Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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593
FXUS63 KDTX 111549
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1049 AM EST Mon Dec 11 2017

.UPDATE...

Continuing to assess potential for accumulating snow across the area
for today into tonight as a clipper tracks through the Ohio Valley
this afternoon. The lead isentropic ascent wing has moved into SE MI
a bit earlier than models had called for but snowfall rates will be
low with only a dusting to a few tenths coming from it. Visibilities
have dropped to around 1 mile with a few small pockets dropping
below that briefly. Although the event now falls in the range of all
the hires models, this is not translating to higher forecast
confidence as they continue to waver with the location of the
afternoon fgen band. It is even visible within individual models as
well with multiple fgen levels lighting up. In the end, we are still
looking at a broad coverage of around 1-2 inches of snow by Tuesday
morning with a band of 2-4 inches possible with the heavier band.
Most likely position of the heavier fgen/snow band will be somewhere
between I94 and I69, which will be pinned down further with the
afternoon package. Timing of the band will be around 00-06Z, with
light snow continuing through 12Z.

&&

.PREV DISCUSSION...
Issued at 550 AM EST Mon Dec 11 2017

AVIATION...

Ceilings should hover in 3-4kft range this morning and lower this
afternoon as first patch of light snow spreads over area within lead
area of isentropic lift. Light/variable winds to become light south.
Snow will peak this evening as FGEN forcing increase with passage of
this system. Difficult forecast to place narrow band of strongest
FGEN, but looks to be in the KFNT/KPTK area 00z-06z with slightly
lighter snow on either side. Still, IFR vsbys can be expected across
area with IFR or borderline IFR cigs. Cold front will sweep the snow
out of area 07-09z, with gusty NW flow to 30 knots or so developing
in its wake late tonight. Cigs will lift back to MVFR/lower VFR
within this drier NW flow.

For DTW...Period of -SN to develop around 17z with potential for
heavier snow band in the 00z-01z time frame. At this time, this best
FGEN banding appears more likely just north of the terminal, but it
will be very close. Generally expect MVFR conditions by midday with
first batch of -SN with IFR vsbys near IFR cigs this evening with
better snow rates. Gusty NW winds to 30 knots and by around 09z with
cold fropa.

//DTW THRESHOLD PROBABILITIES...

* High confidence for ceilings below 5000 feet through tonight,
  lower on Tuesday.

* High confidence in precip type being snow this forecast.

PREV DISCUSSION...
Issued at 255 AM EST Mon Dec 11 2017

DISCUSSION...

Challenging forecast as phasing/merging 6 hr height falls takes
place over the Central/Eastern Great Lakes this evening/tonight.
Lead symmetric upper wave clearly seen on water vapor imagery
tracking through eastern North Dakota, with full head of the steam
toward the southeast. Meanwhile, strong PV anomaly descending from
western Hudson Bay/Northern Manitoba, with trough axis dropping into
Western Great Lakes early this evening, helping to draw the lead
wave over the western Ohio valley this afternoon northeast. Still
conflicting signals in the various models on which FGEN band will be
most active this evening. The low level one toward the Ohio Border
or the mid level FGEN farther north toward the I-69 corridor. 12z
Euro ensemble members also exhibited/revealed this high variability
with respect to the max QPF axis placement as well. As pointed out
by day shift, the longer the two systems stay separate, the better
chance the snow axis will reside farther south, near Ohio border
(see ARW/NMM), but if phasing occurs sooner and low deepens rapidly,
better chance the higher snow axis will be farther north, along and
possibility north of the I-69 corridor (see 00z NAM/GFS). First
things first today, lead isentropic ascent/warm advection/moisture
advection (2.5 G/KG of specific humidity at 700 MB) to lift through
southeast Michigan during the day (around noon), which should be
sufficient for up to 1 inch of snow, as warming thermal profiles in
the 850-700 MB layer diminishes the snow to liquid ratios to
traditional 12:1. More intense snowfall rates arriving this evening
with mid level FGEN flareup, and brief deformation before system
heads off into the Eastern Great Lakes. The 00Z regional GEM spreads
the QPF out a bit more during the evening, as trowal/700 MB Theta-E
axis sinks southeastward as it pulls away, with the aggressive dry
arctic air arriving between 7-12z. As far as snowfall amounts go,
including the light snow from the daytime hours, 1-4 inch totals
still seem reasonable, and will favor the Euro solution highlighting
the middle tier (M-59/I-69) counties with highest totals. This
scenario also has support from the 6z RAP13 through 3Z Tuesday. An
advisory may be needed as 3 inches in six hours is possible, but due
to uncertainty with the exact placement and bulk of activity now
looking to come just after the evening commute, will defer to day
shift to pull the trigger if need be. As stated earlier, quicker
phasing/deepening would tend to put the Thumb under the gun for
higher amounts tonight, but it appears the system will come together
just a bit too late to expect much more than 4 inches. Perhaps more
concerning is the possibility of intense Lake Huron Band(s) on
Tuesday backing/glancing the northern tip of the Thumb region from
Port Austin to Harbor Beach area. Based off 00z NAM 925 MB
lift/convergence, going to be a real close call, and certainly will
be highlighting the potential for accumulating snow along the
eastern shoreline in HWO.

For rest of southeast Michigan, flurries/scattered light snow
showers, wind, and cold will be the story for Tuesday as 850 mb fall
into the -18 to -20 C range. Probabilistic guidance suggest wind
gusts in excess of 30 mph, supported by 16-20 MB 6 HR rise/fall
pressure couplet tracking through early Tuesday.

Although partial clearing is likely Tuesday night, with the airmass
being so cold, a few flurries will likely be around with any
lingering 925 mb moisture around. Winds coming around to the west
with warm advection push late Wednesday may be sufficient for light
snow, but indications is bulk of the snow with clipper system will
be passing south of the State Wednesday night. Still plenty of
opportunities for additional snow late in the work week into the
weekend as active baroclinic zone looks to be straddling the Great
Lakes Region with temperatures residing fairly close to normal
values.

MARINE...

Southerly winds will gradually increase today and veer to the
southwest in advance of approaching cold front this evening. The
cold front is quite strong and will bring much colder air over the
lake, leading to unstable conditions. Will be upgrading the Gale
Watch to a Warning with sustained winds peaking near gale force on
Tuesday morning with gusts pushing 45 knots at times. Gales will
arrive rather abruptly late tonight in the wake of this front and
persist into Tuesday evening before subsiding somewhat.

The combination of these gusty winds, very cold air and frequent
wave heights in excess of 10 feet will lead to areas of freezing
spray as well allow conditions suggest most of this spray will hold
in the moderate category so no warning is anticipated. The highest
waves may reach 20 feet over so by midday Tuesday. At that time, snow
squall activity will also begin to consolidate over the southern
basin of Lake Huron.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 2 AM Tuesday to 5 AM EST Wednesday for LHZ362-363-
     462>464.

     Gale Warning from 4 AM Tuesday to 4 AM EST Wednesday for LHZ421-
     441>443.

     Gale Warning from 1 AM to 11 PM EST Tuesday for LHZ361.

     Gale Warning from 4 AM to 4 PM EST Tuesday for LHZ422.

Lake St Clair...Small Craft Advisory from 3 AM to 11 PM EST Tuesday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 3 AM to 11 PM EST Tuesday for LEZ444.

&&

$$

UPDATE.......DRK
AVIATION.....DG
DISCUSSION...SF
MARINE.......DG


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