287
FXUS63 KDTX 121629
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1129 AM EST Wed Feb 12 2025

.KEY MESSAGES...

- A Winter Storm Warning is in effect for Huron, Sanilac, St. Clair,
Macomb, Lapeer, and Tuscola Counties where 6 to 8" of snow is
expected through tonight with the heaviest snow from late afternoon
to late evening.

- Winter Weather Advisories are in effect for the rest of Southeast
MI. Most areas are forecast to see 4 to 6" of snowfall with the
exception of parts of Washtenaw, Wayne, Lenawee, and Monroe Counties
where a wintry mix will cut into snowfall amounts this evening. 3 to
5" of snow and up to 0.15" of ice accumulation is forecast there.

- Reinforcement of below normal temperatures follows this system for
the late week period.

- Another round of accumulating snow is likely from Friday night
into this weekend. Snow may be heavy at times.

&&

.UPDATE...

Lead arm of isentropic has begun to lift across Lake Michigan with
the Tri-Cities/Thumb residing on its weaker elevated (>800mb) fringe
resulting in a strip of virga radar returns. Weaker elevated ascent
will persist through the rest of the morning though with a
respectable dry slot (-20C dewpoint depression) between 800-850mb
(per 12.12Z KDTX RAOB), this is expected to remain mostly as virga,
save for areas of the northern Saginaw Valley. Deeper isentropic
ascent begins to develop over all of SE MI by afternoon, after
roughly 1-2PM, as the warm conveyor pushes north into the area
supporting accumulating light snowfall. There is looking to be two
windows for enhanced snowfall rates above 0.5"/hr with this system.
First comes late afternoon- evening, ~4-8PM, as the elevated warm
frontal slope sets up across SE MI providing a more focused arm of
forced ascent. This arm eventually settles over roughly north of a
Howell-Port Sanilac line this evening maintaining rates between 0.5-
1"/hr for areas north of said line. The second window comes late
evening-early tonight, ~10PM-2AM, as the surface low reaches Lake
Erie and an attendant jet streak reaches southern lower MI.
Combination of deepening low pressure, sharpening elevated frontal
boundary, and increasing upper jet divergence supports snowfall rates
near 1"/hr for areas south/east of a Lansing- Bad Axe line. Overall
forecast snow amounts still looking largely on track from prior
forecast with the main adjustments to tighten up the 6-8" area given
the signal for more focused bands of higher rates. Opted to upgrade
Lapeer and Tuscola (for the southeastern portion of the county) due
to a slight northward shift in the 7" potential in the EPS/GEFS. The
northeast corner of Oakland could also see similar totals though held
off due to the smaller geographic footprint. Transition to a period
of wintry mix still likely for areas south of I-94 this evening-
tonight, particularly Monroe and eastern Wayne. No significant
changes in 12Z runs this morning to support any headline adjustments
for icing potential. Areas near Toledo could push 0.2" of icing in a
worst case scenario though running forecast remains amounts up to
0.15".

&&

.PREV DISCUSSION...
Issued at 628 AM EST Wed Feb 12 2025

AVIATION...

Winter storm will bring widespread snow to Southeast Michigan
terminals from this afternoon through late tonight. IFR to LIFR
conditions are expected in falling snow. The potential for sleet and
freezing rain will be possible at the Detroit terminals between 22Z
and 5Z. The heaviest snow rates in Metro Detroit may occur in the 4-
8Z time window.

For DTW...Accumulating snow is forecast to overspread DTW this
afternoon. Sleet and freezing rain will be possible between 22Z and
5Z with heavy snowfall between 4-8Z.

DTW THRESHOLD PROBABILITIES...

*  High for ceiling at or below 5000 feet after 17z Today.

* High precipitation type as all snow this afternoon. Moderate in
  precipitation type as sleet and freezing rain Wednesday evening.

PREV DISCUSSION...
Issued at 526 AM EST Wed Feb 12 2025

DISCUSSION...

Today`s winter storm is taking shape across the Plains this morning
as a mid-level wave exits the Four Corners and engages with the
baroclinic zone in place from the Southern Plains into the Ohio
Valley. The surface low will emerge out of the southern Mississippi
Valley and lift into the Ohio Valley by this evening, eventually
tracking just to our east overnight. This places SE Michigan within
the warm conveyor and associated isentropic ascent this afternoon
and early evening. Sfc-700 mb specific humidity right around 3 g/kg
and deep lift through the column will bring light to moderate
snowfall rates after saturation commences shortly after noon. Model
soundings reveal a DGZ well above the surface near 15 kft AGL, with
a deep isothermal layer between -4 and -8 C beneath. This will
support SLRs near climo at around 12:1 for much of the event. The
initial snowfall is anticipated to progress across all or most of
southeast MI with rates of up to 0.5"/hr possible at times through
the afternoon, leading to accumulations of roughly 1 to 3" by 00z
this evening.

