367 FXUS63 KDMX 211729 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1129 AM CST Tue Feb 21 2017 ...Updated for 18z Aviation Discussion... .SHORT TERM.../Today through Tonight/ Issued at 348 AM CST Tue Feb 21 2017 Confidence: Medium to High. Main concerns today will be the extent of morning fog over the northeast/west and then recovery in temperature for the afternoon hours. Trough continues to pull east of the area early today with lingering clouds across the east in the boundary zone. Boundary layer moisture remains high with light winds and clear skies over much of the north/northwest/west. Temperatures have been hovering near the dewpoints and expect that near sunrise patchy fog now will become more widespread...especially over the north and west. A weak ridge of high pressure will move over the area early today...resulting in light winds and little mixing during the morning hours. HIRES models break out fog across the northeast and west to a lesser extent. Some uncertainty as to how widespread it will be during morning commute...will most likely first apply an SPS to the areas of concern and then headlines if needed. Models in good agreement to have much warmer H850 temperatures pushing northeast into western/southern areas by 00z tonight. Though afternoon mixing is by no means significant...mixing should increase boundary layer gusts to 15 to 20 mph at times across central and western areas from 18-22z this afternoon...enough to warm the area back to near record highs this afternoon. This afternoon highs should reach about 60 in the far north to the lower 70s over the southwest/south during the afternoon. By tonight another trough will approach from the northwest...but overnight H850 temperatures will actually increase over the region ahead of the trough. This warming aloft should keep fog from developing to any appreciable extent and also keep overnight lows mild...though the far north may dip to just below 40 after slightly cooler highs this afternoon. The south once again should remain in the mid 40s overnight. There may be some high clouds moving over the region tonight but otherwise skies should remain generally clear and winds light over the region. .LONG TERM.../Wednesday through Monday/ Issued at 348 AM CST Tue Feb 21 2017 The unseasonably warm weather will continue on Wednesday as 850mb temperatures climb into the 11 to 15C range. At the surface, winds from the southwest will prevail ahead of a cold front associated with low pressure moving across southern Minnesota. The warm low-level temperatures and warm breezes from the southwest should provide another day of near record to record warmth. Unfortunately, this spring-like warmth in the middle of February will begin to come to an end as the aforementioned cold front passes through the state by late Wednesday. High temperatures on Thursday will still be warm by typical February standards averaging 10 to 15 degrees above normal. Attention then turns to our late week weather system as a long wave trough digs into the Western US on Thursday and the jet energy ejects out onto the Plains and cyclogenesis begins. Ahead of the developing low pressure, isentropic lift will aid in areas of showers forming across the state. While models have tracked the low a bit farther north from previous runs, still fairly consistent with a low moving from western Kansas late Thursday into southeast Iowa by early Friday. This will put southern Iowa in the warm sector and rumbles of thunder are possible as there will be marginal MUCAPE Thursday evening. The big story though is the potential for a significant winter storm with a plowable snow over most of the northwest half of Iowa. Rain will begin to transition to snow over northwest/north central Iowa Thursday night. As the low continues to move to the northeast on Friday, the rain to snow transition will continue to move to the southeast as winds will quicken and blow from the north and northwest pulling colder air into the state. The transition is expected to be rain to snow as the soundings show a quick cooling of the thermal profile. Winds are also a concern. As with the previous forecast shift, have increased winds above initial guidance and this yields sustained winds of 15 to 25 mph with gusts to around 40 mph. This of course will cause areas of blowing snow and have incorporated that into the forecast for Friday and Friday night for areas primarily north of Highway 20 where sustained winds and gusts are expected to be highest. Winds will gradually abate Friday night into early Saturday as the snow ends. Zonal flow takes over for the weekend into early next week. A rather weak system is posed to pass on Sunday with precipitation chances. A push of milder air early next week will also bring another chance of precipitation. && .AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/ Issued at 1126 AM CST Tue Feb 21 2017 Medium confidence in VFR conditions throughout TAF period. High confidence in LLWS primarily at KFOD, KMCW, and KALO. KDSM included in LLWS as well. May need to add fog at KCMW and KALO as IFR possible with 1sm vsby. Confidence not high enough in fog at this time... 00z Wed TAF update may need to include fog. Confidence high in clear to mostly clear skies. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...REV LONG TERM...Ansorge AVIATION...Kotenberg