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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
710 am EDT Friday Aug 29 2014



VFR conditions are expected over the next 24 hours. A warm front
will lift north through the western Great Lakes...but bulk of shower
and thunderstorm activity is expected to remain west of Southeast
Michigan. Light southeast winds (10 knots or less) will persist for
much of the period.

//Dtw threshold threats...

* low confidence in ceilings at or below 5000 feet tomorrow


Previous discussion...issued 337 am EDT Friday Aug 29 2014

Short and tonight

Upper level ridge and dry air (41 c dew pt depression at 700 mb
per 00z DTX sounding) in place over Southeast Michigan early this
morning. The middle levels will begin to moisten up as the day wears
on with the ridge axis drifting east...but the surface flow will
remain out of the southeast...maintaining stable conditions (sfc
lifted indice's on the positive side) through tonight (per 00z nam). It is
not inconceivable a few showers may sneak into the western part of
the County Warning Area as the overall mean moisture will be high...precipitable water values
around 1.75 inches. In addition...upper level pv ejecting from
lower Mississippi River valley will track through the Ohio Valley
and clip Southeast Michigan early this evening.

925 mb temperatures of 19 c this afternoon (21z) support maxes
predominately in the upper 70s. Further warming of the low levels
will take place 925 mb temperatures prognosticated to rise to around
23 c...which coupled with canopy of middle/high clouds expected to hold
mins in the middle 60s.

Long term...

The Holiday weekend weather will include warm temperatures to go
along with an active rainfall pattern to start and possibly to end
the period. The best chance of rain will be during Saturday and
Saturday night with a break most areas Sunday and then a renewed
chance of showers and storms Monday afternoon into Monday night.

Surface low pressure will track from central Wisconsin to the
eastern u.P. During Saturday with the warm sector firmly over Southeast
Michigan. A rich low level moisture axis continues to present heavy
rainfall and thunderstorm potential as the main concerns in an
environment of modest instability but with a robust wind profile.
Model depictions of low level dewpoint indicate readings near 15c
being pumped in by a 30 knot low level jet and 700 mb dewpoint near
3c with similar wind speed at that level on up to 500 mb. Forecast
soundings show surface based cape building into the 1000-1500 j/kg
with low density and mostly in a narrow band within a weak surface
pressure trough where dewpoint reaches 70f. The weak surface trough
is symptomatic of the synoptic scale environment overall as the upper
wave shears to our west and north. The weak large scale forcing is a
concern but the high amount of moisture coupled with daytime heating
should result in at least scattered coverage in support of the upper
end chance probability of precipitation in our going forecast. The strong wind profile
raises a concern for severe potential but negative factors appear to
weigh more against at this point mainly due to the weakly forced
synoptic scale and sub 6c/km 700-500 mb lapse rates through peak
heating. There will be a tendency for mature convection to adopt
linear Mode but with low potential for cold pool development.
Locally heavy rainfall will be more of concern given the deep
moisture profile...especially if activity lines up parallel to the
mean flow...and despite individual cell motion moving along at 30

Saturday afternoon convection will last well into Saturday evening
before the low level Theta-E ridge slides eastward and out of the
region overnight. The weak low pressure system will manage to pull
an equally weak cold front into the south half of lower Michigan by
Sunday morning which will take most of the day to settle southward
and eventually dissipate. Differences in model solutions by that
time are found mainly with regard to the timing and strength of middle
level subsidence trailing the exiting low pressure system. All
present marked drying of mean relative humidity toward Sunday evening but probably
too late to rule out a stray shower or thunderstorm along the front
before it dissipates...especially from the eastern thumb toward the
Ohio border where some late season lake breeze enhancement will be
possible. That being said...expect coverage to be considerably less
compared to Saturday with less confidence in the capability of the
front within the short window of daytime heating and lingering
weak large scale support.

The weak short wave ridge between systems will keep Sunday night dry
as the next low pressure system moves through the Midwest into
northwest Ontario by Monday. This is a larger system and model
agreement is good on the position of the surface low and trailing
cold front. Rain potential to wrap up the Holiday weekend will be
tied to the renewed development of a low level Gulf moisture stream
and the eastward timing of the cold front...both of which favor
afternoon and evening timing at this point. The middle week period then
has potential to remain convectively active if global model
solutions hold and the front stalls near the Ohio border Tuesday
into Wednesday.


Low pressure moving through the plains today will reach central
Wisconsin Saturday morning and then the North Shore of Lake Huron
Saturday night. This system will produce moderate to fresh southerly
flow over all marine areas. Increasing potential for 25 knot gusts
Saturday will be accompanied by a chance of thunderstorms Saturday
afternoon and evening. This will be the most active period of wind
and precipitation during the Holiday weekend. Improved marine
conditions are expected Sunday and most of Monday before the chance
of showers and storms return with the next low pressure system into
Monday night.


DTX watches/warnings/advisories...

Lake Huron...none.
Lake St Clair...none.
Michigan waters of Lake Erie...none.



short term...sf

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