338 FXUS63 KDMX 182320 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 520 PM CST Fri Dec 18 2020 ...Updated Aviation Discussion... .DISCUSSION.../Tonight through Friday/ Issued at 304 PM CST Fri Dec 18 2020 Today and Tonight; Ridge axis quickly raced eastward across the state this afternoon with low-level flow turning southerly underneath. This advected warmer air into the region, and temperatures reach the lower to mid 40s across the southern forecast area. Stratiform cloud cover gradually broke apart throughout the day, with the greatest clearing in western and southwestern Iowa. As a result, peak day time heating with clear skies gave temperatures a really good boost across this area. The source of the ridge's fast propagation is an H5 trough trailing immediately behind it, with the axis just entering the High Plains early this afternoon. It appears that this deeper trough will develop a northern axis and southern axis, with the north axis moving faster starting this evening. Lee cyclogenesis has been ongoing, and surface cyclone is currently moving across the central Plains. With the jet streak rounding the base, cyclone should remain in phase and continue to deepen as the northern short-wave trough propagates eastward. For the next few hours, this will send a weak warm front across the state. Isentropic lift will slowly reintroduce cloud cover this evening and overnight. Cold front will begin to push through late this evening, and will be the focus for weak convergence which may bring some light precipitation to areas mainly north of Interstate 80. Model soundings indicate decent saturation near the surface, and a brief period of ice introduction. However, the thermal profile changes rather rapidly with the advancement and passage of the cold front. Therefore, will be looking at a wintry mix of rain, snow, and maybe some freezing rain or freezing drizzle through this evening. QPF though is extremely light, and therefore little to no accumulation is expected, and impacts will be minor. A few elevated surfaces including bridges may become slick, but should not be an overly widespread issue. The biggest uncertainty this evening is fog potential. There is certainly enough moisture to support drizzle and fog development this evening. The main factor will be how long the clear skies remain. If skies remain clear past sunset for any particular area, radiational cooling will quickly kick in, allowing the atmosphere to saturate and fog in. Where cloud cover quickly redevelops, temperatures should hold steady enough to hold back fog, but still may see some minor reductions in visibility if there is enough forcing for drizzle. For right now, eastern portions of the forecast area would look most favorable for fog, as this is where skies are mostly likely to remain clear past sunset. However, tonight's fog potential will require close monitoring of satellite trends. Saturday through Monday: Flow will be predominantly northwesterly for much of this weekend, with several H5 perturbations eventually leading to larger trough pattern for Midwest and Great Lakes Regions. As a result, temperatures will once again lean toward the cooler side. Sunday Night into Monday, vort max moves through northern and northeastern Iowa, which may add some light precipitation to the far northeast. However, the better forcing will reside in eastern Minnesota and western Wisconsin with this feature, and thus, will maintain a dry forecast our counties at this time. Conditions could become quite breezy though when a stronger pressure gradient develops in association with the vort max. Extended: Broad ridge develops late Monday into Tuesday across the central Plains and upper Midwest. This will send temperatures back into the mid 40s for much of the forecast area, and will maintain dry conditions. There is strong short-wave trough though that will follow behind this ridge, and is set to move across the forecast area Wednesday through Thursday. While there will be plenty of forcing associated with this feature, model consensus is not great Wedensday. 12z deterministic GFS is dry, as well as GEFS members. Meanwhile, 12z deterministic ECMWF paints most of the forecast area with at least a tenth of an inch of QPF. NBM starting guidance was dry for this event, but have coordinated with neighboring offices to add in slight chances Wednesday for snow. But overall, uncertainty remains very high for the middle of next week. Bigger story may end just being wind gusts between 35 and 40 MPH. && .AVIATION.../For the 00Z TAFS through 00Z Saturday evening/ Issued at 520 PM CST Fri Dec 18 2020 VFR conditions currently in place across central Iowa. Low cigs return as a stratus deck slides in from the northwest late this evening or overnight. Expect MVFR cigs, possibly dipping into IFR at times. The deck should be fairly progressive and gradually move out from west to east late Saturday morning or afternoon. Low cigs may linger around until closer to 22z or 23z at KALO and KOTM. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION...Krull AVIATION...Martin