Marine Weather and Tide Forecast for Sparta, WI

DISCUSSION
Issued at 205 PM CDT Tue Apr 22 2025

In the wake of this morning's storms, a weak quasi-stationary surface boundary remains draped through central Minnesota. Morning GOES upper level water vapor imagery showed a zonally oriented transient thin, dry trough axis racing across the Northern Plains causing rapid lifting to the low and the attached filament of moisture. The strongest low level moisture axis extends well to the southwest through the Central into the Southern Plains with a 10 to 20 degree dewpoint gradient, subsiding dry air lee of the Rocky Mountains abutting rich Gulf moisture. A quasi-regressive pattern with confluence between two areas of anticyclonic flow amplifies upper level heights from the Central into the Northern Plains which keeps the surface boundary mostly stationary across the southern half of the forecast area over the next couple of days.

Precipitation Chances Tonight:

Initial precipitation chances will be tied to the elongation of a curved low level moisture axis being stretched by pronounced synoptic easterly flow. VWPs through the Central Plains collocate the low level front with early afternoon MRMS, increasing areal extent of a narrow corridor of reflectivity early this afternoon. Mostly will skirt to our south tonight with the aforementioned easterly flow.

Precipitation & Storm Chances Overnight into Wednesday:

Subsequent precipitation and storm chances increase in locally southern counties from northeast Iowa into southwest Wisconsin overnight into early Wednesday morning. The nocturnal low level jet really ramps up moisture transport, flirting the 60 degree isodrosotherm along our doorstep. The anticyclonic curvature to the low level jet and overall easterly flow expected to shunt stronger storms and heavier rain along our southern most counties and then parts east. Due to higher shear values remaining postfrontal, a push/pull of instability over the front limits strong storm concerns. Expect weakening storms as they progress northeast then east as they potentially reach the weak frontal boundary. A very short, transient window of isentropic upglide could advect storms slightly farther north but confidence is low, so have kept highest PoPs to the south.
Northern extent of rain and storms could end up more of an immediate term forecast concern. While a strong to severe storm cannot be ruled out, not overly confident at the current hour given an overall lack of forcing.

Wednesday Storm Potential:

The instability axis builds through central Wisconsin on Wednesday as diurnal heating helps advect a narrow axis of sufficient CAPE from southwest to northeast. Similar to Wednesday morning, forcing looks questionable as overhead ageostrophic descent downstream of the ridge axis is collocated with isentropic descent. Therefore, a narrow axis of storms will be possible through central Wisconsin. LREF cluster analysis (22.00Z) has high agreement for initial collocation of MUCAPE, limited MUCIN, and 30kts of Bulk Shear along our southern half of the forecast area, again.

Any diurnal heating influence hinges on morning storm coverage and timing on exit cloud cover. The best low level stretching and forcing may be overnight which would limit storm concerns.
Have increased PoPs given the narrow corridor of stretching. GFS suggests severe possibility with sufficient 0-3km shear while the ECMWF remains near nil. While overall impacts and confidence is low, Wednesday storms could be something to keep an eye on in coming forecasts as there are a few things to come together yet.

Widespread Rainfall Thursday:

Eventually the global pattern gets a little more progressive, driving a surface low northeast along the low level baroclinic boundary through Thursday. The quasi-stationary front won't budge though, perpetuating precipitation through Friday morning when it eventually shoves east.

Thursday will be the most widespread rain with potential storms as well. Rainfall amounts will be above average but not extreme according to 0.7 ECMWF EFI collected with a 1.25" PW that is 2+ standardized anomalies above model guidance. Global ensemble spread (25-75) suggests 0.5" to 1.75" in 24 hours. Not overly concerning regarding floods. Impacts in southern most counties may be of higher concern, dependent on how the next couple storms shake out.

Severe Storm Potential Next Week:

Rain and storm chances don't go far however as a Rocky Mountain Low quickly forms through the weekend, slowly lifting northeast through the Central Plains. While current confidence in low placement is minuscule, extending from Canada through the Texas Panhandle (EPS/GEFS), most long term global ensembles surge 2000-4000 J/kg of CAPE somewhere from the Northern Plains through the Upper Mississippi River Valley. SPC Convective Outlook places us in a Day 7 for potential severe storms.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/
Issued at 619 PM CDT Tue Apr 22 2025

VFR conditions anticipated for the next 24 hours. Light winds at less than 10 kts out of the south will continue for much of the overnight period before veering to the west/northwest late tomorrow morning into tomorrow afternoon. Showers and storms should remain south of the terminals overnight along a stalled frontal boundary with activity building north through the day tomorrow. Confidence on the northward extent of these showers and storms remains fairly low but did include PROB30 groupings to account for current uncertainties. Any showers or storms could result in MVFR ceilings/visibilities at the terminals.

ARX WATCHES/WARNINGS/ADVISORIES
WI...None.
MN...None.
IA...None.