textproduct: Detroit/Pontiac

KEY MESSAGES

- Gradually increasing chances for showers and possibly some thunderstorms through tomorrow. Heavy rainfall leading to localized flooding is possible, mainly along and south of the I-94 corridor.

- Showers and storm chances late Tuesday into early Wednesday.

DISCUSSION

Cloud shield has overspread a good portion of Southeast Michigan today. The clouds originated with yet another episode of deep convection across Illinois and Indiana this morning. The persistent ribbon of deep moisture and reduced moist stability continues to lurk just to the south with peripheral influence in the form of a gradual increase in near surface moisture and expanding cloud cover. Meanwhile, the warmest conditions are observed across the Thumb and Saginaw Valley, where sunny skies remain.

A weak zone of stretching deformation below 700-mb is draped along the southern border counties. It is currently supported by unbalanced flow through increasing system-relative cyclonic flow and is maintaining forcing for light shower development, which is expected to continue through the balance of the daylight hours. Furthermore, a weak MCV just south of Lake Michigan will move eastward along this zone - potentially augmenting the ongoing activity into this evening. The response will be muted in intensity, though, due to the lack of instability.

Looking upstream, a shortwave trough over Montana and the southern Prairie provinces will slide toward the region tonight. As this wave approaches, it will start to interact with low-level frontal zone across the corn belt. One result will be the southwestward extension of the upper-jet and the induction of below entrance region ascent. Coincidentally, the moist unstable airmass over the mid-Mississippi Valley will claw northeast across the Lower Peninsula tomorrow morning in a corridor ahead of the wave and the approaching cold front. Some residual upstream shower activity may survive along the leading instability gradient as it advects into Southeast Michigan during the early to mid morning. Otherwise, scattered showers and thunderstorms will develop across Lower Michigan tomorrow afternoon into the evening. The best support for coverage will be closer to the main shortwave across central Lower Michigan into the Thumb region. Instability source depth will be restricted to the lowest 1-km, with not much CAPE depth to govern a higher predictability zone for sustained development. Furthermore, the lack of deep layer shear projects the lack of convective organization - aside from any activity that is forced along cold frontal zone.

In the wake of the cold front, Monday and the first half Tuesday look quiet and seasonable.

A broad region of height falls is expected across eastern Canada into the Lakes region and the northeast going into midweek. All the global ensemble families are trending toward lower with height projections in the context of reduced intra-family spread - garnering greater confidence in a meaningful pattern shift. The onset of the cooler conditions will be prefaced by a cold front with some showers and storms possibly as early as Tuesday afternoon. Chances are lingering into early Wednesday, as the timing of the front is still getting sorted out. An early arrival would offer greater thunderstorm chances during peak heating hours on Tuesday, while a delayed arrival would place the region in the nocturnal minima for convection. The replacement airmass will be cooler and drier with highs in the 70s and dewpoints in the low-mid 50s.

MARINE

High pressure over the Northeast continues to move east over the Atlantic as low pressure tracks across Ontario through Sunday. As the low tracks north of the area, it will push a cold front through the central great lakes region late Sunday. Ahead of the front, south winds prevail for much of Sunday around 10 to 20 knots. Behind the cold front, northwest winds fill in and increase across Lake Huron, and eventually down through Lake St. Clair and the Michigan waters of Lake Erie late Sunday. While the strongest winds are expected across Lake Huron, especially the Open Waters with 20-25 knot winds expected Sunday evening into early Monday morning, will need to keep an eye on the wave forecast and the potential for Small Craft Advisories for nearshore waters across the tip of the Thumb of Michigan. Showers and thunderstorms will be possible as this system moves through Sunday and Sunday night. The best chance for thunderstorms will be along the cold front later in the day and into the evening hours.

PREV DISCUSSION

Issued at 155 PM EDT Sat Aug 15 2026

AVIATION...

High degree of cloud cover with the moisture advection north of the activity associated frontal system across the Ohio Valley is bringing overcast skies at mostly low VFR ceilings. A few scattered light rain showers lifting Lower Michigan this afternoon warranted at least a PROB30 mention for later this afternoon into the early evening. Otherwise, VFR ceilings will become MVFR ceilings late tonight into tomorrow morning with pockets of IFR ceilings becoming increasingly likely. Additional potential for showers exists tomorrow morning with lower ceilings hanging around during the day. Light E to SE winds today will become more S to SW tomorrow morning into the early afternoon. Isolated showers and possible thunderstorms again possible tomorrow afternoon.

D21/DTW Convection... Scattered thunderstorm chances this evening hold near the Ohio border. Additional window for convection is possible Sunday morning and afternoon, but overall thunder coverage is expected to be limited.

DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5000 feet or less this afternoon, high tonight and tomorrow.

* Low for thunderstorms this evening and again Sunday.

DTX WATCHES/WARNINGS/ADVISORIES

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