995 FXUS63 KLSX 011940 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 240 PM CDT Sat Apr 1 2023 .SHORT TERM... (Through Sunday Night) Issued at 240 PM CDT Sat Apr 1 2023 Clouds are departing to the east as the last gasps of our departing low finally exits. It's been quite windy today, especially this morning, as cold advection drove good mixing within the flow on the back side of the low. We've lost the cold advection, though, and we're rapidly losing the pressure gradient, so winds will correspondingly rapidly decrease. In fact, it's likely we go fully calm at least for a time tonight under a clear sky. This should set up good radiational cooling conditions with temperatures falling off into the 30s. The typical cold spots will probably drop below freezing. By morning, though, winds will begin shifting to the south and we'll warm back up again. We'll even get a bit above normal on Sunday afternoon with highs in the upper 60s and low 70s. A cold front drops south into the northern part of our forecast area Sunday night. The northern stream wave that is driving it is pretty low amplitude and the front will lose its push southward as it arrives. In fact, a wave in the southern stream will begin to pull moisture northward and we may see some light showers as early as Sunday night along and behind the front. The primary focus with the southern wave is well to the south across Texas, Arkansas, and Louisiana, but an increasingly moist low level flow into the stalling frontal boundary is what will trigger our rain chances. Kimble .LONG TERM... (Monday through Next Saturday) Issued at 240 PM CDT Sat Apr 1 2023 The next large scale trough is still offshore of the Pacific Northwest. It will dig into the western US on Monday, with a downstream ridge over the central US. The result will be an increasingly warm and moist southerly low level flow across the Plains ahead of a surface trough in the lee of the Rockies. This will shove the front back northward Monday night into Tuesday morning, with more showers or thunderstorms possible along and north of it with the increased low level flow into the boundary. There's some uncertainty on how long the front lingers, but there's a good chance that it moves northward and the focus for overnight showers and storms is further north of our forecast area Monday night into Tuesday morning. There's pretty good agreement that the upper trough will move out into the Plains on Tuesday, with surface low pressure pulling out of the lee of the Rockies northeastward into Iowa. To the south, a dryline spreads very far eastward, as far east as western Missouri. Meanwhile a cold front drops south behind the low, eventually catching up to the dryline in Missouri Tuesday night. Overall this is a very similar setup to what we just saw on Friday, with a few minor differences emerging at this stage. One is the timing as the cold front moves through 6 to 8 hours later than this past Friday's front, with the focus for thunderstorms in our area shifting later into Tuesday evening and overnight as well. We're also much more confident on the existence of strong instability ahead of this wave than with the last one. The moist flow northward to the east of the dryline will combine with temperatures warming into the 80s to produce surface based CAPE values of 1000 to 2000 or even higher. There's also a fairly pronounced cap across the warm sector east of the dryline that should help to prevent thunderstorms from forming earlier in the day, leaving more untapped instability available for storms which do eventually form. Convection is likely to initiate along the dryline in eastern Kansas or western Missouri where the cap is weakest Tuesday afternoon. The terrain of the Ozarks in southwest Missouri and northwest Arkansas could also be a favorable initiation point. In an environment with strong speed and directional shear as well as steering flow across the boundary, we'd expect supercells to be the primary storm mode with all the severe weather hazards that accompany them including large hail, damaging winds, and tornadoes. As the cold front arrives Tuesday night, storms may eventually grow into a broader line with the severe weather threat transitioning to damaging winds and embedded QLCS tornadoes. With a strong pressure system driving the action on Tuesday, we'll once again be dealing with the potential for strong non convective winds. The NBM continues to overestimate the winds and gusts in the warm sector where mixing is much more shallow beneath the cap. However, behind the dryline mixing will be superb, but this likely stays west of our area Tuesday afternoon. Once the cold front moves through Tuesday night into Wednesday morning we'll see better mixing in cold advection and likely the strongest wind gusts observed in our forecast area. Wind Advisory level gusts (40KT or higher) is most likely Wednesday morning mainly in the northern part of our area closer to the surface low. We'll cool back down behind the front Wednesday, with the cooler weather lasting for at least a few days. There's a lot more variability in the flow pattern late in the week and into next weekend, but the general idea of dry and cooler is likely to take us to the weekend. Kimble && .AVIATION... (For the 18z TAFs through 18z Sunday Afternoon) Issued at 1220 PM CDT Sat Apr 1 2023 An area of clouds, mostly VFR but some MVFR at Quincy, will pull away to the east pretty quickly this afternoon. Winds will also be on the decrease and actually go light and variable to calm tonight under a clear sky. Winds become southerly tomorrow morning. Kimble && .PRELIMINARY POINT TEMPS/POPS... Saint Louis 53 38 67 53 / 10 0 5 20 Quincy 50 34 66 49 / 10 0 5 30 Columbia 55 35 70 51 / 5 0 5 30 Jefferson City 56 34 71 51 / 5 0 5 30 Salem 50 35 64 52 / 5 0 0 20 Farmington 54 32 67 50 / 0 0 0 20 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX