448 FXUS63 KDMX 072103 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 303 PM CST Sat Dec 7 2019 .SHORT TERM.../Tonight through Sunday Night/ Issued at 303 PM CST Sat Dec 7 2019 Confidence: Medium to High Few major concerns through at least Sunday night. Warm air advection continues to stream into Iowa today with cirrus moving in from the west overspreading the area this afternoon. Tonight the approaching trough will continue to near Iowa with a thermal ridge advancing over the area. This will keep overnight lows quite mild with some additional clouds expected. Sunday will see an increase in thicker/lower clouds especially in the afternoon over the northwest as the trough nears and in the southeast as another weak system drifts north toward the region. Despite the cloud cover tomorrow, southwest flow at the surface and aloft should allow for at least 10 degrees of recovery into the afternoon over most of the region and will maintain highs in the upper 40s/lower 50s south with mid to upper 30s likely in the north. There is a trend this morning for some delay in onset of precipitation in the NAM and short term models due to a slower arrival of the main Arctic cold front over northern Iowa to just about 12z Monday. This may have a slightly greater impact on overnight low temperatures into Monday morning. The current forecast is near the lower end of the spread of guidance at around the 25th percentile of the output. The latest guidance is warmer overall and even at the 50th percentile values, overnight mins should be some 3 to 4 degrees warmer. With some adjustments in timing of precipitation onset, the main area of light snow is being slowed by about 3 to 4 hours northwest and then progresses southeast through 12z as prior forecast. .LONG TERM.../Monday through Saturday/ Issued at 303 PM CST Sat Dec 7 2019 /Monday through Tuesday Night/... Confidence: Medium As the boundary moves southeast during the day Monday, a general light rain or light snow event is expected with a small chance of a light freezing mix along the transition zone Monday morning which should not cause any significant impacts as soundings continue to show the column quickly saturating. The balance of the precipitation will end by late afternoon. Snow totals continue to be light and generally similar to the previous forecast with around a half inch in the far north and little to any accumulation south of Highway 20. One concern in the current set of HiRes models and the Euro is a secondary band of snow developing and moving along the Arctic front from 14 to about 19z over west Central to Central Iowa in the Highway 20 to I80 corridor, then weakening quickly in the early afternoon. This appears tied to stronger mechanical forcing aloft and also the main frontal boundary aloft between H850 and H700. Though the present risk appears large enough to mention, but with some time to evaluate the chances, the risk is not enough to add any additional snow to the forecast. If the area of snow did develop, it could add about an inch across western to central Iowa during the morning hours Monday. As previously mentioned, the onset of the cold air is likely beginning around 12z in the northwest. Though a minor delay, the impact will be greater on highs Monday as little recovery is anticipated over much of the area once the boundary settles south. Winds will pick up along and behind the boundary during the late morning and afternoon hours though values of 15 to 25 mph are at most expected. Once the first boundary moves south of the area, a secondary boundary will arrive by 12z Tuesday with a stronger push of cold air and scattered flurries. H850 temperatures quickly drop to 15 to 18 below from 18 to 00z Wednesday. Model spread in highs have actually increased for Tuesday. Currently our high is near the mean, though given the uncertainty temperatures might be colder. Temperatures Tuesday night will be quite cold in the single digits above and below zero over the area. /Wednesday through Saturday/... Confidence: Low to Medium The remainder of the week shows more uncertainty as the models begin to diverge in both strength and placement of systems. Wednesday warm air advection returns to the region by afternoon with both the GFS/Euro indicating some lift along a warm front and the potential for a light precipitation event. Beyond Wednesday, the GFS is stronger and faster with a northern stream wave by Thursday with subsequent energy arriving Friday night and again late Saturday. The Euro holds onto the Great Lakes ridge into later Thursday with a weaker northern stream wave Thursday. The next significant system arrives on Saturday. Overall, the general trends in both models that are at least consistent from yesterday include overall milder temperatures and at least one or two chances for precipitation. Highs are still expected to be in the 20s to 30s north and in the 30s to 40s south where any precipitation that falls will be warm enough for either a mix or all rain. && .AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/ Issued at 1136 AM CST Sat Dec 7 2019 Main concern through the period will be increase in MVFR cigs. Approaching weak system southeast and return of boundary layer moisture will increase risk of MVFR cigs south aft 16z Sunday far southeast. In the far north, an increase in MVFR cigs associated with the approaching trough/wind shift will also aft 17z Sunday. Otherwise LLWS increases aft 23z and continues through 10-11z as stronger southwest winds arrive. Main widespread challenges with cigs will arrive aft 18z. /rev && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...REV LONG TERM...REV AVIATION...REV