Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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398
FXUS63 KDTX 230124
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
924 PM EDT THU SEP 22 2016

.UPDATE...

A mid level short wave continues to slowly move across Lower Mi
this evening. A pool of dry air centered around 700MB as noted on
the 00Z DTX soundings has thus far kept showers out of the
forecast area this evening. This region of dry mid level air will
slide east during the overnight, allowing a ribbon of mid level
moisture to move into SE MI. By this time larger scale support
will weaken as the mid level wave exits the region. This moisture,
along with weak elevated instability and the passage of a cold
front overnight may overcome the waning upper level forcing, thus
supporting the current low chance pops in the forecast. In terms
of the cold front, as of 01Z, it has already moved through Oscoda
(KOSC). It should accelerate across southern Lake Huron, entering
Saginaw Bay and the northern thumb prior to midnight. Ample post
frontal low level moisture will support a good deal of stratus and
some drizzle/light showers well into the day Friday. Overall, the
current forecast has this handled well. An update will be issued
simply to make a slight upward adjustment to pops late to account
for some post frontal drizzle and to nudge nighttime temps up a
couple degrees in light of the relatively moist post frontal
dewpoints.

&&

.PREV DISCUSSION...
Issued at 658 PM EDT Thu SEP 22 2016

AVIATION...

The upper level support triggering the showers across portions of
northern Michigan will weaken during the night as the sfc cold
front, now over nrn Lower Mi, pushes south across the Se Mi
terminals. This will result in rather low chances for heavier
showers/thunderstorms. Some brief light showers along the front
itself do remain possible. The cold front is expected to pass
through MBS between 04 and 05z and exit south of metro Detroit
around 10Z. The passage of the front will be marked by a veering of
the wind to the north-northeast. Latest model guidance, with some
support from upstream observations, suggests a low stratus deck (IFR
and low end MVFR) will trail the sfc cold front by a couple hours.
Low level moisture quality may also support some post frontal
drizzle/light showers. A deepening of the low level inversion should
hold the stratus in place through the day Friday, while bases will
only slowly lift as the depth of the low level cold air increases.

For DTW...Generally light winds and high based clouds will keep
conditions rather tranquil until the approach of the sfc cold front.
It is expected to track from north to south across metro around 10Z.
A post frontal north-northeast wind in the 8 to 12 knot range will
then persist through the day Friday. The slight downslope into metro
may be enough to keep the post frontal cigs low end MVFR, although
this does carry some uncertainty.

//DTW Threshold Probabilities...

* High for ceilings below 5000 ft after daybreak Friday.

PREV DISCUSSION...
Issued at 355 PM EDT Thu SEP 22 2016

DISCUSSION...

Afternoon observations support faster timing of the frontal passage
through SE Michigan tonight that has come in with the latest model
run. Mid afternoon surface analysis indicates a weak wave of low
pressure along the front near the straits that is a reflection of
the mature MCV over lower Michigan shown aloft in satellite imagery
and model analysis fields. Modest coverage of showers will be
associated with these larger scale features during the afternoon
into early evening. The upper jet nearby to the north will also
contribute support, along with dynamic forcing on the north flank of
the MCV, to help focus weak convection within and to the south of
the frontal zone before the wave moves east by mid evening.
Thunderstorm potential will be limited by surface based CAPE
hovering around 1000 J/kg feeding MLCAPE only around 500 J/kg, both
of which will continue to fade with the loss of daytime heating.
However, expect shower potential to linger through the night as the
frontal structure remains sharp above the surface with just enough
elevated instability for some weak convection. The otherwise
unorganized pattern of forcing suggests only entry level POPs.
Pressure rises trailing the MCV will apply a southward shove to the
front as it exits the region during the evening, then synoptic scale
subsidence over northern Ontario under strong northwest flow aloft
will do the rest to build high pressure and push the front into Ohio
by early Friday morning.

The main story behind the front will be clouds and cooler
temperatures over SE Michigan. A chance of showers will also linger
within the low to mid levels of the frontal zone as it remains well
organized under neutral to slightly cyclonic flow aloft. There is
actually good model agreement that keeps the 850-700 mb theta-e
ridge overhead through the day and into Friday evening. This raises
the question of how far south/downstream to allow for showers as the
upper level ridge axis builds strongly to our west Friday night.
Prefer guidance that offers minimal coverage near the Ohio border
and dry conditions over the rest of the area. The upper ridge should
continue to be a limiting factor for rain, if not clouds, through
Saturday as well. Lingering clouds and NE low level flow will
reinforce cool air and likely limit high temps to the mid 60s near
Lake Huron to around 70 inland, which are actually readings right
around normal for late September.

Apex of upper ridge looks to be directly overhead Sunday, and
despite a modest increase in midlevel moisture, expect another dry
and seasonable day. For next week, a complex interaction between the
upper low now over the Sierra Nevada and the wave now approaching
the Alaskan Panhandle leads to somewhat low predictability.
Regardless, precipitation potential will increase for early next
week as lower heights approach.

MARINE...

Moderate to fresh northeast wind will develop across area waters
tonight as a cold front slowly settles south. Prolonged onshore flow
may gust to 25 knots at times during the day Friday...especially in
Saginaw Bay. In addition...prolonged onshore flow will build
significant wave heights into the 3 to 5 foot range for the
nearshore zones of Lake Huron late tonight into early Friday night.
Moderate easterly wind will persist through the weekend.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until midnight EDT Friday night FOR LHZ421-422-
     441>443.

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

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...BT/DT
MARINE.......BT


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