345 FXUS63 KGRB 240440 AFDGRB Area Forecast Discussion National Weather Service Green Bay WI 1040 PM CST Sat Dec 23 2023 Updated aviation portion for 06Z TAF issuance .UPDATE... Issued at 806 PM CST Sat Dec 23 2023 No changes anticipated to headlines this evening. Southeasterly flow within a saturated boundary layer continues and similar to last night, visibilities and cloud heights are gradually trending downward. In general, surface observations and spotter reports indicate isolated dense fog is already occurring across the region. The downward trend of visibilities are expected to continue through the rest of the evening, with dense fog gradually becoming more widespread through the overnight hours. Surface winds are a little higher over north-central WI than last night, which may be just enough to keep fog characterized as patchy dense rather than widespread dense. With a tightening pressure gradient, MOS guidance indicate surface winds increasing generally along and west of the I-39/US 51 corridor roughly after 3 am. The increasing turbulent mixing via boundary layer winds could result in ending the Dense Fog Advisory early over western portions of the forecast area. Don't think the eastern parts of the forecast area will have that same potential. Regardless, no changes to the ending time of the Advisory. && .KEY MESSAGES... - A Dense Fog Advisory has been issued for the entire forecast area through 11 AM Sunday. Poor visibility will result in hazardous driving conditions, and may lead to flight delays or cancellations. - Record or near record temperatures expected for Christmas Eve and Christmas Day, then readings will cool throughout the week. - An active pattern containing several shortwaves will bring multiple chances for precip to the forecast area throughout the week. Widespread amounts of 0.40-0.60" are expected with some areas approaching one inch of liquid precip. && .DISCUSSION... Issued at 325 PM CST Sat Dec 23 2023 Short Term...Tonight and Sunday Fog / Precip Trends: Widespread dense fog slowly lifted through the morning hours, with patchy dense fog continuing across parts of the area early this afternoon, especially from Shawano County northwestward, but Oshkosh and Sturgeon Bay were still back to 1/2 and 1/4 mile at times. Dewpoint depressions remain very low this afternoon, so expect the fog to remain/develop once again through the late afternoon and evening. After collaboration with surrounding offices, went ahead and issued another dense fog advisory into Sunday morning. Will start it at 00z as the dense fog will be more patchy late this afternoon, then is expected to expand through the evening. Winds aloft do increase overnight, so the fog may lift slightly into a very low stratus deck, but it being a busy holiday travel time, will lean on the cautious side. Weak lift and WAA could produce some patchy drizzle later tonight into Sunday as well, with a trace to a couple hundredths of an inch of rain expected for most spots. Temperatures: A very mild night is on tap for the area with a strong push of WAA reinforcing the already well above normal temps. Temps won't drop much, with most spots only falling into the upper 30s to low 40s, which is 20-30 degrees above normal. On Sunday, another mild December day is expected. Will lean towards the slightly cooler, better performing guidance with the thick cloud cover in place. But that still brings high into the mid 40s to low 50s, with some records in jeopardy. Long Term...Sunday Night Through Saturday Monday through Wednesday... An active pattern is expected to set up as a northern and southern stream shortwave are progged to phase and develop a closed upper-level low over the central Plains early on Monday. This will result in the placement of a 1001 mb surface low that hovers over western Iowa until it ejects into the upper Mississippi Valley Tuesday afternoon. This system is expected to bring two rounds of precip to much of Wisconsin Monday into Tuesday. The first round will arrive Monday morning in conjunction with a surface cold front. A brief dry period is possible Monday afternoon and evening as the dry slot sets up across central Wisconsin, confining most PoPs during this time to northern and southern portions of Wisconsin. The second round of precip will then wrap in from the deformation zone of the surface low to the south later on Tuesday as an occluded front lifts north across Wisconsin. A plume of Gulf moisture extending into the Midwest on Monday will increase precipitable water values to around five standard deviations above average during this time, placing more widespread QPF across the western Great Lakes on Tuesday. Highest QPF values approaching one inch will be located in regions of western to central Wisconsin with amounts decreasing to the east. Highest values will also be collocated with an area of weak mid- level FGEN associated with the surface occlusion. Temperature profiles ranging from the mid 40s to low 50s indicate that this will be a rain event, with some areas possibly being in danger of breaking record high temperatures on Christmas as south/southeast winds advect mild temperatures into the region. Wednesday night through Friday... As the closed upper-level low exits to the east, another robust shortwave will dig south from Ontario late Wednesday into early Thursday. Some models suggest that this shortwave will briefly become a closed low, although there is still some discrepancy. Precip associated with this system looks to be mostly snow as most of Wisconsin sets up in a cold air advection regime under northerly flow. Potential for lake enhancement across Lake Superior may lead to some accumulations in northern Wisconsin, although it is still too soon to determine exact amounts. The bulk of the precip is expected to fall Wednesday night into Thursday, although lingering northerly flow on the back end of the exiting surface low may lead to some flurries into Friday. Saturday and Sunday... Additional shortwave energy is progged to sweep through the Great Lakes region Saturday into Sunday. Precip chances are possible with this system, although it is still too far out in the forecast period for models to resolve any details. Current model guidance suggests temperatures may be cold enough for any precip to be in the form of snow, although uncertainty is still prevalent. && .AVIATION...for 06Z TAF Issuance Issued at 1039 PM CST Sat Dec 23 2023 Poor flying conditions will continue into Sunday morning with moist, upslope flow continuing. Visibilities and ceilings will deteriorate further into the early overnight hours. Most locations will see near to below airport minima in dense fog again overnight into early Sunday morning. A Dense Fog Advisory has been issued. Visibilities should improve relatively quickly Sunday morning thanks to a tightening pressure gradient and increasing boundary layer winds. MVFR/VFR visibilities are likely to return by early in the afternoon. Ceilings will lag behind somewhat with LIFR lingering across north-central WI through the afternoon, and IFR/MVFR ceilings elsewhere. The chance of drizzle will increase by late Sunday afternoon when saturation depth increases slightly, but low confidence whether the saturation depth will be sufficient for drizzle. Arrival of widespread showers looks slower, most likely after 06z Mon over north-central and central WI. && .GRB WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until 11 AM CST Sunday for WIZ005-010>013- 018>022-030-031-035>040-045-048>050-073-074. && $$ UPDATE.........MPC DISCUSSION.....Bersch/Goodin AVIATION.......MPC