Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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128
FXUS63 KDTX 200344
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1144 PM EDT Wed Apr 19 2017


.AVIATION...

An MVFR based stratus deck has been developing this evening from PTK
south into metro Detroit in the wake of the earlier cold frontal
passage. An influx of moisture above the frontal inversion overnight
will likely support further expansion of this stratus deck. This
moisture advection overnight will support a redevelopment of showers
across the area, perhaps a few thunderstorms, after 08Z.

Strengthening low pressure across the wrn Great Lakes overnight will
then drive the sfc front back north as a warm front Thurs morning.
Moisture inflow into this frontal boundary is likely to create some
fog and low ceilings along/north of the front, with conditions
likely to rapidly improve by early Thurs afternoon once it passes.
Thunderstorm development is then expected during the afternoon,
preceding a cold front.

For DTW...Expect some lowering of the ceilings during the overnight
through Thurs morning as low level moisture increases. A warm front
will lift across metro around 17Z. Prior to this, low clouds and fog
will remain possible. The passage of the front will be marked by an
increase in sw winds. Thunderstorm develop is then expected shortly
after this warm frontal passage.

//DTW THRESHOLD PROBABILITIES...

* High in ceilings below 5000 ft tonight through Thursday morning.

* Low in thunderstorms Thursday morning. Moderate Thursday
  afternoon.

&&

.PREV DISCUSSION...
Issued at 738 PM EDT Wed Apr 19 2017

UPDATE...

The more widespread rain and thunderstorms have exited east of metro
Detroit. The mid level short wave is still tracking along the
southern Mi border and will likely support some lingering showers
into the evening. Otherwise, most of the area will remain precip free
until overnight into early Thurs morning with an influx of deep layer
moisture and elevated instability provides another round of showers
and scattered thunderstorms. Overall the current forecast has this
handled well.

As of 23Z, the cold front stretches from near Owosso to Mt Clemens.
This front will push through the rest of the forecast area over the
next couple of hours. The Lake Huron modified post frontal airmass is
quite cool (Port Huron actually dropped 19 degrees in an hour). An
update will be issued to make some minor adjustment to the
temperature trends to account for latest observations.

PREV DISCUSSION...
Issued at 311 PM EDT Wed Apr 19 2017

DISCUSSION...

Water vapor imagery shows upper level shortwave trough clearing
northern Lake Huron, which will help drive cold front south through
southeast Michigan, before returning north as a warm front tomorrow.
In meantime, the front has struggled to make quick progress south
during the daytime hours, as dew pts range from lower 60s toward the
Ohio border, with upper 40s across Tri-Cities region. Meanwhile, a
weakening upper wave (MCV) riding along the northern Ohio Valley,
which is expected to support shower and thunderstorm development
south of M-59 before 00z, as 18z SPC mesoscale analysis revealing
SBCAPES around 1500 J/KG. Favorable wet bulb zero heights and 0-6 km
bulk shear of 30 knots supports isolated storms potentially reaching
marginal severe limits south of Eight Mile Road, but mid level lapse
rates are weak (less than 6.5 C/KM). Moisture axis/PW values up
around 1.4 inches lined up in West-East fashion along the Michigan
border could bring a localized heavy rain threat if low level
convergence along the front is enhanced, as winds have backed to the
northwest along/north of I-69.

Sfc-925 mb front progged to sink south of the border this Evening,
but strengthening low level Jet (50+ knots at 850 mb) over northern
Illinois will lead to renewed moisture transport and destabilization
overnight, as showalter index goes slightly negative by morning. Spc
day 2/Slight chance for severe storms Tomorrow looks good,
especially if the 12z NAM is right, as MLCAPES builds to 1500 J/KG
by early afternoon, and warm front/triple point is slow to push
north through the day, which could support a tornado or two over mid
and northern section of the CWA with good 0-1 KM shear in excess of
30 knots, as the surface circulation tracks close to Saginaw Bay
late in the day. Otherwise, healthy 0-6 km bulk shear of 40 to 50
knots supporting damaging wind threat with the deep unidirectional
shear from 925 mb and above, as mid level lapse rates look much
better, 7+ C/KM, which will also offer up a hail threat. PW values
around 1.5 inches support heavy rainfall, but progress nature of
system should limit flooding as aggressive mid level (700 MB) dry
slot punches in late, likely ending activity by 00z Friday.

Cold advection through Friday night, as 850 mb temps bottom out in
the negative low single numbers (EURO/regional Gem), to possibly
negative mid single numbers (NAM). That amount of cold cyclonic flow
should support a good deal of clouds during Friday, and certainly
may end up mostly cloudy vs the outgoing partly cloudy forecast.
Assuming clouds dissipate in the evening, frost/freeze concerns will
likely develop as temps fall into the 30s by Saturday morning.

Ridging over the area will keep Southeast Michigan dry this weekend
and for the start of next week.  Overall, temperatures will hang out
in the upper 50s to around 60 degrees through Monday before temps
expected to warm back into the 60s by mid week.  The next chances
for any precipitation look to come on Tuesday as a shortwave skirts
along the northern Great Lakes region.

MARINE...

A gale warning is now in effect Thursday for northern Lake Huron.
Easterly flow will develop ahead of low pressure moving into the
western Great Lakes tonight. Colder air moving into the region on
this wind pattern will generate enough instability over the water to
reach gusts near 40 knots, especially when combined with the long
easterly fetch. The wind pattern will be weaker farther south but
still require small craft advisories remain in effect for Saginaw
Bay and southern Lake Huron. Numerous to widespread showers and
scattered thunderstorms will also accompany the system before it
exits Thursday night. Moderate west to northwest flow is projected
to follow by Friday which could build waves in excess of advisory
criteria before cooler high pressure arrives to start the weekend.

HYDROLOGY...

Showers and thunderstorms will be confined to southern portions of
SE Michigan during late afternoon and early evening, mainly along
and south of the M-59 corridor. Heavy rainfall will be possible in
this activity with localized totals greater than 1 inch producing
ponding of water on area roads and in prone areas.

The associated cold front will exit into Ohio and Ontario by early
evening and take storms out of the area. The front is then projected
to move back north into Lower Michigan as a warm front late tonight
through Thursday. Showers will expand over the region during this
process with a chance of thunderstorms. Coverage is expected to be
greatest near the Tri Cities and northern Thumb early in the day
before developing southward toward Detroit during the afternoon.
Widespread rainfall totals around 0.75 inch are expected with
localized totals over 1 inch again possible in thunderstorms before
activity exits the region by Thursday evening. The potential for
some minor flooding will exist, particularly given the wetter than
average conditions noted in recent weeks.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 4 AM Thursday to 10 AM EDT Friday for
     LHZ421-441.

     Small Craft Advisory from 8 AM to 10 PM EDT Thursday for LHZ442-443.

     Gale Warning from 8 AM to 8 PM EDT Thursday for LHZ361-362.

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

$$

AVIATION.....SC
UPDATE.......SC
DISCUSSION...SF/SS
MARINE.......BT
HYDROLOGY....BT


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



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