Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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772 FXUS63 KLSX 290833 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 333 AM CDT Sat Jun 29 2024 .KEY MESSAGES... - Temperatures in the low to mid-90s will combine with dewpoints well into the 70s to result in heat index values near 105 degrees over sections of southeast Missouri. A heat advisory will go into effect from 12 p.m. to 7 p.m. today. - Isolated to widely scattered thunderstorms this afternoon into this evening could be strong to severe, mainly along and south of I-70. The primary threats are damaging winds and large hail. - Below normal temperatures and low dewpoints Sunday and Monday will result in pleasant conditions. The active, warm pattern returns from mid to late week. && .SHORT TERM... (Through Late Sunday Afternoon) Issued at 332 AM CDT Sat Jun 29 2024 Water vapor imagery shows a broad mass of cloud cover remnant of a decayed MCS that originated over eastern Kansas and tracked into western Missouri overnight. The decay of this complex was a result of rapidly decreasing MUCAPE from west to east with a notable void in theta-e over sections of central and eastern Missouri. Interestingly, MUCAPE increased east of the Mississippi River, where a few showers and thunderstorms occasionally, and briefly, cropped up on radar. Support has been rather limited, which was evident of the short-lived updrafts earlier this morning. What remains now is an elongated, northeast to southwest line of showers with a few lightning strikes in the limited convective support. This activity is expected to continue to erode with eastward travel this morning. The main impact of this initial wave will be the cloud debris that lingers through this morning, which may attempt to break from late morning to early afternoon. The focus then shifts to the northwest with the approach of a slow moving cold front that, as of 08z, extend north to south through central Iowa into eastern Kansas. This front will slowly sink south through the day, encountering dewpoints well into the 70s. HREF ensembles show mean SBCAPE values recovering back to around 2500 J/kg along and ahead of the front late this afternoon into this evening, while some raw guidance suggests these values could be in the 3000-3500 J/kg range. The limiting factors may be the lack of strong, deep ascent with much of the vorticity progs fragmented in nature and largely distributed through the mid- levels. Additionally, 0-6km shear values are modest at 20-30 knots. A weak surface inflection ridge along the boundary late this afternoon into this evening, likely after 22z, setting the stage for isolated to widely scattered showers and thunderstorms, mainly along and south of I-70. Modeled sounding profiles show 6C to perhaps locally 7.5C mid-level lapse rates with respectable CAPE profiles that could utilize afternoon instability for isolated large hail threat. The moisture rich environment, however, show little sign of a dry layer aloft to suggest wind will be a big threat, though the collapse of any strong updrafts could result in a localized 60 mph gust. This also leads to another item to address - the heat advisory over the far southern sections of the CWA. Given the slow progress of the front, there may be enough time for at least some sunshine. The moisture is obviously not an issue with the uncomfortably high dewpoints. Moisture pooling ahead of the front, combined with temperatures in the low to mid-90s will lead to heat index value near 105 in these areas. Therefore, the heat advisory will continue as advertised. The cold front will sink south late tonight into early Sunday. Surface high pressure, which strengthens to near NAEFS climatological maxes, will build into the region for the later half of the weekend. This will bring refreshingly cooler temperatures and lower dewpoints with highs that are 10-15 degrees below normal. Maples && .LONG TERM... (Sunday Night through Friday) Issued at 332 AM CDT Sat Jun 29 2024 Upper ridging begins to build in from the PLains as the anomalously strong surface high centers over the Great Lakes Monday. Easterly surface flow continues to funnel dry air into the region with HREF ensembles showing 60-90% probabilities for dewpoints at or below 50 degrees from eastern Missouri into Illinois. The dry airmass and highs in the upper 70s to low-80s will make a spectacular start to the work week. Surface flow turns out of the south from west to east late Monday into Tuesday as the surface high moves into the New England Region. Warm air advection will mark the end of the more pleasant conditions with a return to the 90s Tuesday. Despite and evident and stark increase in mid-level moisture, the rise in surface dewpoints will be more gradual as mid to upper 50s hang on through the early half of the day. 70 degree dewpoints edge into central and northeast Missouri by early afternoon, while southeast Missouri and Illinois locations remain in the low to mid-60s. Though gradually more moist, conditions will be more bearable over eastern sections of the CWA. The trend continues into Wednesday, when dewpoints in the 70s encompass the entire area. Each afternoon, where air temperatures line up in the 90s and dewpoints reach into the 70s, heat index value could approach advisory levels, barring precipitation chances increase heading into Wednesday. From mid to late week, the pattern begins to look similar to that we are experiencing late this week. The western periphery of the surface to mid-level ridge takes on a southwest to northeast lean. An upper ridge that extends into the Great Lake begins to flatten with the approach of a broad upper trough. As the upper trough closes in, a surface cold front slides underneath mid and upper shortwaves that introduce potential for showers and thunderstorms as soon as late Tuesday and possibly through the end of the work week. 500mb height clusters do show decent agreement in the mean pattern through day 5 with little spread through day 7. I believe the questions will be in the surface to mid-level synoptic layout with regard to the timing and position of the surface front, along with the finer details in the shortwave features. Where and when these features line up could provide a good chance for rainfall with LREF ensembles showing 60-70% probabilities for a return to anomalous PWAT values at or above 2 inches. Maples && .AVIATION... (For the 06z TAFs through 06z Saturday Night) Issued at 1025 PM CDT Fri Jun 28 2024 Main cluster of thunderstorms near the Missouri-Kansas border is expected to move east/southeast and weaken with time. The central Missouri terminals look to be on the (weaker) far northern end. Have therefore removed the previous TEMPO for TSRA and IFR visibilities at KCOU and KJEF. Further north, convective coverage looks less than previously anticipated and also removed TEMPO at KUIN. Maintained some VCTS wording at the metro terminals, but confidence in convection that far east has lessened compared to the previous TAF package. No significant changes thereafter. The cold front is forecast to move through the area from northwest to southeast on Saturday. Thunderstorms are forecast to redevelop along the front by mid-late afternoon and continue through the evening hours. The best chances of showers and storms are in parts of southeast Missouri and southwest Illinois, with metro St. Louis terminals on the northern edge. Gosselin && .LSX WATCHES/WARNINGS/ADVISORIES... MO...Heat Advisory from noon today to 7 PM CDT this evening for Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve MO-Washington MO. IL...None. && $$ WFO LSX