Marine Weather and Tide Forecast for Oconto, WI

DISCUSSION
Issued at 324 AM CDT Tue May 13 2025

Pattern starting to show subtle signs of changing after the prolonged period of dry weather. Upper low drifting north across the Ohio Valley along with weak sfc low will lead to less ridging except for far north central WI today. SE flow to the north of low will transport more moisture into southern and eastern forecast area while the north remains under the influence of drier low- levels and one more day of lower RH values. Light showers have developed in the stream of increased moisture and some of these could impact east-central WI before daybreak into the morning.
Also as dewpoints in the 50s and even lower 60s flow over the still chilly Lake Michigan waters (water temps of 39-42F per the deployed buoys), marine advection fog may expand early this morning. If this occurs, then fog would push onshore along Lake Michigan this morning, but due to mixing on land, probably will not make too much westward progress on the land the rest of the day. Isolated pop-up showers/thunder could occur inland especially over central WI this afternoon where highest dewpoints in the lower 50s, sfc based CAPE reaches 500J/kg. CAPE is skinny and there is minimal effective shear, so just looking at isolated non- severe storms, if they occur. Temps today will be similar to yesterday over far north-central WI with less cloud cover and deeper mixing. Otherwise, highs will trend downward with mid 70s to lower 80s, but only around 60 near Lake Michigan.

Moist, southeast flow persists tonight and this looks to bring in better chances for areas of fog/stratus as marine advection fog expands off Lake Michigan. MOS guidance picking up on the fog as well which makes sense as crossover temps are forecast to drop below afternoon dewpoints over much of the area. Have expanded fog and lingered this into Wednesday morning. Another round of pop-up showers and storms is expected farther inland from lake stabilizing flow on Wednesday afternoon as sfc based CAPES increase to 500 to 750J/kg from central to north-central WI.
Again with limited effected shear, no severe storms are expected.
With more clouds to start the day and limited mixing, highs on Wednesday will come down across the board with readings in the 70s all areas, though staying in the 50s near the Lake Michigan shore.

As has looked for a while, setup becomes *much* more favorable for severe weather Thursday afternoon into Thursday evening as negatively tilted upper trough emerges from the northern plains and lifts across MN and WI. Decent dynamics with the system given height falls from the trough along with 60+ kt H5 jet nosing into WI during peak heating acting upon robust instability with MLCAPEs exceeding 2500J/kg (Highs bouncing back up into the upper 70s to lower even mid 80s) and mid-level lapse rates over 7.5c/km ahead of approaching cold front. Low-levels fairly veered, so wind and hail are the primary concerns. Kind of early to get into these details, but given length of hodographs and relatively low wbzero heights (up to 9kft) with the degree of the instability forecast, if storms remain more discrete, hail could end up bigger than the lower threshold for severe hail (one inch in diameter). Will be something to watch. Some potential that storms could grow upscale, leading to more of a damaging wind and possibly spin up QLCS tornado scenario. A lot of questions to sort out, but SPC does now have all but far northern WI in a slight risk for Day3/Thursday.

Additional showers and maybe thunder occurs on Friday as secondary shortwave slides across the area. As main surface low crosses Upper Michigan and Lake Superior late Friday into Saturday pushing a secondary cold front across the area, some scattered showers remain in the forecast on Saturday, especially over northern WI.
Temps will cool substantially with highs 60s to lower 70s on Friday, but only in the 50s to lower 60s on Saturday behind the secondary cold front. Rest of extended features temps staying at or slightly below normal. With high pressure close by and winds becoming light, Saturday night and/or Sunday night could see patchy frost over the north. Next chance of rain returns on Tuesday.

AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/
Issued at 1248 PM CDT Tue May 13 2025

Isolated showers with a lightning strike or two will be possible this afternoon west of the Fox Valley. IFR ceilings along the Lake Michigan shore are expected to improve to MVFR this afternoon. Further inland, MVFR ceilings should improve to VFR, although there is uncertainty. VFR toward central/north central WI this afternoon and evening.

During the evening, expect the marine fog to spread onshore and then expand inland during the overnight hours. Dense fog looks likely for the lakeshore to the Fox Valley, then patchy dense fog is possible further inland. Just some patchy light fog is possible for north central.

Ceiling and visibility trends will be similar on Wednesday as they were today. The shower (with isolated thunder) coverage is expected to be higher (25 to 40 percent) toward central and north central WI Wed afternoon.

MARINE
Issued at 1019 AM CDT Tue May 13 2025

Expanded the Marine Dense Fog Advisory northward one zone to include areas south of Sturgeon Bay. The marine fog is expected to stick around through at least Wednesday morning due to light, moist easterly flow over the cold open waters. An additional northward expansion to include the Door Peninsula will be possible, but more uncertainty remains.

GRB WATCHES/WARNINGS/ADVISORIES
WI...None.
MARINE...Dense Fog Advisory until 1 PM CDT Wednesday for LMZ542-543.