Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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915 FXUS63 KLSX 181640 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1140 AM CDT Tue Jun 18 2024 .KEY MESSAGES... - Today will be the coolest day of the week, however temperatures will remain above average with peak heat index values ranging from 90-101. There is a chance for afternoon isolated to scattered showers and thunderstorms mainly across southeast and eastern Missouri and southwest Illinois (20-30%). - Above normal temperatures are expected to continue through the weekend with highs in the 90s and Friday and Saturday being the hottest days. && .SHORT TERM... (Through Late Wednesday Afternoon) Issued at 429 AM CDT Tue Jun 18 2024 Current water vapor imagery is showing a mid-level ridge centered over the Mid-Atlantic area which is dominating the eastern CONUS. The associated deep high pressure is resulting in pronounced deep- layered southerly flow across the Mississippi River Valley. This southerly flow continues to transport warm air and Gulf moisture into the region. Today will be slightly less warm, compared to yesterday, with highs in the mid 80s to low 90s area-wide. However, with dew points in the 70s, there will be no issue for heat index values to reach 90-101 degrees. The main catalyst for highs being slightly lower is due a large fetch of increased cloud cover moving into the region. In addition to increased cloud cover a vorticity maximum rounding the mid-level high, currently evident in water vapor imagery over Louisiana, will bring a chance for afternoon scattered showers and thunderstorms. Deterministic guidance is in agreement that the vorticity maxima will traverse across southeast Missouri and southern Illinois. This will help focus afternoon convection and with weak low-level support and SBCAPE values between 500-1500 J/kg where the vort max traverses. Locations that do see a shower/thunderstorm can expect downpours and some gusty winds in the strongest of storms. Should the vorticity maximum traverse further to the north, the footprint of isolated to scattered afternoon convection will be larger. Shower and thunderstorm chances will subside after sundown due to a loss of instability and daytime heating. By Wednesday, the mid-level ridge/high will begin to expand westwards. As it does so, another weak vorticity maxima will round its northwestern periphery and traverse across northwest and north- central Missouri Wednesday afternoon. This will help focus another chance (20-30%) of afternoon showers and thunderstorms. With closer proximity to the center of the high, Wednesday will be another hot day with temperatures a hair warmer than today area-wide with highs in the upper 80s to mid 90s. Despite the slightly warmer temperatures tomorrow, heat index values will not be as high as dewpoints in the 60s are forecast. MMG/Glass && .LONG TERM... (Wednesday Night through Monday) Issued at 429 AM CDT Tue Jun 18 2024 The mid-level high will continue its westward shift through Saturday with surface high pressure dominating the region. The attendant height rises aloft will favor increasing high, temperatures back up into the mid to upper 90s by Saturday. Confidence in this warm-up is further increased as the NBM interquartile ranges only have a 5 degree spread with the lower quartile being in the mid-90s. Due to the dominance of deep high pressure, rain chances will be suppressed Friday through Saturday afternoon and evening. Ensemble guidance is in consensus that a mid-level trough will traverse across the Upper Mississippi River Valley late Saturday into Sunday. As it does so, a shortwave and vorticity maxima will help provide support for a cold front to move across the region. This will provide the next chance for shower and thunderstorm activity. There are differences among the deterministic guidance on the timing of the front, so the exact timing of the shower and thunderstorms are uncertain at this time. However, confidence is increasing that precipitation chances will ramp-up overnight Saturday into Sunday morning with over 70% of ensemble guidance indicating rain during this time frame. Temperatures will remain above normal despite the passage of a cold front on Sunday and continue into early next week as slight ridging builds back in over the central CONUS. Confidence in these above normal temperatures is increasing as the lower quartile of the NBM max temperature distribution is above the average high temperature for late June. MMG/Glass && .AVIATION... (For the 18z TAFs through 18z Wednesday Afternoon) Issued at 1135 AM CDT Tue Jun 18 2024 VFR conditions are expected to prevail throughout the 18Z TAF period. VFR cumulus will impact local terminals this afternoon, and may briefly dip to MVFR levels. A few showers and a weak thunderstorm or two may develop in southeast Missouri this afternoon, and there is a low chance (20%) that a storm could develop near STL. Chances remain too low to include in the TAF, but an amendment may be needed later this afternoon if storms develop nearby. Gusty southerly winds will persist this afternoon, and diminish by sunset. A low chance of valley fog exists at a few terminals overnight, but confidence remains too low to include in the TAF at this time. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX