Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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616 FXUS63 KLSX 130837 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 337 AM CDT Sat Jul 13 2024 .KEY MESSAGES... - Isolated to widely-scattered thunderstorms will once again threaten gusty winds and locally-heavy rain in parts of the region today, but drier conditions take hold Sunday through early Tuesday. - Heat and humidity ramp up Sunday through Tuesday, when heat index values will range from 100-105 degrees. Monday has the highest potential for widespread 105+ degree heat index values. - A slow-moving cold front will usher in renewed rain chances and cooler air starting late Tuesday. While below-normal temperatures stick around to start the weekend, rain chances shift south by Friday. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 335 AM CDT Sat Jul 13 2024 On the eastern periphery of a strong upper-level ridge centered over the Four Corners region, mid-level northwest flow and subtle shortwave impulses continue to stream from the northern Great Plains into the Mid-Mississippi Valley. Amidst this weak diffluence, a broad area of surface low pressure exists in the north-central CONUS. Isolated convection currently firing along a weak surface boundary on the nose of a 40-50kt low-level jet will translate north and east through the day today across the forecast area when the boundary lifts north as a warm front. With heights aloft increasing coincident with the upper-level ridge expanding eastward, synoptic- scale subsidence will keep the coverage and intensity of convection in check. That said, amidst as much as 2000J/kg+ of MUCAPE and precipitable water values over 2.00 inches (not unusual for mid- July), gusty winds and heavy rain would accompany any convection. With the warm front just about clearing the forecast area to the north by sunset, further to our north an area of severe convection is expected to develop and grow into an MCS as it dives southeast through the Upper Mississippi Valley. The nose of the 850mb low- level jet is still forecast to be north and east of the region which, combined with the strengthening mid-level ridge, bodes well for us remaining largely-removed from impactful thunderstorms. There are persistent signals of a composite outflow dropping south during the early morning Sunday and reaching the northern forecast area after dawn. While the chance for thunderstorms to fire along it on Sunday looks low (less than 15% chance), there will more likely than not be some convective debris lingering for part of the morning into the early afternoon. This debris will exist where 850mb temperatures are warmest (22-24 degrees Celsius according to most guidance), which would act to suppress warming Sunday and keep heat index values from rising to more dangerous values. Further south, while temperatures look less impacted by nocturnal convection remnants, mixing may be more efficient with a southwest wind off the Ozarks. This will mute humidity during peak heating, throwing exact heat index values into some degree of doubt in that part of the region. The combined heat and humidity will send heat index values into the low 100s for most, with perhaps the very centers of the St. Louis and Columbia metropolitan areas touching 105 degrees. If this scenario verifies, it falls short of our criteria for a Heat Advisory at this time. It is worth noting that if the outflow boundary that most of this uncertainty hinges on does not impact warming Sunday morning, temperatures and humidity will be sufficient for more widespread dangerous heat. MRB && .LONG TERM... (Sunday Night through Friday) Issued at 335 AM CDT Sat Jul 13 2024 Heat concerns remain Monday and Tuesday as the upper level ridge continues to expand east. Current guidance shows high confidence (90%) that 850mb temperatures will reach at least 21 degrees Celsius with surface dew points near 70 degrees Fahrenheit. These conditions lead to heat index values at or above 105 degrees Monday along and north of I-70 and 100 degrees Tuesday south of I-70. However, certainty exists on Tuesday as the timing of an approaching cold front and cloud cover associated with convection along the front will impact temperatures and thus heat index values. Nonetheless, heat related illness is a concern for Monday and potentially Tuesday if the front`s timing is slower. The dangerous heat won`t stick around too long, as an amplifying upper level trough aloft supporting a low pressure center near Hudson Bay helps establish a cold front in the Upper Midwest on Tuesday. Current guidance suggests that the cold front will enter northern Missouri late Tuesday afternoon into the evening. With this front, convection is expected to form and grow upscale Tuesday afternoon through Wednesday morning along the boundary. As the front pushes south, northerly winds and increased cloud cover will cause temperatures to fall into the mid-80s Wednesday and Thursday. Various rippling shortwaves within the large scale trough are expected to help push the cold front through the CWA by Thursday afternoon. Additional thunderstorm chances (20-30%) exist Wednesday and Thursday with additional shortwaves and vorticity maxima overhead. Behind the front, precipitation chances decrease to below 10% as the upper level forcing continues east, and temperatures are expected to remain in the mid-80s Friday and Saturday. Scott/MRB && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1043 PM CDT Fri Jul 12 2024 The chance for isolated showers and thunderstorms returns early this morning in central and northeast Missouri as a 40-50 kt southwesterly LLJ ramps up in western Missouri. Confidence is highest in showers and thunderstorms passing over the Mid-Missouri terminals after sunrise, though whether they sustain themselves long enough to get there or even travel that far east is still uncertain. The slight chance for thunderstorms will continue through the afternoon, but confidence in exact timing and location of any thunderstorm is too low to mention in the TAFs at this time. Otherwise, largely light winds will increase from the southwest in the morning and produce wind gusts up to 20 kts in central Missouri through west-central Illinois. Jaja && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX