Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 140502

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1202 AM EST Thu Dec 14 2017


Snow is diminishing rapidly late this evening evidenced by lower
reflectivity returns on regional radar mosaic. Expectations are for
MVFR cig heights with flurries for a couple of hours. Low confidence
in what sort of mixed boundary layer light snow could develop in the
cold air advection. No signal to really latch onto, so will go dry
the remainder of the night. Surface ridging and height rises will
support loss of cloud by mid to late morning with VFR conditions.
MVFR cig heights forecasted to develop quickly Wednesday evening in
response to old Lake Michigan plume advecting over the terminals.

For DTW...Surface wind directions turning northerly will allow drier
air to push into the terminal, ending the light snow. MVFR cigs to
largely hold this morning with erosion of low level cloud, VFR skies
late morning. MVFR cigs back into the area Wednesday evening in
response to lake effect cloud pushing overhead.


* High confidence in cigs at/below 5000 ft agl through 12Z


Issued at 950 PM EST Wed Dec 13 2017


Issuing an updated discussion to state that all trends support
allowing headlines to play out until Midnight as-is. Latest RAP/HRRR
output now puts the centers of the 925-850mb low immediately south
of Monroe County as of 0230 UTC. Will see surface wind direction
veer steadily over the southern forecast area during the next couple
of hours. Therefore, will see reflectivities strip out as drier air
advects southward. Light snow with additional light accumulations
still possible through midnight with no additional accumulations
anticipated late tonight.

The one area to highlight remains the shorelines areas adjacent to
Lake Huron in the Thumb. A Winter Weather Advisory remains in effect
until 10Z. Lake enhancement has been ongoing throughout the evening
mainly between Port Hope and Port Sanilac. Current forecast reads 1
to 4 inches possible overnight, which remains plausible.

Issued at 558 PM EST Wed Dec 13 2017


Warm conveyor intersecting frontal zone leading to prolific dendrite
production in an ideal thermodynamic environment. 30 to 40dbz over
Oakland County and other portions of the northern Detroit suburbs
indicative of the well-advertised period of high-end snowfall rates.
1.5" has fallen in the last 30 minutes at WFO DTX, indicating 3"/hr
rates ongoing and now spreading southeast into southeast Oakland
County, northern Wayne, and southern Macomb County. Reports of 5 to
6 inches rapidly begun pouring in with several additional inches
possible in the eastern portion of the northern suburbs. Elsewhere,
moderate to heavy snow of a more tame variety (i.e. up to 1"/hr)
will gradually come to an end from NW to SE as the warm conveyor
exits stage east. The peak of the event will conclude by
approximately 02z with a few additional hours of lingering
deformation thereafter. This period of trailing deformation will
produce a maximu of 2 inches of additional snow. As noted in the
afternoon forecast package, attention will turn to lake effect snow
potential for the remainder of the night. Exeter, Ontario radar
already showing lake effect coming onshore near the Huron/Sanilac
county boundary. Cosmetic changes to the forecast, but headlines
still look good, including the Bay County advisory where incoming
LES potential off of Saginaw Bay will likely end before headline
expiration. Transient nature of LES banding should preclude the need
for any upgrades to warnings.

Issued at 335 PM EST Wed Dec 13 2017


Frontal band is setting up in roughly the expected area today with
the short-lived dry slot quickly filling in with light to mid-range
reflectivities along/south of I-94. The warm conveyor is ramping up
on schedule and is readily evident on the radar mosaic over
northwest Indiana and western Lower. Within the warm conveyor
itself, radar reflectivity is a bit disorganized. Expect episodic
moderate to perhaps briefly heavy snow south of I-94, but
disorganized will preclude higher end totals here. Expect Lenawee
and Monroe to add another 1-2, locally 3 inches. The heaviest snow
will occur at the intersection of the warm conveyor and the frontal
boundary. High-end snow rates perhaps reaching 2"/hr will last maybe
60-90 minutes at any one location, most likely moving through
Saginaw to Oakland County roughly 20-00z. Confidence in higher end
rates elsewhere, such as Detroit city, is low due to uncertainty in
frontal band placement by the time the conveyor arrives.

Backing flow in the wake of the low as it tracks across northern
Ohio will result in onshore flow in the Thumb and downstream of
Saginaw Bay. Some degree of lake effect snow will come ashore and
produce minor accumulations in the 03-09z period. Continually backing
flow as the low pulls away will limit potential both by limiting
fetch length and by limiting duration over any one location. Painted
favored locations in these areas with an additional 1 to 3 inches.
Storm totals including lake effect are still expected to fall
between 3 and 8 inches, lowest in Huron County and near the Ohio
border and highest along a Mount Clemens to Pontiac to Flint line.

Shortwave energy embedded within the high latitudes of the existing
longwave trough will elicit a strong jet response over central
Canada and the northern US Thursday into Friday. Subtle downstream
shortwave ridging within background cyclonic flow will support high
pressure pushing through the CWA on Thursday. With a fresh snowpack,
highs will stay put in the teens while light winds within the ridge
axis will keep wind chills generally above 0. Lows will fall back
into the single digits Thursday night, but clouds and light SW flow
at the surface should prohibit temps from cratering overnight.

Aforementioned jet energy will foster a deepening trough axis that
is progged to pivot across the area late Thursday night through
Friday. Strong ascent is progged by the NAM12 to raise inversion
heights to 10kft. The DGZ will be firmly planted within this layer
coincident with a high level of supersaturation with respect to ice.
Diurnal heating is progged by some guidance to boost SBCAPE to 100
j/kg. Moderate convective layer theta-e lapse rates around -4C/KM
within the presence of such strong lift and developing uniform
westerly flow will be favorable for producing snow squall activity
through the day. Mesoscale details will need to be ironed out over
the next 24 to 36 hours, but attm potential for impactful heavy snow
bands appears to exist.

Saturday will see the start of modest mid-level ridging and the
easing of the longwave trough that has plagued the eastern CONUS.
The associated temperature gradient will also ease as heights over
SE Michigan rise; surface temperatures will rise to a few degrees
above average (mid 30s) by Sunday and persist for the early week. A
chance for precipitation exists Sunday afternoon as a shortwave
moves up the Ohio Valley and brings the potential for wintry mix or
even some rain as temps will be above freezing.

Zonal flow looks to dominate the early week, with a series of
shortwaves passing through leading to slight chances for precip
Monday and Tuesday. Signal is emerging for another digging trough to
drop through the Great Lakes on Tuesday into Wednesday, which will
lead to an increased chance of heavier precipitation and a cold
burst for mid-late next week.


A low pressure clipper system will continue to push southeast across
Michigan throughout Wednesday afternoon and evening, producing
persistent snowfall across the state and its lakes and bays. The
approaching low will increase the pressure gradient over the water,
which will allow wind gusts to peak between 20 - 25 knots through
Thursday morning. The main hazard will come with a shift in wind
direction from the southeast to the northeast tonight, which will
allow wave height to peak between 5 - 7 ft along the shore and open
waters through Thursday morning. Small Craft Advisories are in effect
over the near shore waters tonight into Thursday before winds back
to the northwest, allowing waves to relax across the Michigan shore.
The next significant impact will move in on Friday, where mean west
to northwest flow combined with instability will bring the threat of
snow squalls through the morning and afternoon.


MI...Winter Weather Advisory until 5 AM EST early this morning for

     Winter Weather Advisory until 7 AM EST this morning for MIZ063.

Lake Huron...Small Craft Advisory until 10 AM EST this morning for LHZ421-

     Small Craft Advisory until 5 AM EST early this morning for LHZ422.

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



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