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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1203 am EDT Thursday Sep 3 2015

Aviation...

VFR conditions expect until late night/early Thursday morning MVFR
fog develops. Thereafter...widely scattered rain showers/thunderstorms and rain showers will develop
along weak frontal boundary Thursday afternoon. Will forgo in terminals
at this time given low confidence in coverage. Any activity will dissipate
after 00z Thursday evening.

//Dtw threshold probabilities...

* low for ceiling at or below 5kft after 16z Thursday.

* Low for thunderstorms Thursday afternoon.

&&

Previous discussion...issued 323 PM EDT Wednesday Sep 2 2015

Short term...this evening and tonight

An impressive low level Theta-E potential is in place over lower
Michigan this afternoon as the upper level ridge remains in
residence over much if not all of eastern North America. The most
impressive thing is the surface dewpoint observations being taken
over all areas...but particulalry the Saginaw Valley and central
lower Michigan. There are widespread observations of surface dews in the
middle 70s. Not suprising then to see steep low level lapse rates
both in the 12z radiosonde observations and also in this afternoons mesoanalysis.
There is a general lack of deep layer shear and synoptic scale
forcing for ascent hiding out within the ridge. This will limit the
coverage of thunderstorm development this afternoon and
evening...probably keeping aspect ratio of individual convective
cells on the smaller side. Increased probability of precipitation to a middle-high chance for
the Saginaw Valley/Saginaw Bay vicinity given recent trend on radar.
Current convective available potential energy warrant attention with sbcapes in excess of 4000 j/kg
and the ml variety at 3000 j/kg. Vertical velocity potential from
cape and possible precipation loading supports an isolated strong to
severe wind gust risk this afternoon with the tallest of storms. The
biggest concern with slow cell movement will be a torrential
rainfall.

Model data suggests the shortwave now over portions of northern
Wisconsin and the u.P. Will make a turn to the east and southeast
late tonight as it crests the upper level ridge axis. Timing of this
feature is into the northern lower peninsula between 06-12z tonight.
Destablization of the atmosphere will limit the overall potential
for continued convective development tonight...but focused cva
directly ahead of the feature will require chance probability of precipitation across the
northern County Warning Area through tonight.

Long term...Thursday through next Wednesday

Upper over Wisconsin/Upper Michigan will drop slowly
southeast...with the shortwave axis crossing Southeast Michigan
Thursday. Weak frontal boundary will accompany the wave...with
models also keying in on a surge of low and middle-level Theta-E in
advance of the system. Warm moist airmass should become increasingly
favorable for shower and thunderstorm development as h700-500 upper
cold pool crosses the area and diurnal heating steepen lapse rates
in the low-middle levels. Expecting a fairly good coverage of showers
and thunderstorms tomorrow...especially given warm moist airmass.
Not expecting severe weather given sb cape values only topping out
near 1000 j/kg...but very slow storm motion at/below 10 miles per hour...tall
skinny cape...and precipitable water values of 1.5-1.75 inches will bring a threat
for very heavy rainfall and localized flooding. Increase in cloud
cover and coverage of convection should bring slightly cooler temperatures
tomorrow...but still expecting above normal maximum temperatures well into the
80s.

Forecast models are then showing that the building ridge over
Ontario/Quebec/New England will block the eastward progression of
the Midwest shortwave by Thursday night and Friday...causing it to
remain over lower Michigan...and even retrograde back slightly west
by Friday night. Cooler air aloft and instability associated with
this feature should keep a chance for showers and thunderstorms in
the forecast through Friday evening...before the wave starts to
dampen and daytime heating is lost.

Longwave ridge over the eastern Continental U.S. Looks to ensure a hot and
humid Labor Day weekend. A bit of a weakness in the ridge over the
Ohio Valley/southern Great Lakes will exist Saturday. The associated
cooler midlevel temperatures will allow for marginally unstable conditions
to develop which argues for entry level chance probability of precipitation in the weakly
forced environment. 12z GFS/Euro differ on frontal passage timing
Labor Day or Monday night...but either way best dynamics look to
remain well northwest of the local area so nothing more than entry
level chance probability of precipitation for Labor Day as well.

Marine...

A weak cold front dropping through the central Great Lakes tonight
and Thursday will provide a good chance for thunderstorms. Winds
will remain fairly light through the middle-week period...with a
gradual shift starting over Lake Huron tonight and ending over Lake
Erie by Thursday afternoon to a more northerly direction behind the
front. Shower and thunderstorm coverage will be lower on
Friday...with east winds also building in...in response to an upper
level disturbance settling in over the area.

&&

DTX watches/warnings/advisories...
Michigan...none.
Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.
&&

$$

Aviation.....Hi
short term...bt
long term....hlo/dt
marine.......hlo

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