National Weather Service United States Department of Commerce


696
FXUS63 KDTX 140451
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1151 PM EST Fri Dec 13 2019

.AVIATION...

Extensive mid level cloud will continue to funnel across the region
overnight, downstream of a low pressure system tracking across the
Ohio valley. Some increase in low level moisture will occur through
the morning period, although greater near surface saturation now
appears to remain south of the area. This process in combination
with a light wind field may still support some degree of fog
formation over the southeast Michigan airspace. Greater potential
for MVFR to IFR restrictions will exist across the Detroit corridor
and possibly PTK, with lack of greater moisture depth limiting fog
density northward into FNT/MBS. Upstream observational trends and
recent model guidance continue to suggest fog development versus
stratus. Lingering fog lifts during the late morning period, perhaps
offering a window for VFR conditions into the afternoon. Moisture
will then increase from the west late in the day within colder west-
northwest winds. This will introduce the possibility for light snow
shower development in MVFR/IFR conditions Saturday night.

For DTW...Potential for fog formation Saturday morning remains, but
confidence in visibility dropping below 1 mile is diminishing.

DTW THRESHOLD PROBABILITIES...

* Low for cigs aob 5000 ft late tonight through Saturday afternoon.
High Saturday night.

* Low confidence in visibilities 1/2SM or less and/or cigs of 200 ft
less Saturday morning.

* High for ptype as snow Saturday night.

&&

.PREV DISCUSSION...
Issued at 400 PM EST Fri Dec 13 2019

DISCUSSION...

Several-day period of enhanced westerlies over the eastern Pacific
now culminating as a synoptic scale trough buckles along the lead
edge of the jet presently surging into the Lower Mississippi Valley.
A diffuse surface trough oriented along the northern segment of the
trough has supported widespread low clouds and fog over the western
and northern Great Lakes throughout the peak heating period.
Meanwhile, the surge of warm advection occuring ahead of tonight`s
cyclogenesis episode over the Southeast is funneling shallow moist
air/fog northward through the Ohio Valley. Light/patchy fog will
overspread the area tonight as wind becomes calm within the trough
and cloud bases lower coincident with nocturnal stabilization.
"Areas" of fog has been added to the forecast east of the glacial
ridge as this area will be located along the western periphery of
light southerly theta-e advection originating over the Ohio Valley.
The primary concern for the overnight period then becomes dense fog
potential early Saturday morning. Any precipitation will be high-
based and exceedingly light given poor forcing and dry-mid-levels.
Inability for sprinkles to effectively wet-bulb the column naturally
introduces a low end patchy/transient freezing drizzle type of set
up for locally colder places/surfaces/etc. Given that there will be
zero shear in the boundary layer, any -fzdz will be disorganized,
originate exclusively from high clouds, and will not pose any issues.

As the cyclone deepens over interior New England and lifts
northeast, boundary layer flow over the Lower Peninsula will respond
by backing to WNW through Saturday. Increasingly well-mixed flow
will support a light gust component around 20 kts while temperatures
stagnate in the low 30s beneath overcast skies. Energy embedded
within developing zonal flow aloft will sweep through the central
Great Lakes overnight Saturday providing favorable synoptic scale
support for expanding lake effect snow showers. When considered
along with NAM12 depictions of convergent boundary layer flow
settling into the I-94/I-96 corridors and boundary layer growth to
nearly 6kft (sufficient to generate dendrites within convection),
the potential exists for light lake effect accumulations but any
greater potential will be limited by modest overlake theta-e lapse
rates topping out around 2-3C/KM.

Forecast complexity increases considerably for early next week and
confidence drops off accordingly. NWP have characteristically
bounced around the location of the polar low over northern Canada,
its location sensitive to the behavior of and interaction with the
adjacent closed high over northern Alaska. Further complicating
matters will be the cyclonic wave break resulting from the
developing New England cyclone which will force additional ridging
into the high latitudes and introduce further uncertainty to the
large scale pattern over Canada. Since both the CMC and NCEP
ensembles are demonstrating sensitivity to the relatively low
amplitude development sequence as it occurs at the interface of the
incoming Pacific energy and the confluent wave guide anchored over
Canada both will be sensitive to this process as it unfolds. The
ECMWF likewise demonstrates some uncharacteristic waffling during
this period as its solution space remains sensitive to the same
factors. The 12z ECMWF provides a good illustration as 400mb PV
analysis reveals that subtle changes in the flow result in greater
wave separation (centered over the Dakotas) occuring Monday. The
more progressive northern stream then suppresses the front to the
south resulting in a muted frontal snow late Monday night or Tuesday
morning. Surprisingly, the UKMET resides on the opposite end of the
spectrum allowing snow to develop as early as Sunday night as the
front lifts toward Michigan. An outcome ranging anywhere from 1 to 5
inches, highest near the Ohio border, is plausible attm. Amounts
will be highly conditional on onset time which remains the greatest
source of uncertainty at this time.

MARINE...

Relatively light southerly flow tonight on the south side of a low
pressure system that will weaken as it moves east of Lake Superior
and washes out into southern Ontario. Broad troughing will move over
the Great Lakes this weekend leading to an uptick in west to
northwesterly winds ushering in arctic air. The result will over
some lake instability leading to winds that gradual increase late
Saturday night and early Sunday morning with gusts to around 30
knots. Winds will then decrease into Sunday afternoon as high
pressure begins to build into the region. This will lead to more
favorable marine conditions into early next week with light winds
and low waves. Next chance at unsettled marine conditions will be
Tuesday evening into Wednesday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....MR
DISCUSSION...JVC
MARINE.......AA

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