DISCUSSION
Issued at 154 PM CDT Fri Jul 26 2024

Short Term...Tonight and Saturday

Mostly clear skies are expected tonight across the region. Some modest surface warm air advection will keep temperatures a bit warmer tonight, with lows generally ranging from 55 to 60 degrees.

A stratocumulus deck, seen on satellite imagery over eastern Minnesota, is expected to track east over central and north- central Wisconsin Saturday morning and linger across that area through the afternoon. Further east, mostly sunny skies are expected across northeast and east-central Wisconsin. This deck of clouds should keep temperatures a few degrees cooler across the west, with highs in the lower 80s. Further east, highs are expected to rise into the upper 80s away from Lake Michigan with highs in the 70s near the lake. In addition to the warmer temperatures, dew points will rise into the 60s, allowing humidities to rise across the western Great Lakes.

Long Term...Saturday Night Through Friday

Concern surrounding thunderstorm potential Sunday into Monday with prolonged heat and humidity lasting into the week continue to be the main focus for this forecast period.

Precipitation: By Saturday evening, the ridge aloft partially responsible for our tranquil weather will erode with an amplified but sluggish shortwave drifting into the region from the Mid- Mississippi Valley. This feature will help draw moisture from the Gulf of Mexico north, introducing the potential for showers and thunderstorms to close out the weekend. Chances start increasing more appreciably Sunday afternoon when more noteworthy height fall aloft accompanying the shortwave begin to impact eastern Wisconsin and diurnal instability will maximize. That said, the forcing itself is relatively weak, which will likely keep convection scattered at best Sunday afternoon and evening. Deep- layer shear continues to be relatively weak (15-20kts) among available guidance, which will hinder thunderstorm strength and overall severe weather potential. With precipitable water values approaching 1.50 inches in most deterministic and ensemble solutions, which approaches the 90th climatological percentile, heavy downpours will be a concern in addition to typical thunderstorm hazards.

The stronger mid-level forcing will persist overnight Sunday, but with a loss of diurnal instability, there is likely to be a lull in convective activity overnight into Monday. Another threat for thunderstorms will develop Monday afternoon, by which point the forcing aloft becomes more nebulous. However, a weak cold front approaching from the west will lend better low-level lift and better thunderstorm coverage Monday afternoon and evening into early Tuesday when the front exits. Again, the shear parameter space is rather weak but abundant moisture will threaten heavy downpours. Stronger ridging aloft and a weak surface high take hold later Tuesday into Wednesday, leading to a break in precipitation. Another wave approaches along the periphery of the ridge from Canada late Wednesday into Thursday, introducing another rain threat, though guidance differs wildly on its speed and intensity.

Temperatures: The stretch of near-normal, comfortable weather we've enjoyed will be a distant memory by Sunday, with 850mb temperatures jumping into the low 20s Celsius accompanying an expanding upper-level ridge. Along with the increased humidity streaming north into the Upper Midwest will send heat index values into the upper 80s from Sunday onward, approaching 90 degrees in some spots through the week. This level and longevity of heat and humidity will threaten minor to moderate impacts, particularly to those sensitive to heat. There is some degree of uncertainty surrounding how long the heat will last, owing largely to increased rain chances Wednesday into Thursday. While there is relative agreement on an amplified mid-level wave disrupting the ridge overhead Wednesday into Thursday (and possibly beyond), there is no confidence in the timing or location of this wave. The track and evolution of this wave will factor greatly in how warm we get. If the wave directly impacts us, our current forecast may be too warm. If it is relegated further north and the ridge remains intact, the heat and humidity will persist into the late- week.

AVIATION
for 00Z T AF Issuance Issued at 553 PM CDT Fri Jul 26 2024

Precipitation: None.

Clear skies will continue until around 15z Saturday. Increasing low level moisture will result in stratocumulus clouds developing between 15z and 18z Saturday and continue through Saturday afternoon. A broken deck with CIGS developing between 2500 and 3000 feet during the morning should rise to around 3500 feet during the afternoon is expected at KRHI/KAUW/KCWA/KMFI/KISW. A scattered deck is expected across northeast Wisconsin which should start out around 2800 feet rising to around 3500 feet during the afternoon.

VSBY trends: Could see patchy ground fog in river valleys of far northeast and east central WI late tonight into early Saturday morning, but confidence remains too low to include at GRB/ATW/OSH/MTW.
Low level winds increase tonight across north-central Wisconsin which should prevent any fog from forming.

Smoke Trends: Elevated smoke to remain across the forecast area through the TAF period, but little or no surface impacts are expected.

Winds: Light S-SE winds expected tonight, with S winds gusting to 15-20 kts by late Saturday morning and then continue through Saturday afternoon.

OSH
VFR conditions expected through the TAF period, with only a very low (10%) probability of MVFR vsbys in fog late tonight.
Clear skies will prevail through 15z Saturday, then stratocumulus should develop around 2800 feet between 15z and 18z, rising to 3500 feet Saturday afternoon. South winds around 5 knots should prevail tonight, then increase to around 10 knots with gusts to around 17 knots late Saturday morning and afternoon.

GRB WATCHES/WARNINGS/ADVISORIES
None.