542 FXUS63 KGRB 300437 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1037 PM CST Fri Dec 29 2023 Updated aviation portion for 06Z TAF issuance .KEY MESSAGES... - 0.5-2" of snow possible Saturday night into Sunday as a clipper system moves through the Great Lakes. - Highest snow amounts expected across east-central WI, including the Fox Valley Sunday morning. - Short period of gusty north winds (20-30 mph) as snow ends Sunday afternoon and evening could create minor blowing and drifting snow on east- west oriented roads. && .DISCUSSION... Issued at 207 PM CST Fri Dec 29 2023 Short Term...Tonight and Saturday Main forecast concerns will be on cloud trends late tonight and whether any precipitation could work into parts of north-central WI late Saturday afternoon. The 19Z MSAS surface analysis showed a weak ridge of high pressure that extended from the Upper MS Valley into western Ontario. A weak area of low pressure over extreme northwest MN is a clipper system headed in our direction. Visible satellite imagery indicated skies had cleared over most of the area with some clouds hanging over east-central WI. The surface ridge (and its associated upper ridge) to move into the western Great Lakes region tonight providing for mostly clear skies and a light west-northwest wind. However, little change in the air mass overhead could bring another round of low clouds and patchy fog to parts of northeast WI after midnight. Confidence is low as to exactly where/if these clouds actually develop. Models bring the weak clipper low pressure into western Lake Superior toward daybreak, but a lack of moisture ahead of this system would preclude any precipitation. Min temperatures to range from the lower 20s north, middle to upper 20s east-central WI. This clipper will continue to move southeast and enter northeast WI Saturday afternoon. However, upper support trails back to northwest WI/northern MN where a mid-level shortwave trough to be situated. Since both moisture and forcing are lacking through the day, most if not all of the forecast area should stay dry. We will see clouds increase in the afternoon and have kept a slight chance pop over parts of north-central WI late in the day. Max temperatures on Saturday to range from the lower to middle 30s north-central WI, mainly upper 30s east-central WI. Long Term...Saturday Night Through Friday Snow... Changes: Only slight changes to snow totals from the previous forecast. Did increase northerly winds a few miles per hour for Sunday afternoon and evening as a tighter pressure gradient and increasing winds at 925mb could result in gusts of 20-30 mph, mainly for the Fox Valley and east-central WI. Details: The main period of focus of the forecast is a clipper system bringing a likely chance (60-80%) for snow beginning late Saturday in northwest WI and spreading southeast before ending mid-Sunday afternoon. The primary forcing with this system is an area of mid- level convergence that is progged to pass directly over WI. One factor that my limit snow accumulation is a lack of strong forcing through the dendritic growth zone. Accumulations of about 0.5-2" are expected with the highest totals across the Fox Valley and east- central WI. There does look to be a short period Sunday afternoon and evening during which gusty (20-30mph) north winds could create some blowing and drifting snow, mainly impacting east-west oriented road. Uncertainty: The primary area of uncertainty with this system is whether northeast winds coming down the bay and lakeshore Sunday morning could produce lake enhanced snow for a 3-4 hour period before winds become more northerly by Sunday afternoon. It is likely that given the short duration for which lake enhancement could occur snow totals won't be greatly increased, however, if the low center continues to trend further east snow totals for areas south of the bay and along the lakeshore may need to be increased. Rest of the forecast... Dry conditions are expected to return to the area for much of the first half of next week with a high pressure system over the southern CONUS extending into the region, a cold front associated with a surface low tacking over central Canada could produce light snow Wednesday morning, mainly across the Northwoods, but impacts look minimal at this time. Another clipper type system may also bring some precipitation towards the end of next week, but given the that this system is a week out left NBM PoPs for now. && .AVIATION...for 06Z TAF Issuance Issued at 1032 PM CST Fri Dec 29 2023 The latest models this evening have backed off on the freezing fog potential for portions of central and north-central Wisconsin. There will still be fog, but it not anticipated to be dense. However, with clear skies and lights winds and temps already near the dewpoint temperature, would not be surprised if there was some patchy dense fog in a few spots around sunrise or even a few hours after sunrise. Look for MVFR or lower CIGS to overspread the entire area Saturday afternoon and evening with a round of light snow which should linger into It has been awhile since the area has seen an accumulating snow. && .GRB WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION.....Kallas/GK AVIATION.......Eckberg