Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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880
FXUS63 KDTX 301922
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
322 PM EDT Thu Mar 30 2017

.DISCUSSION...

Flood watch continues into Friday for Detroit Metro Area/South half
of the CWA.

Looks like a one-two punch of additional rain impacting
southeast Michigan, first one (with highest rainfall rates) late
today/early this evening, as main moisture/instability axis lifts
through with PW values around 1.1 inches (per 12z NAM) and 850 mb
dew pts up around 10 C, both values pushing toward the daily record
values for the end of March, per sounding climatology page out of
SPC. Deep enough warm layer to produce efficient rain producers, as
cold easterly flow at the surface allows for good over-running
isentropic ascent with 850 mb low level jet working through southeast
Michigan. Thunderstorms look to be mainly south of I-69 and will
enhance rainfall, but localized nature probably leading to wide
ranging QPF amounts (half an inch to one inch with localized in
excess of 1 inch possible). Small hail possible as one heads toward
the Ohio border as cells taps into some of the instability over the
northern Ohio Valley.

More widespread moderate to briefly heavy rain along/north of I-69,
where rainfall totals this evening/tonight could also reach around 1
inch, but should be able to handle this rain better than urbanized
metro Detroit area. None-the-less, standing water and minor/nuisance
flooding in low lying areas a good possibility.

The second round of potentially moderate to heavy rain looks to be
late Tonight into Tomorrow morning as it appears Trowal/deformation
axis will set up along or south of M-59 corridor, as 500 mb Low
tracks into Ohio Valley and the trough axis goes negative tilt, with
the 12z NAM really bullish, adding another inch of QPF. However, the
main circulation is over southwest Missouri, and if the center
tracks closer to the Ohio River during the day tomorrow, the bulk of
the rainfall/deformation axis may end up just south of the Michigan
border, which is the trend the 12z Euro has come in with. Rainfall
potentially holding on into the afternoon hours, but should be
diminishing, with cooler and drier air rushing in Friday
evening/night. Upper level shortwave ridging and at the surface
allowing for a good deal of sunshine on Saturday, which should allow
highs to reach into the lower 50s, right around normal.

An active weather pattern will persist over the CWA through the
extended period. Low pressure repeatedly develops over the central
and southern plains; sending system after system northeastward.
While most of the weekend will have seen dry weather, by Sunday
night the next weather system over the southern plain states will
have pushed into the Ohio River Valley. Rain is expected to develop
overnight Sunday and linger over the CWA into Tuesday. Another low
moves northeastward on Wednesday and conditions look wet into the
following weekend. As with any extended forecast; the timing is
subject to change. Temperatures are expected to remain mild through
the period.

&&

.MARINE...

Easterly gradient flow will persist this evening in response to the
approach of surface low pressure. A long fetch down Lake Erie has
allowed wind gusts to push up against Gales earlier this afternoon.
The pressure gradient will modestly relax this evening as a surface
trough axis lifts directly northward. This should push greatest
easterly gradient flow to the Lake Huron Basin by this evening.
There remains a high probability that wind gusts will reach 30
knots, with a chance for a brief period of gale force wind gusts
(duration of which should remain less than three hours). The winds
will back toward the northeast tonight into Friday morning before
weakening later in the day Friday as the sfc low exits the region to
the east and weakens.

&&

.HYDROLOGY...

Flood watch through Noon Friday for areas along and south of M-59.

Half an inch to one of rain has fallen Today. Two more main rounds
of rain expected, the first one being now through mid evening, with
another half an inch to one inch of rain expected across the
majority of southeast Michigan. Standing water and minor/nuisance
flooding in low lying areas a good possibility everywhere.

Scattered thunderstorms south of I-69 potentially enhancing
rainfall and leading to more substantial flooding or urban type
advisory, with isolated totals in excess of 1 inch possible.

The second round of rainfall activity looks to be developing late
tonight into tomorrow morning, either along or south of M-59
Corridor. Once again, half an inch to one inch of rain possible,
which will lead to additional flood concerns. It is also possible
this heavier rainfall axis will slip just south of the Michigan
border, and allow for a quicker end to the rain during the day.

Bottom-line...Rainfall totals in excess of 2 inches may cause
flooding issues across the Detroit Metro Area, with low level areas
most susceptible.

&&

.PREV DISCUSSION...
Issued at 242 PM EDT Thu Mar 30 2017

AVIATION...

The upper level low will slowly drift across the state of Missouri
this afternoon which will continue to direct the main axis of
moisture transport into southeastern Michigan. Initial midlevel
deformation and isentropic ascent has peeled northeastward this
afternoon. Additional reorientation to deeper forcing will occur
this afternoon and tonight as energy rotates around the upper level
low. Therefore, looking at a persistence of IFR to LIFR cigs with
vsbys generally holding IFR. Nocturnal processes will then bring an
increased likelihood for br/fg with visibilities potentially
lowering to less than 1 mile. A secondary warm front is expected to
push directly into southeastern Michigan between 22-02Z this
evening. Thunderstorms will be possible during this timeframe for
the KPTK and Detroit terminals.

For DTW...Best elevated instability push is set for 22-02z when
thunderstorm activity will be possible. Otherwise, position of DTW
to deep moisture requires IFR to LIFR cigs/ and IFR vsbys throughout
the night and likely for much of Friday.

//DTW THRESHOLD PROBABILITIES...

* High in ceilings below 5000 ft after 14Z.

* Medium in cigs/vsbys to fall below 200 ft and or 1/2SM tonight.

* Moderate in thunderstorms 22-02Z.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch through Friday morning for MIZ068>070-075-076-082-083.

Lake Huron...Small Craft Advisory until 4 AM EDT Saturday for LHZ421-422-441>443.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.

&&

$$

DISCUSSION...SF/DE
MARINE.......CB
HYDROLOGY....SF
AVIATION.....CB


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at www.weather.gov/detroit.



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