Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 200815
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
415 AM EDT SAT AUG 20 2016

.DISCUSSION...

A significant, deep late summer midlevel trough will swing across
the northern plains today, across the Great Lakes on Sunday
inducing what will become an area of strong 1000-500mb layer
geopotential height falls over the eastern U.P. and the Straits.
Model data has converged on a solution that brings a surface low
to the eastern U.P. in the low to mid 990 mb by this evening...and
stalling out a strong inverted trough in the mid to upper 990 mb
over the far northern Lake Huron basin for the first part of
Sunday. Suffice it to say the area is looking at a regionally
active 24 to 30 hour period at least until the inverted surface
trough can release off of northern Lake Huron sometime late Sunday
afternoon.

The main forecast questions for today center around thunderstorm
onset, timing, and potential severity. As for the questions of
thunderstorm onset and most likely timing, there are 2 solutions
being advertised within the solution space. The first largely holds
off on thunderstorm activity impacting southeastern Michigan
until after 23-00Z, requiring the last of the upper level ridging
to squeeze out of the area and lower tropospheric convergence
arriving with the cyclonic or southwesterly flow trajectories. The
other solution suggests earlier onset of shower and thunderstorm
activity, sometime around 19 to 20Z, along a prefrontal trough
that is hydrostatic reflection at the surface of higher theta e
ridge or lower density aloft. The answer likely resides somewhere
in the middle but the forecast will be more shaded toward the
overwhelming consensus of the hi-res solutions which has some
support from the 20.00Z ECMWF. Bottom line, expecting a chance for
showers and thunderstorms to begin after 19Z this afternoon with
the best time window for thunderstorms occurring between 22-03Z.

As for severity, the most problematic aspect of the setup for
southeastern Michigan is that the greatest instability this
afternoon will be out of phase, or not collocated with the greatest
shear and upper level forcing. In fact, models continue to show that
0-6km bulk shear will remain sub 30 knots for all areas south of M
46 through 21Z, with 0-6km bulk shear increasing rapidly from 30-35
knots north of I69 between 21-00Z. Forecast soundings show that
highest MLCAPEs of around 2000 J/kg will occur earlier in the day
with less clouds/more insolation early leading to a tounge of rich
near surface theta e across far southeastern Michigan. Lowering of
near surface of theta e is expected during the afternoon with the
passage of the prefrontal trough which will knock MLCAPES down into
800-1250 J/KG. This agrees well with the latest Swody1 discussion.
Considering all of the factors the potential will exist for isolated
to severe thunderstorms, particularly north of M 46 after 21Z. The
biggest severe weather threat will be damaging wind gusts to 60 mph
with any line or bowing structures that become orientated orthogonal
to the southwesterly mean flow. The latest swody1 has much of
southeastern Michigan designated as a marginal risk for severe
weather with a slight risk in place over much of northern and
western Lower Michigan which is in agreement with the outgoing
forecast.

Precipitation chances will end rapidly by late Sunday evening with a
robust dry slot pushing directly across the state. No real concerns
with lingering drizzle at this time as deep subsidence is expected
to push to 2500 kft agl which leaves little depth to saturation.

Sunday. The meat of the significant trough will be overhead of
southeastern Michigan which will result in substantial cloud. Model
soundings is showing some backpedaling now on strength of steep low
to midlevel lapse rate pocket that will be in place particularly
over the southern CWA. This leads to some doubt as to exactly how
much shower activity will be around with a boatload of midlevel
subsidence still in place. Will likely need maturation of boundary
layer and deeper mixing for activity and continue to shade later in
the afternoon for the shower activity perhaps persisting a little
into the evening.

&&

.MARINE...

A low pressure system will develop over Wisconsin Saturday and
deepen quickly during the evening and overnight hours over far
eastern Lake Superior. Southeasterly/southerly winds will increase
to 15-20 kt Saturday afternoon and to 25 kt early Saturday night in
the warm sector over much of Lake Huron. The strongest winds are
expected late Saturday night into Sunday as a cold front ushers in
colder air (850 mb temperatures down to around 5C over northern Lake
Huron late Sunday) across the warm waters. The low will be at its
peak strength late Saturday night near 990 mb or possibly lower, with
a very impressive gradient advertised. 925 mb winds of 45kt
advertised by the NAM could mix to the surface Sunday over far
northern Lake Huron where a Gale Warning is in effect. A Gale Watch
is in effect for Central Lake Huron into Saginaw Bay due to some
uncertainty with the exact strength and track of the low. A
Small Craft Advisory may be needed for the southern Lake Huron
nearshore waters. Winds will rapidly diminish Sunday night as high
pressure builds in.

&&

.PREV DISCUSSION...
Issued at 213 AM EDT Sat AUG 20 2016

AVIATION...

Moist conditions and light winds should favor MVFR fog development
late tonight. Some guidance suggests formation of IFR stratus
deck. Will continue to monitor but confidence too low to include
in TAFs attm.

Southerly winds will increase in the warm sector of a strengthening
low pressure system tracking from Wisconsin into the UP Saturday,
gusting to around 25 knots in the afternoon. Showers and
thunderstorms could begin as early as 19Z with lead theta-e surge,
but high-res solutions hold off until the better forcing arrives
along a pre-frontal trough in the 22-02Z timeframe. Opted to handle
the uncertainty in the 19-22Z timeframe with VCSH with prevailing
TSRA 22-02Z. Frontal passage will shift winds to westerly toward
the end of TAF period.

For DTW...Some fog and/or a period of low clouds may impact the
airport in the 09Z to 14Z time frame. Though a few showers and
thunderstorms are possible as early as 19Z, best likelihood for TS
impacting the airspace is 22-02Z. Outside of thunderstorms, sub
5KFT cumulus may become BKN at times Saturday, but feel that SCT
will prevail until 22Z. A period of MVFR cigs is possible after
cold frontal passage turns winds to westerly around 06Z.

//DTW Threshold Probabilities...

* Low in ceilings below 5000 feet Saturday morning, moderate during
  the afternoon and evening.

* Moderate in thunderstorms late Saturday afternoon and evening.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from late tonight through Sunday afternoon FOR LHZ363-
     421-422-441-462.

     Gale Warning from 4 AM to 10 PM EDT Sunday FOR LHZ361-362.

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

$$

DISCUSSION...CB
MARINE.......CB
AVIATION.....DT


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