394 FXUS63 KDMX 261142 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 542 AM CST Sun Dec 26 2021 ...Updated for the 12z Aviation Discussion... .DISCUSSION.../Today through Saturday/ Issued at 201 AM CST Sun Dec 26 2021 Key Messages: - Light precipitation later today, mainly in the form of rain. - Widespread accumulating snowfall possible across northern half or so of Iowa on Tuesday. - Much colder and blustery by Wednesday morning. Currently the region is clear, dry, quiet, and cool with a surface high pressure area draped down across Minnesota and Iowa yielding clear skies and light winds through the Hawkeye state. A few patches of fog have developed in southern Iowa, but otherwise all is quiet as we bid farewell to another Christmas. A large 500 mb gyre has set up over far western Canada with a lobe extending down over the Seattle/Tacoma region, carving out a broad trough over the western U.S. A fast moving shortwave ejecting out of the trough is moving over the Four Corners region early this morning and will shoot over the High Plains today and Iowa by this evening. Its approach will promote cyclogenesis over northeastern Colorado, with the resultant surface low moving near the Nebraska/South Dakota border this evening and northeastward across Minnesota late tonight and Monday morning. Ahead of the low a band of warm air advection will move quickly over Iowa from this afternoon into tonight, generating light precipitation as it passes. However, with the surface ridge in place ahead of this zone and its rapid approach, there will be a lot of dry air to overcome and forecast soundings indicate limited cloud/moisture depth as the WAA moves through, leading to limited accumulation potential. With the system also maturing as it moves overhead, it is likely our southwestern counties will see little to no precipitation, with the potential steadily increasing as one moves northeast through the service area. Precipitation type is problematic, as initially temperature profiles support a snow/wintry mix potential but by the time the column saturates to a sufficient extent for precipitation, strong moisture advection will push temperatures and dewpoints/wet bulbs up to the range where any precipitation will fall as liquid drops with an impressive warm nose signature in forecast soundings. Most of the area will see air temperatures above freezing by that time, though near the Minnesota border there may be a brief window for a light glaze before wet bulbs rise above 32. Even so, the potential for actual ice accumulation is uncertain and limited at best, and it is more likely any hazards would manifest further north/northeast toward southeastern Minnesota and Wisconsin. The precipitation should clear our area by around midnight, except in our eastern and northeastern counties where there may be sufficient lingering low-level moisture to promote drizzle for several additional hours overnight. If temperatures/wet bulbs do manage to hover just below freezing this could lead to light glazing and travel impacts, so that possibility will be monitored throughout the day and tonight. A surface trough trailing from the cyclone will move quickly across Iowa early Monday morning, scouring out any remaining precipitation/drizzle and leading to a sunny and dry Monday, though just a bit cooler and breezy in the morning. Meanwhile, the aforementioned lobe of 500 mb low pressure over the Pacific Northwest will finally rotate eastward and become more progressive during the day Monday, crossing the Rockies Monday night and impacting Iowa and the surrounding states on Tuesday. This system will be stronger and carry more available moisture, leading to an increase precipitation/accumulation potential in our forecast area. Also of note is that the surface low is likely to track further south, near the Iowa/Missouri border on Tuesday afternoon and evening. As a result, the precipitation is more likely to fall as snow early on, around Tuesday morning, before transitioning through a wintry mix to light rain by the afternoon. This will limit any accumulating snow potential in southern Iowa, but in the north widespread light to perhaps moderate accumulations appear probable, depending on how long the profile remains cool enough to promote snow as the predominant p-type, There is also some potential for freezing rain/drizzle, mainly in the north, but that appears limited to the time period just before precipitation ends and at this time any ice accumulations appear sporadic and marginal. We will be watching this system closely over the next couple of days but there is more potential for snowfall amounts in northern Iowa to trend more toward light than toward heavy, as there may be increased melting/compacting by Tuesday afternoon, and also because some solutions have been showing more of a split in the precipitation shield around the cyclone, with the bulk of the warm air advection area shunted just to our southeast, and the northern snow axis trending a bit further northward toward the Minnesota border. Nevertheless, there is likely to be a time window of travel impacts at least in our northern counties, with details coming into better focus over the next couple of days. Somewhat similar to the Sunday system, a surface trough will sweep rapidly across Iowa somewhere around Tuesday evening, clearing out any remaining precipitation and bringing gusty winds and notably colder air into the region by Wednesday morning. In fact, it is likely temperatures will fall below zero in northwestern Iowa by sunrise Wednesday, with wind chill values in the teens below zero in that area, and perhaps bottoming out near or just below zero as far south as the Des Moines metro. While not extreme for this time of year, the season has been remarkably mild overall to this point, so the arrival of this cold and blustery weather may be a bit of a shock to the system on Wednesday. Fortunately it will be short lived as temperatures rebound a bit on Thursday and Friday. At the end of the forecast period, long range models have come into better agreement tonight in depicting a broad and deep mid-level trough moving into the Midwest on Saturday, and perhaps bringing a more widespread accumulating snowfall to parts of the region. Clearly confidence is low at this range, but it is something to keep an eye on in the coming days. && .AVIATION.../For the 12Z TAFS through 12Z Monday morning/ Issued at 542 AM CST Sun Dec 26 2021 Fog will affect OTM at times in the next 2-3 hours, otherwise VFR conditions are expected through at least 18Z. Thereafter, lowering ceilings will reach MVFR levels at times this afternoon, then IFR at times tonight before improving again near or after the end of the 12Z TAF period. -SN or a light wintry mix more likely at MCW late today into tonight may reduct visibility at times, with additional fog development possible at MCW/ALO overnight contributing to higher probability of IFR or lower conditions. Amendments will be likely late today/tonight to account for short term forecast and observational trends. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ DISCUSSION...Lee AVIATION...Lee