Scientific Forecaster Discussion

000 fxus63 kgrb 211723 afdgrb Area forecast discussion National Weather Service Green Bay WI 1123 am CST Thu Feb 21 2019 Updated aviation portion for 18z taf issuance Synopsis... issued at 326 am CST Thu Feb 21 2019 A brief break in the active weather through tomorrow, then a major winter storm will bring widespread precipitation and strong winds to the area this weekend. The large scale pattern was dominated by high upper heights over the northeast Pacific, which was resulting in splitting of the flow downstream over noam. The southern stream was highly amplified and dominant, with a trough over the Desert Southwest and ridging over the southeast states. Subtle but important changes will occur next week as the positive anomaly over the northeast Pacific shifts north-northeast into Alaska and strengthens. Though the developing upper anticyclone will be undercut over the eastern Pacific, the downstream flow across noam is likely to merge back together, with the negative anomaly from the southwest U.S. Upper trough reforming in the Lake Winnipeg region late in the forecast period. The large scale evolution will result in a transition from the current active southwest upper flow regime to a much colder northwest flow pattern by the end of next week. Temperatures will remain at or above seasonal normals through the weekend, then drop below normal next week. Well below normal conditions are likely by late in the week. While it remains in place, energy ejecting out of the southwestern U.S. Trough will bring several opportunities for precipitation, resulting in above normal amounts for the period. The most significant precipitation episode will be with a rapidly deepening cyclone expected to cross the region during the weekend. && Short Friday issued at 326 am CST Thu Feb 21 2019 A relative lull in the active weather will occur during the short-term part of the forecast. Snow showers and flurries will linger for a time this morning (mainly across the north) before ending. Otherwise, the main forecast issue will be cloud trends. The specifics of those are somewhat muddled. For today at least, the main issue is low clouds and those will be more plentiful across the north, with more sun in the south. By tonight and Friday, patches of middle and high clouds will be streaming across at times as well. Trended toward mid-range skycon during those periods. Colder air was advecting into the area early this morning, on the southern flank of the departing cyclone. Temperatures will continue to fall until mid-morning, but should recover some this afternoon. Temperatures tonight will depend on clouds, and thus the low temperature forecast inherits the uncertainty of the specifics of the skycon forecast. With a fresh snow cover and relatively light winds, readings will likely drop sharply during any periods of clear/mostly clear skies. But there is great uncertainty in when/where those conditions will occur. Staked out a middle ground with the low temperature forecast with the expectation of at least some breaks in the clouds occurring. Readings across the north could easily be 10 or more f degrees colder if any prolonged clear periods occur. Stayed close to a blend of top performing guidance products for high temperatures on Friday. Long term...Friday night through Wednesday issued at 326 am CST Thu Feb 21 2019 Southwest flow at 500mb is expected at the start of the period, then should transition to a northwest flow aloft by the middle of next and continue into the weekend of March 2-3. Temperatures are expected to be at or above normal into the weekend, then will run below normal next week. Latest European model (ecmwf)/GFS indicating 850mb temperatures of -20c to -30c, by the first few days of March, thus it appears Spring will take awhile to get here. For Friday night, 850mb warm advection noted on the models which should help produce precipitation late Friday night into Saturday morning. Latest trends indicated a slower arrival of the precipitation, not reaching until east-central Wisconsin until after 12z Saturday. Kept areas near the Lakeshore dry Friday night, while chances of light snow are expected north and west of the Fox Valley. There may be a little freezing rain towards 12z across central Wisconsin. The chances of light snow or freezing rain will continue Saturday morning, especially north and west of the Fox Valley until pavement/air temperatures rise above freezing. The 850mb warm advection shifts east of the state Saturday afternoon. There may be a lull or break in the precipitation until late in the day when the system from the southwest approaches the area. Precipitation should be on the increase again late in the day across central Wisconsin, then overspread the remainder of the region Saturday evening. Models in good agreement in bringing strong storm across the Badger state Saturday night. Although in good agreement, there are still differences in the storm track and how much warm air wraps up into northeast Wisconsin. The GFS was the furthest west with the 850mb 0c isotherm, while the Canadian/European model (ecmwf) had the 850mb 0c isotherm straddling Interstate 41 across the Fox Valley. The subtle difference will have a major impact when precipitation changes over to snow across the Fox Valley. Also, the heavier band of precipitation is expected to move across the area later Saturday evening into early Sunday morning, which could change the rain or wintry mix over to snow sooner than expected. Confidence is growing for a band of heavy snow of 6+ inches north and west of the Fox Valley. Still some uncertainty if this band of heavy snow would make it as far north as Vilas County. Still plenty of time to sort out the details. As the system moves east of the area, BUFKIT soundings indicating wind gusts over 40 mph are possible across northeast and east-central Wisconsin late Saturday night into Sunday. The combination of falling snow and strong winds will make for hazardous conditions for travelers to continue into Sunday afternoon. For Monday, high pressure will move across the area bringing tranquil conditions to the area. Next system is expected to bring more snow to the area Monday night into Tuesday night. Low confidence at this time with the storm track and snow amounts. It will be cold next week with highs mainly in the single digits and teens above zero. && Aviation...for 18z taf issuance issued at 1123 am CST Thu Feb 21 2019 Outside of a few flurries, fairly quiet weather expected through the short-term portion of the forecast. The biggest concern will be low to mid cloud potential through tonight. VFR/MVFR conditions are noted at most taf sites this morning; however, rhi has been generally staying around 1500 ft ceilings. That patch of clouds extends farther west int northwest WI, so the ceilings will likely linger for a good portion of the afternoon. County Warning Area and auw could end up seeing some MVFR ceilings at times, as there are some lower clouds upstream, but they have had trouble advancing very far east this morning. The rest of the taf sites will likely remain VFR. && Grb watches/warnings/advisories... none. && $$

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