Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 112023
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 1223 PM EST Wed Jan 11 2017

AVIATION...

Conditions will remain dry through the majority of the afternoon as
subsidence up to 10 kft continues to hold.  Changes start to come in
late afternoon as deeper moisture is advected northward into the
area.  This is due to low pressure lifting through the central Great
Lakes and moving northeast towards the region.  As forcing and jet
dynamics increasing, this will allow for a frontal boundary to set
up across Southeast Michigan tonight.  Ceilings are expected to drop
into the IFR to LIFR this evening as rain chances increase around
23Z.  Peak of the rainfall looks to be around the 03Z to 08Z
timeframe.  A slight chance does still exist for a thunderstorm, but
confidence remains to low to include in the TAFs.  Uncertainly also
exists with regards to the potential for fog development overnight
due to winds remaining mixed discouraging any fog to develop.

//DTW Threshold Probabilities...

* Low for ceiling below 5k ft through the afternoon. High late
  afternoon into Wednesday night.

* Medium for ceiling heights of less than 200 ft or visibility of
  less than 1/2sm from mid evening through the overnight hours.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...MR/JD
MARINE.......MR
AVIATION.....SS

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at www.weather.gov/detroit.



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