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Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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394 FXUS63 KLSX 141945 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 245 PM CDT Sun Jul 14 2024 .KEY MESSAGES... - Dangerous heat and humidity will will continue at least through Monday. Heat index values Monday afternoon are expected to reach 105 to 110 degrees.. - The temperature forecast for Tuesday remains uncertain due to the possibility of scattered showers and cloud cover as a cold front moves into the area. - Multiple rounds of thunderstorms are forecast along the slow- moving front late Tuesday through Thursday which could lead to flooding. Much cooler weather is expected for Wednesday through Sunday. && .SHORT TERM... (Through Late Monday Night) Issued at 242 PM CDT Sun Jul 14 2024 The upper level ridge over the Four Corners region continues to dominate the weather pattern. Many locations have already reached 100-105 heat index despite mid level clouds and a few sprinkles earlier today. The heat and humidity will only intensify over the next 24 hours as southwest flow will continue to prevail ahead of the next system. All signs point to Monday being dangerously hot and humid as 850mb temperatures approach 25C which mixes down to right around 100F at the surface. Dew point temperatures this afternoon are mostly in the 73-76F range and are expected to remain in that range if not increase slightly tomorrow. This will produce dangerous heat index values Monday afternoon ranging from 105 to 110. There is a very small chance that the tail end of an MCS over Illinois may graze parts of west central and southwest/south central Illinois early Monday morning. If this does occur, the outflow could blunt some of the worst of the heat in those areas. However, current thinking is that the MCS will stay well north of the area and the outflow will wash out due to the prevailing southwest flow. Carney && .LONG TERM... (Tuesday through Next Sunday) Issued at 242 PM CDT Sun Jul 14 2024 Models are still advertising a dramatic pattern shift beginning on Tuesday. An unusually strong mid-upper level trough will dig through the Midwest starting Tuesday, and continue southeast all the way through the Mississippi Valley to the Gulf Coast by Thursday. This trough will drag a slowly moving cold front into Missouri and Illinois Tuesday. It`s still uncertain where the effective boundary will be on Tuesday due to potential outflows from convection Monday night. Indeed, several CAMs show the tail end of an overnight MCS making it well into the northern 1/2 of the CWA by 12Z, and the NAM pushes it south of the I-70 corridor in central and eastern Missouri into southwest Illinois by mid-Tuesday morning. The uncertainty with the position of the boundary continues to make the temperature/dew point forecast difficult at best. Therefore we will continue to hold on extending the Heat Advisory. That being said, there is potential for 105+ heat index Tuesday afternoon south of the cold front particularly if higher dew points pool ahead of it. Thunderstorm chances during the day on Tuesday along and ahead of the front remain in doubt as there is a significant capping inversion in place. There is also a possibility for a few severe storms capable of producing damaging winds during the afternoon and evening (if they can break the cap) as guidance suggests it will be quite unstable with more than 3500 J/Kg SBCAPE ahead of the front. Thunderstorms look more likely on Tuesday night, mainly after midnight, as the southwest low level jet strengthens to around 30-35kts, overrunning the now stalled front and producing moderate to strong moisture convergence over Missouri and Illinois. The low level jet is forecast to veer a bit with time, which is a good set up for multiple rounds of training storms. P-WAT values should be around 2 inches, and forecast soundings show the warm cloud depth to be in excess of 13Kft. All this adds up to a good potential for heavy rain and possibly flash flooding. Where the heaviest rain falls will depend on the position of the front, but current thinking is somewhere near or south of the I-70 corridor. The front is expected to begin slowly moving again on Wednesday and Wednesday night as the trough aloft continues digging into the Mississippi Valley. There may be another round of thunderstorms capable of producing heavy rain Wednesday afternoon as what looks like an MCV generated from convection over the Plains Tuesday night phases with the trough right along the front. It`s unclear at this point how likely this is to happen as the GFS seems to be contaminated with grid-scale convective feedback, but if it does occur it would certainly exacerbate any flooding from the previous night. Cool Canadian high pressure will build into the region behind the front on Thursday. Ensemble cluster analysis shows very good agreement with the pattern shift and position of the trough over the eastern 1/2 of the CONUS for Thursday through next Sunday...although there are some differences in the strength of the trough by the end of the period. This lends good confidence to the forecast of below normal temperatures for Thursday through Sunday. Carney && .AVIATION... (For the 18z TAFs through 18z Monday Afternoon) Issued at 1232 PM CDT Sun Jul 14 2024 VFR flight conditions and southwest flow are expected to prevail through Monday afternoon. There is a very small (<15%) chance that the tail end of a complex of thunderstorms, which is expected to move across parts of northern Illinois, may make as far south as west central and south central Illinois after 06Z. If this actually does occur, the storms should be weakening and are not expected to impact the majority of the area. Carney && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory until 8 PM CDT Monday for Audrain MO-Boone MO- Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO- Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO- Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO- Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO- Shelby MO-Warren MO-Washington MO. IL...Heat Advisory until 8 PM CDT Monday for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL- Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL- Pike IL-Randolph IL-Saint Clair IL-Washington IL. && $$ WFO LSX