804 FXUS63 KDMX 180431 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1131 PM CDT Tue Aug 17 2021 .DISCUSSION.../Tonight through Tuesday/ Issued at 359 PM CDT Tue Aug 17 2021 Forecast Highlights: -- Warm, humid through Thursday. -- Smoke aloft Wednesday in Northwest and Northern IA. -- Thunderstorm chances return later in the week, severe storms possible Friday. TODAY THROUGH WEDNESDAY: A few clouds in central to eastern Iowa along with the return of wildfire smoke to northwest Iowa can be seen on GOES-16 Satellite this afternoon. With a weak mid-level ridge in place and the remnants of the upper-level trough pushing off to the east through the day today, mainly quiet weather continues. Moisture return this afternoon aided by southerly low level flow has caused dew points to surge into the 70s in western Iowa with mid to upper 60s dew points elsewhere as of 2pm. The return of the humidity has pushed heat indices back into the 90s in western Iowa as of 2pm. High temperatures this afternoon are expected to climb into the mid to upper 80s with heat indices topping out in the upper 80s to low 90s across the area. With a thermal ridge continuing to build off to the west and moving eastward into tomorrow and Thursday, a stronger push of theta-e advection will raise temperatures a few degrees on Wednesday with high temperatures near 90 and heat indices topping out in the low to mid 90s on Wednesday afternoon. Breezy southerly winds are expected on Wednesday afternoon as the pressure gradient tightens with 10 to 20 knot wind gusts becoming common. Some scattered convection could still be possible on Wednesday afternoon, mainly associated with peak heating and in southern Iowa, but any storm chances look to be isolated at best. The formation of diurnal cumulus is likely with with the warm and humid airmass in place and an unstable boundary layer, but forecast soundings in southern Iowa remain mostly dry at the surface and mid levels so any updrafts that do form will be difficult to maintain. With all these caveats and scattered/isolated nature of any possible convection, kept forecast dry for Wednesday. The return of smoke from western wildfires today in northwest Iowa will continue into Wednesday with the higher concentrations aloft moving somewhat east overnight into tomorrow impacting northwest into northern Iowa through the day on Wednesday and with smoke aloft making its way towards central Iowa overnight into Thursday, as of the latest HRRR model output. Added additional sky cover to account for smoke aloft Thursday in the sky grids, but no visibility impacts expected as of current model trends. THURSDAY AND BEYOND: The mid-level trough that will bring our next system pushes east from the the Inter-Mountain West Thursday into Friday. An associated surface cold front will be move through the area on Friday into Saturday with precipitation chances ramping up late Thursday into Friday morning ahead of the aforementioned cold front. Although scattered showers and a few storms are possible throughout the day Friday, the best chances for storms looks to be Friday afternoon into early Saturday morning. Some strong to severe storms are possible with most models suggesting the trough will take on a negative tilt allowing for the surface cyclone to deepen as it approaches the area. A ribbon of high instability will be ahead of the cold front though deep layer shear may be a lacking ingredient. 0-6km shear away from the boundary is 20-25 knots but higher shear values could be along, but more likely behind the boundary. The SPC has introduced a 15% probability of severe storms over much of the northwest half of the state, so we will continue to monitor these trends over the coming days to further evaluate the severe weather potential. In terms of temperatures, Thursday will be similar to Wednesday with the warm and humid conditions continuing. High temperatures will once again be near 90 with heat indices climbing a degree or two warmer across the area, but again in the low to mid 90s. The precipitation/storms and cloud cover on Friday will provide some temperature relief dropping temperatures back into the 80s and the cold front passage helping to drop temperatures a bit more on Saturday with locations in northwest Iowa dropping back into the upper 70s with low to mid 80s elsewhere. There are additional chances for precipitation late Sunday into early next week, but details will continue to be ironed out in the coming days. && .AVIATION.../For the 06Z TAFS through 06Z Wednesday night/ Issued at 1131 PM CDT Tue Aug 17 2021 Quiet aviation weather is expected for much of the forecast with generally widespread VFR conditions. Still some threat of patchy low lying fog during the early morning hours with light winds overnight. South to southeast surface flow will persist for the duration. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ DISCUSSION...KCM AVIATION...Cogil