textproduct: Detroit/Pontiac

KEY MESSAGES

- There is a chance of showers and thunderstorms today with an isolated risk for strong to severe storms through 10 PM.

- Additional chance for showers and embedded thunderstorms tonight into Sunday as a low pressure system drifts south of the state. There is an even lower chance for isolated strong to severe storms.

- There is a localized heavy rainfall threat today and Sunday.

- Heat and humidity remain to start the weekend but at more manageable levels with highs in the mid 80s and heat indices in the low to mid 90s today.

DISCUSSION

Active holiday weekend is underway across SE Michigan as heat and humidity fuel thunderstorm chances today and Sunday. Extended stretch of above normal temperatures has generally come to an end with temperatures now in the mid 80s, and even cooler high temperatures (low 80s) Sunday through Tuesday as a weak cold front drops across Lower Michigan rest of today and tonight. This occurs as high pressure expands south into Ontario.

The front will be the main driver of scattered thunderstorm chances this afternoon-evening. Have already seen activity light up along the convergence axis, which will migrate south between now and roughly 02z (10pm) tonight. The main limiting factor to convective vigor is the marginal instability profiles, as mid-level lapse rates are only about 5.0-5.5 C/km. That said, if the outflow from these storms can constructively merge into an organized cold pool this could offset some of the instability limitations. Latest thinking from a severe standpoint is that these storms will be capable of gusts 40-60 mph as they track south-southeast through the evening hours. Will also need to keep an eye on the MCS that is developing over northern Illinois. Models have shifted slightly, now hinting at the MCS clipping into lower MI later tonight. If this occurs, still looking at wind as the main threat but would mean higher PoPs continuing across the south overnight.

The perturbation driving this feature is forecast to move across the Ohio Valley late tonight, with several additional waves then forecast to follow in its footsteps until the main trough axis arrives Sunday afternoon. As this occurs, the mid level deformation axis eventually forms into a closed low. All of these waves will be capable of engaging the moisture-rich, unstable airmass to keep showers and storms going overnight, particularly south of M-59. Much of this will be ongoing upstream activity that advects into the area, similar to what we have seen over the past 24 hours.

Arrival of the synoptic trough Sunday afternoon strengthens the signal for an extended period of deformation over SE Michigan. Plan- view wind plots show potential for a convergence axis to develop over Ontario, which would be capable of sending multiple rounds of slow-moving showers/storms into the Detroit Metro region Sunday. This combined with the slow storm motion (~10 knots), a moisture- rich environment (PWAT of at least 1.8 inches, dewpoints in the low 70s), and modest instability all point toward a heightened heavy rainfall risk starting Sunday and ending when the deformation axis pushes east. There will also be an isolated damaging wind threat due to precipitation loaded cores. When this deformation axis dislodges is inconsistent in the current model suite, ranging sometime between Sunday night and Tuesday. This equates to a low predictability setup for the early week period which could include persistent rain chances. More details on the heavy rainfall threat in the Hydrology section.

Cooler temperatures expected early next week, followed by another heat spell mid-week as high pressure/upper level ridging build in. NBM high temperatures once again approach 90 degrees, although SE Michigan will be on the eastern periphery of the ridge which tends to be more open to the storm track. This expecting an active late week period.

MARINE

Ongoing progression of a cold front which has now cleared the Saginaw Bay and southern Lake Huron will continue to progress south this evening across Lake St. Clair and Lake Erie. This will pivot wind direction from the northeast and will bring the continued chance for isolated/scattered showers or thunderstorms. Outside of any thunderstorm development, ambient winds remain on the lighter side this weekend and into early next week as high pressure system builds in. Additional shower and thunderstorm chances exist again tomorrow across the southern Great Lakes, but otherwise the early week period will remain dry.

HYDROLOGY

Several opportunities for showers and thunderstorms exist through the holiday weekend within a favorable environment for heavy rainfall. A swath of 1+ inch rainfall fell over the I-94 corridor last night with half inch reports in many other locations. Any additional storms that develop within this environment will be slow- moving and capable of rainfall rates in excess of an inch per hour. A heightened opportunity for heavy rainfall exists Sunday and Sunday night with potential for a convergence axis to develop over Ontario and establish a band of training showers and storms into the urban areas of the Detroit Metro region. This could result in localized storm totals in excess of 2 inches. Confidence in this type of pattern however is low and the model signal is weak, but if this does occur there is increased potential for flash flooding especially in urban, low-lying, and flood prone areas.

PREV DISCUSSION

Issued at 1235 PM EDT Sat Jul 4 2026

AVIATION...

Ample boundary layer moisture has resulted in a widespread MVFR based strato cu field early this afternoon. Ongoing diurnal heating/boundary layer growth will allow ceilings to trend toward VFR during the afternoon. The heating in combination with the moist boundary layer will result in weak to moderate instability this afternoon/evening. A southward moving cold front this afternoon/evening will enhance low level convergence, providing a good chance for convective initiation. This is most likely in the KFNT/KMBS area with the potential for the convergence to sink southward across metro Detroit this evening. Weak mid level short wave energy may support a few showers/isolated thunderstorms across the metro Detroit area again overnight.

D21/DTW Convection...Scattered to possibly numerous clusters of thunderstorms are possible this evening, mainly between 22Z and 03Z across the airspace.

DTW THRESHOLD PROBABILITIES...

* Moderate in thunderstorms this evening. Low overnight and Sunday morning.

* Moderate in ceilings aob 5000 feet today.

DTX WATCHES/WARNINGS/ADVISORIES

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