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fxus63 kdtx 131730 
afddtx

Area forecast discussion
National Weather Service Detroit/Pontiac Michigan
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 Michigan/in
Michigan/Ohio 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 southeast to NE tonight, and finally to northwest 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.

*

&&

Previous discussion...
issued at 952 am EST Wed Dec 13 2017

Update...

Lead isentropic ascent producing patches of moderate snow this
morning. Snow will be continuous, but mostly light, for the next
several hours as the column slowly saturates. Minimal accumulations
of an inch east of US-23 by 18z with 1 to locally 2" to the west.
Clipper is still on track to unfold as forecast. A morning update
will be issued to fine tune the position and magnitude of the heavy
snow band. The biggest change to the official forecast will be to
lighten snow on the front and end of the event and ramp up snowfall
during the peak. 1 inch per hour rates are a certainty with a brief
period of 2"/hr rates possible. The event peak is still forecast to
begin around 20z. There remains some uncertainty along the southern
periphery of the heavy snow band. As it stands, it is highly likely
that snow will come to an end in part or all of Lenawee and Monroe
County in the 18-21z period before the warm conveyor ramps and allows
suthern areas to fill back in. Any minor deviation in storm track
from forecast expectations will cause this afternoon snow void to
shift north or south.

Previous discussion...
issued at 337 am EST Wed Dec 13 2017

Discussion...

Heavy snow to impact the evening commute.

Strong clipper system entering the western Great Lakes this morning,
with the 990-992 mb low tracking through far Southern Lake Michigan
this afternoon, as the two strong upper level waves/500 mb height
fall centers are already in lockstep and beginning to merge, with
the consolidated Max height fall center tracking close to the
southern Michigan border early this evening.

Excellent low level jet (55+ knots at 850 mb) over the Ohio Valley
nosing into Southern Lower Michigan during the day, leading to
intense lift/isentropic ascent/fgen lined up over The Heart of the
County Warning Area 18-23z, the worst possible time for the evening commute. Overall
moisture content still not very impressive, as specific humidities
rise to around 2.25 g/kg, but across a fairly deep layer. Still,
pristine thermal profiles with the best lift in the dgz should be
conducive for good snow to liquid ratios 15:1 far south to 18:1
north (on average through most of the event). Expecting about a 3-4
hr window of snowfall rates at or above 1 inch, and if the 00z NAM
negative epv in the 850-700 mb layer verified, then could see rates
approach 2 inches, however most other models are bit more stable.
Real good model consensus depicting Max quantitative precipitation forecast slightly better than 4
tenths of an inch. The 00z Euro, which seems to be lined up with
fairly well right now, trended a bit farther north. Thus, some big
concerns with the mid level dry slot impinging north this afternoon
across the southern Michigan border, shutting off precipitation for
a period, possibly as far north as the I-94 corridor. None-the-less,
local probabilistic guidance (heavily influenced by sref) also
indicating high likelihood of 7 inches across Wayne County, and with
possibility of a little moisture flux boost/low level convergence
off Lake Erie after 21z as sfc-925 mb winds come around to the south
and then southeast, and went ahead and upgraded Wayne and Washtenaw
counties. Otherwise, farther north, feel pretty good with a swath of
5 to 8 inches (isolated higher totals possible), and upgraded the
majority of the counties to a Winter Storm Warning (15z-5z). Went
with an advisory for Bay County and thumb region as residual dry
airmass holds on longer, but winds shifting to the north this
evening should allow for some Lake Huron enhancement continuing past
midnight, allowing those counties to be in the 3 to 7 inch range.
Went with advisory for Lenawee and Monroe for 3 to 6 inches with the
dry slot concerns and being so close to the surface low, along with
the diminished snow to liquid ratios.

Clearing out late tonight with winds diminishing, and should be able
to dip into the 5 to 15 degree range with the fresh snow cover,
which will likely hold maxes into the 15 to 20 degree range for
Thursday as 850 mb temps reside in the negative mid teens.

Digging trough axis swinging through Thursday night into Friday
looks to provide additional light snow, especially with the help
from Lake Michigan with the westerly flow. Possible messy wintry
mix or even rain scenario for the end of the weekend as low pressure
is forecasted to track through the western Great Lakes (per 00z
euro), with a good shot of temperatures climbing into the mid to
upper 30s.

Marine...

Winds and waves will subside steadily early this morning and have
allowed Gale Warning to expire. A storm system will then bring
moderate snow from across Southern Lake Huron southward today. As
this low pressure passes to the south, winds will increase from the
northeast tonight and then back to northwest on Thursday. While gale
conditions are not anticipated, the strong onshore flow will lead to
building waves and the need for small craft advisories over the near
shore waters tonight into Thursday before winds back to northwest
and takes the higher waves east away from the Michigan shore. Cold
air will help produce bands of lake effect snow on Lake Huron
Thursday. Another system will help back the flow to the southwest
for Friday while bringing more snow activity to the lakes. At this
time, winds are not expected to reach gales again at least through
the upcoming weekend.

&&

DTX watches/warnings/advisories...
Michigan...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 from 7 PM this evening to 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.....Am
update.......jvc
discussion...sf

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