Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 290718
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 1213 AM EDT Thu SEP 29 2016

AVIATION...

Lightning strikes have diminished considerably since late evening
over areas upstream of the terminal corridor while light to moderate
rain continue to expand south of FNT. The rain, along with
increasing easterly flow off Lake Erie will combine to produce a
downward trend through MVFR into IFR during the night. This will
also be supported by the proximity of the low near the Ohio border
for an extended period of time. That also being the case, FNT to MBS
stand the best chance for low end VFR to return during the day being
farther north into drier air. For now, plan to bring ceiling back
into MVFR from PTK to DTW during the afternoon assuming modest
daytime heating will help with some improvement.

For DTW... Late evening radar trends suggest increasing frequency of
moderate showers producing MVFR restriction but little or no
coverage of thunderstorms over the D21 air space. The main concern
will be increasing easterly wind combining with the rain to produce
IFR ceiling during the night through the morning.

//DTW Threshold Probabilities...

* High for ceiling 5000 ft or less through the day.

* Low for thunderstorms during the night.


&&

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

Lake St Clair...Small Craft Advisory from 8 AM this morning to midnight EDT Friday
     night FOR LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 8 AM this morning to midnight EDT Friday
     night FOR LEZ444.

&&

$$

DISCUSSION...SF
MARINE.......CB
HYDROLOGY....CB
AVIATION.....BT


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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