textproduct: Detroit/Pontiac

KEY MESSAGES

- Warm and dry during the day on Saturday, then showers and thunderstorms move in Saturday night into early Sunday.

- Drier and cooler conditions early next week before another active pattern commences midweek.

DISCUSSION

Post-frontal airmass characterized by surface dewpoints falling into the 50s and a 10C reduction in 850 mb temperatures have led to more seasonable conditions across Southeast Michigan. Column winds organize out of the west while geopotential heights rise aloft, ensuring continued subsidence and dry weather. 12.12Z KDTX RAOB revealed a deep layer of dry air from 0.5-8.0 km AGL, with a max dewpoint depression of 46C near 4.5 km AGL. Broad 1016 mb surface high pressure rolls eastward from The Plains into the lower Ohio Valley by this evening backing winds WSW. Afternoon gusts of 25-30 mph drop off after the boundary-layer decouples by nightfall. Clear skies tonight with lower dewpoints allow for efficient longwave cooling, with lows dropping into the 50s by Saturday morning.

Low cloud fraction to start and backing southwesterly Gulf-modified flow within the low levels will culminate in Saturday being the warmest day of the forecast period. 850 mb temperatures rebound into the teens with ample insolation lifting 2 m temperatures into the mid-upper 80s. A quasi-Rex Block pattern emerges over the eastern half of CONUS with southern Lower Michigan residing near the stronger kinematics within the unified jet axis. Favorable mixing profile leads to another day of diurnal gustiness, generally southwesterly at 25-30 mph. An enhanced low-level jet response over The Thumb and Tri-Cities could favor gusts approaching 40 mph at times before sunset.

A medium wavelength trough of Pacific origin digs into the Upper Midwest, phasing with an embedded shortwave revolving around the Ontario upper low. This leads to a packet of height falls moving through the Great Lakes late Saturday into Sunday. As the composite wave works through aloft, it forces a cold frontal passage overnight into Sunday morning. Pre-frontal theta-E advection will be present helping to destabilize the frontal slope with steepening mid-level lapse rates resulting in MLCAPEs in excess of 1 kJ/kg. However, 0-6 km bulk shear will be falling during the evening and overnight hours as the front arrives, limiting the overall intensity of convection, while instability weakens. At this time, a mix of gusty showers and a few thunderstorms are most likely to progress through the region following some pattern of linearity with low potential for hazardous/severe wind.

The progression of overnight convection should slow, thus, lingering decaying showers could remain over some eastern locations Sunday morning. Once the front finishes clearing out, noticeable cooler air backfills into the forecast area through the rest of the day. Highs top out in the 70s with dewpoints eventually retreating into the 40s by the end of the day. Dry, cool, and breezy weather persists Monday and into Tuesday morning with a zonal flow configuration aloft. The next chance for rainfall returns for the second half of Tuesday with the next speed max. A much stronger wave could impact Lower Michigan mid-late week with strong geostrophic flow and upper level divergence.

MARINE

High pressure is building across the southern Lakes today while the next cold front starts moving toward western Lake Superior. This will help keep a fairly tight pressure gradient in place across the region today. Winds will remain elevated from the southwest offering a period of gusts to 25 to 30 knots for Saginaw Bay and the tip of the Thumb during the afternoon Saturday so we'll issue a Small Craft Advisory. The front then sweeps east by Sunday morning. Winds will stay elevated around 20 knots, but out of the northwest Sunday.

HYDROLOGY

Showers and thunderstorms move through from west to east Saturday night into Sunday ahead of a cold front. Total basin rainfall amounts of quarter to a third of an inch expected. No widespread flooding concerns exist, although heavy downpours with rainfall rates up to an inch per hour may be possible. Localized flooding of low-lying or poor drainage areas possible.

DTW THRESHOLD PROBABILITIES

* None.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory from 10 AM to 10 PM EDT Saturday for LHZ421- 422-441.

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