478
FXUS63 KDTX 212245
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
545 PM EST Thu Nov 21 2024

.KEY MESSAGES...

- Scattered to numerous snow showers diminishing into this evening.

- Scattered to numerous lake effect rain showers return overnight
  into Friday, mainly for areas east of U.S. 23.

- Dry weather is then expected for most of the weekend.

&&

.AVIATION...

A region of rain and snow showers now moving across metro Detroit
will eXit to the south shortly after TAF issuance, although some
additional light rain may continue to advance into Se Mi from Lake
Huron this evening. There is ongoing weak low level warm air
advection from the northeast which will warm the boundary enough to
trend precip toward all light rain. Observational trends suggest
some degree of low level drying in the wake of this region of light
rain, warranting a prevailing VFR cloud deck into the overnight.
Given some additional moisture advancing in from Lake Huron, some
intervals of MVFR remain possible. A better surge of low level
moisture from the northeast is forecast Friday morning, after 09Z.
This will trend ceilings toward low end MVFR, with some intervals of
IFR not out of the question. Weak short wave features pivoting
around the upper low to the east may also support some uptick in
morning rain/drizzle. There will be an increase in the NNW gradient
on Friday. Despite limited mixing depths, the strengthening low
level wind fields will warrant forecast wind gusts in the 25 to 30
knot range Friday afternoon.

DTW THRESHOLD PROBABILITIES...

*   Moderate in ceilings aob 5000 ft tonight. High on Friday.

*   Moderate for wet snow mixing in with rain this evening. High for
    precip type being all rain Friday.

&&

.PREV DISCUSSION...
Issued at 345 PM EST Thu Nov 21 2024

DISCUSSION...

Scattered rain and snow showers will wind down into the evening as
low pressure over Lake Michigan weakens and sweeps south into the
Ohio Valley. At the same time, secondary low pressure developing
along the eastern seaboard will retrograde back into far eastern New
York state. This will allow Atlantic moisture to wrap back around
the overall upper level low pressure to our east and pivot back
through Lake Huron into parts of SE lower MI overnight tonight into
Friday. Best rain chances will focus over the Thumb where northerly
flow from the lake will aid in the great Lake Effect coverage. Rain
chances will drop considerably westward with little/no rain from
U.S. 23 west. Temperatures will moderate into the mid 40s as the
coldest air associated with this system shifts southeast.

Generally dry conditions are expected over the weekend as this low
continues to shift east away from the region. Temperatures will hold
in the mid to upper 40s as a wavy zonal upper flow pattern becomes
established. A passing shortwave trough over southern Canada will be
shunting just north and east of the area as this generally westerly
flow overspreads the Great Lakes, but its close approach to the area
is only enough to limit warming into the 50s. A mid level shortwave
trough will translate through the developing zonal flow and bring
the next chance of rain showers Sunday night into Monday as warm air
advection in advance of this feature overspreads the area into late
Monday and Monday night as the system itself crosses the western to
central Great Lakes.

Rain and snow showers may linger into Tuesday depending on just how
trailing shortwave energy from Canada feeds southeast into this main
wave. In its wake, dry and colder conditions will become established
with highs edging closer to 40 for Tuesday and back into the 30s
into the midweek period.

MARINE...

An expansive upper low continues to govern Great Lakes conditions
today, with the surface low currently over southern Lake Michigan.
Locally the pressure gradient is relaxed with light northerly winds
over northern Lake Huron and southwest winds over the remaining
waterways. As the surface low drifts into the Ohio Valley tonight,
winds flip to the north across all waterways while the pressure
gradient tightens in response to strengthening New England low
pressure. By mid-Friday morning, gusts flirt with gale force
especially toward the southern end of the Lake Huron fetch and
around the tip of the Thumb. The latest guidance still keeps
frequent gusts just below 34 knots, noted in local probabilistic
guidance where there is nearly a 60% chance for peak gusts of 30
knots, but less than a 10% chance for gusts to 34 knots. Have opted
to maintain high-end Small Craft Advisories for now, but there is
still variability in model guidance right around decision thresholds.
Regardless of where frequent gusts fall in the 30-35 knot range,
northerly flow will be gusty on Friday especially over southern Lake
Huron. The presence of the upper low will also maintain rain-snow
showers tonight, changing to all rain showers for Friday and
Saturday. A brief ridge of high pressure then builds in Sunday ahead
of an unsettled early week period.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...None.
Lake Huron...Small Craft Advisory from 1 AM to 10 PM EST Friday for LHZ421-422-
     441>443.

Lake St Clair...Small Craft Advisory from 1 AM to 10 PM EST Friday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 1 AM to 10 PM EST Friday for LEZ444.

&&

$$

AVIATION.....SC
DISCUSSION...DG
MARINE.......MV


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