Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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743
FXUS63 KDTX 291645
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1245 PM EDT THU SEP 29 2016

.AVIATION...

SE Michigan airspace will remain positioned within the moist
northerly periphery of low pressure slowly circulating over the Ohio
Valley through this TAF period. This will sustain a pattern of
persistent shower development within an extensive canopy of low
stratus.  Conditions largely at MVFR through the remainder of the
daylight period, although brief disruptions into IFR remains
plausible /particularly DTW-YIP/ as heavier showers roll through.  A
more organized area of rainfall will then work toward the region
again tonight/early Friday.  Timing remains challenging, but
generally looking at a window for prevailing IFR to emerge
overnight.  Embedded thunder a possibility at any point, but
certainly not worthy of a mention given the high degree of
uncertainty.

For DTW...Radar trends support at least periodic disruptions to
visibility /low MVFR-IFR/ as heavier showers move through
during the afternoon period.  Persistent MVFR stratus outside of
these showers.  Continued moderate northeast wind will maintain
northeast flow operations through the Friday.

//DTW Threshold Probabilities...

* High for ceiling 5000 ft or less through Friday.

* Low for thunderstorms through Friday.

&&

.PREV DISCUSSION...
Issued at 1042 AM EDT Thu SEP 29 2016

UPDATE...
Focus remains on current band of moderate to heavy showers and
thunderstorms across the Detroit Metro area and potential for
additional activity through the rest of the day. Flash flood
warning has been extended for parts of Wayne, Oakland, and Macomb
counties through 215pm due to persistent band of thunderstorms
over the area. Some locations have already received between 2 and
3 inches of rainfall and an additional 1 to 2 inches will be
possible through the early afternoon before this band breaks up.
Radar trends show some signs of weakening but the corridor of
forcing will move very little with the good feed of moisture off
the Atlantic continuing keeping the flood threat going. Hires
models do show a lull in activity this afternoon for a short
period before the next wave rotates around the low and into SE MI
this evening. Attention will then turn to precipitation potential
for this evening and overnight.

PREV DISCUSSION...
Issued at 318 AM EDT Thu SEP 29 2016

DISCUSSION...

Occasional showers persisting right through the weekend, but key
point is there will be plenty of dry periods intermingled at any
given point in southeast Michigan.

Latest Water Vapor shows our very large upper level low centered
over central Indiana, with continued southward progression during
the day into Kentucky, and likely wobbling around and holding into
Friday in that Vicinity. Little if any change in the magnitude
expected, as the center holds around 564 dam at 500 MB.

The Detroit Metro area is in a good spot for rainfall this morning,
possibly heavy with the stout upper level divergence and modest
instability (slightly negative showalter index) axis in place. With
the Atlantic moisture feed and max cape density around and on the
positive side of the zero isotherm, looking at efficient rain
producers, especially with any isolated thunderstorms which develop.
Will be monitoring radar trends and amounts for possibly advisory
type flooding, but transient nature of activity thus far is not
leading too much of a concern. The good news we should dry out some
for the afternoon hours (still probably enough low level moisture to
support light rain showers) as we get negative Upper level PV
advection and instability axis slides just west and south of the
CWA.  This modest instability looks to return/slip north of the
Michigan border on Friday. Little in the way of isentropic ascent
(290 K) indicated this afternoon, but the waves of isentropic ascent
then look to return Tonight/Friday.

The Atlantic Moisture feed will continue into southern Lower
Michigan through Friday, with indications the axis/850-700 mb Theta-
E ridge will be North Of I-69 by Friday, and that is where heaviest
rainfall is anticipated on that day. 00z Euro indicated an
impressive 850 mb dew pt approaching 11 C.

The system then looks to unravel as we head into the weekend, with
the ribbon of moisture circling the center expanding outward. None-
the-less, with the 500 mb low moving into Lower Michigan late
Saturday, still should be able to support some instability showers
despite the leaner moisture profiles. Model consensus Tonight is for
a slower departure on Sunday, and showers also seem like a good bet
as wrap around moisture axis returns, but in a diminished state,
with pw values nearing 1.25 inches. With the clouds and showers
around, diurnal temperatures swings will be muted, with 50s for lows
and 60s for highs.

MARINE...

A diffuse pressure gradient has been in place over southeastern
Michigan tonight as the upper level low continues to work southward.
This has lead to a relatively weaker wind field so far. The upper
level low will eventually center over Kentucky today allowing the
easterly winds to increase in speed. The upper level pattern will
remain fairly stagnant into the weekend leading to prolonged period
of northeast winds in the 15 to 25 knot range. Small craft
advisories remain in effect for all marine areas into late Friday
night due to large waves and gusty winds. The area of low pressure
will lift back northward into Michigan late Friday night into
Saturday. This will decrease the surface pressure gradient and turn
winds more to the east, allowing wind speeds and wave heights to
decrease overnight.

Moisture will meanwhile continue to wrap around the area of low
pressure into the weekend, providing numerous showers but only a low
chance for thunderstorms. The cool airmass will provide a chance for
waterspouts again today as the colder air resides over the warm lake
waters.

HYDROLOGY...

A large area of low pressure will remain stalled out south of Lower
Michigan through the remainder of the week and for much of the
upcoming weekend. Periods of showers will be slung back to the west
along the northern periphery of this feature off of Lake Erie and
directly into southeastern Michigan. Total QPF amounts of 1.50 to
2.50 inches is expected through Saturday night. Highest amounts are
forecasted to the east of the glacial terrain, including the Detroit
Metro area. The rain is expected to cause rises on area creeks and
streams, and may allow some ponding to develop on roadways and low-
lying areas.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Friday evening FOR MIZ060>063-068>070-075-076.

Lake Huron...Small Craft Advisory until midnight EDT Friday night FOR LHZ422.

     Small Craft Advisory until 5 AM EDT Saturday FOR LHZ421-441>443.

Lake St Clair...Small Craft Advisory until midnight EDT Friday night FOR LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until midnight EDT Friday night FOR LEZ444.

&&

$$

AVIATION.....MR
UPDATE.......DRK
DISCUSSION...SF
MARINE.......CB
HYDROLOGY....CB


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