Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 300358
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 PM EDT THU SEP 29 2016

.AVIATION...

IFR ceiling will be widespread over SE Michigan during the night and
through the morning. This will be mixed with mostly MVFR restriction
in rain showers as they increase in coverage from Ohio. Nightfall
and infusions of moisture into the boundary layer from rain will
promote persistence of the low ceiling. MBS will have the best
chance to waver between IFR and MVFR within the farther north
reaches of the low pressure system and where the frequency of rain
will be lower until later in the day. IFR visibility will be more
dependent on coverage and duration of heavy rainfall as the surface
wind will remain moderate from the east and prevent much fog
component. Determination of coverage and duration of heavy rain
remains a challenge to pin down outside of the 0-3 hr time range in
this type of low pressure system. This is also the case for
otherwise low thunderstorm potential which will remain out of the
initial forecast and defer to later updates as needed. Ceiling and
visibility conditions are expected to progress upward into MVFR
assuming modest daytime heating during the afternoon will be
sufficient to lift ceiling outside of heavier showers.

For DTW... Ceiling on the cusp of IFR during the evening will settle
below 1000 ft during the night and remain there through the morning.
Visibility restriction remain mostly above 3 miles in moderate
showers. The main concern in the absence of thunderstorms will be
continued moderate northeast wind that will force traffic operations
to the northeast through Friday evening.

//DTW Threshold Probabilities...

* High for ceiling 5000 ft or less through Friday evening.

* Low for thunderstorms through Friday evening.

&&

.PREV DISCUSSION...
Issued at 1028 PM EDT Thu SEP 29 2016

UPDATE...

The large and energetic low pressure system continues to swirl
over the Ohio Valley tonight. It remains fully capable of sending
another round of heavy rainfall into SE Michigan overnight through
Friday and the flood watch will remain in effect.

The main forecast challenge will be to determine timing and
location of the heavy rain areas which lose predictability very
quickly outside of 3 to 6 hours. The latest observations indicate
the next batch of heavy rain organizing over Ohio/Pennsylvania at
press time which is then expected to rotate over lower Michigan
during the night through Friday morning. The watch area appears
correctly positioned to cover the bulk of the overnight pattern
and allows a trend toward greater impacts near the I-69 corridor
through Friday morning as indicated in the most recent mesoscale
models and an early look at the 00Z NAM. This trend will be
monitored closely though as any additional heavy rainfall in the
Detroit metro area would quickly reproduce some degree of urban
flooding.

PREV DISCUSSION...
Issued at 336 PM EDT Thu SEP 29 2016

DISCUSSION...

Much attention remains on the stalled upper low over the Ohio River
Valley and resultant potential for several rounds of showers and
thunderstorms through Sunday. Early this morning into the afternoon,
a persistent band of showers between I94 and M59 resulted in between
2-4 inches of rain across the area. With all models advertising
several more rounds of showers this evening, Friday morning and
Friday afternoon there is concern for some areal flooding across SE
MI. For that reason a Flood Watch has been issued and is in effect
now through Friday evening.

The upper low dug further south this afternoon which fortunately
pulled the lingering band of showers further south away from the
locations already dealing with some urban flooding issues. The
earlier wave worked west which lessened the coverage of the moderate
to heavy showers, but the steady plume of moisture with deep layer
easterly flow off the Atlantic has kept showers going through the
day. 12Z DTX sounding shows saturation up to nearly 20kft with PWATS
nearing 1.5 inches hence even though the stronger forcing shifted
east, showers were able to persist through the afternoon. Models
keep the stronger 850mb flow going through Friday evening as the low
slowly lifts back north. SE MI will remain on the moist northern
fringe of the low through Friday evening with several waves to
rotate around the low through the area. It is difficult to time
these waves as hires models are all over the board with timing and
location. The first surge of moisture will work into the area this
around 21Z this afternoon. Looks like a good coverage of showers but
lower intensity. Then the next wave may lift NW into the area after
03Z tonight. The most concerning aspect of the showers will be early
Friday morning into the afternoon as the low lifts northward sending
the next lobe of vorticity and forcing aloft over the area. There is
potential for another east-west oriented band to set up over the
same locations as this morning. Will continue to monitor and adjust
any flood headlines accordingly. Overall consensus is for an
additional 1-3 inches of rain to impact parts of SE MI now through
Friday evening.

The low will continue north, becoming centered over lower MI by
Saturday evening. This will push the deeper moisture off to our
north and east by Saturday ending the threat of the heavy showers.
Showers will persist none the less as cooler air aloft leads to
increasing low/mid level lapse rates aided by diurnal processes.

One more day of unsettled weather Sunday as the midlevel circulation
lingers, though the local area will be cut off from the better
moisture feed so likely just some lighter showers. Ridging at the
surface and aloft will exert control early next week into midweek,
leading to dry weather and a slight warming trend.

MARINE...

Upper low slowly drifting toward the region will sustain firm
northeast winds through the end of the week.  This will setup a
prolonged period of small craft conditions as winds of 15 to 25
knots result in large wave development.  Gradual improvement in
conditions through the weekend as the upper low drifts overhead,
allowing wind speeds and wave heights to decrease with a subsequent
reduction in the pressure gradient.

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 tonight as the colder air resides over the warm
lake waters.

HYDROLOGY...

Multiple rounds of showers are expected to pivot across Southeast
Michigan through the weekend.  This rainfall could become moderate
to locally heavy at times.   The highest rainfall amounts are
forecast to remain south of the I-69 corridor, where an additional 1
to 2 inches will be possible.  Earlier rainfall in metro Detroit,
where as much as 3 to 4 inches fell, will make this region
particularly vulnerable to additional urbanized flooding.  In
addition, rapid rises of rivers and creeks already swollen from
recent rainfall may occur. A flood watch is in effect through Friday
evening for areas between the I-94 and I-69 corridors.

&&

.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.....BT
UPDATE.......BT
DISCUSSION...DRK/DT
MARINE.......MR
HYDROLOGY....MR


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at www.weather.gov/detroit.



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