Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 132316
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
616 PM EST Wed Dec 13 2017


.AVIATION...

Moderate snowfall is expected to persist through the 01Z to 02Z time
frame as the clipper system, currently stationed over the MI/IN
border, continues to push southeast into Ohio. Wind direction is
expected to quickly change throughout the evening and overnight
hours from the SE to the NE and eventually the NW closer to midnight
as a result of the low pressure system progressing east of the
terminals. This will allow cold, dry air to filter in across
Michigan and will act to end moderate to heavy snow chances late
this evening, with light, lingering snow showers possible,
approaching Midnight. Expect LIFR visibilities to persist with the
moderate to heavy snow, with IFR cigs sticking around through at
least the early morning. A brief period of northerly flow should
help advect dry air into Michigan tomorrow afternoon which will
allow for partial clearing of cloud cover, however, westerly flow
will then advect moisture off the lake, bringing the returned chance
for cloud cover.

For DTW...IFR to LIFR for visibility concerns due to falling snow
will continue through 01-02Z time frame. The clipper system will
continue to move east into Ohio which will act to diminish snow
intensity, returning visibilities to P6SM after Midnight, however,
MVFR to IFR cigs will hold steady through most of the morning as NE
to N flow helps advect moisture off of Lake Huron.

//DTW THRESHOLD PROBABILITIES...

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

&&

.PREV DISCUSSION...
Issued at 558 PM EST Wed Dec 13 2017

UPDATE...

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.

PREV DISCUSSION...
Issued at 335 PM EST Wed Dec 13 2017

DISCUSSION...

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.

MARINE...

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.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Warning until midnight EST tonight for MIZ047-053-
     060>063-068>070-075-076.

     Winter Weather Advisory until midnight EST tonight for MIZ048-082-
     083.

     Winter Weather Advisory until 5 AM EST Thursday for MIZ049-054-055.

Lake Huron...Small Craft Advisory until 5 AM EST Thursday for LHZ422.

     Small Craft Advisory until 10 AM EST Thursday for LHZ421-441>443.

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

$$

AVIATION.....CB
UPDATE.......CB
DISCUSSION...JVC/TF
MARINE.......AM


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at www.weather.gov/detroit.



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