Scientific Forecaster Discussion

000 fxus63 kgrb 191007 afdgrb Area forecast discussion National Weather Service Green Bay WI 407 am CST Tue Feb 19 2019 Forecast discussion for routine morning forecast issuance Synopsis... issued at 404 am CST Tue Feb 19 2019 An active weather pattern with a round of snow tomorrow and the potential for a major winter storm this weekend. The large scale pattern is currently dominated by high upper heights over the eastern Pacific. The downstream flow over North America is somewhat split, with the bulk of the energy in a strong, dominant, amplified southern stream. The mean trough position is over the Desert Southwest, with a ridge over the southeastern states. The large scale pattern is likely to remain fairly stable through the period, and favors an active storm track from the Southern Plains to the Great Lakes region. There will be numerous opportunities for precipitation as disturbances ride along in the southwest flow, so above normal precipitation amounts are likely. The two most significant systems will be a disjointed cyclone with somewhat limited moisture expected to affect the area tomorrow into tomorrow night, and a powerful cyclone that will eject northeast from the plains this weekend. The southwest flow will result in at or above normal temperatures after tonight. && Short term...today...tonight...and Wednesday issued at 404 am CST Tue Feb 19 2019 Quite a few forecast details to work out this morning. Nearly ideal radiational cooling conditions under a sprawling Canadian anticyclone were resulting in sub zero readings across most of the area early this morning, with a few spots likely to approach 20 below zero by daybreak. Plentiful sunshine today with the improved mid-late February sun angle should still allow most locations to recover to within a few degrees of normal despite the cold start. Clouds will overspread the area quickly late today and this evening. It will be a race between the incoming clouds and nightfall, but it looks like another period of favorable radiational cooling conditions are likely across the far north and northeast early tonight. So sharpened up the temperature gradient by raising mins a little in the south and chopping then several degrees in the north. Snow will begin to overspread the area late tonight and early Wednesday, though the exact timing remains in question. The trend on the guidance has been to slow the arrival of the snow. Dprog/dt displays off the models show dry air around 850 mb holding on longer. Observed data also suggest a slower arrival as the primary surface front is way south into the Gulf, and little return flow of moisture was detected at 925 mb or 850 mb. In addition, unlike some substantial snow systems earlier this winter, the highly amplified southern stream does not favor a push of Pacific moisture directly in from the west either. So the bottom line is that it's going to take significant lift and time to achieve saturation and get precipitation to develop. The 06z run of the NAM now barely gets precipitation into Wisconsin by 12z Wednesday. Did not hold it off that long, but don't expect snow to reach more than the southwest part of the area before 12z. It should overspread the area fairly quickly thereafter as stronger forcing sweeps across from the southwest. Snowfall totals in the forecast continue to support an advisory for the whole area, but given the delay it is unlikely travel conditions will deteriorate until during the day on Wednesday-- into the third forecast period. When considering that, and the fact that a further delay is still possible, opted to hold off on posting an advisory with the morning issuance. We will almost certainly eventually need an advisory, but holding off posting one now will give US a much better chance of coming up with the appropriate timing. Middle level moisture getting stripped out during the afternoon could result in freezing drizzle during lulls in precipitation over central and especially east-central Wisconsin during the afternoon. Long term...Wednesday night through Monday issued at 404 am CST Tue Feb 19 2019 500mb trough across western North America with a ridge across the southeast United States will bring an active weather pattern through the end of the month. First system expected to exit the region Wednesday night, the second system will affect the area on Feb 23-24 and a third system on Feb 26-27. Temperatures at the start of the period will be or slightly below normal on Thursday, with temperatures trending toward above normal levels for the weekend before cooling off early next week. For Wednesday night, light snow is expected to taper off and end for most. There may be some freezing drizzle across portions of central and east-central Wisconsin. Any freezing drizzle should have a minimal impact on roads as they will be treated or snow covered. Additional accumulations of an inch or two are possible across far northeast Wisconsin with less than an inch expected south of a Merrill to Sturgeon Bay line. Light snow may linger into early Thursday morning across north-central Wisconsin. Partly cloudy skies will prevail on Friday with highs in the upper 20s to lower 30s. The next chance of light snow arrives Friday night with weak system approaching from the southwest. The main story over the weekend will be the main system Saturday night and Sunday. Enough warm air across east-central Wisconsin for the potential for rain, freezing rain or sleet Saturday evening. The warm pocket collapses after midnight across east- central Wisconsin, probably not reaching Manitowoc County until closer to 12z. The model runs tonight bring copious amounts of qpf with the European model (ecmwf) showing a band around an inch from southwest into northeast Wisconsin. If this trend continues, there will be a band of heavy snow (6+ inches) in the above mentioned location of the heavy qpf axis. Light snow should come to an end Sunday night with tranquil conditions expected on Monday. Next system moves across the region on Tuesday, Feb 26th with the snow ending on Feb 27th. && Aviation...for 12z taf issuance issued at 404 am CST Tue Feb 19 2019 Generally good flying conditions are expected today into tonight as a large anticyclone shifts across the region and then off to the east. Conditions will begin to deteriorate starting late tonight as snow overspread the region from the southwest. && Grb watches/warnings/advisories... none. && $$

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