821 FXUS63 KDMX 261745 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1245 PM CDT Sun Mar 26 2023 ...Updated for the 18z Aviation Discussion... .DISCUSSION.../Today through Saturday/ Issued at 315 AM CDT Sun Mar 26 2023 Key Messages: - Narrow band of accumulating snowfall anticipated this morning, with several inches of snowfall possible in some areas and little to none elsewhere. - Fairly quiet from later today through Wednesday, with some fleeting precipitation chances and steady temperatures. - Large, dynamic spring storm system set to impact the region later in the week, around Thursday and Friday. All eyes are on the precipitation approaching the area early this morning. So far things have been setting up more or less as expected. An initial band of scattered precipitation pockets has move from southwestern toward central Iowa, producing light rain/snow/mix but with little accumulating and nearing the Des Moines metro at this time. Meanwhile, snow is spreading across eastern Nebraska and regional radar imagery shows the frontogenetical forcing axis beginning to set up our anticipated WSW-ENE band within the last hour or so. Thus far it has not matured and snowfall accumulations in that area appear modest, but high-resolution near-term models continue to indicate it will get its act together quickly over the next hours in the east/southeast Nebraska to west/southwest Iowa region, with rapid snowfall rates of an inch-plus per hour possible before 12Z or so. The expected location of this threat falls within the ongoing advisory, and no changes are being made at this time. As the band matures this morning, we will monitor closely in case any partial cancellation or expansion of the advisory becomes warranted. Overall forecast snowfall amounts have not changed appreciably from previous, however with such a narrow feature likely to have sharp cutoffs and pockets of enhanced snowfall, near-term adjustments will be likely throughout the morning. It should also be noted that most of the high-res runs since Saturday evening have consistently showed a fading of snowfall intensity as the band works eastward later this morning, a combination of waning forcing and post- sunrise warming/melting, though there are some exceptions including recent HRRR runs. For now, it is likely the highest snowfall accumulations will fall in a swath from roughly Omaha to Audubon to Boone, which again lines up with the current advisory. The band of precipitation will move quickly eastward out of our area during the late morning and early afternoon hours, after which a surface high pressure ridge will build down across the region through Monday. This will keep things cool, dry and quiet to start the coming week. Depending on how much snow falls this morning and how much of it melts this afternoon, any lingering swath of snow cover tonight could lead to potential for some fog and temperature variations, but these would be localized and if most or all of the snow melts this afternoon not a factor. Another mid-level shortwave will move over the region late Monday night into early Tuesday, but has considerably less focused forcing or available moisture than this mornings system and the forecast remains dry other than some low POPs in our south/southwest with little accumulation or impact. Another fast-moving but more robust impulse will then mover across the Upper Midwest around Tuesday night into Wednesday, likely generating a swath of precipitation across southern Minnesota and Wisconsin during that time frame. By the time this encroaches on northern Iowa much of the lift associated with the shortwave will already be moving eastward into the Great Lakes, and another surface high building in from the north behind it will bring dry air intrusion, the combination of these effects being that the precipitation should fade out as it enters Iowa. However, the timing/location of this will be close, and some model runs do paint light to moderate accumulations in our northern counties late Tuesday night/Wednesday morning. Have held only 20 POPs for now, but this bears watching as an increase might be needed in coming forecast updates. The most impactful weather period/system of the coming week, after this mornings focused snow band, will come from around Wednesday night through Friday night. On Wednesday a deep 500 mb trough will round ashore over California, turning downstream flow more southwesterly. 500 mb ridging over Iowa will thus come to an abrupt end as leading ripples ejecting out of the western trough, coincident with surging warm air/moisture advection, will bring an initial round of transient precipitation likely in the form of rain to our service area roughly around Thursday morning. Subsequently, as the large western trough progresses eastward it will result in Lee cyclogenesis with a deepening surface low developing over eastern Colorado and extending a sharpening inverted trough/front northeastward over Iowa in a rather classic synoptic setup. This will promote strengthening flow/surface winds and a rapid northward surge of moisture and surface dewpoints south of the boundary, leading to burgeoning instability and thunderstorm potential by late Thursday and Friday. As the surface low finally kicks out into the Midwest on Friday it will then likely generate additional precipitation, ranging from thunderstorms on the southeastern flank of the cyclone to snow on the northwestern flank. However, the details of this evolution are where the models really show disagreement, namely with the GFS being considerably stronger with both the 500 mb and reflective surface low, and correspondingly a bit further north with its track, while the EC and GEM show more of an open wave at 500 mb and a relatively weaker surface low moving a bit further south. These variations will have significant impacts on sensible weather and hazards for Iowa, so we will be monitoring the evolution of the forecast this week as theoretically severe weather and winter storm conditions could both be possible in parts of Iowa around Friday. && .AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/ Issued at 1245 PM CDT Sun Mar 26 2023 Area of snow is pushing through central into eastern Iowa early this afternoon and initial LIFR/IFR conditions will improve into VFR through the afternoon or early evening from west to east. While not in the official forecast at this time, there is the possibility of fog development tonight due to the recently fallen and melting snow from a DNS/CIN to ALO line. As the fog dissipates in the morning, will need to monitor for the potential of borderline MVFR ceilings, though have left out since this is nearing the end of the period. && .DMX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 1 PM CDT this afternoon for IAZ034>039-044>050-057>059-070. && $$ DISCUSSION...Lee AVIATION...Ansorge