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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1200 am EDT Friday Aug 1 2014

Aviation...

//discussion...

There remains a low chance for a shower or thunderstorm during the
early morning hours as the next middle level trough enters the region.
Recent upstream radar would suggest the greatest potential at
fnt...but confidence currently remains too low to include a
specific mention. This will also provide a period of thicker middle
cloud into early Friday. Favorable environment will exist for some
degree of shower and thunderstorm development with the aid of
daytime heating Friday afternoon. Uncertainty in both timing and
coverage will preclude a specific mention at this stage. Light
south to southwest winds through the period.

//Dtw threshold threats...

* low confidence in ceilings below 5000 feet on Friday.

&&

Previous discussion...issued 341 PM EDT Thursday Jul 31 2014

Short term... through tonight

The upper low over Canada still resides over eastern Ontario but the
pivoting sheared vorticity maximum will tighten about the center of the
low with that momentum...and proximity to the jet...helping to lift
the system NE overnight. Heights will stall with slight tendency to
increase through the night as the pivoting motion prevents greater
response aloft. While this is going on...several short waves seen on
water vapor imagery will track through the trough and Southeast Michigan. The
first wave...currently over Iowa...should dive just to the south of
Michigan later this evening but the wave surging south...just north of
Lake Superior...will swing through the area tonight.

This evening will feature some scattered showers and thunderstorms
as good diurnal response in the cumulus field with the building
instability...cape pushing 800 j/kg. There is a bit of a cap around
650mb as seen in the 12z DTX sounding which will act to keep
convection more scattered in nature. With a boundary draped across
northern Michigan and SW flow resulting from the changing orientation of
the trough...nocturnal moisture advection...seen in 850mb
dewpoints...feeding into the region with the approaching wave will
warrant the continued mention of slight chance probability of precipitation through most of
the night. Cloud cover will be a bit challenging tonight as the loss
of diurnal heating will tend to clear skies out as we have seen for
a couple evenings now...but the wave will bring clouds back into the
area. The question will be if and for how long will the potential
clearing last? This will mostly affect minimum temperatures tonight
which should largely be in the upper 50s. Overall the night should
be pretty uneventful for most locations with such low coverage in
precipitation.

Long term...

The dominance of the upper level ridge over western North America
will keep the downstream Great Lakes region ripe for lower heights
and a large wavelength upper level trough. This remains the case
over the next couple of days even as the large pv anomaly and closed
low pressure circulation pulls out of eastern Canada. Atypical
because what should be a period of building upper level heights and
increased ridging instead...remains downstream of the western United
States drought and feedback ridge structure.

Persistent northwesterly gradient flow will change very little in
orientation over the next few days. Forecast Theta-E along with
moisture from an relative humidity perspective suggests little to tangible
advection will occur. Thus...looking at a persistence forecast plus
some incremental change with some modest recovery of near surface
dewpoints due to evapotranspiration effects. Forecast soundings are
bullish in comparison to today/yesterday/Tuesday with decent
700-600mb lapse rates contributing to 500-750 j/kg. Little to no
shear with marginal convective instability supports very little
strong thunderstorm risk. Looking at some rainers.

Saturday is in an interesting forecast with some competing
interests. Specifically...the state will be directly underneath an
organizing upper level trough/axis. However...the preexisting
airmass that will be undergoing the chance/caught in backside of
developing cyclonic circulation will be a chunk directly off of
the upper level ridge from portions of central Manitoba. This
airmass will need to undergo a significant amount of modification
to support much precipitation. With that said...the area will be
under cyclonic flow. The models are likely overdoing
evapotranspiration...but there is enough there to go with a chance
of showers and thunderstorms by Saturday afternoon. A decent
signal in the models that terrain will play an important role in
where the activity develops.

Low amplitude upper longwave trough will continue to be located over
the Great Lakes from Sunday into early next week. However...Sunday
actually looks to feature some middle/upper level height and
temperature rises as the deepest portion of the trough shifts east.
Nwp still showing negative lifted indices and long skinny cape over
The Mitten...however...with lack of forcing will go along with
neighboring offices in keeping Sunday dry for now. Ill-defined
disturbances embedded in weak/moderate midlevel westerlies will
spell daily shower/thunderstorm chances into early next week...with
temperatures remaining near normal.

Marine...

A weak area of surface high pressure will build across the Great
Lakes region throughout the upcoming weekend. A very weak wind
profile will be in place leading to little to no surface winds or
organized wave growth. The surface feature will remain very weak
causing afternoon chances for showers and thunderstorms to persist.
The better potential for showers and thunderstorms Friday and
Saturday will remain over inland areas.

&&

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

&&

$$

Aviation.....Mr
short term...drk
long term....cb/dt
marine.......cb

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