textproduct: La Crosse

KEY MESSAGES

- Building heat through Tuesday with highs pushing into the upper 80s to mid 90s and heat indices ranging from mid 90s to around 100.

- Heat gradually wanes Wednesday and beyond with small shower/storm chances (20-30%) returning Friday into the weekend.

DISCUSSION

Issued at 201 AM CDT Mon Jul 13 2026

Through Tuesday: Building Heat

A toasty first half of the week is in store as anomalous upper ridging migrates from the Northern Plains to right over the forecast area. Soundings from 13.00Z show 500mb heights of 599 dam at Bismarck, Aberdeen, and Riverton. Of note, Rapid City looks to have preliminarily tied their all time maximum 500mb height of 600 dam, which underscores the unusual magnitude of this ridge and associated heat for mid July. Further diving into how potentially unusual this event will be for the Upper Mississippi Valley, the ECMWF Extreme Forecast Index for maximum temperatures ranges from 0.8 to 0.95 across our area by Tuesday, with values nearing 1 along the north and south shores of Lake Superior. This indicates most of its ensemble members suggest a rare magnitude of heat that falls outside the model's climate record back to 2005. This signal has been consistent over the past several runs. However, we are not anticipating a record- breaking heat event locally, as shift of tails values are around 0, indicating a lack of extreme maxT values from any of the ensemble members.

Nevertheless, as the ridge builds overhead through Tuesday, 850mb temperatures are progged to reach 24-25C per various deterministic guidance and ensemble means. With the thermal ridge axis extending from northern Minnesota into the Upper Peninsula, 850mb temperatures will locally be warmest to the north of I-94. There have been some minor north-south oscillations of the thermal ridge axis over the past few runs, but overall impacts haven't changed much. Just to reiterate, humidity will not be as big of a factor with this heat event, unlike the oppressive dewpoints we had in late June pushing into the upper 70s. This time dewpoints in the mid-upper 60s to lower 70s are more likely.

Still looking like the river valleys and west-central Wisconsin will be the warmest for today with highs pushing the low to mid 90s. By Tuesday more of the forecast area, especially near and east of the Mississippi River, is likely to climb through the 90s. That may necessitate an expansion of the heat advisory for more of the area on Tuesday, but will let later shifts assess. Other than maybe some thin passing cirrus clouds at times, we'll be under full sunshine both days. Daytime mixing lends some forecast complexity in how surface temperatures and dewpoints will ultimately respond, given forecast soundings depict a very dry boundary layer both days with fairly light boundary layer winds. Weak to modest mixing could yield warmer temperatures than currently forecast, but would also likely result in lowered dewpoints, so there is some uncertainty in resultant impact to heat index values.

Wednesday and Beyond: Heat Gradually Wanes, Shower/Storm Chances Return

With the stout upper level ridge in place through at least mid-week, warm temperatures stick around the upper Midwest, although slightly cooler compared to earlier in the week as the thermal ridge continues southward. Along with the continued warmth, little potential for showers/storms will exist across our area while this ridge remains in place. Heading into the late week though, the upper level ridge begins to break down as various shortwave troughs ride over its northern extent. Plenty of uncertainty in how the upper level pattern will ultimately evolve at this point as the various ensemble members suggest many different solutions, but precipitation potential increases again into Friday and again at times over the weekend (20-30%).

AVIATION /12Z TAFS THROUGH 12Z TUESDAY/

Issued at 553 AM CDT Mon Jul 13 2026

Light southwest winds at 5 to 10 mph expected for today. Winds then transition to light and variable for the overnight period. Mostly clear skies and VFR conditions through the TAF period with the lone exception being some fog in the river valleys, not including the Mississippi River Valley.

ARX WATCHES/WARNINGS/ADVISORIES

WI...Heat Advisory from 11 AM this morning to 9 PM CDT Tuesday for WIZ017-029-032>034-041>044. MN...Heat Advisory from 11 AM this morning to 9 PM CDT Tuesday for MNZ079. IA...None.