textproduct: Green Bay

UPDATE

Issued at 417 AM CST Thu Dec 26 2024

Impacts increasing as reports of slippery roads are emerging over parts of the area. Since temps remain below freezing through late morning and through that time there will be intervals of drizzle, have issued a winter weather advisory for patchy freezing drizzle until noon. Not sure on extent of the freezing drizzle over far northern WI this morning, so have kept those areas into far northeast WI out of the advisory for now. Thus far most of the drizzle has stayed west of the lakeshore, so will also leave lakeshore counties out for now as well. Will continue to evaluate both those areas through the morning for further expansion of the advisory.

KEY MESSAGES

- Patchy drizzle this morning, before steadier light rain arrives this afternoon into tonight, but especially on Friday. Below freezing temperatures for all but the immediate lakeshore will result in freezing drizzle and potential of light icing (20-40 percent chance of over 0.01 inch of ice) this morning. Risk of icing will persist over northern Wisconsin tonight into early Friday morning.

- The combination of a mild and moist airmass flowing over a snowpack will result in areas of fog at times, beginning late this morning or early this afternoon and continuing at times into Saturday morning. The fog may become dense with visibilities around a quarter of a mile or less. - Above normal temperatures are expected to continue through the end of the year with colder temperatures returning during the first week of January. There is some uncertainty in the magnitude on how cold it will get after January 1st. The Dynamic Ensemble based Scenarios output indicated a large temperature spread on January 2-3.

DISCUSSION

Issued at 318 AM CST Thu Dec 26 2024

Short Term...Today...Tonight...and Friday

Typical hazards that occur when winter takes a break with snowcover still present are appearing this morning; drizzle, freezing drizzle and fog. This scenario sticks around into the upcoming weekend.

Precipitation...Radar and local obs point to some drizzle already making its way northward across the area early this morning as southerly flow increases with high pressure departing east. Thus far, reports indicate the drizzle is very light and not resulting in any icing. Appears better chance that sufficient low- level lift and saturation team up to result in more formidable drizzle and/or light rain occurs late today through tonight. By that time, sfc temps will have risen above freezing for all but far northern WI. Thus the freezing aspect/icing potential from the drizzle will be minimized over most of the area. Even as air temps rise above freezing later tonight even over northern WI, road sfc forecasts are still shown to be near freezing. Continued to carry mention of rain/freezing rain over northern WI as the lingering cold ground/road temps may still lead to some slippery spots on untreated roads. By mid morning Friday, risk of icing should be over as forecasts of road temps rise above freezing even over northern WI. Next area of more widespread rain will arrive Friday afternoon as southern stream system lifts across the area. This wave will result in the most rainfall over the next two days, with amounts by late Friday afternoon up to 0.10 inch over northeast WI and pushing toward 0.25 inch over portions of central WI.

Fog and potential for dense fog...Certainly already becoming foggy over southern WI currently and based on projection of dewpoints at or above 32F lifting in from the south across the snowpack late this morning into early afternoon, expect areas of fog to expand from south to north across our forecast area. The fog may become dense the rest of the day. 00z NAM and GFS MOS guidance failed miserably at forecasting the dense fog currently over southern WI so have started to mentally lower its persistent, but light, fog forecasts across our area later today into tonight. This matches upstream obs, past climo and majority of high-res guidance/HREF ensembles which point to probabilities of visibility less than one-quarter mile increasing to over 80 percent this afternoon into tonight over almost the entire area. Looking likely we'll need a dense fog advisory eventually, but with uncertainty on start timing, will not issue that right now. Once it forms, areas of dense fog likely will persist well into Friday. Normally, the rain moving in on Friday afternoon would scour out the fog, but with this being more of an advection fog over melting snowpack, it may just lead to thicker and more widespread fog. Additionally, as dewpoints continue to rise to around 40 over Lake Michigan, marine fog will likely develop later today and that along with onshore flow could also expand/sustain the fog developing over land areas. Seems not a question of if but when concerning fog. Once it forms, it may stick around even into the start of the weekend.

Long Term...Friday Night Through Wednesday

A negatively tilted 500mb trough is expected to swing across the area Friday night. This system has a 70-90% chance of bringing a tenth of an inch of rain across the entire area and 30-50% chance of a quarter of an inch across northeast Wisconsin. Along with the rain, areas of fog are expected which is expected to be dense at times with a visibility of 1/4 mile or less. On Saturday, the system will move away which is expected to bring diminishing chances of rain. The passage of the front along with increasing winds should help diminish and end the fog.

Dry conditions should prevail Saturday and for most of Sunday. Low confidence in the small chance of rain across northeast Wisconsin on Sunday. Monday is expected to be dry before the next system moves across the region on New Years Eve Day. An interesting wrinkle into the forecast has been introduced tonight. The GFS continues to be consistent with a fairly decent precipitation event for northeast Wisconsin. The ECMWF on the other hand keeps the system well south of the area. The wrinkle in the forecast is from the Canadian model which now has shifted considerably north from previous runs. The model output tonight would bring some precipitation to our southern counties. There is a small chance of rain/snow to account for the shift of the Canadian.

Above normal temperatures are expected to continue through the end of the year, then turn colder after the 1st. There is some uncertainty in the magnitude of the cold air as there is a fairly large spread in temperatures on January 2-3 at Green Bay off the Dynamic Ensemble based Scenarios output. With the northwest flow pattern in early January, snow shower activity may sneak into the far north-central Wisconsin at times, thus small chances of snow over the far north Tuesday and Wednesday.

AVIATION

for 12Z TAF Issuance Issued at 551 AM CST Thu Dec 26 2024

LIFR/IFR cigs will prevail through the TAF period. Thus far, mainly seeing IFR/MVFR vsby, but as fog over southern WI spreads north this morning that will change. Most locations will experience VLIFR/LIFR vsby this afternoon through end of the TAF period as fog becomes more dense. Areas of drizzle and freezing drizzle lifting through the area through midday will change to mainly drizzle and light rain late today into tonight. The precipitation will be intermittent, but once the fog develops, it will persist through the rest of the TAF period and on into Friday as well.

GRB WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until noon CST today for WIZ018>021-030- 031-035>039-045-048-049-074.