Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 271854
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
254 PM EDT TUE SEP 27 2016

.DISCUSSION...

Southern edge of moisture associated with upper low pressure over
the northern Great Lakes will generally shift slowly southward into
the forecast area late today into tonight as this low begins its
anticipated wobble south along Lake Michigan. Various vorticity
centers circulating about the main low will complicate the advance
of showers into/through the area as initial push of moisture into
the northern forecast area late this afternoon/early this evening
will stall as one vort center pivots east of the area while the next
in line encroaches from the west.

The passage of this second lobe will eventually lead further shower
development south into the middle of the forecast area overnight.
Will maintain a slow expansion of precipitation chances southward
into the area into the overnight hours with a good chance of showers
over the Saginaw Valley and northern Thumb tapering to chance pops
I-69/M-59 to dry conditions from roughly metro Detroit south.

As the upper low pressure center drops south southeast into the Ohio
Valley on Wednesday, a feed of moisture begins to wrap northwest
back into Southeast Lower Michigan as lobe of vorticity pivots back
around the low center. Between the renewed influx of low level
moisture from the southeast and daytime heating under -20c H5 cold
pool, anticipate an expansion of showers with embedded thunder over
much of the area from midday into the late afternoon/early evening.

Modest 0-6km bulk shear values of 30 knots or so, at least within
areas of enhanced directional veering associated with pivoting vort
lobe, suggest a few storms could actually garner some organization
assuming CAPE values can climb into the 500-750 J/kg area in pockets
as NAM12 suggests may occur. Also, with precipitable water readings
reaching an inch or better, pockets of heavier rain would also not
be a surprise as the feed of moisture from the southeast increases
during the afternoon within the beginnings of a warm conveyor belt
around the east/northeast side of the upper low.

A similar setup holds into Thursday into Thursday night as well as
this warm conveyor strengthens and precipitable water values climb
to 1.25 inches across most of Southeast Lower Michigan. Expect a
likelihood of showers with pockets of heavier rain possible. With
the upper low sinking south into Kentucky, the pocket of modest low
level instability expected by midday will shift a bit south and
limit slightly better storm chances to areas along/south of I-94.
Otherwise, shower activity should be rather widespread again during
diurnal instability maximum from midday into the early evening.

While the warm temperatures that have been common this month will
not return in this pattern, relatively mild conditions can be
expected within this increasingly moist airmass. High temperatures
in the mid to locally upper 60s will be common through the middle of
the week. Meanwhile, low temperatures will build from the upper 40s
tonight back into the lower/mid 50s overnight Wednesday night.

Cooler temps and unsettled weather will continue through the
upcoming weekend as low pressure meanders back northwards across
lower Michigan.  The track of the low is still uncertain although
models are hinting at a basic north/northeast track back up across
the state before the low moves away from the area and towards the
New England area on Monday. Conditions dry out starting next week as
upper level ridging finally builds back in.

&&

.MARINE...

Small craft advisories remain in effect into this evening for all
marine areas of southeastern Michigan as southwest flow around an
upper low remains gusty through the daylight hours. Wave action will
begin to diminish late today, but remain in the 5 to 7 foot range
over the open waters of central Lake Huron this this evening.

This low pressure will then stall south of the Great Lakes for the
rest of the week. Increasing east to northeast flow around this
system will lead to the return of small craft advisory conditions by
Wednesday night. These conditions will then basically persist for
the rest of the week with wind gusts to 25 knots or better at times
and high waves within the onshore flow.

A steady flow of moisture around the east and northeast side of this
upper low will maintain very unsettled conditions across the central
Great Lakes with widespread shower activity and embedded thunder at
times. Waterspout chances will also increase into Wednesday as the
chance for convection expands. In fact, there appears to be a pretty
decent chance at a few waterspouts over parts of the area Wednesday
as over water instability becomes rather impressive. This seems most
likely over parts of Lake Erie where the coverage of showers and/or
storms will be most robust.

&&

.PREV DISCUSSION...
Issued at 100 PM EDT Tue SEP 27 2016

AVIATION...

Southwesterly gusts around 25 knots will be common for the
remainder of the daylight hours today. Low clouds over northern
Michigan will gradually edge south through the TAF period as a deep
upper low moves south along Lake Michigan. A few showers may make
inroads to MBS/FNT this afternoon into the overnight hours, but most
of the terminals will remain dry and VFR through much of the TAF
period. Better coverage of lower clouds, showers, and perhaps a few
thunderstorms expected Wednesday afternoon, but the majority of the
restrictions are expected to lie just outside this TAF period. Have
included a PROB30 from PTK southward after 15Z Wednesday to account
for some guidance indicating an earlier start to shower activity.

For DTW...Deeper moisture will remain north of the area through the
overnight hours leaving VFR conditions. Gusty winds around 25kt this
afternoon will diminish after sunset. Showers will increase Wednesday
afternoon, possibly as early as 15Z but more likely after 18Z. There
is a chance of a thunderstorm or two, but probability is too low to
include attm. Winds will back from southerly to easterly Wednesday with
direction and speed both nearing, but currently expected to stay
outside of, noise abatement thresholds.

//DTW Threshold Probabilities...

* Medium in ceilings at or below 5000 ft after 15Z Wednesday.

* Low in thunderstorm potential after 15Z Wednesday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 10 PM EDT this evening FOR LHZ421-422-
     441>443.

Lake St Clair...Small Craft Advisory until 10 PM EDT this evening FOR LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening FOR LEZ444.

&&

$$

DISCUSSION...DG/SS
MARINE.......DG
AVIATION.....DT


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