Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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349 FXUS63 KLSX 190334 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 1034 PM CDT Sat May 18 2024 .KEY MESSAGES... - Seasonably-warm temperatures persist through Tuesday, with potential for some to see 90 degrees Sunday and Tuesday. - Other than the low (15-20%) threat for thunderstorms over parts of the region tomorrow, rain chances stay at bay until at least Monday. - An active pattern with multiple thunderstorm chances is a near- certainty for next week. The ingredients for severe thunderstorms are most likely to converge over our region on Tuesday, but another threat Wednesday will depend on the speed of a cold front. && .SHORT TERM... (Through Late Sunday Night) Issued at 219 PM CDT Sat May 18 2024 Clear skies gave way to diurnal cumulus development earlier this afternoon amidst weak southerly surface winds and warm temperatures. A cold front is slowly approaching the region from the northwest attending a surface low tracking across south-central Canada. Directly along it, brief and meager convection from earlier this morning is no longer evident. A ridge aloft is gradually impinging on the region from the southwest and working with the weak surface convergence to inhibit much convection. As the front approaches and clips northeast Missouri and west-central Illinois, there is a very slim (less than 15%) chance for a thunderstorm early this evening. However, with synoptic-scale subsidence in place, anything that manages to develop would quickly collapse. The front itself won`t make much progress into the region overnight, mostly due to the surface low associated with it being so far north. Ahead of it, the weak winds and generally- clear skies will allow for another round of efficient radiational cooling. Despite mixing today, low-level moisture will still be in relative abundance from recent rain and dewpoints near the 75th climatological percentile. This will lead to another threat for fog overnight across most of the region (save for areas along and behind the front). Patchy dense fog will threaten river valleys from roughly midnight to about 8AM Sunday, by which point surface heating will cause the fog to dissipate. The weak boundary lingers across northern Missouri and west-central Illinois during the day Sunday, but temperatures on both sides of it jump into the mid/upper 80s. A few spots, St. Louis and Columbia in particular, will make a run at 90 degrees thanks in part to the impact of the urban heat island. As instability builds through the morning and early afternoon, with SBCAPE values forecast to exceed 2000 J/kg, the front has potential to serve as a focus for isolated thunderstorms. That said, mid-level subsidence will still be strong and work with meager shear to keep any convection in check. If a storm can manage to deepen as a weak shortwave approaches, it would threaten a marginally-severe hail threat in northern Missouri. This is an outside potential at this time, so we are not messaging a severe thunderstorm threat. The thunderstorm chances diminish with sunset and loss of surface- based instability. Meanwhile, flow aloft begins to amplify and southerly surface winds strengthen. Temperatures overnight Sunday will stay warm for mid-May, and further promote seasonably-warm temperatures into Monday. MRB && .SHORT TERM... (Through Late Sunday Night) Issued at 219 PM CDT Sat May 18 2024 Clear skies gave way to diurnal cumulus development earlier this afternoon amidst weak southerly surface winds and warm temperatures. A cold front is slowly approaching the region from the northwest attending a surface low tracking across south-central Canada. Directly along it, brief and meager convection from earlier this morning is no longer evident. A ridge aloft is gradually impinging on the region from the southwest and working with the weak surface convergence to inhibit much convection. As the front approaches and clips northeast Missouri and west-central Illinois, there is a very slim (less than 15%) chance for a thunderstorm early this evening. However, with synoptic-scale subsidence in place, anything that manages to develop would quickly collapse. The front itself won`t make much progress into the region overnight, mostly due to the surface low associated with it being so far north. Ahead of it, the weak winds and generally- clear skies will allow for another round of efficient radiational cooling. Despite mixing today, low-level moisture will still be in relative abundance from recent rain and dewpoints near the 75th climatological percentile. This will lead to another threat for fog overnight across most of the region (save for areas along and behind the front). Patchy dense fog will threaten river valleys from roughly midnight to about 8AM Sunday, by which point surface heating will cause the fog to dissipate. The weak boundary lingers across northern Missouri and west-central Illinois during the day Sunday, but temperatures on both sides of it jump into the mid/upper 80s. A few spots, St. Louis and Columbia in particular, will make a run at 90 degrees thanks in part to the impact of the urban heat island. As instability builds through the morning and early afternoon, with SBCAPE values forecast to exceed 2000 J/kg, the front has potential to serve as a focus for isolated thunderstorms. That said, mid-level