Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 291544
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1144 AM EDT FRI JUL 29 2016

.UPDATE...
Not much new detail to add to the forecast, just a few tweeks to
pops to account for recent trends. Southern lower MI is embedded
in a wide band of high theta e air with some enhnaced regions of
isentropic ascent embedded within that. Shower activity across mid
MI is now waning with the loss of noctural forcing and aid of low
level jet. Best activity remains over southwest lower, located
lower down the frontal surface and slowly tracking southward for
the better surface based instability with heating commencing.
Shortwave ridging seen on water vapor overhead will wane as the
trough appraoched this afternoon. Should be just enough to keep
activity overhead minimal. With scattered showers across our
north, just to our west, and now popping up over Ontario, see no
reason to deviate from the inherited chance pop. Any shower or
thunderstorm that does develop will have the opportunity to
produce heavy rainfall as pwats remain high and storm motion is
relatively slow.

&&

.PREV DISCUSSION...
Issued at 703 AM EDT Fri Jul 29 2016

AVIATION...

Mainly mid clouds (5-15 kft) persisting through forecasting period.
Scattered showers and possible thunderstorm activity will attempt to
push ceilings/visibilities toward MVFR, with best chance of activity
looking to be toward 00Z south of I-69. Although, coverage and
timing looks to be rather chaotic through the period. Have included
prob30 for thunderstorms across southern taf sites, with scattered
showers likely persisting much of the night as upper level
trough moves overhead.

For DTW...Chance for showers or possible thunderstorm during most of
the forecast period, but highlighted the best chance late
today/early this evening with prob30. Good chance for scattered
showers to persist tonight, but uncertain in direct impact to
terminal. Greater coverage of showers will yield better potential
for cigs to reach or fall below 5000 feet.

//DTW Threshold Probabilities...

* Low for thunderstorms impacting kdtw this afternoon and evening.

* Low for cigs aob 5kft.

PREV DISCUSSION...
Issued at 357 AM EDT Fri Jul 29 2016

DISCUSSION...

The large scale pattern over North America features a broad upper
ridge centered over Arizona and a low over northern Quebec. In
between is weak westerly flow from the Pacific coast of Canada into
the Great Lakes. Upper air analysis indicates a positively tilted
trough between the Quebec low and a primary short wave over the
northern Plains into the Midwest states during the morning. The
combination of the short wave and larger scale trough is supporting
a loosely organized pattern of mid level deformation from Wisconsin
and Illinois into central lower Michigan. This deformation pattern
will be the main weather feature affecting conditions in SE Michigan
today through tonight while the short wave slides slowly along the
south flank of the zone.

The forecast challenge in this scenario will be to identify
localized areas of greater rainfall concentration as forcing will
result in clusters of convection this time of year rather than a
broad pattern of rain that might occur outside of the summer season.
As is the nature of weakly forced convection, specificity on details
of timing, coverage, and intensity will drop off with time while
some greater refinement can occur in the short term 0-6 hr
period. This is already being demonstrated during the early
morning where radar indicates clusters and bands of showers/storms
expanding from southern Wisconsin into central lower Michigan
within the mid level deformation axis/theta-e ridge. Weak elevated
instability is fueling the convection which should continue to
expand eastward as nocturnal enhancement progresses before
sunrise. CAM solutions like the ARW and NMM do not have a good
handle on this even in the short term. The HRRR/RAP does handle
the eastward expansion reasonably well in favor of the synoptic
scale models that all support at least likely POPs in the Saginaw
Valley and northern Thumb through the morning. Scattered POPs to
the south down to the Ohio border will cover the stray
showers/storms that have been bubbling up since midnight and
which is expected to continue in a more disorganized fashion
throughout the morning.

Expect elevated convective processes to weaken by early afternoon as
is typical during the upswing in the diurnal cycle. Additional
convection will become surface based and favor the area near the
Ohio border during afternoon through evening. Mid chance POPs from
guidance are OK there, if not lower, as the stalled surface front
will be south of the border where the bulk of daytime activity will
be focused. There could be some inverted troughiness back into far
southern lower Michigan but surface based instability will be quite
limited by larger scale NE flow and residual cloud cover within the
mid level deformation axis/theta-e ridge. This will lead into a
repeat performance later tonight into Saturday morning as the upper
level short wave moves just slowly eastward and pulls the
deformation axis a little farther south. As mentioned above, the
resulting weak convection is difficult to pin down outside of the
short term. For now, the highest chance POPs will be over the
south half of our area while evaluating potential to increase in
upcoming forecasts.

This system will still be around by Saturday night as the 00Z model
runs are in good agreement with slowing and sharpening the upper
level short wave. The result is some uncertainty regarding the dry
conditions in the forecast for Sunday. Plan to hold the line here in
favor of upcoming model trends before larger scale high pressure
arrives to start next week. The surface high is projected to produce
warm temperatures for the start of August but with lower humidity
through Monday and most of Tuesday. There are signals for a round of
MCS type activity during the second half next week and possibly the
next front by Friday.

MARINE...

Persistent northeast winds, up around 20 knots, focused over central
and southern sections of Lake Huron today into tonight will allow
for elevated wave heights in the nearshore waters of Lake Huron
through Friday. Small Craft Advisories are in effect for waves in
excess of 4 feet. Northeast winds will continue into the weekend,
but should hover more around 10 knots, allowing for waves more in
the 2 to 4 foot range.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 10 PM EDT this evening FOR MIZ048-049-
     055-063.

Lake Huron...Small Craft Advisory until 10 PM EDT this evening FOR LHZ421-422-
     441>443.

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

$$

UPDATE.......DRK
AVIATION.....BT
DISCUSSION...BT
MARINE.......SF


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