Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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337 FXUS63 KLSX 130350 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1050 PM CDT Wed Jun 12 2024 .KEY MESSAGES... - Temperatures are expected to run 5-15 degrees above normal for several days. Thursday, Sunday, and Monday continue to look like the warmest days with highs well into the 90s and several nights with lows no cooler than 70 degrees. - Severe thunderstorms are possible Thursday evening, mainly over northeast Missouri and west-central Illinois. The primary threats will be large hail and damaging winds. && .SHORT TERM... (Through Late Thursday Night) Issued at 249 PM CDT Wed Jun 12 2024 Surface ridging centered over the mid-Atlantic extends southwest through Texas with light south-southwest flow over the mid- Mississippi River Valley this afternoon. Clouds are far less represented today than yesterday as the closest system passes over the Upper Midwest. The southern extent of this system will send high clouds near the Iowa/Missouri border with little impact farther south. Local conditions are better characterized by an increase in moisture from east to west with surface dewpoints in the low-60s in Illinois to the mid over central Missouri. Ample sunshine has allowed temperatures to respond in kind with observations well into the 80s with low-90s largely confined to urban areas. The initial focus will be on increasing moisture and heat tonight into Thursday. South and southwesterly flow is expected to continue to feed moisture northward out of the western Gulf of Mexico into the central Plains and Upper Midwest. Guidance has been consistent with the surge of 20-25C air being transported from the central Plains into central and northern Missouri. The warmest mid-level air extends through central and northern Missouri, leading to a setup with higher temperature dispersed across the northern two-thirds of the CWA, generally along and north of I-44 in Missouri and I-64 in Illinois. Widespread 90s are expected with the warmest reading reaching the mid to upper 90s, especially around urban heat islands. Heat index values approach 100-103 over central, east-central and northeast Missouri into west-central Illinois. Thunderstorms come into the picture late Thursday as a cold front slowly drifts in from the north. Aside from the NAM`s occasional north/south bounce in frontal position, guidance has been in relatively good agreement. The front is expected to stretch east to west through central Iowa late Thursday afternoon (20-22z). A strong capping inversion is apparent ahead of the front, which should limit convective initiation to the areas of best convergence along the boundary. Guidance has been consistent in producing peak SBCAPE indices around 4000 J/kg, mid-level lapse rate around 8C, and high moisture content with dewpoints in the mid-70s pooling along the front. 0-6km bulk shear has ticked up a bit in the last couple of cycles with 40-45 knots now looking more likely. The limitations largely stem from the northern track of the upper wave over northern Illinois, relying on diurnal trends and surface convergence to break through the inversion. General agreement among the CAMs is for convective initiation to commence north of the Iowa/Missouri border before shifting south through the evening. The conditions suggest that once the cap breaks, whatever is able bust through could be explosive with initial discrete cells including large hail and damaging winds. Mean mid-level flow runs parallel to the front suggesting discrete development will quickly become linear and transition the primary threat to damaging wind, some which could be strong enough to support gusts near 70 mph. While hail could be as large as golf ball size, this transition happens rather quickly, limiting this threat to the far norther tier of counties in the CWA. Timing places the front in the far northern sections of the CWA (northeast Missouri/west- central Illinois) after 00z, well past diurnal heating. Timing brings to question how long thunderstorms survive given the last of upper support this far south and west of the upper level shortwave and with the lack of a strong low level jet. Weakening is favored as storms track south, limiting the best potential for severe weather over northeast Missouri and west- central Illinois with the threat all but entirely diminishing along the I-70 corridor. The time to watch will be 00z-04z with much of the potential to low to consider much of a threat thereafter. Maples && .LONG TERM... (Friday through Next Wednesday) Issued at 249 PM CDT Wed Jun 12 2024 (Friday through Tuesday) A stalling cold front is expected to extend along or just south of the I-70 corridor early Friday morning. A remnant shower or isolated thunderstorm from overnight convection cannot entirely be ruled out, but most activity will have dissipated. The front drifts into southern Missouri and Illinois by Friday afternoon with potential for an isolated, diurnally driven shower or thunderstorm. Chances were held below 15 percent with the lack of deep ascent. An upper level ridge begins to translate eastward late Friday into Saturday before an upper level shortwave ejects northeast from the central Plains into the Upper Midwest. This eastward translation of the upper ridge, in tandem with the upper level shortwave, will motivate mid-level warm air advection to lift into central Missouri Saturday morning. Meanwhile, surface high pressure over the Great Lakes is expected to result in easterly flow at the surface with dewpoints in the upper 50s to low-60s over the eastern half of the CWA to near 70 from central to southeast Missouri. The surface boundary remains stalled until later in the day, when the surface high moves far enough east to result in return flow. Temperatures remain warm, but relatively comfortable compared to what comes later in the weekend. The general theme is for gradual increase in moisture from the southwest late in the day. One question that lingers is the chance for an isolated shower/thunderstorms Saturday afternoon as the warm front begins to lift northeast. The core of the upper support remains far enough north/west that chances remain low enough to keep PoPs out of the forecast, but I would not be surprised to see trends justify low end chances in subsequent forecasts. The warmest air continues to stretch from late this weekend into early next week. It will be markedly more humid with dewpoints well into the 60s to low-70s, pending the evolution of the mid and upper ridge. Dp/Dt and GEFS upper level pressure contours show an ever-so- slight southeastward shift in the ridge. While this sounds small, shortwave energy continue to revolve around the western/northern periphery of the upper ridge as the western side of the surface to mid-level ridge feeds moisture northward out of the Gulf of Mexico. If this shift continues, it may lend more questions than answers, introducing the potential for diurnal convection late Sunday into the early part of next week. Warm air seems to be solidly in place on south/southwest flow with high in the 90s late this weekend into next week. Sunday and Monday have the best promise for some mid to upper 90s with an increasing spread heading into midweek. The caveat will be the magnitude of warmth as it is dependent on these trends and any shift in the ridge. Maples && .AVIATION... (For the 06z TAFs through 06z Thursday Night) Issued at 1040 PM CDT Wed Jun 12 2024 VFR conditions will prevail through approximately 00Z Thursday night before a cold front threatens convection at the terminals Thursday evening into the night. The best potential for thunderstorms exists at KUIN during the early evening, with the threat gradually decreasing as the convection moves further south. VCTS was added at all TAF sites (except for KSUS and KCPS, since the convection will arrive after the TAF period) during the time convection would most likely be near the terminals. However, there is little confidence that any terminal will be directly impacted. Following the convection, a cold front will pass south of the I-70 corridor and bring a drier, northerly wind to the region. MRB && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX