Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 092020
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
320 PM EST Mon Jan 9 2017

.DISCUSSION...

Copious energy coming onshore in California and the Pacific
Northwest will be steered across the Great Lakes and Ohio Valley for
the next several days as a cold polar low sets up invof Hudson Bay
and ridging over the Southeast US remains strong. The result will be
a SW-NE oriented storm track with several opportunities for rain and
mixed ptype. General warmth aloft, and occasionally at the surface,
will limit snow potential through this time.

Per upper air analysis, the first wave to watch is the PV reservoir
anchored off the PacNW coast, which extends southward roughly
parallel to the CA coastline at 18z this afternoon. Tightening mass
gradient already underway downstream of the Rockies as evidenced by
the jet streak positioned over the northern Great Plains. This is
supporting a narrow band of moderate snow that will translate into
Lower Michigan by midnight local time tonight. Current radar
appearance and relative steady state forcing between now and then
suggest no major evolution. The band will migrate east of the area
by about 09z, leaving around an inch of snow in its wake.

Forecast quickly becomes more challenging in the 07-18z period. Wave
currently over the west coast will undergo rapid amplification over
the Mississippi on Tuesday afternoon. Low-level jet and theta-e
advection will ramp up rapidly in advance of this process, forcing
precipitation to fill in rapidly over Lower Michigan during the pre-
dawn hours. Low static stability and strong forcing will support 6+
hours of moderate to occasionally heavy snowfall rates. Warmer
column will keep snow ratios in check, but an additional 2+ inches
will be possible areawide by 12z. Rapid warming of the column then
commences as the 50-60kt LLJ continues to work through the area and
a strong southerly gradient at the surface rapidly warms temps. 12-
15z will see a changeover to rain between the Ohio border and
approximately M59. Strong low-level convergence along the upstream
side of the the veering low-level jet and deepening dynamic forcing
aloft will provide a window between 15-18z for the northern segment
of the advisory to pick up an additional 2-3" before transitioning
to rain. Southerly gradient not at all favorable for lengthy mixed
ptype. Thus, another justification for raising snow totals has
simply been the shortened transition period. Only guidance with a
known surface cold bias over the snow, such as the NAM/NMM/Canadian
support more than an hour or so mixed precip at any one location.
All told, expect 2 to 6 inches, lowest at the Ohio border and
highest in the Saginaw Valley and Thumb. Given coincidental timing
with morning rush hour, a winter weather advisory was an easy call.

Defined dry slot will surge into the area beneath the shortwave
aloft. Rain will subsequently transition to light showers or drizzle
in the aftn prior to aggressive cold fropa. Cold surface bias once
again plays havoc 21-06z Tuesday evening as several models fail to
handle boundary layer depth, high temperatures, or isentropic
descent following fropa which will drive strong winds across the
area. Preferred guidance for these variables is the ARW/ECMWF, which
suggest raising highs several degrees into the mid to upper 40s. ARW
indicates deep isentropic ascent 274-284k capable of dynamically
forcing wind advisory criteria gusts to the surface 21-06z, with
peak potential centered around 03z. A wind advisory may be needed
for this period.

Next wave ejects out of the west in similar fashion as it is steered
northeast between large Canadian gyre and strong SE US ridging. Much
less potential for amplification this time around, ensuring an
fgen/overrunning scenario for someone Wednesday evening into
Thursday night. Far too early to wade into the details here given
sensitivity to position of the surface baroclinic zone and unsampled
upper air features. Suffice it to say that icing potential will
exist over the NW third or half of the CWA with rain an increasing
likelihood further south, including Metro Detroit. At this time,
snow potential appears minimal. Fairly warm scenario overall, so all
rain for everyone also not out of the question.

.High pressure building in from the Northern Central Plains into the
Great Lakes region late Thursday into Friday will keep conditions
dry, as as temperatures return back to seasonal norms. The next
chance for precipitation will come Saturday into Sunday, as a
stationary front initially sets up across the Ohio River Valley.
Overrunning of warm air along the front will initially bring the
chance to see snow throughout the afternoon hours, with the brunt of
the precipitation staying south of the county warning area, along
the front. The better chance to see precipitation will arrive as a
developing low over ArkLaTex pushes northeast into the Great Lakes
regions. Confidence remains low regarding the timing of this system,
with the GFS solution pushing the low into Michigan throughout
Monday, while the ECMWF solution moves it through the region on
Sunday. In either solutions, this system exhibits strong WAA which,
at the present time, favors more of a rain solution over snow.
However, additional model runs will be needed to increase confidence
in timing and precipitation type.

