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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
132 PM EDT Tuesday Sep 16 2014



Aviation...

//discussion...

VFR conditions expected through the taf cycle as high pressure
builds into the region. Diurnal cumulus field forming as boundary layer
moisture tries to mix out but is struggling a bit more than
expected. This is especially true for fnt/ptk which are on the
higher terrain. Loss of heating will allow cumulus to quickly clear out
tonight leading to clear skies. Minimal coverage expected tomorrow
due to increased mixing.

//Dtw threshold threats...

* low confidence in ceiling below 5kft this afternoon or tomorrow
afternoon.

&&

Previous discussion...issued 327 am EDT Tuesday Sep 16 2014

Short term... today and tonight

Southeast Michigan will reside within a lower amplitude and weakly cyclonic
middle level northwest flow over the next 24 hours. This positioning
will leave the region within a gradually developing low level
anticyclonic flow emanating from the 1022 mb high currently centered
over the middle MS valley. Slow Post-frontal drying of the column
under way early this morning...recent satellite depictions showing
increasing breaks in the residual stratus canopy. While this
process will take a greater hold with time...enough evidence
exists in the latest relative humidity fields to suggest the sub-inversion layer
will continue to carry a modest degree of moisture until the
boundary layer becomes fully mixed out late today. This points
toward seeing a combination of some degree of maintenance of the
existing stratus and/or the reemergence of a broad midday stratocu
deck. High temperatures account for these expectations in
insolation potential...850 mb temperatures of 4 to 6c supporting
highs lower 60s range.

Loss of heating will bring an abrupt clearing trend this evening as
stable low level anticyclonic flow takes a stronger hold. The
existing airmass under a clear sky but a persistent light
northwest gradient will yield a spectrum in lows ranging from
around 40 degrees in the coldest locales to upper 40s within the
urban influences of Detroit.

Long term... Wednesday through Monday

The mean longwave trough axis is forecasted to remain in place over
eastern North America through Friday morning. This will keep
southeastern Michigan safely under northwesterly/cyclonic flow
through this same period. Expect below normal temperatures to
persist Wednesday through Friday.

The combination of two upper level jetlets pivoting around the very
impressive potential vorticity anomaly near James Bay will cause the
main trough axis to swing directly through the central Great Lakes
on Wednesday. Southern Lower Michigan including semich will remain
positioned under a dead zone for upper level winds. This signifies a
lack of overall forcing. Model moisture profiles and upward vertical velocities supports a
dry airmass...one born out of prior subsidence. Model data does not
show much for active subsidence. Models in prior days was suggesting
that a weak near surface front would coast southward possibly
impacting the Saginaw Bay Wednesday afternoon. Latest iterations of
the nwp now paint a picture where the weak frontal boundary is
expected to remain well north of the County Warning Area...where the strongest
850-700mb gradients will sharpen. Best of the forcing across the
north will come in the form 850-700mb fgen on the cool side of the
front. Just left a slight chance in the forecast for the far
north.

In response to strong 1000-500mb geopotential height rises swinging
across southern Canada...a very stable anticyclone is forecasted to
drop southward through the Great Lakes Wednesday night and Thursday.
With the strong surface high and associated compact pressure
gradient...winds are expected to respond aggressively out of the
northeast Wednesday night and Thursday. Moisture fluxing off of Lake
Huron in addition to a Stout northeast wind went ahead and kept lows
Wednesday night relatively buoyed. Northeasterly winds should
continue chilly conditions into Thursday as well with lake clouds
difficult to dislodge. Temperatures Thursday could very well end up
12-15 degrees below normal. Also...went aggressively cool for lows
Thursday night (sided with the coolest of guidance).

More and more uncertainty has been entering the picture in regards
to exactly how the weekend system will play out. Confidence is
actually high there will be a wavelength system to impact the
Great Lakes this weekend. However...intramodel continuity hasn't been
great at all which causes some heartburn. As far as timing...model
solutions are now suggesting that classic shortwave ridging could
protect the area for a time from precipitation. So...do not want to
dive headfirst with likelys for this weekend. Plenty of time to sort
this out.

Marine...

Marine conditions will be favorable in terms of wind and waves
despite the passage of two cold fronts during the week. The first is
passing through this morning which will have just a weak wind shift.
The second will bring a chance of light rain Wednesday and renewed
surge of cooler air over Northern Lake Huron by Thursday with
modest northerly flow possibly ramping up waves a bit more over the
relatively warm water of Lake Huron. After that...the next stronger
low pressure system will bring warmer air Friday and Saturday but
possibly stormy conditions later Saturday and Sunday as the
associated cold front moves through the region.

&&

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

&&

$$

Aviation.....Drk
short term...mr
long term....cb
marine.......cb



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