Marine Weather and Tide Forecast for Sparta, WI

DISCUSSION
Issued at 142 PM CDT Mon May 12 2025

Elevated Fire Conditions into the Evening

Temperatures were into the 80s across much of the area this afternoon with southerly winds gusting to near 20 mph. The lowest relative humidities will be north and west of La Crosse and at 18Z, these values were in the lower 20%s. Thus, elevated fire danger through the early evening is expected.

Increasing Clouds, Moisture and Rain Chances

An upper level low pressure system located over Mississippi will slowly move toward the Ohio Valley and into the eastern Great Lakes by Wednesday. On Tuesday, east/southeast winds ahead of the low will bring moisture east across IL/WI and into the area. By Tuesday afternoon, some marginal instability will be present with broken sunshine allowing for the development of showers and a few thunderstorms during peak heating. Coverage will increase toward the southeast, with those further north and west seeing more sun and less/no shower activity (near/west of a Medford-Rochester-Charles City line). Would expect southwest WI to experience similar to what central/nrn IL are seeing today with showers on radar. Some chances will again be present across WI /bordering IA/MN Wednesday afternoon as the area remains on the westward extent of the influence of the low centered near Michigan. Plenty of dry hours are expected both days with hard to predict, transient showers/storms moving from east to west.

Severe Thunderstorms Possible Thursday

Synoptic scale ingredients still look to be coming together over the Upper Mississippi river valley for the potential for severe thunderstorms on Thursday. Good consensus exists on ample low-level moisture ahead of an ejecting, negatively tilted tropospheric-deep low progressing across the the area. Forecast guidance really struggles this time of year with low-level dewpoint forecasts however, so the magnitude/abundance of the moisture, and thus instability, is still a bit unclear. As evidence of the struggles, model dewpoint forecasts in the 60s are forecast for 7 pm tonight across the area in the 12.12Z NAM/GFS, nearly 10-15F too high in a 12 hour forecast. It appears ample MLCAPE will be present with even conservative measures (25th percentile 1000-1500 J/Kg) ahead of an approaching mid-level wind max of 60-70kts and strong dynamical lift. A stronger capping inversion looks like it would limit pre-frontal storms and initiation would occur right on the front. But details in the cap strength southward and maybe most important, the timing of the wave and cold front, still need to be resolved. It appears the higher probabilities suggest mainly a Wisconsin risk, while there is a 20% chance for a slower frontal timing, bringing severe storm risks westward into MN/IA further /based on the 12.00Z Grand Ensemble, 12.12Z NAM/GFS. A consistent ECMWF deterministic signal is a slower timing in the last three runs /12.00-12.12Z/.

The risk probabilities for this event will increase from west to east across the area and mainly target Thu afternoon. The AI probabilities have a bit of spread in them too but do indicate this day needs to be monitored. SPC is currently highlighting much of WI and bordering MN/IA counties with a 15% risk which would likely translate into a level 2-3 of 5 risk which still looks appropriate for the latest model guidance. Large hail, damaging wind and tornadoes would be possible with orthogonal storm motion to the front favoring more discrete storms / supercells. With details yet to be resolved, but consistent larger scale ingredients, will continue ongoing level of messaging.

AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/
Issued at 630 PM CDT Mon May 12 2025

VFR conditions are forecast for tonight into Tuesday morning, with a broken 3-4 kft high MVFR/low VFR cumulus deck developing by midday east of the Mississippi River and persisting into the early evening. There is a 30-50 percent chance of showers and thunderstorms in this same area Tuesday afternoon, though aviation impacts look minimal. Winds remain generally from the southeast through the period, decreasing to 5 kts or less overnight and increasing to 5-10 kts for Tuesday.

FIRE WEATHER
Issued at 142 PM CDT Mon May 12 2025

Relative humidities will see some recovery overnight, and with increasing dewpoints on Tuesday, min RH values should increase into the 30s to 40% north and west of La Crosse, higher elsewhere. There are periodic wetting rain chances as the week progresses.

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