&&

.MARINE...

Southerly winds increase tonight as the gradient strengthens
downstream of an organizing winter storm system.  Gusts to gales
beginning late tonight and continuing into Tuesday for portions of
southern and central sections of lake Huron.  Strongest period of
winds focused late Tuesday into early Tuesday, as post-cold frontal
southwest winds respond to the inbound colder air.  Winds are then
expected to drop off precipitously late Tuesday night. A very warm
airmass is then forecasted to build into the Central Great Lakes for
Wednesday and Thursday. This will likely lead to very stable
conditions over the adjacent waters. Uncertainty remains high with
regards to the exact positioning of a very strong stationary front.
Adjustments in future forecasts both with respect to wind direction
and windspeeds for the middle and end of the week forecast are
likely with any changes to the positioning of the front.

&&

.HYDROLOGY...

Low pressure will lift through the upper midwest into the northern
Great Lakes and Ontario on Tuesday. An area of snow will expand into
the region overnight before changing to all rain on Tuesday. The
transition will occur from south to north, beginning near the Ohio
and Michigan state line around sunrise, then finally reaching the
Thumb during the afternoon. Snow amounts will range from 2 inches
near the Ohio border to  nearly 5 inches over parts of the northern
Thumb and Saginaw Valley. Total rain and rain equivalent is expected
to range between one half and three quarters of an inch between
midnight Tuesday to midnight Wednesday.

&&

.PREV DISCUSSION...
Issued at 1234 PM EST Mon Jan 9 2017

AVIATION...

Favorable trajectory of the existing lake Michigan moisture plume
under southwesterly flow will maintain a high coverage of lower
stratus through the evening hours from PTK northward.  Marginal MVFR
conditions during this time as ceiling heights remain at 3 to 4 kft.
Ensuing pattern of warm air advection tonight will then translate
into an increasing coverage of snowfall across SE Michigan.   A lead
band of snow sweeps through 03z-06z.  A greater coverage and
intensity of accumulating snowfall then develops from 08z-09z
onward, with widespread IFR conditions development at that time. The
warmer air will bring a transition to all rain from south to north
late Tuesday morning. There is the potential that a brief wintry mix
of freezing rain/sleet will accompany this transition. This
potential currently remains too low to include a defined mention.

For DTW...Onset of accumulating snow 03z-05z, with greatest
intensity 08z-12z.  Total accumulation 2 to 3 inches.  Transition to
rain may result in a brief wintry mix late Tuesday morning /12z-
15z/. Low confidence in the possible occurrence of sleet/freezing
rain, therefore no defined mention at this stage.

//DTW Threshold Probabilities...

* Low in ceilings below 5k ft prior to 03z, then high overnight.

* High in snow as ptype through 12z Tuesday morning, then very low
  confidence in the brief occurrence of freezing rain/sleet before
  transitioning to rain late Tuesday morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 11 PM this evening to 10 AM EST
     Tuesday FOR MIZ075-076-082-083.

     Winter Weather Advisory from 11 PM this evening to 1 PM EST Tuesday
     FOR MIZ047>049-053>055-060>063-068>070.

Lake Huron...Gale Warning from 4 AM Tuesday to 7 AM EST Wednesday FOR LHZ362-363-
     462>464.

     Gale Watch from Tuesday afternoon through late Tuesday night FOR
     LHZ421-422-441>443.

     Gale Watch from Tuesday evening through Wednesday morning FOR
     LHZ361.

Lake St Clair...Gale Watch from Tuesday afternoon through Tuesday evening FOR
     LCZ460.

Michigan waters of Lake Erie...Gale Watch from Tuesday afternoon through Tuesday evening FOR
     LEZ444.

&&

$$

DISCUSSION...JVC/AM
MARINE.......MR
HYDROLOGY....MR
AVIATION.....MR


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