Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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259
FXUS63 KDTX 110236
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
936 PM EST Sat Dec 10 2016

.UPDATE...

The lake effect snows diminished as they reorganized with a
southern flow over Lake Michigan this evening. This has left just
a few flurries in the remnant plume over the PTK and FNT areas.
Outside of that...stratocumulus as dissipated, but plenty of mid
and high cloud to keep the mostly cloudy conditions.

Later tonight, the forecast is on track. Light snow has reached a
SBN to Auburn IN line at 02z. This light snow should over spread
much of the forecast area between 06z and 08z, holding closer to
12z for the thumb region. The moderate snowfall will hold off
closer to daybreak. 18z model runs were in line with their 12z
runs, so do not see a need to change much to the ongoing advisory
or expected amounts.

&&

.PREV DISCUSSION...
Issued at 656 PM EST Sat Dec 10 2016

AVIATION...

Lake effect snow showers were diminishing this evening and do not
expect them to affect any taf sites for the reminder of the
evening. The attention turns to the next snow storm which should
bring back MVFR cigs and IFR visibilities by 06z to 08z. Both
cigs and vsbys will lower around 09z. Further lowering to 1/2 or
3/4 mile vsbys with cigs around 500 to 600 feet between 15z to
18z. Those conditions will be persistent through Sunday.

FOR DTW...with lake effect snow showers ending will have a few
hours of VFR cigs above 10000 feet. Then the snow from the next
storm moves in around 06z and will quickly lower both cigs and
vsbys to IFR which will continue through Sunday.

//DTW Threshold Probabilities...

* High for ceilings below 5000 feet after 06z.

* High for ceilings below 1000 feet after 16z, but low for
  ceilings below 200 feet.

* Moderate for visibility below 1/2 mile after 18z

* High for snow as precipitation type through Sunday.

PREV DISCUSSION...
Issued at 324 PM EST Sat Dec 10 2016

DISCUSSION...

12z dtx sounding indicated 850 mb temp of -16 C, with inversion
height around 800 mb which has been able to support some
disorganized snow shower activity from Lake Michigan working into
southeast Michigan. 12z nam suggests a more focused band (seen in
925 mb omega) in between M-59/I-69 corridors will be lifting north
(toward Saginaw Bay) through the night as winds back to the south.
This band will have potential to produce light localized accumulations
ahead of the main over-running snow shield arriving late tonight.

Upper level energy/wave now onshore, tracking through the Pacific
Northwest. 12z NAM/Canadian/GFS consensus indicates strengthening
mid level wave/500 mb trough reaching the Upper Mississippi River
Vally by 00z Monday, with max 6 hr 500 mb height fall center and
surface low tracking over/near Green Bay, and through far northern
Lower Michigan/Straights Sunday night. This is farther north and
west compared to 24 hours ago. This could impact distribution of the
snowfall across southeast Michigan, as nam suggests higher amounts a
bit farther north, as 700 mb FGEN Band looks to straddle I-69
corridor late in the day on Sunday. Nam also shows 4 G/KG of
specific humidity within the 850-700 mb layer reaching the southern
Michigan border late in the day. Unfortunately, placement and exact
southwest-northeast orientation of the developing 700 mb JET (50-70
knots) Axis over the Central/Eastern Ohio Valley varies
significantly as an important weaker wave exits the Southern Plains.
Thus, GFS and UKMET suggest active frontal zone will reside along
and south of the Michigan border, robbing southeast Michigan of some
moisture.

Isentropic ascent/fgen is very good, but relatively short lived for
any given location. Warm advection leading to average 850-700 MB
temps of -3 to -6 C south of M-59 Sunday afternoon, with -5 to -9 C
across the north, which will support higher snow ratios, and thus
thinking more uniform/widespread snowfall, as the lower qpf in north
is off set by higher ratios. Snow to qpf ratios quickly lowering to
10:1 down south is expected to cap higher totals. Have been debating
all day whether we are looking at more of 5 to 8 inch snowfall, vs 6
to 9 inch. Based off the 12z Euro still remaining more subdued with
moisture advection, only 3 g/kg of specific humidity in the 850-700
mb layer sneaking just past the southern Michigan border and 24 hr
qpf advertised to be in .3 to .6 inch range, prefer to go out with
high end advisory for 5 to 8 inches as suspect mid level fgen will
not stay parked over one particular location too long, but would
expected highest totals in around the Detroit Metro area toward the
border/along triple point. Backed surface-925 mb flow coming off
Lake Erie/Lake St. Clair could even give a modest boost to the low
level convergence as winds reach the higher terrain of the glacier
ridge. Can`t rule out isolated/slightly higher totals exceeding 8
inches and possible upgrade to warning criteria, but uncertain on
location this far out. It continues to look like the
accumulating snow will be over before the Monday morning commute
as the aggressive dry slot punches in. Temperatures also look to
be around freezing mark Monday morning, so long as roads are
plowed and salted, should not have big issues on main roads.

Tuesday through Sunday Extended Forecast: Scattered snow showers
will be possible mid-week as upper level Canadian low rotates
multiple weak waves through the Great Lakes Region. It will also
help usher the descent of cold arctic air into southeast Michigan.
Temperatures will fall well below the norm, with high`s in the teens
and overnight lows plunging into the single digits. On Friday,
models show a large, complex area of low pressure spreading eastward
across the central U.S. At present time, this low is expected to
reach the western Great Lakes Friday morning and southeast Michigan
later that afternoon. Timing adjustments are likely as the low
develops and progresses.

MARINE...

Moderately gusty westerly winds this evening will gradually diminish
tonight, shifting to southerly by Sunday morning in advance of
approaching low pressure.  Any ongoing lake effect squalls will
subsequently shift east into the Canadian waters through this time.
Southerly winds turn gusty late Sunday and Sunday night as the low
advances into the Great Lakes.  A brief period of gusts into the 25
to 30 knot range over southern sections of Lake Huron during this
time.  Otherwise, the main element with this system becomes a
widespread long duration accumulating snowfall that starts late
tonight and persists through Sunday night.  Gusty westerly winds
develop behind this system Monday.  The passage of an arctic front
Monday night into early Tuesday will reinforce these winds through
the mid-week period, potentially leading to a period of gales during
this time.

HYDROLOGY...

Low pressure lifting across the area will spread a long-duration
snowfall into Southeast Michigan starting late tonight and lasting
through late Sunday night. Total snowfall of 4 to 9 inches is
expected at this time, with highest totals from the I-69 corridor
southward. Snow intensity is expected to peak during the afternoon
and early evening. Rainfall equivalent of the snow that will fall is
expected to range between one-third and three-quarters of an inch.
Snow will end very early Monday morning.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 7 AM Sunday to 4 AM EST Monday FOR
     MIZ047>049-054-055.

     Winter Weather Advisory from 4 AM Sunday to 4 AM EST Monday FOR
     MIZ053-060>063-068>070-075-076-082-083.

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......RBP
AVIATION.....RBP
DISCUSSION...SF/DE
MARINE.......MR
HYDROLOGY....MR


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