387 FXUS63 KLSX 252336 AFDLSX Area Forecast Discussion...Updated Aviation National Weather Service Saint Louis MO 636 PM CDT Wed Mar 25 2020 .SHORT TERM... (Through Late Thursday Night) Issued at 315 PM CDT Wed Mar 25 2020 Early this afternoon, quasi-zonal flow aloft continued to prevail across much of the CONUS. However, a deep and strong storm system was located just offshore from California and the storm system that affected our area yesterday was now over the Mid-Atlantic states, defaulting to a weak RIDGE over the central CONUS. Surface high pressure had gone through our region last night and now extended from the Lower Ohio Valley through the Lower Mississippi Valley, resulting in a gradually strengthening S-SE flow over our region. A cold front was draped from western Kansas to near KMSP. The increasing low level WAA and sun angle has taken its toll on the thinning stratus deck that had overspread our region, quickly dissipating it and resulting in mostly sunny skies. Temperatures were rebounding this hour through the 50s and beginning to hit 60 in a few spots. The cold front to our northwest is expected to edge into northern MO and central IL late tonight and will result in increasing cloudiness for much of our region. The largely dry column in place over our region is not expected to moisten too much considering the lack of an open flow from the Gulf of Mexico and much in the way of upper support, but there is some evidence for a low chance for a shower near the front, and have placed slight chance PoPs late tonight for portions of northeast MO and central IL. Thanks to the increasing clouds and southerly surface flow, a mild night temperature-wise lies ahead with readings bottoming out in the upper 40s and lower 50s. The strong storm system out west is expected to continue to deepen as it moves over California on Thursday, resulting in a slight amplification of the upper RIDGE over us, and this will result in a flow aloft that will increasingly work to stall the surface frontal boundary moving through. By late Thursday afternoon, the surface cold front is expected to stall somewhere just north of Interstate 70. Low clouds should make their return to much of the area north of the front, and with increasing WAA aloft and a moistening column, bumped up PoPs for most of northern MO and central IL for what should be a period or two of scattered showers moving through during the day. South of the front, enough sunshine should be seen to result in a large temperature gradient in the frontal zone, where max temps may range from the low 60s in northern MO and central IL to the mid to upper 70s for areas to the south. The frontal boundary is anticipated to remain quasi-stationary on Thursday night, somewhere near the I-70 corridor, with a general increase in pcpn chances as upper level disturbances from the strong storm system out west begin to get ejected in our direction and the column they are acting upon is increasingly moist and unstable. Look for shower and thunderstorm chances to ramp up late Thursday night for most of our region. The SPC Day 2 Marginal Risk applies to this event, where elevated hailers approaching severe may be possible in this environment. Another mild night temp wise, with mins ranging from the upper 40s to around 50 north of the front to the mid 50s to around 60 south of the front. TES .LONG TERM... (Friday through Next Wednesday) Issued at 315 PM CDT Wed Mar 25 2020 The GEFS H500 and surface pressure means shows the strong storm system out west tracking through the Four Corners region on Friday and just north of our region on Saturday. This will be occurring while the quasi-stationary surface front pushes north as a warm front late Friday and Friday night, leaving our region in the warm sector for Saturday with a cold front moving through Saturday afternoon and evening. The convective breakdown for this period shows a preference for elevated convection continuing into Friday for much of northern MO and central IL, while the potential for more surface-based convection will begin to show its hand heading into Friday afternoon and possibly into the evening near the frontal boundary which will begin to move north by this time. The increasing prevalence of what should be surface based convection will underlie what will be a waxing severe severe storm threat heading into Friday afternoon and early evening. Moderately steep mid level lapse rates and strong bulk shear values overlaying mid 60s surface dewpoints will support large hail and damaging straight line wind gusts as the main threats. These severe threats could go fairly deep into Friday night as well. On Saturday, with the main upper support and frontal zone to our north, another round of thunderstorms looks expected, but coverage should be more scattered and likely focused more on the timing of the cold front from midday through early evening, with possibly another round of strong-severe with a similar environment in play with regards to mid level lapse rates and shear. The high expected coverage of rain on Friday should keep temps slightly cooler than persistence, but still much above normal with 60s and 70s. A higher confidence warm sector setup on Saturday should give most areas their warm day of the week, with readings in the 70s areawide, possibly making a run on 80F in a few areas. Not to be ignored, there is also good GEFS member clustering around one inch QPF for a widespread area for Friday through Saturday. As with any convective event, of which this should have plenty, there is also decent potential upside to the QPF for any localized training of convection, of which a quasi-stationary frontal boundary for the first half of this event presents the opportunity. This may result in a re-aggravation of minor flooding along area rivers, many of which already had a minor flood event recently. It looks like we will see a dry period for most of Sunday and Monday, as a weak upper RIDGE builds in with surface high pressure in control. Temperatures look to return to normal during this time. After Monday, uncertainty in any one solution ramps up considerably, with most components showing another system affecting our region but the timing is off by a lot and also what form the system will take. The forecast largely slides back towards a CLIMO posture for early next week, meaning max temps around 60 and PoP in the 23-30% range until this comes into better focus. TES && .AVIATION... (For the 00z TAFs through 00z Thursday Evening) Issued at 625 PM CDT Wed Mar 25 2020 KUIN: VFR flight conditions are expected to dominate much of tonight. There could be some light fog for a period near daybreak. Otherwise a cold front will slide through the area close to 12z bringing stratus and IFR to MVFR flight conditions, and perhaps a few showers in the afternoon. KCOU: VFR flight conditions are expected to dominate much of tonight. There could be some light fog for a period near daybreak. Otherwise a cold front will be approaching from the north and could result in a period of MVFR flight conditions/CIGS on Thursday morning as well a few showers. KSTL/KSUS/KCPS: VFR flight conditions are expected to dominate tonight into Thursday. I can't rule out a spotty light shower from late Thursday morning into the afternoon but right now the probability looks to low to mention in the TAFs. Glass && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX