Marine Weather and Tide Forecast for Deerfield, WI

UPDATE
Issued 1035 AM CDT Wed Apr 23 2025

Though this afternoon's thunderstorm potential remains as stated in the SHORT TERM discussion, we have noticed a downward trend in chances and intensity. The remnants of a would-be MCV continue to track eastward from Darlington / Madison WI, but are expected to continue a weakening trend as the upper-air pattern outpaces the convection, leading to an area of subsidence.
Generally free of lightning with this activity through the morning. Many CAMs struggle to resolve convection in the wake of this activity, and the present cloud cover suggests it will be tough for the airmass to recover for afternoon convection. As such, the afternoon precip chances have been lowered to 20-45% for most areas.

One thunderstorm currently approaching Oshkosh / Lake Winnebago is expected to continue due east. You may hear some rumbles of thunder in northern Fond du Lac and Sheboygan County over the next hour, though it should move over Lake Michigan thereafter.

Sheppard

SHORT TERM
Issued 313 AM CDT Wed Apr 23 2025

Today through Thursday:

Main focus for today will be the waffling stalled frontal boundary currently sitting just south across IA/IL. Have been seeing convection develop along the boundary overnight with an occasional taller storm producing severe hail back in IA.
Overall there remains limited if any upper-level support for this activity with the core of the upper-level jet strung further north across the Dakotas and MN border and mid-level vorticity max with the shortwave trough along the MN/Canadian border. The ongoing scattered storms are mainly being driven by convergence along the surface boundary and additional development is likely being triggered by mesoscale forcing such as outflows from nearby decaying convection. So will monitor these trends through the early morning hours as the better forcing remains south of the WI/IL border.

Expecting this stalled boundary to gradually lift northward through daybreak as a shallow low-level (700mb) and surface trough is progged to lift northward up the Mississippi River Valley along with a modest 30kt 850mb jetlet, where the latest convective development is occurring. As the surface boundary pushes north of the Cheddar Curtain into southern WI, expecting the scattered shower and few thunderstorm chances to follow suite through the morning. Again this activity lacks upper-level dynamics/support, but there will be enough low-level convergence paired with dewpoints climbing into the 50s for off- on shower/storm chances through much of the day. Thus today is not going to be a complete wash out as there will be periodic dry periods.

The 00z CAMs/HRRR members agree with this trend of storms chances gradually lifting northward through the morning. Again the main driver for convection will be the surface boundary itself along with any nearby mesoscale forcing mechanisms such as remnant outflows or even potentially the lake breeze this afternoon. While not expecting widespread development of stronger storms, cannot not rule out seeing a few isolated storms grow more upscale and even become severe as it taps into the modest deep layer (0-6km) shear around 30kt along with SBCAPE progged to top off between 500-1000J/kg by the afternoon/evening. Mainly would be hail threat if any stronger/severe storms develop, but could see some gusty winds mix down given some hints of steeper low-level lapse rates approaching 7C/km. Additionally if any storms train over the same area for extended period of time, then we cannot rule out a localized flash flooding potential, but given the boundary/convection looks to be fairly progressive as it moves across southern WI may limit the concern.

As we lose daytime heating and as the frontal boundary is progged to lift northward this evening, expecting a decrease in shower/storm chances overnight into Thursday. Thursday does look to be a bit drier through the morning and early half of the afternoon with the front north of the area. However additional scattered shower and thunderstorms chances will return later on Thursday as the frontal boundary looks to shift back southward as high pressure builds across the Upper Midwest and as a ripple of mid-level is progged to trek across the region. As the boundary begins to shift south, there continues to be hints of a backdoor cold front to works its way down across Lake Michigan to help shift the boundary southward and bring cooler temps for the lakeshore areas later Thursday afternoon/evening.

In between the shower/thunderstorm chances, southern WI is looking a a warmer stretch today and Thursday with highs in the upper 60s and low 70s for inland portions. Meanwhile areas look to be a bit cooler by the Lake Wednesday and especially on Thursday with the potential of the backdoor cold front feature.

Wagner

LONG TERM
Issued 313 AM CDT Wed Apr 23 2025

Thursday night through Tuesday:

The active pattern will continue overnight Thursday as another round of showers and storms will be possible ahead of an incoming system tracking up from the Plains. An elongated sfc trough is expected to close off into a surface low along the Wisconsin and Illinois border Friday morning. The GFS and NAM struggle to produce MUCAPE over the region, given saturated conditions aloft, so most of the activity Friday morning should be thunderless. The low will progress east through the day, with rain coming to an end Friday afternoon/evening. High pressure will build into the Upper Great Lakes Saturday and Sunday with a slight cooling in temps to the low 60s Saturday and then mid to upper 60s Sunday as warm advection starts over the region as a large upper ridge shifts east from the Plains.

Monday and Tuesday will then see enhanced warmth and humidity building into the upper Mississippi Valley as the high amplitude ridge moves overhead. With southwesterly flow aloft and passing weak shortwaves, chances for thunderstorms are expected to begin Monday and linger through Tuesday until a sharp trough digs and swings through the Upper Midwest Tuesday night. Given the overlap of CAPE and shear over our region during this time, severe weather does appear possible, but it is too early to assign a discernible area/threat magnitude to any storms we see at the end of the extended period.

CMiller

AVIATION
Issued 1040 AM CDT Wed Apr 23 2025

Showers from elevated convection expected to track east from Darlington / Madison WI, bringing off / on rain to southern WI terminals this morning through early this afternoon. Generally expecting a lull in activity for a few hours after it passes, then additional 20-45% chances for showers and thunderstorms later this afternoon through this evening. Light south to southwest winds through said period. Cloud ceilings generally VFR, with the exception of any substantial showers / storms if applicable (MVFR cigs possible).

A similar story for tonight, with chances for scattered showers / storms. Southeastern WI terminals are largely spared from this activity (generally less than 20% precip and storm chances late tonight along and south of a Janesville to Milwaukee line).
North of there, expecting 30-40% chances. Light and variable winds (due to a surface frontal boundary stalled over the region). Cloud ceilings generally remaining VFR, MVFR possible in the event of showers / storms. Seeing some potential for fog to develop overnight (low confidence), will include in the next TAF issuance if confidence is high enough.

Sheppard

MARINE
Issued 313 AM CDT Wed Apr 23 2025

Low pressure continues to slowly weaken as it lifts north of MN into south central Canada. Light and variable winds persist over Lake Michigan as a weak warm front is expected to lift north through the morning and continue northward through this evening into Thursday. Expecting periodic shower and thunderstorm chances along the frontal boundary as it slides north. While winds remain fairly light expecting more easterly winds north of the warm front with more southerly winds south of the boundary. Expecting to see the front to waffle back south and gradually wash out into Thursday as high pressure builds across the Upper Great Lakes. Winds look to be variable through the end of the week before gradually increasing into Friday as another surface low works it way across the Midwest from the Plains.

Wagner

MKX WATCHES/WARNINGS/ADVISORIES
WI...None.
LM...None.