Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 091720
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1220 PM EST Sat Dec 9 2017


.AVIATION...

Widespread light snow and fog is expected to persist through the
early and mid portions of the afternoon as long as southerly
component to the flow exists ahead of surface low/trough now coming
onshore in far southwest lower Michigan. Bulk of heavier
precipitation should be closely associated with forcing near the
surface low as it moves southeastward across northern Indiana and
into Ohio during the afternoon. At the same time, surface
trough/front extending from southwest lower Michigan to near Saginaw
Bay will gradually move southeastward as well passing MBS around
19z; FNT around 20z; and the far southeast Michigan TAF sites around
21z. As precipitation will continue ahead of the trough, but weaken
this afternoon.  Also, expect visibilities to gradually become MVFR,
before going VFR shortly after frontal passage.  Likewise cigs
generally in the low MVFR range will gradually go VFR later this
afternoon and evening.

//DTW THRESHOLD PROBABILITIES...

* High confidence for ceilings at or below 5000 feet this afternoon
  and evening, low for ceilings after 04z tonight.

* High confidence in visibility below 3 miles through mid afternoon.
  High confidence for visibility aoa 6 statute miles aft 00z this
  evening.

&&

.PREV DISCUSSION...
Issued at 936 AM EST Sat Dec 9 2017

UPDATE...

The first widespread accumulating snow event of the season remains
underway across Se Mi. Given the rather weak nature of the forcing
and rather poor moisture quality, snowfall totals will remain on the
light side. Thus far this morning, available snowfall reports suggest
anywhere from a couple tenths up to an inch and a half have fallen
across the forecast area. The better mid level ascent has also
remained largely above the good dendritic growth layer, thus flake
size has been for the most part small.

A rather compact mid level short wave now over Chicago will track
southeast across Indiana this afternoon, with the associated sfc low
tracking toward Fort Wayne by late afternoon. A broad region of mid
level isentropic ascent, focused mainly from Flint to Mt Clemens and
points south, will persist through the remainder of the morning,
then gradually weaken during the course of the afternoon as the upper
wave slides farther south. This will lead to a gradual decrease in
the areal coverage of the snow. Low to mid level lapse rates will
however steepen during the course of the afternoon as the mid level
trough axis and associated cold pool slide overhead. While the sfc
low will remain southwest of the forecast area, some enhanced sfc
troughing will reside over Se Mi. The increased boundary layer
convergence within the steepening low level lapse rates will support
some higher intensity snow showers with better flake size, likely to
be brief. So in short, the widespread light snow will transition
toward intermittent brief snow shower activity during the afternoon.
Recent CAMs remain supportive of a brief intense lake enhanced snow
band impacting the eastern thumb region during the evening hours. The
brevity of which should keep additional accums an inch or so in this
region.

Additional snow accums for the remainder of the day should be a half
inch to an inch across the southern half of the forecast area, with
just a dusting at best north, aside from the thumb/Port Huron
region.

PREV DISCUSSION...
Issued at 251 AM EST Sat Dec 9 2017

Discussion...

Area of ascent in advance of digging shortwave continues to steadily
expand eastward into the forecast area early this morning. Initial
plume(s) of light snow within southwest flow in advance of trough
axis will work into the area during the rest of the morning. Rather
dry low levels below H85 within this west-southwest flow (using the
KGRB and KILN soundings as proxies) are certainly cutting into snow
rates to some extent on the leading edge of this area of light snow
as better visibility restrictions have been slow to expand east from
the shores of Lake Michigan where low level moisture is maximized.

Still, low levels over the area should continue to slowly saturate
and allow for light accumulating snow before sunrise. Trajectory of
the moisture plume(s) within this southwest flow will largely bypass
areas of southeast MI along and south/east of I-94 through 10z-12z.
As was noted by previous shift, moisture quality with this system,
aside from the mesoscale influences of the Great Lakes, is very
poor. So, as vorticity max at the base of this shortwave trough digs
southeast into the Ohio Valley by early afternoon, and better
synoptic scale lift is lost, expect the area of light snow across
the region to become very splotchy in character with time (thus
rather inefficient in producing accumulating snow).

Overall, this suggests that snow within areas that receive steady
light snow within southwest flow may total 1 to 2 inches of snow by
late this afternoon. Meanwhile, far southern portions of the area
will probably fall short of an inch as start of accumulating snow is
delayed several hours. Hires models still depict an interesting
period over the Thumb by early evening as northerly flow around
secondary low pressure that develops as upper trough axis reaches
this area of relative "warmth" over the lake waters. Essentially,
lake effect banding over the lake today is forecast to buckle and
pivot into area within some form of mesoscale low feature. Will
maintain a peak of near 3 inches, but must admit the confidence in
eventually achieving that much is low. With all that said, an
advisory is not needed today.

Otherwise, the deep eastern NOAM upper longwave trough is expected
to remain in place for the remainder of the forecast period, perhaps
flattening slightly heading into next weekend. This will maintain
this cold period of weather, which will intensify somewhat as cold
airmass deepens over area in wake of the next clipper system. At
this time expect, temperatures to bottom out in the Tue/Wed time
frame with highs in the lower/mid 20s and lows in the single digits
to teens.

This aforementioned clipper low will bring the next chance of
widespread light snow from Monday into Monday night. This will cross
the Gulf/Alaska (as opposed to the current system which dove sharply
SSE from the northern Alaska) so moisture quality, while likely on
the low side, should be a bit better. Current indications suggest
several inches of snow will be possible during this time frame.

MARINE...

Low pressure tracking through lower Michigan today will lead to
increasing northerly winds over Lake Huron, with gusts of 25 to 30
knots late today into this evening. These stronger northerly winds
will lead to large waves over the southern Lake Huron basin,
impacting the nearshore waters as well where small craft advisories
have been issued. Cold airmass will promote snow showers over the
lake, along with winds remaining elevated into Sunday out of the
northwest-west. Another low tracking through the area on Monday will
allow for light winds, but northwest winds on the backside of the
low on Tuesday will be strong, potentially reaching Gale force over
Lake Huron.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 7 AM EST Sunday for LHZ421-441>443.

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

$$

AVIATION.....DW
UPDATE.......SC
DISCUSSION...DG
MARINE.......SF


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