648 FXUS63 KDMX 271742 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1237 AM CDT Sat Apr 27 2019 .SHORT TERM.../Today through Tonight/ Issued at 416 AM CDT Sat Apr 27 2019 Compact surface low and associated shortwave trough over Nebraska will continue to quickly push east across the state today. Models are in good agreement with the overall synoptic pattern and timing. There are still some subtle differences wrt to the surface low track, but at least all agree the surface low tracks along the vicinity of the Iowa/Missouri border today. The biggest changes were to the higher snowfall totals shifting a bit further south and have a swath of 3-6" across the north to northeast. Also, added some light accumulations as far south as Fort Dodge to Marshalltown areas, but still expecting a sharp gradient of heavier snow amounts. Confident to expand the Winter Storm Warning further west and south for the higher snow amounts coupled with strong winds and visibility concerns (more on this below). The time frame of most concern is from 11-12z to 18-20z today across northern Iowa. Strong moisture transport and impressive isentropic lift along the 290K to 305K surfaces into the state this morning before quickly waning and moving east past 18-21z this afternoon. Intense frontogenetical forcing from northwest Iowa to east-central part of the state from 850mb to 700m suggesting the potential for heavy banded snow. Strong omega cross-hairs in the dendritic layer and with some negative EPV present to potential release CSI and lead to upright convection and thus still cannot rule out a few rumbles of thunder across the north to intensify the snow rates. As mentioned, moisture is certainly not the issue today and confident the precipitation type will be either rain or snow as no significant warm layer aloft or refreezing is present today. The tricky and million dollar question is when the rain switches over to snow across northern Iowa and in turn when will it begin to accumulate. Certainly with the strong omega within the dendritic layer and the amount of large scale forcing across the state, the snowfall rates should be intense enough to overcome the warmer surface temperatures (even as warm as 38F). Road temperatures across the north are in the lower 50s as of around 08z, but once the rain begins to fall and then the snow, these temperatures should drop fairly drastically this morning. Likely to see a short duration of the rain/snow transition with large, wet dendrites this morning, from 10-13z from west to east over the north. The snow ratios will increase from around the 6:1/8:1 at the start of the snow to 10:1 and maybe as high as 13:1 by midday into the early afternoon. Thus, the "concrete" snow is likely for much of the morning hours and the difficult part is how much will actually accumulate on the hard surfaces. The HREF, SREF, and HRRR suggest snowfall rates of 1 to 2 inches per hour from 12-18z this morning over the warning area and although it'll take a little time to cool the hard surfaces, these types of snow rates have proven in the past to overcome this obstacle and allow snow to accumulate. Thus, the sloppy, concrete snow mess coupled with the strong winds is going to make for difficult travel conditions. Plus, with the large dendrites anticipated, visibility is likely to be less than a mile for much of northern Iowa past 11-12z this morning. The hires models are suggesting this trend with the RAP/HRRR the most bullish wrt to visibility restrictions. In summary, the heavy, wet, concrete snow to develop past 11-12z this morning and persist through 20-22z across north-central to northeast Iowa today. Expanded the Winter Storm Warning further west and south and added a few counties to a Winter Weather Advisory on the southern sharp edge of the heavy band of snow. Certainly cannot rule out a few locations from Mason City to Waterloo seeing near 8+", but more confident in the widespread 4-6" in this area. .LONG TERM.../Sunday through Friday/ Issued at 416 AM CDT Sat Apr 27 2019 Fairly progressive flow will prevail through the period with cool WNW flow initially transitioning into WSW flow into the middle of next week bringing several windows for precipitation. After the precip ends today our first concern will be cooling temperatures and frost/freeze potential overnight into Sunday morning. High pressure should drop into the state with mostly clear skies and light winds allowing for decent radiationally cooling, especially north where some snow is expected to persist into the night with temps dropping below freezing. Have omitted frost wording north where lingering snow cover will be problematic, but have included central and south where readings at or below freezing and high RH should be quite favorable for frost. No headlines yet, until current snow headlines clear and snow cover can be better defined, but fully expect frost and/or freeze headlines of some degree later today. Projected lows are also approaching record lows for Sunday. This will be followed by precip chances increasing once again as the current Pacific NW short wave reaches the northern Plains and Great Lakes Sunday Night into Monday. Much of the kinematic forcing should remain to our north, but another surge of strong warm/theta-e advection will pass through bringing additional showers and a few thunderstorms. Confidence decreases somewhat looking toward the next system and the middle of next week. The GFS is faster with the MO Valley short wave late Tue while the ECMWF waits until Wed. Both depict a period of appreciable precip and passage of more mature surface reflection across MO, but the timing differs by 6-12 hours. This will bump PoPs up, especially south, with at least some light precip anticipated in most areas through that broader window. The end of the work week looks mainly dry with seasonal temperatures, but confidence in details is again low with the 00z GFS and ECMWF depicting varied strength and timing solutions regarding Rockies ridging. && .AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/ Issued at 1237 PM CDT Sat Apr 27 2019 Strong surface low currently over sw/scen IA will continue to move off to the ESE. Precip across northern TAF locations will be ending between 21Z and 00Z. MVFR with local IFR cigs and occasional IFR vsby will persist through that time. VFR cigs will quickly move in behind the system after 00Z with clearing by 06Z. Strong northerly surface winds will remain in place through 01Z or so before diminishing and going light and variable after 03Z. VFR cigs will return Sunday with southeast surface winds at 10-20kts. && .DMX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 7 PM CDT this evening for IAZ004- 015-024-025-037-038-050. Winter Storm Warning until 7 PM CDT this evening for IAZ005>007- 016-017-026>028-039. && $$ SHORT TERM...Podrazik LONG TERM...Small AVIATION...FAB