Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150458
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 PM EST Thu Dec 14 2017


.AVIATION...

Low level thermal plume off of Lake Michigan is currently spreading
across southeastern Michigan this evening. Temperatures of -10 to -
13C reside within the could bearing layer and is generating a
relatively good amount light returns on radar. Very transient
activity could result in a quick tenth or two this morning. Moisture
and shortwave energy arrives over the central Great Lakes after 12Z
Friday. Very good synoptic scale support for ascent with a
dampening, but direct CVA, some left exit region dynamics, and
midlevel theta e advection. Steepening lapse rates Friday afternoon
will then lead to a period of snow squall over the area. The
outstanding question remains how widespread and/or transient the
snow squalls will be.

For DTW...Expecting cigs heights below 5000ft by 04Z this evening,
continuing throughout the remainder of the forecast period. Will
watch very light returns to drift southward towards the airfield.
Latest indications are bulk of these flurries/light snow showers
will remain north of DTW. Question is on how widespread in coverage
snow squall will be 18-00Z Friday.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs aob 5000 ft agl around 04z tonight.

&&

.PREV DISCUSSION...
Issued at 343 PM EST Thu Dec 14 2017

DISCUSSION...

NEAR TERM / REST OF TODAY THROUGH FRIDAY

As of 340 PM EST...Much quieter weather day across the region after
the strong clipper that brought significant snowfall to much of
southeast Michigan yesterday, in which snow totals ranged from 3-5
inches in the northern Thumb and far southern areas to 5-9 inches
across the rest of southeast Michigan. Ridge of high pressure
building into the region centered over Iowa has allowed for dry
weather outside of diminishing lake effect snow activity over
southern Lake Huron and mostly sunny skies. The fresh snowpack, cold
airmass in place (850 hPa temps around -15 C), and shallow boundary
layer has only allowed high temperatures to warm into the teens, with
a few lower 20s readings along and south of I-94. Dry weather will
continue through midnight in advance of another clipper system.

This clipper system will be associated with a deep northern stream
PV anomaly diving southeastward from Manitoba, which will help
enhance a midlevel shortwave impulse that will cross central
Michigan during the day Friday. Clipper low itself will track north
of southeast Michigan across Lake Superior and far northern Lake
Huron, with the trough axis swinging through the region late morning
through mid afternoon. Enough lake enhanced moisture will be present
in the westerly flow downstream of Lake Michigan to promote
scattered light snow showers after midnight and into early Friday
morning, mainly west of US 23.

As daytime mixing commences, the boundary layer will become quite
unstable in a low-level environment containing specific humidity
values of 1.50-1.90 g/kg, with mixing heights approaching 5 kft in
most places. Static stability will be low allowing for steep low-
level lapse rates in a fairly uniformly mixed mixed layer (30-35 kts
at top of layer), coupled with 25-50 J/kg of SBCAPE. Deep layer of
saturation with respect to ice through the boundary layer, with
omega maximized in the DGZ will also be present, maximized in the
late morning through early evening hours. All of these factors in
play lend to a highly favorable thermodynamic profile capable of
snow squalls.

There still remains a few questions to the snow squall potential,
mainly concerning the orientation, which will be highly dependent on
the near-surface flow which looks to waver roughly between westerly
and west/southwesterly. Exact orientation will be critical, as
several major highways/interstates (US 23, I-75, I-96, I-94, I-69)
will all be in play for snow squall potential. Given the
convectively unstable environment, coupled with the larger scale
forcing from the passing midlevel shortwave, the late morning
through early evening may potentially feature localized hazardous
and treacherous travel in the snow squalls, which will feature a
quick half inch to perhaps greater than an inch of snow
accumulation, wind gusts of 30-35 mph, and near whiteout conditions.
The other concern is a weak boundary layer capping inversion which
may allow for more dry air entrainment, but thermodynamic profile
and updrafts should largely compensate for this potential limiting
factor.

Outside of the snow squall activity, general flurries will be
possible along with gusty west/southwest gradient flow of 20-30 mph.
With the high snow ratio fluffier snow content on the ground, and
little time for settling/compaction since the recent big snowfall,
patchy blowing/drifting snow will be possible especially during the
afternoon hours. With the favorable mixing profile, high
temperatures will be able to warm well into the 20s despite abundant
cloud cover.

SHORT TERM / FRIDAY NIGHT THROUGH SATURDAY NIGHT

Despite clipper passing by to the northeast, cold front associated
with it will linger northwest to southeast over central southeast
Michigan Friday night into Saturday, gradually weakening with time
as PV max exits east and diffluent upper-level flow impinges
upstream. Still, enough lingering moisture and FGEN forcing will
promote additional light snow potential, with another inch or two
possible, bringing total accumulations Friday through Saturday in
the 1-3 inch range. Given the spread out nature of the snowfall, not
anticipating any advisories at this time. Frontal boundary continues
to weaken and slowly lift northward with snow showers ending by
Saturday evening as flat upper-level height rises build in from the
south. Temperatures will moderate during the short term, with highs
Saturday approaching or slightly exceeding the freezing mark for the
southern two-thirds of the forecast area.

LONG TERM / SUNDAY THROUGH WEDNESDAY

The biggest change for the extended period will be the return of
seasonal to slightly above seasonal temperatures as a persistent
upper-level troughing pattern pushes out of the Midwest. A weak
upper-level ridge and south to southwesterly flow will allow
temperatures to peak in the mid to upper 30s for a high on both
Sunday and Monday. The ECMWF and GEM models also exhibit a weak warm
front push through Michigan late Sunday into Monday morning which
will bring the slight chance for precipitation during this time
frame. Confidence remains low regarding the precipitation type that
would be seen if the GEM and ECMWF model runs pan out as
temperatures straddle along the freezing mark in the lower levels.
At this time a rain to wintry mix solution looks to be the most
probable outcome, however, additional model runs will be needed to
narrow down precipitation type. Additional precipitation chances
will move through on Tuesday as an upper-level trough and embedded
shortwaves move in across the Great Lakes, bringing the chance for
snow showers. Breezy conditions throughout Tuesday and Wednesday
look more certain as cooler air above promotes mixing throughout the
day. This upper-level trough will bring cooler air to the region by
Wednesday as temperatures struggle to hit the freezing mark for a
high across the state, however, long range model runs bring another
warm front through the region Thursday and Friday, returning
temperatures into the upper 30s to lower 40s. This warm front will
also be our best chance of widespread precipitation as low pressure
pushes near the state. Models have not yet converged on low pressure
strength and location at this time, thus confidence is low regarding
precipitation type. The ECMWF and GEM model runs push the low just
north of the Metro region which would result in a rain transitioning
over to snow solution, while the GFS shows a less developed low to
the south of Michigan, bringing more of a snow solution. This system
will be something to keep an eye on heading into early next week.

MARINE...

Winds will remain on the lighter side out of the southwest around 10-
20 knots through this evening as a surface high pressure will drift
across the Ohio Valley. Waves tonight will also decrease with some
elevated waves remaining in the Outer Saginaw Bay, until the next
shortwave/clipper system arrives tomorrow bringing some widespread
unsettled weather conditions. Snow squalls are possible across Lake
Huron starting tomorrow as low pressure system enters the northern
Great Lakes tomorrow morning. Winds will increase to the 25-30 knots
range out of the southwest as the system approaches and then turn to
the northwest behind the system. Widespread gales are not
anticipated at this time. Winds decrease and turn out of the east to
start the weekend a high pressure system drifts just north of the
Great Lakes.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 AM EST Friday for LHZ421.

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

$$

AVIATION.....CB
LONG TERM...AM
MARINE.......AA


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