Scientific Forecaster Discussion

000 fxus63 kfsd 222111 afdfsd Area forecast discussion National Weather Service Sioux Falls South Dakota 311 PM CST Fri Feb 22 2019 Short term...(this evening through sunday) issued at 304 PM CST Fri Feb 22 2019 Active weather pattern just keeps going and going. For the rest of today, light snowfall will continue to focus across the western half of the forecast area, especially west of the James River in the vicinity of the 700 mb frontogenesis. By early evening however, the the active southwest flow aloft directs the next subtle shortwave into the Missouri Valley then southwest Minnesota overnight. The light snowfall will begin to advance eastward this evening, however drier air in the low/mid levels clouds across the eastern half of the forecast area could slow in initial progress of the snowfall. Loss of saturation in the dendritic zone this evening with persistent dry air in the mid levels will likely allow precipitation to mix with light freezing drizzle at times. The drier air aloft should limit any additional snowfall amounts, especially late this evening and overnight. Models do suggest that some frontogenesis develops across our far southeast late afternoon and evening, which could potentially bring some greater snowfall amounts. Think that this area could see 1 to 3 inches of snow, while elsewhere less than an inch is expected. Even with a greater threat for freezing drizzle, icing accumulations will also be relatively light, with only a light glaze of ice expected. While the initial wave exits the area late tonight, lower clouds and broad lift will keep patchy freezing drizzle in the region into Saturday morning. Models are in pretty good agreement with the timing and track of the next storm system. The main shortwave trough arrives in the Central Plains on Saturday, becoming a closed circulation over Kansas as it lifts northeast through the afternoon. Light snow will start spreading across the Missouri Valley around late morning into midday, spreading north and east in the afternoon. The best potential for and inch or more of snowfall remains across the southeastern half of the forecast area, largely southeast of a Yankton to Marshall line. Strong forcing and lift sets up near the southeastern corner of the forecast area (just south of Storm Lake and Ida grove) in the afternoon, and will allow for heavy snowfall rates to develop in our extreme southeastern counties. There remains some uncertainty in how far north the heaviest precipitation falls, so have decided to go with a Winter Weather Advisory for northeast Nebraska and much of northwest Iowa for Wednesday afternoon and evening, roughly following where 3 inches or more of snow is expected. On the higher end in bv and Ida counties, amounts could approach 6 inches, but with uncertainty in how far north the heavier quantitative precipitation forecast pushes, will leave in an advisory for now. In addition to the snowfall, winds will be on the increase, creating further reductions in visibilities. The strongest winds, however, hold off until the end of the falling snow. The surface low becomes very tightly wrapped as it tracks through northern Missouri and central Iowa. Winds will increase through the day on Saturday, with gusts increasing to 30 mph. Northerly winds increase further in the evening, with gusts reaching as high as 40 to 50 mph by midnight across much of southwest Minnesota and northwest Iowa. Although the snowfall will have ended, expect ground blizzard conditions of near white-out conditions from blowing snow where the winds are strongest. Will upgrade the weather storm watch to a Blizzard Warning, largely for ground blizzard conditions from midnight early Sunday through Sunday afternoon. May need to look into the potential for a Winter Weather Advisory for blowing snow further west, but will leave it to just the warning with this event over 24 hours out yet. Am also concerned how much the freezing drizzle could create a crust on the existing snowpack to the west of the warning, therefore limiting the blowing snow potential. Windy conditions continue into Sunday, with the strongest winds remaining across our eastern half. Long term...(sunday night through friday) issued at 304 PM CST Fri Feb 22 2019 With the focus on the winter weather tonight into Sunday, did not make much adjustment to the extended grids. It does look like a thermal boundary sets up across the northern plains for the first half of the week, bringing precipitation chances around the Sunday night time frame and again Monday night. The screaming message remains: very active and wet pattern continues. && Aviation...(for the 18z tafs through 18z Saturday afternoon) issued at 1138 am CST Fri Feb 22 2019 Periods of MVFR to IFR conditions are expected through most of the tafs as snow showers and freezing drizzle move through the region. Light to moderate snow will be on the increase late this afternoon and will continue through the evening hours at all sites. Precipitation is expected to change over to freezing drizzle during the overnight hours into late Saturday morning. Highest ice accumulations/impacts expected across ksux. && Fsd watches/warnings/advisories... South Dakota...Winter Weather Advisory until midnight CST tonight for sdz038- 050-052-053-057>060-063>065-068. Minnesota...Winter Storm Watch from late Saturday night through Sunday afternoon for mnz071-072-080-081-089-090-097. Iowa...Winter Storm Watch from late Saturday night through Sunday afternoon for iaz002-003-013-014-021-022-032. NE...none. &&

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