Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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136
FXUS63 KDTX 251603
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1203 PM EDT THU AUG 25 2016

.UPDATE...
A few updates to pop and temp trends as we usher out the weakening morning
MCV and look forward to the cold frontal passage this afternoon.
The cold front now extends from northern Saginaw Bay down through
the southern Lake Michigan basin. Frontal passage timing looks to
be around 18-22Z this afternoon. This timing should give our
southern counties, generally south of I-94, time to destabilize a
bit more than the cloud covered mid Michigan region. Models have
dropped off a bit with coverage of thunderstorms this afternoon,
but the cold front will move through a very moisture rich region
(pwat 1.90 inches on the DTX 12Z raob and surface dewpoints in the
70s) with high shear (bulk shear values near 40 knots) so some
organization to the storms cannot be ruled out. CAPE values are
tempered by warm mid/upper levels, but will still reach near 1000
j/kg. Scattered thunderstorms will develop through the early
afternoon, first over the Saginaw Valley along the front, then
expanding south and east. Main threats are heavy rainfall and
straight line winds due to mostly unidirectional wind shear, but
yesterday the atmosphere proved that there is enough backing of
the winds at the surface with these low LCLs to keep a mention of
a possible brief tornado in the forecast. Thermal profile shows
the threat of hail is negligible.

&&

.PREV DISCUSSION...
Issued at 702 AM EDT Thu AUG 25 2016

AVIATION...

The pressure gradient has remained strong enough and conditions have
been effectively mixed to keep low stratus preferred over fog. Any
remaining fog will burn off quickly at the start of the period.
Humid air and moisture transport ahead of the slow moving cold front
will bring the potential for showers and thunderstorms today. The
first activity is tied to a remnant MCV along a warm frontal feature
now pushing in from the Grand Rapids area. This activity should
largely wane this morning while affecting the northern taf sites.
Deeper theta e reservoir or ridge will advect across lower
Michigan and set the stages for some additional convective
initiation development later this afternoon during the 20-23z time
window. Weak system overall will limit much of a wind response
tonight as well as degree of cold air advection.

//DTW Threshold Probabilities...

* Moderate for ceiling 5000 ft or less through afternoon.

* Medium for thunderstorms mid to late afternoon.

PREV DISCUSSION...
Issued at 345 AM EDT Thu AUG 25 2016

DISCUSSION...

Moisture plume (analyzed at 00Z between 1.75 and 2.0 inches) can be
seen on water working up from Mexico into the southern Great Lakes
between a large upper high over the southeastern U.S. and an upper
trough sinking from central Canada into the Northern Plains. Several
shortwaves/MCVs are evident within this plume stretching along a
warm front from the Plains into the Western Lakes. These waves and
the front should work into Southeast Michigan during the morning and
afternoon per latest model guidance. First round of showers/tstorms
triggered by these waves is now working across Lake Michigan. This
activity is expected to weaken as it crosses Southeast Michigan
during the mid to late morning as low-level jet support wanes. Some
of the higher-res guidance then suggests that instability will
struggle to recover enough to support much in the way of
thunderstorms later in the day, while other runs support enough
recovery for strong storms. This places us in a very familiar
situation of waiting to see how much the atmosphere can recover from
early convection before stronger forcing works in during the
afternoon. Strengthening mid-level wind field will increase 0-6km
bulk shear values to 35-40 knots by this afternoon, while forcing
from smaller waves is bolstered by surface trough and initial cold
front. Warm start to the day should help temps rise into the 80s,
with dewpoints near 70 helping to generate SB CAPE values likely
around 1000 J/kg or more. This will support a threat for severe
weather if instability can be realized from mid-afternoon through
early evening, with fairly unidirectional wind profile making
damaging wind gusts to 60 mph the biggest threat. Very high moisture
content will also bring the threat for very heavy rainfall and
localized flooding. Drier air will arrive tonight behind the surface
trough, and behind a stronger secondary front that will arrive
overnight. This will end the threat for rain and allow skies to
clear a bit.

Weak shortwave ridging will allow surface high pressure to build
over Michigan on Friday. Larger-scale subsidence should keep the
area dry even as a weak shortwave riding the periphery of the upper
high centered over the southern U.S. lifts up into Michigan. This
should allow for an increase in mid/high level cloudiness by late in
the day, with GFS/NAM model forecast soundings keeping the lower
levels dry. A warm front will lift through the western and central
Great Lakes Saturday into Saturday night in advance of a stronger
trough working through the Northern Plains and Upper Midwest. This
will bring increasing clouds (mostly cloudy skies) and chances for
showers and thunderstorms from at least the I-69 corridor northward.
Upper trough, favorable right entrance region forcing, and cold
front working through Saturday night and Sunday will expand chances
for showers/tstorms across all of Southeast Michigan. Temperatures
for the end of the week have been adjusted upwards as warm air
advection look much more robust in latest model runs. Highs both
Friday and Saturday look to reach near 80 to the mid 80s.

MARINE...

Modest west to southwesterly winds in the 10 to 20 knot range will
persist ahead of a slowly approaching cold front. Ahead of this
front, the potential continues to exist for showers and
thunderstorms from Saginaw Bay southward, particularly this morning.
The cold front is expected to track through the central Great Lakes
tonight. Overall, very little wind increase is expected behind this
weak system. Modest west winds are expected again for Friday before
shifting to the south on Saturday.

&&

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

$$

UPDATE.......DRK
AVIATION.....CB
DISCUSSION...HLO
MARINE.......CB


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