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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
720 PM EDT Thursday Aug 28 2014



Aviation...

//discussion...

Deep column ridging will keep conditions quiet for southeastern
Michigan through tonight. Only thing to mention for the overnight
is the increase of convective debris high cloud spilling into the
state. Removed the shower mention that was previously a prob30
group at mbs. Forcing for ascent is expected to be too poor to
overcome deep dry air that is now in place. At this time...the
initial wave of precipitation is expected to lift and impact
locations near Northern Lake Michigan.

//Dtw threshold threats...

* none

&&

Previous discussion...issued 331 PM EDT Thursday Aug 28 2014

Short term...through tonight

Dome of high pressure encompassing Michigan beneath upper level
confluent northwesterly flow has allowed for benign but seasonably
cool conditions today. Upstream...sharp upper trough is located over
eastern Colorado with a lead shortwave ejecting over MN/IA. This shortwave
is prognosticated to continue northeastward...flattening the upstream upper
ridge as the large-scale wavetrain progresses eastward. Deeper
moisture /E.G. 1.85 inch precipitable water observed on 12z dvn sounding/ will
increase in advance of this feature...allowing for an increase in
high/midlevel clouds from west to east into the local area mainly
from 06-12z tonight. Isentropic ascent will increase in concert with
this feature as well...with the best combination of ascent and
forcing remaining north and west of the local area overnight...but
possibly enough to brush the Saginaw Valley area with some light
precipitation toward 12z. However...low level dry air will be stubborn...so
likely nothing more than a few sprinkles.

Other forecast concern is low temperatures and the potential for fog
tonight. Surface high becoming re-centered over New York by 12z Friday
will allow for at least a very weak gradient to develop...but it
should be weak enough for winds to decouple. The influence of the
departing high looks to be greatest over The Thumb...which will also
remain cloud-free the longest. As a result...expecting chilly lows
in the middle-40s with patchy shallow fog also possible. Did not have
enough confidence in depth of the low level moisture to put fog in
the grids at this time though. Elsewhere...look for lows in the
lower 50s in the Saginaw Valley region to the upper 50s in the
Detroit heat island...possibly rising a few degrees late due to the
advancing cloud shield.

Long term...

Attention for the long term turns to precipitation potential
associated with the open wave lifting out of Colorado this
afternoon. Important synoptic scale features are easily located on
today's National radar mosaic. The associated warm front is
supporting a narrow band of showers as it lifts into the western
Great Lakes while the low level jet and the primary moisture
transport are supporting an equally narrow band of showers extending
from southern Oklahoma through northwest Missouri.

From a dynamic standpoint, little will change with this system as it
progresses eastward and shears into the prevailing westerly flow on
Friday. A brief period of zonal moisture transport parallel to the
front will bring an outside chance of showers to the northern 2/3 or
so of the forecast area on Friday morning, but otherwise expect that
forcing features will remain too far upstream to have any
considerable influence on sensible weather. As a result, Friday through
Friday night are forecast to be largely dry, even if a couple spots
measure a couple hundredths within prevailing warm air advection regime between now
and then. Veering southerly flow will bump highs up several degrees
to around 80 on Friday and limit low temperatures the middle to upper 60s on
Friday night.

Late Friday night into early Saturday will see an eastward
progression of the low level jet that will support gradually
increasing chances of showers heading into the daytime Saturday. The
most widespread coverage of rainfall will remain to our north tied to
the good dynamic forcing, but steady height falls in the presence of
a strengthening 30-35kt low-level jet are sufficient to warrant a
mention of high chance probability of precipitation for the balance of the day Saturday,
primarily in the afternoon in advance of the cold front. By this
time, the upper low will have been cutoff from the prevailing flow
for so long that there will be essentially no cold air aloft to be
found. The resulting paltry middle-level lapse rates will limit severe
potential. Nonetheless, a low density cape profile combined with
moderate shear of 35 knots will support an outside shot at updraft
organization and attendant strong/severe threat on Saturday
afternoon. It is Worth noting that the potential exists for low
level stratus to form beneath the frontal inversion early Saturday
morning. Soundings are not overly supportive attm, but it is
something Worth watching as it will potentially impact temperatures and/or
destabilization potential. Highs should rise into at least the middle
80s by late Saturday afternoon given deep mixing to 850mb temperatures of
16-18f and ongoing warm air advection within steady southwesterly flow through the low
levels.

Potentially active Labor Day weekend in store with a moist airmass
in place and periodic disturbances traversing the local area. Upper
air pattern will feature an amplifying trough advancing from the
Pacific northwest to the upper Midwest while an upper ridge holds
firm over the southeastern US. There is also a good signal of a feed
of Gulf moisture into the upper Midwest this weekend. Models have
trended faster with the main upper longwave trough/cold front
working through by Monday/Monday night...though more run-to-run
consistency is needed. At present...will keep showers in the
forecast through early Tuesday morning with decreasing probability of precipitation through
the remainder of the work week.

Marine...

Moderate to fresh southerly flow will develop Friday evening into
Saturday as a cold front approaches the waters. Gusts will be
limited due to a stable warm air regime...although there will be a
chance for thunderstorms Friday night through Saturday night
associated with this system. Conditions will remain unsettled with
continued moderate flow through the weekend as another system
quickly approaches on the heels of the first.

&&

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

&&

$$

Aviation.....Cb
short term...dt
long term....jvc/de
marine.......jvc



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