Expert Forecast Opinion

173
FXUS63 KDTX 181950
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
350 PM EDT Sat Apr 18 2026

.KEY MESSAGES...

- A Frost Advisory is in effect tonight for most of Southeast
Michigan with lows in the 30s.

- Some rain and/or snow showers possible Sunday afternoon and evening
with gusts above 30 mph at times; a rumble of thunder cannot be
ruled out.

- A Freeze Watch is in effect Sunday night for most of Southeast
Michigan with lows in the 20s.

- Temperatures warm back toward normal Tuesday, climbing well above
normal through the rest of the week.

&&

.DISCUSSION...

An extensive and potent cold front, extending from 980 mb surface
low pressure stalling over southern Hudson Bay, has cleared east
into the southern Ontario peninsula this afternoon. As expected, 2m
temperatures dropped in the wake of the boundary, from morning
readings in the 60s to afternoon readings in the 40s/50s. Post-
frontal winds veered accordingly, and should persist from the west
overnight with a slow decay of the pressure gradient. The ensuing
cold advection has steepened low-level lapse rates and allowed for
efficient mixing of the still shallow boundary-layer winds. Gusts of
20-30 mph should taper off overnight as a weak nocturnal inversion
emerges, while prevailing winds drop below 10 mph, late. A Frost
Advisory was issued for tonight until Sunday morning for all
counties where the growing season has begun. Temperatures drop into
the mid 30s, but could briefly drop below freezing around sunrise,
should skies fully clear out and winds trend near calm. After a week
of intermittently heavy rainfall, high soil moisture may help slow
the rate of cooling for near-surface vegetation.

The composite longwave trough axis will bisect Lower Michigan Sunday
morning. Although the jet streak and its strongest dynamics will
have departed, a cyclonic flow regime will persist overhead
throughout the day. Evidence of low-level convergence passing across
Lake Huron and perturbed mid-level geopotential heights support
scattered shower activity during the day. Moisture advection is
unremarkable, and the lake contribution to moistening should be
minor given relatively cold water temperatures. However, forecast
soundings indicate deep surface-based mixing potential with the
cloud-bearing layer residing within a portion of the DGZ, generating
areas of precipitation. While temperatures will climb into the 40s
during the day, drying within the lowest 3-5 kft AGL may favor
evaporation/shedding of meltwater on snowflakes upon descent.
Sounding data varies greatly with the height/depth of the saturated
layer which has consequential impacts on final hydrometeor state.
Should the maximized DGZ and deeper/drier boundary layer solutions
win out, any snowfall would be non-accumulating, melting on contact.
Additionally, potential exists for a few rumbles of thunder given
the steepness of low- and mid-level lapse rates. This also supports
blustery diurnal conditions with winds veering northwesterly and
gusting above 30 mph at times.

Showers should quickly wane after sunset Sunday, ahead of a very
chilly night by mid-late April standards. High confidence exists in
overnight lows dropping into the 20s as 850 temperatures plunge
below minus 10C late Sunday night. Issued a rare Freeze Watch for
all areas in which the growing season has begun given the heightened
sensitivity of early season vegetation and well-advertised cold
airmass.

Height rises build in Monday with longwave ridging spreading east of
The Rockies and a shorter wavelength ridge axis dropping across the
Upper Midwest from the northern stream jet. Surface high pressure
races southeastward with the northern anticyclone on Monday, with
central high pressure of 1030 mb settling over Lower Michigan Monday
afternoon. The surface high moves east Monday night, leading to a
return flow setup and above normal temperatures. A cold front stalls
out just to the south Tuesday night and Wednesday offering a chance
for showers and/or thunderstorms near the Michigan/Ohio border. Much
warmer to close out the week with highs in the 70s and dewpoints
back into the 50s. Isolated showers/storms possible before expanding
in coverage with the next FROPA Friday or Saturday.

&&

.MARINE...

Cold air advection is gaining traction this afternoon which will
maintain higher mixing depths and persistence of gusty W to NW winds
to 20 to 25 kt tonight. A secondary cold front passes through on
Sunday, resulting in an uptick in wind magnitude during the
afternoon and evening. Small Craft Advisories have been issued for
western Lake Erie and Lake St. Clair with this forecast and
additional advisories are likely for the Lake Huron nearshore late
Sunday into Sunday night as wave heights build over 4 feet within
gusty NW flow to 25-30 kt. High pressure builds directly into the
Great Lakes on Monday allowing winds and waves to subside. Gusty
south wind then develops Monday night into Tuesday ahead of the next
cold front tracking across the upper Midwest.

&&

.PREV DISCUSSION...
Issued at 101 PM EDT Sat Apr 18 2026

AVIATION...

Cold front is exiting the airspace at forecast time. Low MVFR cloud
conditions are place within the moisture rich frontal band. There
are gradual improvements noted in upstream observations over West
Michigan, but it will take several hours for this drier air near the
surface to arrive. Upon doing so, ceilings will slowly lift and mix
out to a predominant VFR condition this evening. There are enough
signs in upstream cloud field west of Lake Michigan to still offer a
window of nearly clear skies late tonight and early Sunday morning.
The incoming cold airmass will support unstable boundary layer
conditions by mid/late morning - quickly establishing a robust
cumulus field, some embedded showers, and gusty westerly winds.

DTW THRESHOLD PROBABILITIES...

* High for ceilings aob 5000 ft through this afternoon into the
evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Frost Advisory from 2 AM to 10 AM EDT Sunday for MIZ053-060>063-
068>070-075-076-082-083.

Freeze Watch from Sunday evening through Monday morning for MIZ053-
060>063-068>070-075-076-082-083.

Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422.

Lake St Clair...Small Craft Advisory from 10 AM to 8 PM EDT Sunday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 10 AM to 8 PM EDT Sunday for LEZ444.

&&

$$

DISCUSSION...KGK
MARINE.......TF
AVIATION.....Mann


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