Marine Weather and Tide Forecast for Sparta, WI

DISCUSSION
Issued at 1203 AM CDT Thu May 21 2026

Today - Friday: Cool and Dry Weather Continues

Fairly benign weather continues today and Friday as surface high pressure slowly meanders eastward under zonal upper level flow lingering across the Upper Midwest. A weak shortwave ridge develops this afternoon within this flow, which is expected to favor dry conditions through Friday.

However, before this ridge develops, weak shortwave trough energy reflected in the 700-500hPa layer traverses the zonal flow, reinforcing cloudiness today. Some potential for light showers exists late this morning into the afternoon as the wave propagates through the Upper Midwest. However, cloud bases sit around 10kft through the day while dry air dominates the sub-cloud layer which may prove challenging for the weak synoptic scale forcing to overcome. As such, the 20.18z REFS suggests probabilities for measurable precipitation sits at <15%, highest south of I-90 where slightly more favorable moisture profiles exist.

With little in the way of airmass change expected in the next 24-48 hours, cooler temperatures are expected to linger into the weekend.

Friday Night - Sunday: Periodic Showers and Storms

Greater precipitation chances come Friday night into Saturday associated with two shortwave troughs moving near/through the Great Lakes region. The first wave is along more of a northern stream, propagating along the northern Great Plains and the second is along more of a southern stream, propagating through the Ohio River Valley. Moisture is a bit of a question during this period across our region though. The northern stream wave takes on a negative tilt with time, resulting in the strongest 850hPa moisture transport focusing over central to western Minnesota while the southern stream wave maximizes moisture transport south of our area. The 20.19z NBM certainly highlights the these two areas, northwest and southeast of our area, as the most favorable for rain Friday night into Saturday (60-80%), leaving relatively lower probabilities across our area (40-60%).
Deterministic guidance even suggests that our area may primarily remain dry with only scattered showers. Little to no instability is depicted with these waves, owing to the overnight timeframe and warming temperatures in the 850-700hPa layer associated with warm advection ahead of the waves. As such, currently not anticipating thunder.

One final shortwave trough swings through the region Saturday afternoon, bringing another, albeit low, shot at scattered showers (10-20%). Some thunder potential exists during this timeframe as the 20.18z GEFS and EPS depict upwards of 500 J/kg of SBCAPE and MUCAPE, respectively. However, 20.18z GFS soundings suggest instability will fail to reach the -10C isotherm, greatly reducing the risk for lightning development. Adequate moisture is also an issue during this period, leading to the overall lower confidence in precipitation chances.

Next Week: Warming Temperatures

Height rises associated with an anomalously (95th percentile per the 20.12z LREF) high amplitude upper level ridge building across the central United States next week opens the doors for increasing temperatures across the Upper Midwest. A warming trend is noted in all of the global ensembles including AIWP ensembles. While there remains some cold outliers, the interquartile range of the aforementioned ensembles is impressively low at this point, and suggests temperatures climb into the upper 70s to low 80s by early next week.

AVIATION /18Z TAFS THROUGH 18Z FRIDAY/
Issued at 1216 PM CDT Thu May 21 2026

VFR conditions are expected throughout the TAF period with easterly winds of around 8-12 kts. Easterly winds will increase during the late morning on Friday to around 10-15 kts with gusts up to 25 kts.
Upper-level clouds this afternoon transition to mid-level clouds this evening and into the daytime hours on Friday.

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