Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 230805

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
305 AM EST Mon Jan 23 2017


Surface trough is in the process of gradually sharpening and
pivoting to the west as low pressure lifts east of the Appalachians.
As a result, light northerly gradient is gradually being replaced by
a drier easterly component as well as diminished boundary layer
forcing overall. Dense fog advisory has been dropped as 1/4sm
readings have become increasingly sparse and conditions are expected
to further improve from east to west during the morning hours.

Very weakly forced light QPF scenario will unfold during the
remainder of the day. Considering existing moist boundary layer is
already proving sufficient for drizzle production, developing weak
isentropic ascent and mid-level deformation around the back side of
the aforementioned low should be effective for producing at least a
few hundredths areawide today. Though already light, rain intensity
can be expected to further diminish as it slowly propagates east-to-
west across the CWA this aftn. However, approach of the shear max
along the lead edge of approaching subsidence layer tonight is
modeled to provide just enough ascent to perk rates up again over
the Saginaw Valley. As a result, trended pops upward overall but
made no changes to rainfall amounts. With no change in airmass,
highs will once again settle 10 to 15 degrees above normal.

Hemispheric water vapor imagery reveals Rossby wave train
characterized by a deep low off the PacNW coast and another over the
Aleutian Island chain at 07z this morning. 00z global NWP hold firm
on the overall pattern evolution as a powerful 200kt jet extension
works across the Pacific during the midweek period. Aleutian low is
progged to close off in the persistent left exit region and form
some semblance of a new Alaska vortex. Downstream ridge will be
forced into the western US and the PacNW wave will open up and work
across the Great Lakes on Wednesday.

This wave is shown to be rather dynamic with strong shortwave
forcing and left exit dynamics aloft. Subsequent LLJ response will
funnel another round of warm air into SE Michigan Tuesday night into
Wednesday. Ptype will still be rain for all of the CWA, but trailing
deformation may allow a changeover to light snow over the Saginaw
Valley and thumb late Wednesday. Only minor accums possible attm.
Highs Tues and Wed still stuck in the 40s until airmass until cooler
air is finally ushered in late Wed.

Pacific energy will continue to feed around the periphery of the
western US ridge, carving out a persistent longwave trough over the
eastern US that has potential to hold through the remainder of the
forecast period. A return to more seasonable NW flow with daily high
temps closer average values around 30 degrees and the occasional
chance for snow can be expected during this time.



Fog will gradually dissipate over marine areas during the day as
easterly wind increases and as patches of light rain move through
the region. The wind speed will be low enough for a minimal wave
response on Lake Huron, especially the south half. Weak high
pressure will then settle over the Great Lakes for Tuesday before
the next low pressure system arrives Tuesday night through
Wednesday. This system will be large enough and strong enough to
produce moderate southeast flow over all marine areas but the
incoming air mass will be warm enough to limit wave growth over Lake
Huron. The low center is then projected to move through Lower
Michigan and Lake Huron Wednesday into Wednesday night. Colder
northwest wind that is more typical for late January will follow
behind the system with speed expected to be just below gales through


Issued at 1149 PM EST Sun Jan 22 2017


Weak gradient flow from the east/northeast has helped keep boundary
layer slightly mixed during the mid to late evening, alleviating the
worst of the vsby restrictions. Still, expect vsbys to oscillate in
the 1/4-1/2SM range much of the time. KDET/KMBS probably stand the
best chance at periodic vsbys nearing 1/8SM or less given relatively
mild ENE flow off of ice cover on Lake St. Clair and Saginaw Bay
respectively. KPTK`s higher elevation nearer the stratus layer will
likely maintain 1/4SM or less vsbys much of the time.

While guidance suggests poor visibilities well into the day again
Monday, short term model field suggest most improvement in the
10z-14z time frame, which coincides with thickening ceilings above
the VLIFR stratus and eventually -RA which will pivto NNW into
area during the morning.

For DTW...Gradient flow around 5 knots has been sufficient to mix
out the 1/16-1/8SM fog from earlier this evening. While dense fog on
the order of 1/4SM remain likely at times, do not expect widespread
vsbys approaching zero again. Ceilings will remain very low though
around 200 feet, at least until rain showers work into area Monday
morning. IFR conditions are expected by that time.

//DTW Threshold Probabilities...

* High for cigs aob 5kft.

* High for cigs around 200 feet and medium for vsbys near 1/4SM
  through 12z or so.


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



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