Scientific Forecaster Discussion

000 fxus63 karx 261138 afdarx Area forecast discussion National Weather Service La Crosse WI 638 am CDT Fri Apr 26 2019 Short term...(today and tonight) issued at 324 am CDT Fri Apr 26 2019 Main forecast concerns for today are on breezy northwest winds and afternoon relative humidity values. Weak high pressure edges toward the region today, but the area remains in a pressure gradient producing breezy northwest winds. Afternoon relative humidity values are expected to drop into the 20s but with the widespread rainfall yesterday, there shouldn't be any major fire weather concerns. Highs today will be on the cooler side ranging from the mid 50s to around 60. Long term...(saturday through thursday) issued at 324 am CDT Fri Apr 26 2019 Focus quickly turns to the potential for heavy snow with possible travel impacts on Saturday. A fast moving, but potent low pressure system moves through the region during the day on Saturday bringing a band of heavy snow to the area. Strong frontogenesis noted in the NAM from 900 through 600 mb. Negative epv also noted above this frontogenesis zone, so there is the potential to see heavy snowfall rates under the band. The band is expected to be narrow in nature, only around 2 counties wide. Forecast models have been trending south with the highest confidence for heavy snow over portions of southeast Minnesota, northeast Iowa, and extreme southwest Wisconsin. At this time, snowfall amounts of 4 to 9 inches appear possible under the band, with amounts tapering off to around 2 inches a County north and a County south of the band. There is the potential for a shift in the exact location of the band so will have to keep a very close eye on this as we get more into the window of mesoscale models, increasing our confidence on where a warning may be needed. Another positive for heavy snow is drier air in place near the surface, with a northeast feed of drier air during the event. Expect evaporative cooling to switch the precipitation over to snow quickly, if not right away early Saturday morning as it spreads from west to east across the area. Expect slushy snow accumulations on roadways, leading to travel difficulties. Valley locations may see more slushy snow accumulations where warmer surface temperatures are in place, but there could still be significant travel impacts. The higher snowfall rates could easily overcome the warmer ground temperatures. The snow should then taper off Saturday night as cold high pressure builds in. A widespread freeze is looking likely Saturday night as temperatures drop into the 20s. A freeze warning will most likely be needed with impacts expected to cold sensitive crops and plants. We will see a brief break in precipitation chances on Sunday then rain chances, and possibly a little snow, return Sunday night into Monday. Cooler and active weather then continues through much of next week with periodic chances for rain. && Aviation...(for the 12z tafs through 12z Saturday morning) issued at 637 am CDT Fri Apr 26 2019 High pressure will provide mostly clear skies today. As a low pressure area moves out of the northern plains, mid and high clouds will increase. Rain will move into krst around 27.09z and close klse around 27.12z. && Hydrology... issued at 324 am CDT Fri Apr 26 2019 Rain and snowmelt from saturdays system looks to produce 1/2 to 1 inch of liquid equivalent precipitation. With antecedent moist ground conditions, expect runoff and more river rises, especially along and south of I-90. && Arx watches/warnings/advisories... WI...Winter Storm Watch from Saturday morning through Saturday evening for wiz041-053>055-061. Minnesota...Winter Storm Watch from Saturday morning through Saturday evening for mnz079-086>088-094>096. Iowa...Winter Storm Watch from Saturday morning through Saturday evening for iaz008>011-018-019-029-030. && $$

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