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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
410 am EDT Friday Apr 25 2014

Short and tonight

Dual midlevel low pressure circulations embedded within fairly
amplified midlevel trough axis is moving through the upper Midwest
this morning. Separation of the features has been maintained with a
pseudo occlusion frontal boundary serving as the conduit between the
twin upper level features. It is this occluded front that will pivot
northeastward into southeastern Michigan this morning bringing the
rain chances. Looking at system dynamics...not really a whole lot
with virtually no upper level jet forcing aloft and very little 290k
system relative isentropic ascent. What will be the case is a quick
shot of warm air and moisture advection as the convergence axis of
the trough slides directly overhead. It is also appearing likely
that favorable low level (850-700mb) deformation will develop
immediately ahead and on the cold side of the southern low
circulation sliding over the far western end of Lake Erie at 18z.
The watermark on probability of precipitation ultimately comes down to the overall chances
of developed precipitation sliding into southeastern Michigan in the
backdrop of surface dewpoint depressions that are running some 20 to
25 degrees currently. So there is quite a bit of dry air that will
need to be overcome. However...given upstream radar mosaic feel
there is more than enough to raise probability of precipitation to categorical for most of
the area. The best potential for precipitation amounts of .25 inch
is south of M59. Timing of probability of precipitation looks 11-17z Tri Cities...14-19z
Detroit/down river...16-21z far east.

Subsidence and surface ridging will build into southeastern Michigan
this evening...carrying into the overnight. Overall degree of
clearing remains uncertain as lingering upper level shortwave energy
from Minnesota/Lake Superior is shown to eject and release through the
northern lower peninsula between 6-12z. The amount of clearing is
important as can foresee some potential for fog late tonight...aided
by recent rainfall today. Confidence in this is low at this time.


Long term...

Strong height fall center digging into the southwest Continental U.S. Tonight
will bring a steady downstream transition toward higher amplitude
and less progressive flow well into next week. Upper ridging takes
a firm hold this weekend locally as this process unfolds upstream...
572 dm heights anchoring overhead by late Sunday. Post-frontal cold
air advection well under by early Saturday...with conditions through
the weekend increasingly marked by a cool/dry veering low level
anticyclonic flow emanating off high pressure centered over Hudson
Bay. Flow eventually takes on a favorable trajectory off the Cold
Lake Huron waters...effectively magnifying the cooling along the
Lakeshore areas. Highs both Saturday and Sunday will hold in the
40s in these locales...elsewhere looking at mainly low to middle 50s.
Middle level moisture working north of the defined low level baroclinic
zone draped across the Midwest/Ohio Valley will translate into a
thickening higher based cloud by Sunday. Moisture quality may be
just adequate enough to bring a few sprinkles/brief showers...but
still favor a dry forecast at this stage given the existence of a
deep dry layer and lack of supportive forcing.

Main pv anomaly will evolve into a closed middle level circulation into
early next week as increasing ridge amplification upstream
effectively cuts off this system from the mean flow. Southeast Michigan
resides along the eastern flank of this system through at least
midweek. This evolution will eventually allow the low level frontal
zone to nudge northward and into the region. This in conjunction
with a period of increasing isentropic ascent and cva will result in
an increasing chance of rain...centered on the Monday night/Tuesday
periods. Beyond this point...looking at continued cooler and
unsettled conditions with the middle level circulation still the



A brief increase in easterly winds across Lake Huron early today as
the pressure gradient strengthens north of low pressure tracking
through the Ohio Valley. Modest northerly winds take hold on
Saturday as colder air funnels in. Winds will shift to an easterly
component by Sunday and Monday. The increasing onshore flow will
allow waves to build along the western shoreline and may support
small craft conditions by Monday.


Aviation...issued 1200 am EDT Friday Apr 25 2014


Middle and high clouds will continue to stream into the region ahead of
a low pressure system approaching the western Great Lakes. Models
continue to bring the system in late tonight and early Friday
morning...therefore VFR conditions are expected through 12z Friday.
A period of rain is expected roughly 12-18z. A dip into IFR
ceilings/visible is possible during this time frame...especially from
ptk southward...however upstream observations indicate this dip is
very brief and transient.

For dtw...VFR conditions will prevail through tonight with
east-southeast winds around 10 knots. A period of rain is expected
after 12z Friday...with ceilings and visible dropping into the MVFR
range...and possibly IFR between 14-18z. Conditions will improve
after 18z as the low pressure system moves eastward.

//Dtw threshold threats...

* high confidence in ceilings dropping below 5000 feet 12-18z Friday.


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



Short term...cumulonimbus

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