Scientific Forecaster Discussion

000 fxus63 kgrb 222010 afdgrb Area forecast discussion National Weather Service Green Bay WI 210 PM CST Fri Feb 22 2019 Forecast discussion for routine afternoon forecast issuance Short term...tonight and Saturday issued at 210 PM CST Fri Feb 22 2019 The latest rap analysis and satellite/radar imagery show high pressure stretching from the northern Mississippi Valley to the eastern Great Lakes early this afternoon. Meanwhile, light snow is occurring from northern Kansas into North Dakota within a warm advection zone and ahead of shortwave troughing moving over The Rockies. The primary system remains in the base of the trough located over the Desert Southwest. However, cloud cover is shifting northeast into Minnesota and Iowa and poised to move into Wisconsin later today. As warmer air aloft surges into the state tonight into Saturday, potential snow and ice accumulations are the main forecast concerns. Tonight...clouds will continue to increase from the southwest and lower at the same time as warm advection aloft moves from west to east across the state. With increasing mid-level moisture transport and decreasing stability, many of the higher resolution models indicate a band of light snow surging northeast across north-central WI after 09z. Meanwhile, guidance has slowed down the arrival of the snow into central WI almost to 12z Sunday. Therefore lowered accumulations some across the I-39/Hwy 51 corridor, where a half inch to an inch of accumulations will be possible by 6 am Saturday. Temps will most likely steady out in the evening before warming a few degrees overnight, particularly over eastern WI where wind directions will become east off Lake Michigan. Saturday...precipitation will slowly shift east during the morning, though models indicate low level dry air will hold off precipitation arriving over eastern WI until 15z or later. Precip should be mainly snow until this time, before warmer air aloft starts to invade from the south to north across central to northeast WI. This will lead to an increasing threat of freezing rain particularly over central WI and the southern Fox Valley by mid to late morning. Because temps start to approach freezing, and the Road temperature forecast calls for readings to warm above freezing after around 10 am, the warming temperatures will mitigate the risk of freezing rain somewhat. Higher uncertainty will be over east-central WI due to the expected later arrival of the precip. The freezing rain/drizzle threat may not ramp up until late morning or early afternoon across northern WI where temps struggle to reach the freezing mark. Models, in general, increased qpf through the day, which increases the threat of hazardous travel. Due to the potential for 1-3 inches of snow and some ice accumulations, will issue a Winter Weather Advisory for the morning for central WI, and from late tonight through the afternoon for northern WI. Long term...Saturday night through Friday issued at 210 PM CST Fri Feb 22 2019 Once our main storm departs Sunday night, the main story for next week will be the well-below normal temperatures as an upper high builds toward Alaska and displaces a chunk of Arctic air southward into the northern plains/Great Lakes. Snow chances next week appear confined to Tuesday/Tuesday night with the passage of a weak clipper to our south and perhaps next Friday with a stronger system moving through the Great Lakes. Surface low is forecast to lift northeast into central or east- central WI Saturday evening and intensify in the process as the accompanying negatively-tilted shortwave trough moves across the Midwest. Besides strong lift from the deepening surface low, northeast WI to reside in the favorable left exit region of the upper jet for additional lift. Moisture will not be a problem with models showing precipitable water values of 0.50 to 0.75" which is about 200 percent of normal. In the mid-levels, models are indicating strong q-g forcing over the entire forecast area, while the strongest frontogenetical forcing exists across northern/parts of central WI. Thermal profiles show enough warm air across east-central WI to keep precipitation as all rain, while north-central WI to be all snow with a wintry mix in between. By later Saturday night, the surface low tracks toward eastern sections of Upper Michigan with snow everywhere except east-central WI where the wintry mix to be located. Snow accumulations by daybreak to range from 2 to 5" central/far northeast WI, 6-9 inches north-central WI. Due to impending headlines for Saturday, prefer to hold on to the Winter Storm Watch for now to avoid additional headlines with the caveat that winter storm warnings will eventually be needed. Snow will continue Sunday morning as the shortwave trough moves into the Great Lakes and a cyclonic flow to be in place. Additional accumulations of an inch or less can be expected central/east-central WI, 2 to 5" for northern WI. The main story for Sunday will be the strong west-northwest winds with a sustained 20 to 30 mph and gusts of 40 to 50 mph. Even as the snow either ends or becomes light Sunday afternoon, anticipate plenty of blowing/drifting snow especially on north-south roads. Visibilities would be significantly impacted and traveling would be hazardous. Max temperatures to be in the upper 20s to around 30 degrees central WI, lower to middle 30s eastern WI around midday with falling temperatures in the afternoon. Any lingering light snow over far northern WI will end Sunday evening as the system pulls farther away and high pressure begins to build east from the northern plains. Skies should begin to clear during the overnight hours with strong cold air advection sending 800 mb temperatures down into the -18 to -22c range by 12z Monday. Winds are forecast to weaken a bit (still 10 to 20 mph) and when combined with min temperatures of zero to 5 below central WI, 5 to 10 above zero eastern WI, wind chills of -15 to -25f are expected with possible headlines for central WI. This high pressure to stretch from southern Saskatchewan to the western Great Lakes region on Monday and help to keep sky conditions generally partly cloudy through the day. Max temperatures for Monday to be much colder than previous days with readings only from 5 to 10 above central WI, 10 to 15 degrees eastern WI. Models continue to struggle with a system headed toward WI on Tuesday. The main culprit for the indecision appears to be the strength of a rockies upper ridge. The Gem and GFS are rather weak, thus shortwave energy takes aim at WI, along with an inverted surface trough. The GFS starts up light snow across northern WI Tuesday morning, while the Gem holds off on any snow until Tuesday evening. The European model (ecmwf) has the stronger upper ridge, thus shortwave energy dives to our south and we remain dry on Tuesday. Prefer to focus higher chance or likely pops from Tuesday afternoon into Tuesday night, however there is too much uncertainty yet to mention accumulation potential. The snow chances would then carry over into Wednesday morning until the shortwave/surface trough exit the area. Improving conditions are then expected for Wednesday afternoon as another high pressure starts to build into WI. Max temperatures for Tuesday and Wednesday to remain well-below normal (teens on Tuesday - upper teens to lower 20s on wednesday). The high pressure is forecast to move into the Midwest Wednesday night and across southern sections of the Great Lakes on Thursday. Quiet and continued cold conditions to persist with Max temperatures again in the upper teens to lower 20s. Models are all over the place with the handling/timing of the next system for late Thursday night into Friday. The GFS is faster/ stronger with the system, the Gem is faster/weaker with the system and the European model (ecmwf) is slower/stronger (holding off precipitation until Friday night). For simplicity sake, will follow the consensus solution which brings a small chance for light snow into northeast WI on Friday. Max temperatures for Friday to be in the lower to middle 20s north-central WI, upper 20s to around 30 degrees east- central WI. && Aviation...for 18z taf issuance issued at 1115 am CST Fri Feb 22 2019 Good flying weather will persist into the evening. Ceilings will lower after midnight with light snow arriving over northern WI mostly after 09z and mostly after 12z over central WI. Vsbys and possibly cigs will lower to IFR over central and north-central WI on Sunday morning. && Grb watches/warnings/advisories... Winter Weather Advisory from 6 am to noon CST Saturday for wiz020-021-030-031-035>037-045. Winter Storm Watch from Saturday evening through Sunday afternoon for wiz005-010>013-018>021-030-031-035>037-073-074. Winter Weather Advisory from 3 am to 6 PM CST Saturday for wiz005-010>013-018-019. &&

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