835 FXUS63 KDTX 051643 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 1243 PM EDT Sat Jul 5 2025 .KEY MESSAGES... - Hot and muggy conditions through this weekend with highs in the lower 90s today into Sunday. - Next chance for showers and thunderstorms on Sunday, then drier with less humidity starting Monday. && .AVIATION... Diurnal VFR Cu developing this afternoon with southwest surface winds aoa 10 knots. Very low chance of a thunderstorm with daytime instability, but southeast Michigan still under the influence of departing upper level ridge, which will generally suppress development. Modest drying in the low to mid levels this evening into tonight supporting mostly clear skies. However, clouds will rapidly increase toward sunrise Sunday as deep moisture advects into the region tomorrow. Advancing frontal boundary will lead to low level convergence and likely MVFR/low VFR cigs slowly advancing northwest to southeast, along with blossoming showers and scattered thunderstorms during late morning and Sunday afternoon. For DTW/D21 Convection... Thunderstorms are not forecast through tonight, with chances gradually increasing during Sunday. THRESHOLD PROBABILITIES... * Low for thunderstorms Sunday morning and early afternoon, then moderate. * Low for cigs aob 5000 Sunday. && .PREV DISCUSSION... Issued at 405 AM EDT Sat Jul 5 2025 DISCUSSION... A 500mb ridge axis marches across the Great Lakes today with hot and muggy conditions held in place beneath. 850mb temp of 17C observed in the 00z DTX sounding increases toward 18 to 19C this afternoon which supports highs into the upper 80s to lower 90s today. Morning fog resulting from a combination of firework smoke from last evening and marine moisture from Lakes Erie and St. Clair should clear up this morning as light wind organizes out of the southwest after sunrise. Dew point currently in the mid 60s to near 70 gains a slight boost from this flow, but then mixes back downward into the mid to upper 60s this afternoon. This will keep heat index relatively in check, but we can still expect a few hours of heat index in the mid 90s today. Can`t rule out a stray shower or rumble of thunder this afternoon, but overall the environment will tend to suppress convection. Mid- level lapse rates will be extremely poor as 500mb temps peak near -4C - near climatological max - which should cap convective chances for most of the area. Highest chances (~15%) exist in the Thumb and near Saginaw Bay where HREF guidance hints at a weak pool of SBCAPE owing to dew points that are slower to mix out. This paired with subtle height falls working in late in the day may be sufficient to trigger a few weak updrafts. The ridge begins to break down on Sunday but the heat and humidity will hold on for another day around Metro Detroit. After lows dip only into the 70s Sunday morning, highs there are forecast to again reach the lower 90s, with dew points around 70 holding heat index in the mid 90s. Across the northwest forecast zones, earlier arrival of clouds and showers/storms with the inbound cold front will dampen the heating potential - highs mainly in the 80s there. The front will be driven in by a positively tilted 500mb trough arriving from the west by midday, which then washes out overhead by Sunday night. As a result, the front will lose some forward momentum and definition by the time it works in during the day Sunday. Still, ample moisture pooling ahead of it should support scattered to numerous showers and thunderstorms. Mid-level lapse rates are again weak but model soundings indicate tall skinny SBCAPE of 1000 to 2000 J/kg is attainable. Relatively weak unidirectional wind through the column holds deep layer wind shear at around 20 kt which precludes an organized severe threat. The main hazard with storms will be lightning and localized heavy downpours given PWAT nearing 2.00", slow storm motion, and deep warm cloud layer. HREF LPMM guidance suggests a few pockets of 1"+ rainfall will be possible but with little confidence on location. WPC has included most of SE MI in a marginal outlook for excessive rainfall. The front looks to stall in the vicinity Sunday night with additional shortwave energy arriving from the Midwest to maintain at least a chance of continued shower/thunderstorm activity into early Monday. Northerly post-frontal flow brings relief from the heat on Monday with highs in the 70s/80s and notably lower humidity. The daytime precip forecast carries lower confidence given uncertainty with frontal placement, but eventually high pressure looks to favor dry conditions into Tuesday. The next disturbance in the low amplitude zonal flow pattern looks to arrive on Wednesday offering the next opportunity for rain chances. Temperatures in this pattern are likely to stay near seasonal norms through next week with highs in the 80s and lows in the 60s. MARINE... Low pressure ejecting out of the northern Plains leads to a tightening of the local gradient and strengthening southwest winds today. Strongest gusts, around 20kts, occur over the central portions of Lake Huron. Due to shoreline effects, the Saginaw Bay has the potential to reach/exceed 25kts this afternoon-evening with waves, particularly in the outer portions of the Bay, being able to approach 3-5ft. While borderline to meet Small Craft Criteria, given the holiday weekend and the rougher waters even if winds/waves come up a bit short, Small Craft Advisories have been issued for these waters. A cold front sagging south from the aforementioned low now sliding over the northern Great Lakes gradually crosses late tonight through Sunday. In advance, scattered to numerous showers and storms are expected. Severe storms not generally expected however an isolated strong storm can`t be ruled out. A few showers/storms look to linger into Monday for the southern Great Lakes as the front is slow to fully vacate. High pressure then briefly follows late Monday through Tuesday before unsettled weather returns midweek. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...None. Lake Huron...Small Craft Advisory until 10 AM EDT Sunday for LHZ421-422. Lake St Clair...None. Michigan waters of Lake Erie...None. && $$ AVIATION.....SF DISCUSSION...TF MARINE.......KDK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.