The peak in the event is expected to begin mid to late evening and
last into the early morning hours. This occurs as upper shortwave
energy consolidates ahead of the approaching trough, eventually
causing the trough to take on a negative tilt Thursday morning as it
passes to our east. This reorientation of the height field overhead
will cause a southwesterly jet streak to accelerate just ahead of
the trough, focusing a corridor of solid left exit region dynamics
directly over the Great Lakes. As this forcing develops overhead,
sharpening of the resident frontal boundary brings a focused region
of ascent via 800-900 mb fgen that slides northeast across the area
in the 10pm to 4am time frame. This burst of precip is likely to
focus south and east of a line from Lansing to Bad Axe. This occurs
as the surface low tracks across northwest Ohio toward the southern
Ontario peninsula and for most of the area it will result in heavier
snowfall rates. The proximity of the surface low to our southern
counties brings the likelihood for a wintry mix to develop there as
a wedge of warm air attempts to lift in ahead of the 850 mb low.
This continues to be a focus area for the forecast as precip type,
accumulation totals, and impacts hinge on where the snow/mix line
sets up and how long it lingers.

About 50% of 12.00z HREF members produce at least 0.01" of icing for
areas along and east of a line from Adrian to Port Huron, so would
not be surprised to see at least a brief period where sleet and/or
freezing rain mix with the snow for any of these areas. Hi-res
soundings tend to have a sleet-heavy flavor here as warm nose temps
struggle to rise above 1 C which brings question to the potential
for full melting of hydrometeors before refreezing occurs. Attention
remains focused on eastern Monroe and Wayne Counties where all HREF
members are on board for a period of wintry mix - many leaning
toward a prolonged episode where ice accumulations may become more
impactful. Soundings here offer a brief window for warm nose temps
to reach 2 C which may be sufficient for a more robust freezing rain
response. With little change in latest FRAM output for this area,
maintained forecast ice accumulations up to 0.15". Worth mentioning
that as peak fgen slides overhead, strong enough precip rates could
in theory cool the lower column enough to support a burst of heavy
snow instead. So a considerable amount of forecast uncertainty still
exists for this area even as we close in on roughly 15 hours before
the period of interest. As the low zips across Lake Erie after
midnight, the column cools and brings a likely changeover back to
snow, which could be briefly heavy before rates begin to taper off
between 07z and 10z.

There is still some movement with model QPF output, but will lean
the forecast grids within LREF (ENS/GEFS/GEPS) grand ensemble
interquartile range of 0.30 to 0.50" for most of the CWA west of I-
75. This will be supportive of snowfall totals mainly in the 3 to 6"
range. For the Lake Huron counties, expect higher QPF of 0.60 to
0.75" owing to the late night storm intensification as well as
shoreline convergence effects amidst the onshore flow of around 25
mph. This pushes snowfall totals more solidly into the 6 to 8"
range, prompting an upgrade to Winter Storm Warning for Huron,
Sanilac, St. Clair, and Macomb Counties. Latest HREF shows about
10 to 20% chance to exceed 8" right along the lakeshore, and a
95th percentile snow amount of 9". Hi-res models also show a
component of inland orographic QPF enhancement which brought
warning considerations for northeastern Oakland, Lapeer, and
eastern Tuscola Counties as well but confidence is not quite there
to pull the trigger. Similarly, consideration was given to
upgrade Lenawee, Washtenaw, Wayne, and Monroe County for potential
for a localized stripe of 7+" amounts wherever a heavy burst
occurs in the late evening, but confidence in occurrence and
location remains too low at this time.

The system departs east on Thursday, its deepening resulting in a
brisk northwest wind that ushers in a renewed arctic air mass
through the day. After a lull in precip during the morning, expect
scattered to potentially numerous snow showers to develop within the
lake moisture plume spreading across the state during the afternoon.
Additional minor accumulations will be possible. Temperatures
holding in the 20s will feel more like the teens with the persistent
wind reaching 25 to 30 mph at times.

Benign but cold weather expected Friday as high pressure tracks
across the Ohio Valley. Friday night then presents the beginning of
our next episode of more active winter weather as accumulating
isentropic ascent snowfall spreads across the state. The parent
upper wave will outpace a deeper trough digging across the southern
Plains which may bring a brief lull in snow for parts of Saturday.
The next low tied to that deep upper trough then lifts up the Ohio
Valley Saturday night into Sunday, possibly unfolding into a similar
scenario to tonight with system deformation bringing heavy snow to
parts of the area. Model consistency has been relatively high with
this system but with plenty of room for adjustments to the axis of
highest QPF as we get closer. Relatively high confidence exists for
a period of much below normal temperatures early next week.

MARINE...

Flow will veer east this morning and afternoon in anticipation of
the next approaching winter system which will strengthen the
pressure gradient, bringing breezy conditions, while producing
widespread snow. Gust potential will remain sub-gales, with
sustained winds peaking over Lake Huron around 20 knots, gusts to 25
knots. Will preclude any issuance of Small Craft Advisories due to
ice cover across the nearshore. Widespread snow will expand across
the Great Lakes late this afternoon, the heaviest of which will be
found over southern Lake Huron and locations south. The path of the
low will track from the Ohio Valley into east Ontario, which will
then back wind direction to the northwest tomorrow as the low
departs. Cold air advection increases with the northwest flow,
expanding mixing depths and allowing breezy conditions to continue
through the day tomorrow. While widespread snow will taper off
tomorrow morning, the boost in over lake cold air will support some
lake effect snow showers.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 7 AM EST Thursday for MIZ047-048-053-
     060-061-068-069-075-076-082-083.

     Winter Storm Warning until 7 AM EST Thursday for MIZ049-054-055-062-
     063-070.

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

$$

UPDATE.......KDK
AVIATION.....CB
DISCUSSION...TF
MARINE.......AM


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