Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 110755
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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 frequest
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.

&&

.PREV DISCUSSION...
Issued at 1154 PM EST Sun Dec 10 2017

AVIATION...

Some fluctuation in ceiling heights anticipated during the overnight
and morning periods, as modest northwest wind provides some low
level drying but may prove insufficient to completely clear MVFR
restrictions. Attention then turns to advancing low pressure that
will bring a period of snowfall to the region Monday and Monday
night. Initial period of lighter intensity snowfall expected to
develop during the midday period 16z-20z, where conditions largely
remain at MVFR restrictions outside of the heaviest bursts. Snowfall
intensity will increase during the evening period, with peak
intensity and accumulation potential centered between 7 pm and
midnight. There remains some uncertainty yet in the placement of the
heaviest snowband, but generally expect all locations to witness a
period of IFR during this time.

For DTW...A period of chaotic ceiling heights likely to define the
overnight period, with some residual MVFR restrictions giving way to
generally low VFR stratus...with some pockets of clearing below 5000
ft still not out of the question during the period. Initial burst
of light snow expected to lift through during the early afternoon
period, providing some minor accumulation of up to an inch.  A
second period of potential heavier snow will focus during the late
evening period /00z-05z/, where an additional inch or two of
accumulation will be possible under IFR conditions. Winds remain
modest from the south through tomorrow evening.

//DTW THRESHOLD PROBABILITIES...

* Medium confidence for ceilings aob 5000 feet overnight through
  Monday morning, then high Monday afternoon in developing light
  snow.

* High confidence in precip type being snow this evening and
  again Monday.


&&

.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.

&&

$$

DISCUSSION...SF
MARINE.......DG
AVIATION.....MR


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