at 1231 PM CDT Fri Jul 12 2024 Dry and VFR flight conditions are expected through the TAF period. Thunderstorms across southeast Missouri and southern Illinois are expected to remain south of the St. Louis Metropolitan terminals. There is another chance for isolated showers and thunderstorms area wide Saturday but confidence in coverage and timing is too low for a mention in TAF. Otherwise, light southerly winds will persist before winds increase and become more southwesterly Saturday morning. MMG/Kimble && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX