Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 132258
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

PREV DISCUSSION...
Issued at 1230 PM EST Wed Dec 13 2017

AVIATION...

A strong clipper system will continue to push across the Great
Lakes, bringing widespread snow to Southeast Michigan. Moderate
snowfall has and will continue to bring IFR to LIFR visibilities
ranging between .5 to 1 mile across most terminals for the early
afternoon. A dry slot is currently making its way across the MI/IN
MI/OH border which is expected to bring brief relief from moderate
snow across the Metro terminals, and may act to bring a quick period
of MVFR to VFR conditions between 19 - 21Z. Any lull in snow will be
short-lived as the heaviest forecasted snow of the day, fueled by
the approaching low pressure system and enhanced moisture, moves in
through 21Z - 00Z, bringing heavy snow. LIFR conditions for
visibility concerns are expected during this time frame as
visibilities diminish down to a quarter-mile. As the clipper pushes
southeast into Ohio, wind direction will start to quickly change,
backing from SE to NE tonight, and finally to NW overnight. Snow
showers will taper off around Midnight.

For DTW...IFR to LIFR for visibility concerns due to falling snow.
Brief lull in snow possible between 19 - 21Z as a dry slot moves
through which would bring MVFR to VFR conditions, however,
confidence is low to moderate regarding if the dry slot will reach
far enough north to bring the brief lull in snow. If the dry slot
reaches the Metro terminal, MVFR to VFR conditions will be short-
lived as peak of the event is expected between 21-01Z with
VLIFR/LIFR in heavy snow possible.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs at/below 5000 ft agl for the Taf period.

* High confidence in heavy snow with visibilities less than 1/2sm
  21-01Z. Moderate confidence in cigs at or less 200 ft agl 21-01Z.

*

&&

.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.
&&

$$

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


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