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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1107 am EDT Tuesday Jul 29 2014


The lead edge of the cloud field now advancing into metropolitan Detroit
Marks a gradient in low level moisture moving from northwest to
southeast across the area. The showers within this region of
moisture have thus far been very light and widely scattered.
Coverage of showers will increase this afternoon during the peak
in the diurnal heating cycle. Colder air aloft will continue
arriving from the north during the afternoon. Coverage and
intensity of showers should increase by early afternoon across the
Saginaw Valley and thumb. Metropolitan Detroit and points south should
see this increase middle-late afternoon with the slower arrival of
the low level moisture and colder air aloft. A surface moving across the Saginaw Valley will also traverse metropolitan
Detroit later this afternoon. The added low level convergence will
certainly enhance coverage of late day convection. The going
forecast still has a good representation of expected afternoon and
evening conditions. Other than some very minor adjustments to forecast
maximum changes will be made to the going forecast.


Aviation...issued 703 am EDT Tuesday Jul 29 2014


A broad area of low pressure centered over northern Ontario will
continue to funnel cool air into Southeast Michigan today. The leading edge
of colder air aloft will spread over the region in the form of middle
level clouds with some light rain and ceiling generally above 5000
feet during the morning. This air mass will then destabilize as
daytime surface heating ramps up during the afternoon resulting in
showers capable of MVFR restriction becoming numerous across the
terminal corridor. There is potential for thunderstorms but coverage
is expected to be low enough to hold off on a mention at any one
location...but trends will be monitored through the day for the
chance of greater coverage. The shower activity will take much of
the evening to diminish and may linger into the overnight hours in
the dtw area.

//Dtw threshold threats...

* medium confidence in ceiling at or below 5000 feet in showers during the

* Low confidence in an isolated thunderstorm during the afternoon.


Previous discussion...issued 358 am EDT Tuesday Jul 29 2014

Short term... today and tonight

A lobe of short wave energy will slide over the Great Lakes from
northern Ontario and affect the weather for Southeast Michigan today and
even much of tonight. Satellite and radar imagery indicate this
feature is supporting shower activity in the absence of any diurnal
contribution. There were even some lightning strikes as late as
midnight over northern Minnesota. Upper air analysis over the
Midwest and central Canada shows 850 dewpoint running around
7c...itself impressive given the strong meridional flow...and more
than enough moisture to combine with the upper level cold pool
averaging -17c at 500 mb to maintain sufficient elevated instability
for modest convection. DTX upper air climatology points out 850 mb
dewpoint is about average for late July but 500 mb temperature near
record lows. Model output then shows the upper wave sliding over
lower Michigan during peak heating which will help scattered morning
activity become numerous. Model soundings depict a skinny cape
profile that appears underdone when surface heating is added to what
was forced in early morning observations upstream. A 72/52 surface
parcel yields modified cape values in the 500-750 j/kg range which
should result in scattered coverage of thunderstorms among numerous
showers over lower Michigan during the afternoon through middle
evening. Freezing level averaging 9500 feet is also notable as it
will make any stronger updraft capable of some pea size hail.

Early morning observations also tip The Hand for tonight as residual
elevated instability will be adequate for showers to linger well
into the overnight hours. With little change in the low level
moisture profile...the NAM offers 850 mb Li near -1c over south
sections of our area throughout the night which will be good for
scattered shower coverage...and a rumble of thunder...through
sunrise Wednesday.

Long term... Wednesday through Monday

Showers associated with the upper level wave and cold pool moving
over Southeast Michigan today and tonight will linger over the region near
the Ohio border Wednesday morning. The rest of Wednesday will then
come under the influence of localized subsidence that will present a
less favorable environment for showers as daytime heating ramps up.
A chance pop will be maintained Wednesday and during the second half
of the week mainly to account for the low predictability between
upper level features and the overall background of instability
supported by the upper level trough...both of which make it
difficult to rule out some shower activity. Model solutions depict
upper level features that are smaller in scale and less organized
compared those leading up to the system today which makes it only
possible to use them in a general sense. There should be plenty of
timing and strength adjustments over the next few model cycles which
will allow refinement of rain chances in the shorter term. On the
other hand...the expectation of below normal but slowly moderating
temperatures remains solid over the next few days. The low level
thermal trough will remain fixed over James Bay with periodic
reinforcements of cooler air in deep northwest flow carrying slightly less
impact each day Wednesday through Friday.

The latest global model solutions indicate a gradual deamplification
of the long wave pattern during the weekend. There is quite a bit of
dispersion in the 500 mb pattern by Sunday...but the starting Point
of such anomalous amplification during the second half of the week
favors solutions with a slower evolution. The main result will be
temperatures making a return to normal in between a continued chance
of showers.


A cool but light northwest wind will persist over Lake Huron and
lower Michigan during the middle week period. Clusters of showers will
move over the marine areas and hinder otherwise favorable marine
conditions. The best chance of rain will occur today and tonight
along with a chance of thunderstorms but primarily near the
shorelines and over Lake St Clair and Western Lake Erie. The wind
will remain light through the end of the week and tend to shift
toward the southwest by Friday with increasing influence of weak
high pressure by the weekend.


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




You can obtain your latest National Weather Service forecasts online
at www.Weather.Gov/Detroit (all lower case).

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