textproduct: Detroit/Pontiac

KEY MESSAGES

- Air becomes sufficiently cold for lake effect rain/snow activation late tonight through much of today.

- Minor slushy snow accumulation is likely across the Thumb, mainly near Saginaw Bay and near the Lake Huron shoreline, southward toward the I-69 corridor.

- Rain/snow showers gradually shift offshore tonight and Saturday with little additional snow accumulation.

DTW THRESHOLD PROBABILITIES

* High for ceilings at or below 5,000 feet through this afternoon.

* High for p-type as rain through much of tonight. Medium for rain- snow with any precip towards mid morning into the early afternoon.

PREV DISCUSSION

Issued at 358 PM EST Thu Nov 12 2026

DISCUSSION...

There are no big changes to expectations on the forecast for the overall lake effect event late tonight through Friday. The inverted surface pressure trough overhead this afternoon moves southward as the sharp mid level trough moves east this evening. NE low level wind trailing the system carries colder air into Lower Mi, colder but still on the mild side in lake effect context. This leaves the larger scale thermal environment dominated by warm Lake Huron temperatures, warm adjoining SE Mi land areas, and the air mass itself as supported by observations upstream across Ontario this afternoon. This is expected to extend the transition timing of rain to rain/snow mix to all snow that is otherwise on track for mainly after midnight through sunrise. Slushy accumulation up to 2 inches toward the Thumb shorelines is the outcome with greatest forecast confidence.

As lake effect instability ramps up late tonight and Friday morning, model solutions still offer dominant band locations off the south shore of Saginaw Bay and along the Thumb shoreline of Lake Huron down to about the I-69 corridor. Convective depth in model soundings peaks around 10 kft with high CAPE density that enables updrafts induced by lake instability reaching well through the DGZ and in excess of -20C. Along with a stray lightning strike, this amount of instability and high moisture content produces a greater than normal cellular/convective character within the areas of dominant band coverage defined by the mean NNE wind field, leading to some spatial and temporal variation on the most intense lake effect snow showers. The 12/12Z NBM cycle seems to reflect some of this variation with a slight downward trend on max QPF, max snow accumulation, and some broadening of the QPF/Snow footprint. As suggested by hi-res models and ensembles, a window for accumulation overachievement is still in play given the chance of a colder profile caused by the high precipitation intensity. The lake effect is highly convective, but also highly variable, which considerably reduces predictability and better monitored for short fused forecast adjustments in later updates. The best forecast message for this update carries forward an inch or two of slushy accumulation with a chance for localized higher totals until bands shift closer to the shoreline or offshore during Friday afternoon.

A transition toward a NW flow configuration across Lower Mi begins Friday night to further the offshore lake effect trend. Additional activity grazes the Thumb shoreline while thermal profiles remain relatively warm and convective depth gradually diminishes after midnight through Saturday morning. The best chance for additional light, slushy accumulation is during the diurnal minimum Friday night and Saturday morning. However, boundary layer thermal profiles warm sufficiently in model soundings for an all rain precip type and really don't cool back to near or below freezing before bands shift more fully into southern Lake Huron Saturday afternoon.

The highly amplified mid/upper air pattern shifts slowly eastward Sunday into early next week. Low level cold air is carried out of the Great Lakes as high pressure moves overhead, deactivating the Lakes and decreasing clouds Sunday night. Normal high temperatures are now in the mid to upper 40s which is in the range of advertised guidance high temperatures that gradually warm back above normal toward the mid week period.

MARINE...

Northeast to north flow takes hold today in the wake of a departing low pressure system which will usher in cooler air across the seasonally warmer waters of Lake Huron. This will continue to maintain breezy conditions across the Great Lakes where gusts around 20 to 30 knots are expected. Localized stronger gust potential will be possible through the Saginaw Bay this evening given the more favorable northeast fetch. Some isolated gusts to gales will be possible during this time. Otherwise, Small Craft Advisories remain in effect for all nearshore locations given the elevated gusts, in addition to the elevated wave heights found within the Saginaw Bay and Lake Huron shoreline.

Wind gusts then decrease slightly overnight, but the maintenance of northerly flow supporting cooler air temperatures will retain gusts around 20 to 25 knots across Lake Huron. Additionally, a couple of prominent lake effect bands are expected to develop overnight across the central and southern Lake Huron basin. Precipitation type will likely be rain over the water given the warmer lake temperatures, but a rain/snow mix will be possible closer to the shoreline. An isolated waterspout remains possible with rain shower activity. High pressure to build in on Sunday, relaxing wind speeds and ending lake effect rain chances.

DTX WATCHES/WARNINGS/ADVISORIES

MI...None. Lake Huron...Small Craft Advisory until 10 PM EST Saturday for LHZ421-422- 441>443.

Lake St Clair...Small Craft Advisory until 4 AM EST early this morning for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 4 AM EST early this morning for LEZ444.