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

Short term... afternoon and tonight

Moderate southwest wind and plentiful sunshine boosted temperatures
into the middle and upper 40s during the afternoon. The exception being
the Tri Cities and northern thumb "only" making it into the lower
40s due to extra cloud cover from Lake Michigan. Even lower 40s are
a solid 10 degrees above normal for December 26. From there...
temperatures will fall back toward the upper 30s during the evening
before rising overnight as clouds increase and continued southwest
wind pumps even milder air into the region with patchy drizzle and
fog late.

Afternoon satellite imagery points to a clear contribution of lake
moisture to the stratocu covering central and northern lower
Michigan. A slight diurnal component along the southern fringe shows
a hint of the high relative humidity around 925 mb indicated in model data that is
otherwise excessive over the rest of Southeast Michigan into the Ohio
Valley. At the same time...while observations point to a high bias
in model relative humidity over our area...legitimate Gulf moisture is streaming
rapidly northward through the middle Mississippi Valley leaving an
interpretation of the high model relative humidity in advance as an Avenue for
rapid propagation into our area tonight. This supports the going
forecast that calls for clouds and drizzle to develop but with onset
timing a few hours later into the night. This will give temperatures
even more more time to rise into the lower 40s to eliminate
precipitation type concerns through sunrise Saturday.


Long term...

Deep trough over the 4 corners region and broad persistent ridging
over the southeast third of the United States have created a
favorable zone for strong shearing over the central United States.
This will result in the southwest U.S. Trough opening up and rapidly
shearing to the northeast during the day Saturday. Attendant height
falls and dynamic forcing will Foster a weak surface low that is
forecast to lift from the quadrant cities to The Straits by 18z Saturday.

The preceding wind field will continue to be characterized by deep
layer southwest flow that will result in high temperatures in advance of
the associated cold front several degrees higher than today with
highs near 50 possible over most of the area. Forcing will not be
particularly strong nor will system moisture be especially
impressive. However, the developing low-level jet response will be
sufficient to support a northward surge of light showers across
the County Warning Area 12-18z worthy of a higher chance pop. Convergent axis
along the front will then bring another chance for light showers
during the evening. The primary change to the forecast during this
time was to broaden probability of precipitation through the day to account for better
preceding isentropic ascent/low level jet support. Respectable pop of wind
with gusts perhaps approaching 25 kts will be possible as cold
advection ramps up behind the front. 850 mb temperatures will fall toward
-10c by Sunday morning which will keep temperatures Sunday
suppressed closer to normal values in the middle 30s as weather stays
tranquil beneath confluent flow aloft.

Cold air advection on the backside of an exiting low pressure system
Sunday will drop daytime highs back into the normal range on Monday.
As the week progresses temperatures will drop below normal with highs only
reaching into the 20s as we move into the New Year. Colder than
average temperatures are expected to persist through Friday before
squeakingup to near normal for the weekend. Breezy west winds
kick in new years evening day and continue into early Friday...which
could result in some biting wind chills if forecast holds. As for
any present conditions trend toward some light
snow showers over the northern thumb for Tuesday night into early
new years evening morning. Chances for scattered snow showers look to
expand southward...encompassing the remainder of Southeast
Michigan New Years Day through the end of the extended period.



Light winds will become fresh and southerly in advance of an
approaching low on Saturday. Passage of the low across The Straits
will push a cold front across the waters that is forecast to shift
winds to a northwesterly direction on Saturday night. Evidence for
gusts reaching marginal gale force over Northern Lake Huron is
increasing. A gale watch has therefore been issued for Saturday
night. Winds and waves will then gradually subside Sunday into
Sunday night.


Aviation...issued 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.


DTX watches/warnings/advisories...


Lake Huron...gale watch from Saturday evening through late Saturday night for

Lake St Clair...none.

Michigan waters of Lake Erie...none.


long term....jvc/de

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