Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 071651
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1151 AM EST Thu Dec 7 2017


.AVIATION...

Lake effect clouds and snow showers/flurries will persist through
the afternoon under continued west-northwest flow across Lake
Michigan. This flow regime will continue to keep the higher
concentration of snow showers and MVFR clouds south of PTK. Hi res
model solutions suggest the snow showers releasing off Lake Mi this
evening, with the back edge of the snow showers pushing through Se
Mi in the 01 to 03Z time frame. This will be due to a surge of drier
air across the lake, which will then be followed by a backing of the
low level winds toward the southwest. The result will be not only
for a decrease in the snow showers, but also a decrease in the sub
6k ft cloud deck.

For DTW...Radar data along with the latest model guidance suggests
some break in the snow shower activity is possible during the early
afternoon. An enhancement in low level convergence however may lead
to some higher intensity snow showers advancing back into metro
later this afternoon and persisting off and on into the evening. The
most probable time window for this to occur will be between 20Z and
02Z. A dusting of dry powdery snow is possible during this time
frame.

//DTW THRESHOLD PROBABILITIES...

* High for ceilings at or below 5000 feet this afternoon and
  evening. Low overnight into Friday morning.

&&

.PREV DISCUSSION...
Issued at 400 AM EST Thu Dec 7 2017

DISCUSSION...

NEAR TERM / TODAY THROUGH TONIGHT

As of 400 AM EST...Lake effect regime is fully in place this
morning, with abundant lake effect activity east of Lake Michigan.
Across southeast Michigan, a thick stratus deck is in place, and a
sharp inversion just above 850 hPa looks to keep plenty of clouds
around throughout the day, although increased mixing this afternoon
will help scour out some of the capping inversion moisture to allow
for a few peaks of sun in spots. Scattered flurries are occurring
across portions of the area, with more persistent flurries noted on
radar north of I-69. These flurries will largely be unimpactful as
the near-surface layer is quite dry, but high snow ratios around 20:1
and some lake-enhanced moisture from Lake Michigan could squeeze out
a patchy coating to perhaps a tenth or so in spots that see the
longest duration of flurries.

For today, the flurry activity will continue across all areas. High-
res guidance continues to indicate a more persistent band developing
downstream of Lake Michigan as the boundary layer flow orients
itself more west/northwesterly. With the abundant near-surface dry
air in place, it will initially be hard for this band to have any
meaningful impact across southeast Michigan as it gets substantially
frayed with eastern extent. Later this morning into the afternoon an
embedded piece of shortwave energy will rotate through the flow,
which may provide enough lift to better organize the band and have
it`s eastern extent reach into far southern areas. Latest low-level
trajectory analysis favors the band to set up mainly south of I-94 in
Lenawee, Monroe, and far southern Washtenaw counties between 15z-
21z. Should this band materialize, however, not expecting too much in
the way of accumulations, a coating to perhaps a half of an inch as
the band should be rather transient in nature. Further limiting snow
accumulation potential will be relatively low equilibrium levels
around 5 kft, and some directional shear in the lowest 3 kft to
disrupt better organization.

The band quickly wanes in eastward extent and intensity this evening
as the flow backs to more of a southwesterly direction as the
shortwave passes by to the east. Flurries will continue to be
possible through the evening before increasing anticyclonic
vorticity advection shuts down activity tonight. High temperatures
today will be around freezing with low temperatures tonight falling
well into the 20s, with some upper teens not ruled out near the Ohio
border where the potential for clearing late is possible.

SHORT TERM / FRIDAY THROUGH SATURDAY NIGHT

Increasing potential for a clipper system to bring the first
widespread snowfall of the season to southeast Michigan Friday night
into Saturday. Model guidance is starting to converge on the track
of the clipper system, which will be accompanied by a vigorous piece
of shortwave energy/PV rotating through the longwave trough.
Guidance is shifting slowly southward with the surface low track
with time, and confidence is increasing that most, if not all of
southeast Michigan will be placed on the favorable poleward side of
the low. Moisture will be limited with this system, and antecedent
dry low-levels will likely delay onset of snow until after 06z
Saturday. QPF amounts look to be between one and two tenths of an
inch liquid equivalent, and with snow ratios likely around 20:1,
this equates to a light snowfall of generally 1-3 inches. Still some
uncertainty with the strength of the low, with the ECMWF now being
the bullish solution with the low getting down to around 1003 hPa,
still modest for a clipper system. Trends will need to be watched
closely over the next 36 hours for potential higher amounts which
may result in the issuance of a Winter Weather Advisory, but for now
will start ramping up the message in the Hazardous Weather Outlook.

Clipper quickly exits to the southeast during the day Saturday, with
northwest flow in its wake. This will favor lake effect snow
activity to pick up downwind of Lake Huron, with potential minor
accumulations mainly in the Thumb region. Cold temperatures continue
through the short term period with highs below freezing and low
temperatures in the teens and 20s.

LONG TERM / SUNDAY THROUGH WEDNESDAY

Active northwest flow regime will prevail through the long term
period, with broad longwave troughing in place. Several pieces of
shortwave energy will dive southeastward through the flow in
response to surges of northern stream PV energy in the polar jet.
These features will lead to the potential of clipper systems moving
somewhere in vicinity of southeast Michigan for additional periods
of light accumulating snowfall. The first system may impact the
region Monday into Monday night with the second Tuesday night into
Wednesday. Too early to tell the exact track of these systems as it
will all depend on where the trough axis sets up and subsequent
trajectories. In between these systems will be the potential for
lake effect snow showers downwind of Lake Huron. Depending again on
the flow, portions of the Thumb have the potential to see several
inches of lake effect accumulation. It will continue to be cold
through the long term, especially early next week when highs will
struggle to reach the mid and upper 20s with lows into the teens.

MARINE...

A weak front will reinforce cold air over the Great Lakes today. It
will produce an uptick in snow showers over Lake Huron but will not
bring much additional wind impact through tonight. A stronger front
will follow during Friday, associated with low pressure moving
through northern Ontario, both of which will increase southwest wind
over marine areas. A short period of gales is possible over central
portions of Lake Huron during the day Friday, diminishing as the
front approaches and then stalls and weakens by Friday night. This
boundary will stall as a larger low pressure system develops over
central Canada and moves through the central Great Lakes Saturday.
Snow will become widespread followed by moderate northerly wind over
marine areas as the system exits Saturday night. Yet another front
is expected to move in and maintain elevated wind and waves Sunday
night into Monday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late tonight through Friday afternoon for LHZ362-
     363-421-441-462.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....SC
DISCUSSION...IRL
MARINE.......BT


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