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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan
1258 PM EST Friday Dec 26 2014


Moderate southwest wind will continue today and tonight ahead of the
stalled frontal boundary extending from the northern Great Lakes
into the Southern Plains. Mild but dry air already in place will
then become even milder as the southwest wind brings in a stream of
warm and moist air from the Gulf of Mexico. Afternoon observations
indicate a Clear Lake contribution to the cloud field over northern
lower resulting in MVFR ceiling that may brush mbs...but the other
terminal sites will have to wait for the Gulf moisture stream to
arrive from the middle Mississippi Valley before ceiling develops and
settles into MVFR with some light drizzle and fog during the night.
The latest forecast represents about a 3 hour later adjustment on
that timing plus a push of IFR into Saturday afternoon as the front
moves closer to the region.

For dtw... other than some shallow cumulus expect just a mix of high
clouds to stream over dtw today through about middle evening. After
that...MVFR ceiling with light drizzle and fog will spread into the
terminal area. Temperature will be in the 40s and ensure
precipitation remains liquid form before drizzle increases to rain
showers during Saturday morning.

//Dtw threshold probabilities...

* high for ceiling below 5000 feet tonight through Saturday.


Previous discussion...issued 342 am EST Friday Dec 26 2014

Short and tonight

A trough over the western Continental U.S. Will undergo continued deepening
today which will aide in the downstream amplification of the ridge
over the eastern Continental U.S....which is building north behind the exiting
middle week system. SW flow will prevail over the area today and
tonight as lower Michigan resides near the interface of the ridge/trough.
SW flow and building heights with the upper level jet maximum to our
north and west will all result in another warmer than normal day for
late December with temperatures in the 40s...even with increasing cloud
cover through the day. Deep layer of dry air above 900mb...with the
exception of patches of cirrus...will start the day off with a good
deal of sun for many locations but moisture will be on the rise as
the trough sends some shortwaves up through the western Great Lakes.
The main shortwave that will affect Southeast Michigan in the short term looks to
get picked up over the gom out of the southern stream of the jet and
sent over lower Michigan between 04-08z tonight. Moisture advection will
be ongoing through the day...more so to the west...with clouds
increasing through the afternoon but this wave will introduce the
nose of a ll plume of moisture. Soundings indicate the bulk of the
moisture will be trapped below 850mb with deep dry layer between it
and the cirrus debris from upstream. This would indicate a better
setup for drizzle than rain...especially as the moisture gradient
just begins to lift through. Earlier forecaster already leaned in
that direction so see no reason to change it at the moment. The
timing of this potential precipitation well as cloud trends
with the moisture plume to the west and surge overnight will be the
main concerns for the day shift.

Long term...

Closed middle level circulation now noted on water vapor across the
southern rockies will open up and shear northeast toward the Great
Lakes over the next 36 hours. Extended period of gradually
strengthening middle level southwest flow in advance of this wave will
establish an increasingly moist and milder low level environment
heading into Saturday. Recent sounding data and low level moisture
fields still lend support for some pockets of light drizzle/rain to
exist within an ongoing Wing of weak isentropic ascent Post-daybreak
Saturday morning. Warm sector airmass takes residence in the wake
of this initial ascent into the midday period...leaving an
environment briefly absent of any tangible forcing and marked
primarily by the extensive cloud cover and warmer temperatures.
Highs will make a run toward the middle and upper 40s.

Main piece of the upper wave will translate northeast and across
northern lower Michigan by Saturday evening. Greatest forcing will reside
across this corridor under favorable exit region upper jet support.
Best ascent locally occurs with the attendant frontal convergence
that sweeps through 20z-02z late Saturday. This will support the
development of light rain showers through this time. Cold/dry air
advection commences beyond this point within the Post-frontal
environment. Opportunity for a brief transition to light snow
showers Saturday night will be limited given the apparent rapid
reduction in moisture quality/depth per recent model guidance.
Lows bottoming out within a couple degree either side of freezing.

Relatively benign pattern sets up Sunday and Monday...the Great
Lakes within the periphery of broader troughing. The existing
airmass will likely result in the usual cloud trend issues...
particularly as the low level flow veers to northeasterly with a
cold frontal passage on Monday. This may present an opportunity for
a few flurries/light snow showers...but a lack of both better moisture
quality and upward vertical motion within the column points toward a
limited potential. Trending colder for the middle and latter half
of next week as the mean trough grows in scale...a stronger shot of
Arctic air projected to encompass Southeast Michigan by new years


Lighter southwest winds will remain across the region through
tonight before strengthening into Saturday. Winds will then shift
to northwest with the passage of a cold front Saturday night. A
moderate northwest wind will continue through Sunday. Gusts remain
forecast to remain below gales through this time.


DTX watches/warnings/advisories...

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


short term...drk

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