Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 031709
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1209 PM EST Sat Dec 3 2016


.AVIATION...

Very little low level mixing has occurred through 17z, which is no
surprise given model forecast soundings of taking decent inversion
already in place around H85 this morning and strengthening/lowering
it into this evening as high pressure builds east into area. Only
after 06z does some drier air work into the sub H85 layer which in
itself it a bit suspect as the low level inversion only weakens
slightly overnight. Will maintain MVFR to lower VFR stratus over the
whole area through 06z and will likely be slightly more pessimistic
after 06z as well given the extent of low level moisture over the
region. The next weather system to affect the area lies just beyond
the forecast period as a surface trough encroaches from the west and
bring snow or rain/snow mix after 21z Sunday.

For DTW...Ceilings will remain anchored around 3500 feet during the
afternoon with some chance of lower ceilings in the 2500-3000 foot
range as this lower layer of stratus just south attempts to spread
slowly northeast. It is difficult to ascertain the amount of low
level clearing to expect overnight, but given cloud trends so far
today will lean a bit more pessimistic. Further reductions in cigs
and vsbys will hold off until after 21z Sunday as rain/snow spreads
into area within region of isentropic lift in advance of trough.


//DTW Threshold Probabilities...

* High in ceilings below 5000 ft through this evening...medium
  overnight.

&&

.PREV DISCUSSION...
Issued at 418 AM EST Sat Dec 3 2016

DISCUSSION...

Weak but steady geopotential height rebound and deep column
subsidence is supporting a broad area of surface high pressure over
much of central North America this morning. Some modest increase in
shortwave ridging over the western Great Lakes will force the
surface ridge axis through southeastern Michigan this evening.
Really looking at another prototypical, quiet early December day,
one of persistence off of Friday. The only question outstanding is
cloud trends as IR imagery has shown a few breaks and some thinner
stratocumulus in place overnight over much of the cwa. Modeled RH
plan view suggests that subsidence at 700mb is being successful in
some dry entrainment downward through the inversion base. Despite
continued dry RH aloft with upper level confluence, moisture and
saturation is forecasted to hold strong at 850mb throughout the day.
Thus, preference is maintain a mostly cloudy forecast today.
However, will not be surprised at all if substantial breaks were to
develop especially later in the afternoon.

The anticyclonically curved exit region of a strong upper level jet
core pushed inland over the PACNW in advance of the 00Z raobs.
This entrance region is forecasted to nose across the intermountain
West this morning harvesting upper level energy out of the vast
Potential Vorticity reservoir that is attempting to cutoff over much
of the western United States. Models suggest some magic takes place
by this evening as leeside troughing and some linkage to higher
midlevel PV occurs with higher moisture content lifting northward
out of Texas. In short, expecting some fairly rapid intensification
to a shortwave over central Plans and Nebraska late tonight before
ejecting rapidly northeastward Sunday. Models continue to show a
shortwave that will be going to a rapid negative tilt while
accelerating through southeastern michigan. A notable change in the
00Z suite was to speed up onset timing of precipitation and
shortwave late Sunday afternoon (bulk could occur between 21-23Z)
while ending the event a little earlier (possibly as early as 03Z).
Overall characterization of the event has not changed. Looking at
bulk of dynamics being driven by direct cyclonic vorticity advection
of shortwave pivoting aggressively into the cwa. Weak equivalent
isentropic ascent does exist late Sunday afternoon on 302K surface
but the more apt description is rather just straight moisture
advection within weak static stability. This thinking matches well
with the forecasted QPF pattern, one that shows widespread modest
precipitation with a lack of any mesoscale definition. The
acceleration of the wave has increased in this solution suggesting a
shorter duration of the event. With much of saturation in the -2 to -
5C window, and the metrics off of the in-house ratio calculator,
feel that much of the snow will fall at 6:1 or 7:1 with a crystal
habit of plates and columns. Given the marginal surface temperatures
of 31 to 33 F, the accumulation factor is expected to be on the low
side particularly for the higher surface pressure locations south
and east of the terrain and the Detroit urban heat island. Add
everything up, looking at an event that may be able to grind out an
inch between 20-03Z. In-house probabilistic snowfall rate
information is also struggling to show any snowfall rate potential
greater than 0.2 inches per hour. Attm, expecting some accumulation
in grassy areas but not expecting much impact on the roadways. That
could change if rates verify higher than expected.

The next precipitation event is expected to occur late Tuesday
afternoon and Tuesday evening as the previous northern Mexico cutoff
become reacquainted to the westerlies by being kicked northeastward
through the Ohio River Valley. Models are not surprisingly warm with
this feature, suggesting 850mb temperatures preceding the
precipitation at +4C. Possible that precipitation type could change
over later Tuesday night, but it appears that drier air would be
needed to wet bulb down. However, drier air would promote an end to
the precipitation. Alot of room to move on this forecast as northern
Mexico energy is notoriously difficult to capture.

MARINE...

Area of high pressure will settle over the Great Lakes today
bringing light winds and waves to the region. A surface trough will
then sweep across the area Sunday into Monday bringing a period of
rain (or a rain/snow mix) Sunday evening and overnight. Winds will
increase out of the southwest as the through axis passes but gusts
will remain below 20 knots through the event.

&&

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

$$

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


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