Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 231138
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
738 AM EDT FRI SEP 23 2016


.AVIATION...

IFR Ceilings and increasing northeast winds will work southward this
morning, but cloud heights will tend to rise during the daytime
hours, and expectations are for an MVFR deck to arrive across the
southern TAF sites (dtw/yip/det) by Noon. High MVFR/Low VFR deck
then looks to hold during the afternoon, and then potentially
scatter out during the evening hours. A few sprinkles or isolated
light rain showers will likely be around today as well, but coverage
and duration too limited to include in tafs. If skies do in fact
clear out this evening, a redevelopment of low clouds or fog is
possible. At this point, preference is to hold onto low VFR/high
MVFR deck.

For DTW...Low clouds (MVFR/possibe brief IFR) are expected to arrive
toward Noon, coincident with increase in northeast winds, which look
to top out around 10 knots. Clouds and wind holding during the
afternoon, with confidence in ceilings diminishing
this Evening/Tonight.


//DTW Threshold Probabilities...

* High for ceilings below 5000 ft by Noon, continuing through
  afternoon hours, medium confidence this evening/low overnight.

&&

.PREV DISCUSSION...
Issued at 357 AM EDT Fri SEP 23 2016

DISCUSSION...

Shortwave energy that passed through the northern Great Lakes on
Thursday has allowed a weak cold front to settle southward along the
northern edges of the CWA at press time. Geopotential height rebound
following a deep midlevel trough over portions of eastern Canada
will allow surface high pressure to build into the region today.

The question at the onset of the forecast period is what will the
character and coverage be of any light precipitation along this
frontal boundary. Much of the developed shower activity has pushed
out across Lake Huron and what remains is some areas of fog
(patchy dense) across portions of the Tri Cities and northern
Thumb. With a number of observing sites not reporting fog there is
some uncertainty as to exactly how much it will fill in, not to
mention whether or not the sluggish cold air advection will be
able to offset the urban environments. Based on the data, the
front by 14Z will settle southward between the I 69 and M 59
corridors, across northern portions of Oakland and Macomb
counties. For this area and to the north, introduced areas of fog
along with a slight chance shower or drizzle wording. This is not
an ideal sounding for a long duration drizzle episode, but much of
the dynamics for any shallow lift will be in the lower troposphere
tied directly to the frontal zone. Less than optimal wind
trajectories with backdoor northeasterly flow off of Lake Huron
will lead to persistence clouds and overcast today. The 850mb
temperature gradient is forecasted to stall out over southern
Lower Michigan denying any good midlevel dry airmass transition.
As a result expecting clouds to hold in tough right through
tonight. Clouds and differential heating will lead to a range in
temperatures, from the middle to upper 60s across the far north to
the 80 degree mark south of Detroit.

With the frontal zone slowly inching southward by the late afternoon
there is a slight chance of a thunderstorm over portions of southern
Macomb and Lenawee counties. Support for a thunderstorm may occur
between 21-00z along or near maximized convergence tied in some
fashion to flow off of Lake Erie. There is guidance that suggests
some activity out over Lake Erie will be possible tonight. If this
were to develop could see a shower track inland over Monroe
county, but confidence is too low in this potential for tonight.

Surface ridge axis will pass over southeastern Michigan during the
first part of Saturday. This will be quickly replaced with a very
pronounced and amplified mid to upper level ridge for both Saturday
and Sunday. There is high confidence in dry weather across all of
southeastern Michigan with one small caveat is a low chance for a
shower to track inland off of Lake Erie Saturday as the remnants
of a midlake convergence axis rotates northward anticyclonically.
There is guidance that is suggesting an extremely pronounced
anticyclonic vorticity maximum that will roll directly through
southeastern Michigan Saturday night and on Sunday which will be
high end support for subsidence. There is a very good signal for a
sneaky warm day on Sunday and increased highs a degree or two into
the middle 70s.

The highly amplified mid to upper level ridge axis will not pass east
of lower Michigan until Sunday evening. Not until then will the
chance for showers and thunderstorms begin. Timing of the next
system appears to have slowed down to the point where the fair
question to ask is when the bulk of the precipitation will fall
Sunday night or Monday. There are a number of other question
regardings this system, the most pertinent being how occluded will
the system be when it tracks through lower Michigan. No question
though this will finally be an autumn storm system, the type we
have not been accustomed to recently.

MARINE...

A cold frontal passage early this morning will lead to prolonged
northeast winds today, generally around 20 knots sustained over Lake
Huron, with wave heights building into the 3 to 5 foot range across
nearshore waters of Lake Huron, including Saginaw Bay.
Small craft advisories continue through this Evening. Gradually
diminishing winds Tonight, dropping at or below 10 knots, with these
lighter winds persisting Tomorrow as cold advection ends. Veering
winds to the southeast Saturday Night into Sunday, but remaining on
the lighter side, in the 10 to 15 knot range.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until midnight EDT tonight FOR LHZ421-422-
     441>443.

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

$$

AVIATION.....SF
DISCUSSION...CB
MARINE.......SF


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