subsidence will still be strong and work with meager shear to keep any convection in check. If a storm can manage to deepen as a weak shortwave approaches, it would threaten a marginally-severe hail threat in northern Missouri. This is an outside potential at this time, so we are not messaging a severe thunderstorm threat. The thunderstorm chances diminish with sunset and loss of surface- based instability. Meanwhile, flow aloft begins to amplify and southerly surface winds strengthen. Temperatures overnight Sunday will stay warm for mid-May, and further promote seasonably-warm temperatures into Monday. MRB && .LONG TERM... (Monday through Next Saturday) Issued at 219 PM CDT Sat May 18 2024 As we enter the next work week, southwest flow aloft will return to the region. After a week-long respite from widespread active weather in the region, this all-too-familiar setup will bring a renewed threat for severe thunderstorms. On Monday, there are some indications that a decaying convective complex from the central Plains will develop an MCV that tracks across the northern forecast area during the morning. If this occurs, and if it can tap into instability during the late morning, additional thunderstorms are possible as it passes through. Regardless of the MCV, a warm front across the Missouri/Iowa border will be a focus for scattered thunderstorms in the afternoon. South of the front, temperatures will remain at least seasonably-warm, climbing to the mid/upper 80s areawide. Depending on how unstable the air is and whether upper-level support will exist for deep convection, some marginal threat for severe weather would exist. We aren`t currently messaging the threat due to the low confidence in its occurrence and potential intensity. The warm air advection becomes much more pronounced on Tuesday amidst deep southwest flow over the Great Plains and ahead of a surface low ejecting from the Rockies into the Upper Mississippi Valley. As a result, warm air advection will intensify and send 850mb/925mb temperatures to the 90th climatological percentile or higher. This will bolster temperatures into the upper 80s to low 90s, as we`ll have no issues mixing to those levels, but the bigger story will be the threat for severe thunderstorms during the afternoon and evening. A cold front and upper-level shortwave (with some favorable, albeit weak, jet dynamics) approach the region through the afternoon and early evening, with copious instability available in the open warm sector. There are still slight timing differences in the overall system speed that linger, but they make little difference in the grand scheme of this threat. Particular attention will be needed to the northern half of the forecast area, where confidence is higher in the sufficient shear, unstable profiles, and stronger upper-level ascent needed for severe thunderstorms. Like many of our events this year, convective initiation looks to begin along a pre-frontal convergence zone during the afternoon. Deep-layer shear orientation looks sufficiently perpendicular to the initiating boundary to support discrete supercells at first, growing upscale over time into a line as those shear vectors back out of the south. While a tornado threat certainly will exist with any discrete thunderstorms, model hodographs are not particularly noteworthy to suggest it being the main concern. Strong low/mid- level lapse rates and vigorous instability will lead to a very real hail/wind threat initially. As the thunderstorms grow upscale, a wind and QLCS tornado threat becomes predominant with 30- 45kts of 0-3km shear in place. The threat won`t completely end until whatever complex that forms exits the forecast area, mainly because the effective cold front will still be lagging back to the northwest. Again depending on the speed of the front, we may be dealing with another round of strong to severe thunderstorms on Wednesday as well, primarily along and south of the I-44 (in Missouri) and I-64 (in Illinois) corridor. At this stage, however, there are appreciable differences among ensemble guidance regarding available instability, front timing, and upper-level ascent. That said, what happens on Tuesday in conjunction with how the synoptic system evolves will play key roles in the severe weather threat Wednesday. There looks to be a brief lull in precipitation chances late Wednesday into Thursday behind the front, which will also cool our temperatures closer to normal. Before too long, another wave amplifies and approaches the forecast area and leads to more thunderstorm chances Friday into the weekend. MRB && .AVIATION... (For the 06z TAFs through 06z Sunday Night) Issued at 1020 PM CDT Sat May 18 2024 VFR conditions are expected throughout the 06Z TAF period for most terminals, with the possible exception of fog overnight tonight through early tomorrow morning. Patchy fog in low lying areas and river valleys may once again impact typical fog prone terminals like SUS and CPS with visibility reductions to MVFR. Brief periods of lower visibility are also possible, even to LIFR, but confidence is too low to include in the TAF at this time. Otherwise, a limited potential for thunderstorms will exist at UIN tomorrow afternoon and evening. A brief period of VCTS was added to the TAF to reflect this, but confidence in direct impacts to the terminal remain low. BRC && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX