Marine Weather and Tide Forecast for Oshkosh, WI

DISCUSSION
Issued at 450 AM CDT Mon Apr 28 2025

Short Term
Today
Tonight...and Tuesday

Scattered convection with small hail was poised to move into our western counties early this morning. This activity was forming on the leading edge of the elevated instability gradient, where WAA was increasing on the nose of a 35 kt low-level jet.

Elevated convection will increase mainly west and north of the Fox Valley this morning through early afternoon, with forcing provided by a short-wave trough and increasing WAA as the low- level jet cranks up to 50 to 55 kts. Instability will be marginal and elevated, with MUCAPE of 300-800 j/kg. However, deep layer shear will increase quickly, reaching 50 to 60 kts by early afternoon. Wet-bulb zero heights will be favorable, in the 9-10K range, so storms will have potential for small to marginally severe hail.

Subsidence behind the departing short-wave, possible low clouds north of the warm front, and some capping may suppress thunderstorm activity during the mid to late afternoon. However, the arrival of surface-based instability into central WI, combined with the impressive shear parameters (SRH 400-600 m2/sec2) in place, suggest that any isolated/scattered discrete storms that develop could quickly become severe and possibly tornadic, especially in the southwest part of the forecast area.

Storms should take on more of a linear mode (squall line/QLCS) as a cold front approaches in the evening. This line is expected to arrive in NC/C WI early in the evening (01z-02z/Tue), and track ESE through the forecast area. SBCAPE of 1000-1500 j/kg and 0-3 km CAPE of 500-100 j/kg, continued impressive shear parameters, STP values of 3-8 and strong Updraft Helicity indices support high-end severe potential. Damaging winds and embedded tornadoes will be the primary threats as this line moves across the forecast area between 8 pm and 1 am. The severe threat should diminish overnight as surface-based CAPE gradually weakens. SPC has kept most of GRB CWA in an Enhanced Risk of severe storms, with part of central WI in a Moderate Risk. Nearly the entire forecast area is included in either a 10% or 15% hatched tornado threat area, indicating significant potential for strong tornadoes. Think that the greatest threat for strong tornadoes will over NC/C WI, with a lesser threat farther east.

Precipitation chances will be lower late tonight into Tuesday morning, through a small thunder threat will likely persist until the cold front moves through Tuesday morning. Dry conditions return Tuesday afternoon.

South winds will increase today, with gusts reaching 25 to 35 mph during the afternoon and evening. The winds will turn west to northwest as the strong cold front moves through Tuesday morning, with gusts continuing in the 25 to 35 mph range until mid afternoon. After a mild/warm day today, strong CAA will result in much cooler temperatures on Tuesday.

Long Term...Tuesday Night Through Sunday

High pressure will move through the western Great Lakes late Tuesday through Wednesday, with light/variable surface winds Tuesday night gradually becoming southeasterly by Wednesday afternoon. The presence of mostly clear skies and light surface winds will result in cool overnight lows Tuesday night, with most locations in NC/far NE WI expected to fall below freezing. High temps will rebound some on Wednesday afternoon, though onshore flow will keep temperatures considerably cooler near Lake Michigan.

The next disturbance approaching from the Northern Plains will attempt to phase with low pressure tracking northeast toward Lower MI during the Wednesday night through Thursday night period. This will result in a good chance of showers, especially late Wednesday night into Thursday. Relatively mild temperatures will allow for all rain during this system's passage. A few showers may linger into Friday as an upper trough moves through, but dry conditions return for the weekend.

AVIATION
for 12Z TAF Issuance Issued at 641 AM CDT Mon Apr 28 2025

Have focused on the ongoing convection over the CWA, a line of storms moving rapidly through MN and another line of storms expected to move through the forecast area this evening into the early overnight to determine timing of thunderstorms in the TAFs.
Given the messy pattern, overall confidence in the evolution of this event is low. This morning's convection will mainly have a hail threat, but any storms that form later today into this evening will have a damaging wind and tornado threat.

Ceilings will drop to MVFR west and north of the Fox Valley this morning, and could remain there for most of the day. Partial clearing may occur in central and east central WI as the warm front lifts through later in the day. Low ceilings will also accompany the squall line this evening, and IFR ceilings could arrive in NC WI as the cold front moves through late tonight.

LLWS is expected across much of the area through the TAF period, due to a low-level jet increasing to 50 to 55 kts. Surface winds will also be strong, with south winds gusting to 25 to 30 kts this afternoon/evening, then turning west to northwest as the front move through the northwest part of the forecast area late tonight.

GRB WATCHES/WARNINGS/ADVISORIES
None.