429 FXUS63 KDMX 291950 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 250 PM CDT Wed Jul 29 2020 .DISCUSSION.../Tonight through Wednesday/ Issued at 250 PM CDT Wed Jul 29 2020 Synoptic Overview: Closed low over Hudson Bay continues to move eastward, and has now moved far enough to remove most of Iowa from the influence of cyclonic flow. A weak H5 ridge has developed across Iowa in the past 12 hours, sandwiched between the aforementioned closed low and another short-wave trough moving toward the southeast from the northern Plains. This short-wave will quickly become the focus for activity this afternoon and evening. At the surface, weak troughing extends from the central Plains into Central Iowa. A stationary boundary analyzed along U.S. Hwy. 20 at 12z this morning, and a warm front across northeast Missouri will work with the short-wave trough to generate some convection and numerous rain showers this afternoon. The formerly mentioned stationary boundary will likely have more of a role in the activity as it is forced by the weak surface high pressure over Minnesota. In the long term, looking at predominantly northwest flow which will keep conditions cooler for the next 7-10 days. Today and Tomorrow: Focus for today will be on the short-wave propagating southeastward from the northern Plains. Although weak, the northwesterly flow associated with this will also push the surface high pressure southward, forcing the stationary boundary to move southward through this afternoon. South of the boundary, the airmass is rather warm, and moist with dewpoint temperatures in the mid 60s. GOES-16 imagery has shown some cloud development on the warm side, but still plenty of breaks allowing for insolation to continue boundary layer destabilization. Most CAM guidance favors convection initiation after 18z this afternoon, and continues development through this evening. The bulk of the activity will be confined to areas mainly south of U.S. Highway 20 as the boundary moves southward. SBCAPE values on the warm side will be pushing 3500 J/kg around peak heating time. In this weak flow regime, deep layer shear is almost absent. However, model soundings are starting to hint at the potential for stronger downdrafts, some points reach DCAPE values between 1200-1400 J/kg. If boundary layer mixing intensifies throughout the afternoon, a favorable environment for severe winds could be realized. However, if the cloud shield currently over northern Missouri expands, mixing would be more limited especially across the south and minimizing the already minimal severe threat that this weak flow environment poses. For now, will keep just general thunderstorms in the forecast. Later this afternoon if CAM trends continue to point toward increasing severe potential, the forecast will be updated accordingly. Thursday, the short-wave will continue to propagate southeast out of the area, but expect the stationary boundary to lag behind it. This will continue to provide some weak convergence throughout Thursday, mainly for areas south of Interstate 80. This will more likely be stratiform shower activity, as most of the instability will be used for Wednesday afternoon's activity. One other minor concern with convection over the next 24- 36 hours would be the hydrological picture. With a slow moving boundary and slow storm motions, efficient thunderstorms will be capable of producing locally heavy rainfall at times. This will be very welcomed in the west-central portions of the forecast area in drought conditions. Overall, most of the area with greatest chances for precipitation will be able to handle moderate amounts of rainfall. 3-hour flash flood guidance currently sits at over 3 inches for most of the forecast area. QPF for an event total in most locations is below this, thus concern over flooding is minimal. Extended: By Saturday the short-wave trough axis digs into the lower Mississippi River Valley, and H5 ridge amplifies west of the Rockies. This will force Iowa into a predominately northwest flow regime that will continue into the middle of the next work week. A few short-waves will likely move across southern Canada, which will likely provide weak perturbations to move through the northwest flow. Expect spotty chances for rain showers and thunderstorms with this. 12z model guidance runs this morning are not showing much in the way of strong surface based forcing through the middle of the weak. With respect to temperatures, this pattern will supply a semi- steady stream of cooler Canadian air across the upper Midwest. In addition, will also be drier. It will be difficult for dewpoints to reach above the lower 60s. Overall, outdoor conditions will be quite comfortable if no precipitation is ongoing at a particular moment. Typically the month of August is a lot more humid and afternoons rather uncomfortable. But as the amplified ridging pattern remains in tact for an extended period of time across the western CONUS, this August will start off cooler across Iowa. && .AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/ Issued at 1232 PM CDT Wed Jul 29 2020 Stationary boundary will move southward this afternoon. Rain showers and thunderstorms are developing in extreme southern Iowa as of 18z. The coverage of storms will continue to expand through late this afternoon and evening. Expect rain showers to reach the DSM and OTM terminals. There may some impacts with vicinity thunderstorms at the northern terminals, but confidence not quite high enough at this point to place thunderstorms directly at MCW and ALO at this time. CIGs expected to remain at least MVFR even with rain showers and thunderstorms. Localized showers may have the ability to drop visibility to IFR for a few minutes, but would not expect those conditions to last long if they occur at all. Thus, will keep visibility at the terminals at MVFR in the 18z TAFs. Shower activity will continue for southern portions of the forecast area through Thursday early afternoon. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION...Krull AVIATION...Krull