Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 112357
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
657 PM EST Wed Jan 11 2017

.AVIATION...

A wave of low pressure will move through the center of SE Michigan
and bring a wide range of aviation weather to the region tonight
through Thursday. This will start with southerly flow bringing in a
moisture laden air mass that will produce borderline IFR/LIFR
restriction in low clouds, fog, showers, and a few thunderstorms.
Ongoing activity will move quickly eastward and open the door for
another surge of showers/storms from the Chicago area. This will
follow the surface low into and through lower Michigan. Expect
precipitation to diminish to drizzle or end as the low moves
eastward and the associated cold front settles southward through the
region. Colder and drier northwest wind will lift ceiling into MVFR
which is expected to persist through at least mid Thursday
afternoon.

For DTW... Monitoring additional coverage of upstream thunderstorms
for potential impact at DTW or in D21 airspace. Otherwise, IFR/LIFR
restriction could break into MVFR briefly as the surface wind turns
southwest before the low slides north of the terminal. The trailing
cold front will move through overnight shift the wind to northwest
through Thursday morning. Additional precipitation possible after
sunrise is expected to be a cold rain as temperatures remain above
freezing.

//DTW Threshold Probabilities...

* High for ceiling 5000 ft or less tonight through Thursday.

* Borderline low/medium for ceiling less than 200 ft or visibility
  less than 1/2sm tonight.

* Moderate for thunderstorms to affect DTW airspace tonight.

&&

.PREV DISCUSSION...
Issued at 323 PM EST Wed Jan 11 2017

DISCUSSION...

Lower amplitude southern stream mid level west-southwest flow of
central pacific origin to largely govern conditions across SE
Michigan over the next 24 hours.  Cold frontal boundary currently
easing through the Upper MS Valley on track to lift into lower
Michigan tonight, this process gaining greater momentum as a steady
stream of shortwave energy funneling through this energetic flow
begins to carve out a greater height fall center upstream.  In the
meantime, defined upper jet maximum and parallel strengthening low
level jet anchoring this flow establishes a solid component of
warm/moisture advection within the downstream warm sector.
Broadening corridor of moist isentropic ascent will drive the
underlying forcing initially to support a northeast expanding area
of rainfall, with an entry locally expected from southwest to
northeast between 22z and 02z this evening.  This process occurs
coincident with the arrival of the frontal slope over the Tri-
Cities/Thumb.  Low level thermal gradient sharpens with time,
solidified by a firm warm air advective component to the south.  925
mb temperatures projected to push 11-12C across the south by 03z,
while values over the  Tri-Cities approach the freezing mark.  This
will actually place the daily high temperature for many locales
during the first half of the night, as readings climb into the
middle and upper 40s south of the front.  Favorable intersection of
left exit region upper jet dynamics with the inbound frontal slope
will establish a stripe of stronger fgen across the 850-700 mb layer.
Frontal placement favors the Tri-Cities/thumb with this forcing.

Timing will present a window for freezing rain/drizzle mainly across
the Tri-Cities/northern thumb overnight, this setup favoring a
cooling near surface layer first.  Temperatures are expected to dip
to/below freezing during the early morning period just as the deeper
forcing/moisture axis strips to the east, suggesting the window for
more meaningful ice accretion will be limited.  A very light glaze
of ice certainly possible, but both the coverage and intensity of
precipitation looks to be rapidly decreasing during this time.  A
rumble of thunder not out of the question for points to the south,
neutral stability across a deep layer augmented by a high theta-e
air mass could support isolated thunder.

Forecast for Thursday becomes more muddled owing to less defined
forcing potential.  Frontal boundary simply eases southeast early on
Thursday as the front remains largely parallel with the mean flow.
With that said, nearly all guidance suggests the boundary does push
clear of the forecast area by midday.  Upper jet max still anchored
overhead may continue to perpetuate weak mid level fgen north of the
exiting surface reflection early Thursday.  Mixed signal across the
model spectrum as to the degree /or even existence/ of ascent, but
this will position areas largely along/southeast of the glacial
ridge with at least a chance of additional light precip.  Forecast
continues to highlight a mixed precip mention along the northern
periphery /M-59 to I-69 tier/, as the northern edges of this
moisture could intersect a supportive but marginal thermal profile.
To the north, mid level wave projected to pivot across central lower
MI will work beneath the left exit region of the existing jet
streak.  Elongated strip of deeper ascent across this intersection,
largely displaced just northwest of the Tri-Cities.  Glancing shot
of forcing, particularly Midland/Bay/Huron counties, will provide a
window for accumulating snow mid-late morning Thursday. Accumulation
below an inch.

Colder but benign weather conditions to finish the week.  Extended
period of modest cold air advection Thursday and Thursday night, the
advective process landing 850 mb temperatures in the -8 to -12C
range by 12z Friday morning. Drying northwest flow and increasing
subsidence takes hold into Friday as a 1040 mb high builds in.
Afternoon temperatures to arrive roughly 5-7 degrees below average.

High pressure will remain over the Great Lakes through the weekend
keeping dry, calm, and cold conditions in place. The bulk of
precipitation should remain to the south of Michigan along a
stationary front in the Ohio Valley. Low pressure moving
northeastward from the Southern Plains then brings the next chance
of precipitation early next week. Initial band of precipitation
could start as a wintry mix before southwesterly flow boosts
temperatures above freezing changing precipitation to rain over
Southeast Michigan by Tuesday. Both the GFS and ECMWF show the low
tracking just west of Lower Michigan on Tuesday keeping Southeast
Michigan in the warm sector. Overall milder pattern then remains in
place next week as the low departs the Great Lakes.

MARINE...

Winds will be weaker compared today compared to yesterday as that
low pressure system continue to quickly move away from the region.
Lake Huron will see northerly flow develop in response to a surface
front slowly moving down and eventually south of Detroit.  The
northerly wind response will remain faint due to the lack of surface
pressure rises and pressure gradient.  There remains the chance to
see a response in westerly winds across Northern Lake Huron Thursday
night as a weak shortwave swings across Lake Superior.  This may
allow winds to reach gales for a period during the overnight hours
on Thursday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...MR/JD
MARINE.......MR


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