textproduct: Detroit/Pontiac

KEY MESSAGES

- Chance of showers and thunderstorms will persist overnight. Locally heavy rainfall and gusty winds possible with any activity.

- While temperatures will be less hot and more seasonable this weekend, the chances for showers and thunderstorms, with an isolated risk for strong to severe storms, will continue Saturday and Sunday.

DTW THRESHOLD PROBABILITIES

* Medium for thunderstorms early Saturday morning and again Saturday afternoon.

PREV DISCUSSION

Issued at 300 PM EDT Fri Jul 3 2026

DISCUSSION...

The ridge flattens out over the Great Lakes today as the center of the surface high drifts towards the mid Atlantic coast and a Canadian trough slides across Ontario into Quebec. This places southeast Michigan in zonal flow, but still under 500mb height to 590dam. The 850mb temperatures this afternoon have been tempered slightly by the suppressed heights, but still reach to near 20C supporting the continued heat. Humidity levels remain elevated with dewpoints in the upper 60s to 70s, which still support heat indices to around 100 degrees this afternoon under mostly clear skies. Will allow the heat headlines to run their course until the current evening expiration. The heat and humidity will also provide fuel for potential thunderstorms this afternoon and into this evening. Repositioning of the jet axis with the adjustment to the ridge puts southeast Michigan in a favorable spot for convective chances. Weak waves passing through this flow atop a baroclinic zone will provide focus for convection. The environment this afternoon into early tonight will be supportive with plenty of instability available. CAPE values of 2000-2500 J/kg are forecast with mid level lapse rates around 6-6.5 C/km. The better 0 to 6 km shear (30+ kts) appears to be displaced across I-69 and north. A convective line at press time looks fairly organized as it approaches western Michigan and expectation is for the line of storms to continue eastward into the area. Will have to keep an eye on any convergence zone to drive any earlier convection prior to the convective cluster/system upstream as it continues into the area more likely towards 22-00Z. One note is that AI models have increased the probability for wind across the southern half of the CWA with the 12Z suite, which somewhat agrees with what some of the latest CAMs solutions depict. A Slight Risk for severe weather remains in effect along and south of I-96/I-696 corridor and points south with a Marginal Risk for the remainder of the area. Confidence has also increased enough for a Severe Thunderstorm Watch to now be in effect through this evening along and south of the I-69 corridor. Wind and hail will be the main threats. Some isolated flooding concerns also exists given the high PWATs of 1.75-2.00 inches and the potential for repeating rounds of west to east moving thunderstorms if convective activity can get generated ahead of the approaching line.

Slight relief from the heat and humidity to start the weekend with heat indices forecast to fall below heat headline criteria, though mid to upper 90s heat index values still possible across the urban areas of metro detroit down to the southern MI border. Greater relief comes Sunday and Monday before a return to near 90 degree temperatures toward mid-week with dewpoints in the 60s. Convective chances remain over the weekend as a low pressure system takes shape over the upper Midwest and slides across the central Great Lakes Sunday night. Deep shear will be lacking with moderate lapse rates, but enough instability will be available to support at least isolated marginally severe thunderstorm potential.

MARINE...

Main focus for today and tonight will be the ongoing development and progression of a line of thunderstorms now located across Lake Michigan. This thunderstorm complex is poised to enter Southeast Michigan and portions of the southern Great Lakes through the evening hours. Locations most susceptible for thunderstorm development based on current observations will be across Lake Erie and Lake St. Clair, where a Severe Thunderstorm Watch is in place. However, upscale growth can have thunderstorm chances extending into the Saginaw Bay and southern Lake Huron. Strong to severe thunderstorm development will be capable of producing wind gusts aoa 50 knots. The issuance of Special Marine Warnings may be warranted tonight. Otherwise outside of thunderstorm development, ambient winds will light tonight into tomorrow. A weak cold front does sweep through on Saturday which will pivot wind direction from the northeast, with the frontal boundary stalling around southern Michigan. Additional shower and thunderstorm chances will continue through the weekend given the warm and humid airmass. Some stronger storm development will again be possible tomorrow, favored across the southern Great Lakes.

HYDROLOGY...

Clusters of thunderstorms have developed upstream this afternoon. Current forecast indications point towards locations along and south of the I-69 corridor having the better potential for thunderstorms. Thunderstorms may have potential to develop ahead of an approaching line of storms, resulting in some training and repeated thunderstorms if they do develop. There is expected to be a wide range in rainfall totals across the region. Localized areas of one inch or more of rain is possible in areas that see the persistent thunderstorm activity. Rainfall rates in these areas are likely to reach 1 to 2 inches per hour. If these storms set up over urbanized areas, localized flooding is possible. Additional periods of showers and thunderstorms are forecast Saturday and Sunday, which will add to rainfall totals. Higher amounts will again be localized.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...None. Lake St Clair...None. Michigan waters of Lake Erie...None.