Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 041002
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
502 AM EST Sun Dec 4 2016


.AVIATION...

Southwesterly winds will continue to disrupt the low level
inversion enough to cause a brief break in the MVFR stratus across
the region. Cirrus debris will start drifting over the region in
advance of the trough that will lift through this evening and
overnight. Models are in good agreement with a period of IFR
CIGS/VSBYS this evening as the deeper ribbon of moisture arrives
producing snowfall. Exact onset is still in question but looks good
to hold off til after 21Z. Abrupt end to the snow will work through
around 06Z. The character of the snow will be wet and with sfc temps
in the 30s and relatively warm pavement temps, the snow will
struggle to stick to paved surfaces.

For DTW...

The onset of snow still looks like it will be mainly after 00Z Sun
evening at metro, likely not ending until about 06Z.

//DTW Threshold Probabilities...

* Medium in ceilings below 5000 ft this morning. High this afternoon
  through tonight.

* Low for CIGS/VSBYS to drop below 200ft and/or 1/2SM tonight.

* Medium in precip type being mostly wet snow this evening.

&&

.PREV DISCUSSION...
Issued at 419 AM EST Sun Dec 4 2016

DISCUSSION...

The well defined anticyclonically curved exit region of +130 knot
Pacific jet streak is currently pushing off of the Rockies and
advancing out into the central Plains over Nebraska. MSAS analyses
this morning shows that some eastward push of a leeside surface
trough has occurred. Meanwhile, water vapor imagery suggests some
ongoing deepening of the system with phasing and slight leafing of
southern stream moisture over portions of Kansas and Nebraska. As
this noted moisture lifts further northward during the next 6 hours,
models suggest the enhanced midlevel potential vorticity will react
with deep midlevel trough pushing off of the Rockies to cause some
spin up and dramatic increase to absolute vorticity that will pivot
around the increasingly dominant western Canada lower height
anomaly. The significant pivot of the shortwave or extremely rapid
change from positive to neutral to negative tilt will occur from 12Z
this morning by NE/KS through Chicago around 00Z and through lower
Michigan 00-06Z this evening.

The 04.00Z numerical weather prediction suite arrived this evening
with a few surprises in the solution space compared to output from
24 hours ago. The biggest surprise was a fairly significant uptick
in the QPF with this system. Bulk of the model output, chiefly the
NAM, is now suggesting that a widespread .20 inch will fall across
the southern 2/3rds of the CWA, while the northern 1/3rd of the CWA
could see as much as .30 inch. The overall governing dynamics of
this event really have not changed much. The event at the big
picture will be driven by direct differential cyclonic vorticity
advection from the negative tilt shortwave described earlier in this
discussion and warm air advection processes. As far as the warm or
theta e advection, the nam has certainly evolved much more bullish
on the ascent potential. The reason for this is earlier nam
solutions were a little stronger with the preceding midlevel
anticyclone that pushed across Lake Huron into Ontario today. This
stronger localized midlevel cyclone kept system relative flow much
more easterly through southeastern Michigan right into the event.
Now, models fade out this anticyclonic circulation keeping system
relative flow southeasterly today. This southeasterly flow in
advance of the system then becomes much more in phase and orthogonal
to isentropes in the theta e ridge as it pivots into southeastern
Michigan. At first, one thinks this just contributes to more
sustained gradual ascent, however, nam cross section views does
support a period of upright ascent across the area.

However, that is not the end of the story. The regional 10km
Canadian was the first to suggest a solution that carries a dynamic
lift response farther south through the Ohio River Valley. The cause
of this southern solution is that a favorable jetlet structure
develops on the forward jet streak over the Ohio River Valley.
Models are now suggesting that favorable left exit region dyanmics
develop on this jetlet which also becomes increasingly phased with
the deep midlevel trough. Best nose convergence off a low level jet
would occur down there as well, with the best of the large scale
moisture transport almost releasing due eastward, and getting
pinched off or occluded before it surges northward in and ahead of
the trough. The experimental HRRR has really latched onto this
solution. Additionally, the 06Z 4km WRF is onto it as well. What
happens over southeastern Michigan is more of straight inverted
trough, convergence, response in the presence of high quality,
deep, moisture. There is at least, qualitatively, support for this
solution in the deterministic NAM mass fields with a dominant
midlevel low ejecting eastward across northern Indiana to the Lake
Erie/Ohio shoreline this evening.

Out of respect for the very high quality of moisture, direct CVA,
and more organized southeasterly system relative flow, the decision
was made to increase QPF to around .20 inch for all areas. The
second surprise of the 00Z suite was to push the timing back again to
the 22-05Z timeframe. Bottom line, expecting a mix of light snow and
rain to overspread western lower Michigan today, before advancing
into southeastern Michigan during the late afternoon hours.
Surface temperatures will rise into the upper 30s today, before
wet bulbing down into the lower 30s as precipitation begins. Given
the later timing and the overwhelming consensus of the
environmental thermal profile suggested in the forecast soundings,
think there is little doubt that much of the precipitation over
southeastern Michigan will change over and fall in the form of
snow. The exception at onset and for a couple of hours will be the
higher surface pressure areas south and east of the terrain and
the Detroit urban heat island, where surface temperatures will be
above 2C with a surface dewpoint at or just above freezing. There
a light rain will try to hold on longer. Keying off of the direct
CVA, belief is that height of event will occur in the 23-03Z
window. Big question outstanding is what snow ratios can be with
surface dewpoints then actually rising to the 32F as the heart of
the inverted surface trough settles in. In-house probabilistic
snow guidance is slightly more favorable now for .2 of an inch
per hour rates for a 3 or 4 hour period but not more than that.
Given a stable warm near surface sounding with lack of low static
stability through the DGZ, still expecting needles and columns
with a best case snow ratio of 8:1. Summing it all up, 1 to 2
inches of a wet snowfall will be possible in grassy areas with
less than 1 inch for Detroit. Better amounts are expected to the
north, more because of slightly cooler thermal profile than any
max bullseye in qpf.

Surface high pressure will grip southeastern Michigan on Monday as
shortwave ridging amplifies over much of the Great Lakes.
Preference sides again with a cloudier forecast given time of
year, westerly flow, and moisture content progged under an
inversion at 900 mb. Temperatures will be right at average.

The next system will arrive Tuesday afternoon as the Mexico cutoff
low gets kicked northeastward. This is one that is really going to
warrant some attention over the next couple of days as there is some
dramatic differences between the thermal profile of the NAM (cold)
and the ECMWF and GFS (warm). The nam if it were to verify suggests
an accumulating snowfall for all of the southeastern Michigan. One
wonders if the nam is its normal self in failing to activate the
frontal surface down lower in the column. The other solutions have a
more definitive sharp northern cutoff to the precipitation shield.
With the warm origin of the system appearing to be at least
preserved in the ECMWF soundings and GFS soundings, will defer to
the previous forecast calling for a predominate cold rain Tuesday
afternoon.

MARINE...

Light easterly flow early this morning will increase slightly out of
the southeast today in advance of an approaching surface trough.
Winds will veer to the southwest behind the trough tonight. Lack of
stronger pressure gradient will keep wind gusts below 20 knots
through the event though the trough will present a period of light
rain/snow this evening and overnight. Southwesterly flow will
continue into Monday while decreasing as high pressure builds back
in from the south.

&&

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

$$

AVIATION.....DRK
DISCUSSION...CB
MARINE.......DRK


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