Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 280358
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 PM EDT TUE SEP 27 2016

.AVIATION...

Showers expanding into the FNT to MBS area have been light enough
for VFR to hold and this will continue through the night before the
pattern shifts back to the west. There could be a brief occurrence
of MVFR ceiling at MBS considering recent observations from MOP but
there really should not be a window for development much more to the
east of MOP. Rain and lower clouds will then consolidate near the
south end of Lake Michigan during the morning before moving back
into SE Michigan during late morning through afternoon. This will be
due to low pressure developing over Indiana which will be reflected
by wind backing and increasing from the east over the region. The
moisture surge combined with daytime heating will make thunderstorms
possible but coverage is expected to be too low to include in the
longer time ranges of the forecast.

For DTW... VFR above 5000 ft will persist through the morning.
Showers are then expected to increase in coverage during the
afternoon which will make MVFR restriction more likely through the
day. Plan to continue holding off on mention of thunder at longer
time ranges in the forecast although isolated coverage remains
possible over D21. Light and variable wind becoming light northeast
during the morning is expected to approach 10 knots while shifting
toward E-SE as a potential negative factor for preferred SW traffic
operations.

//DTW Threshold Probabilities...

* Medium for ceiling at or below 5000 ft after 16Z.

* Low for thunderstorm potential after 19Z.

&&

.PREV DISCUSSION...
Issued at 1037 PM EDT Tue SEP 27 2016

UPDATE...

Nudged POPs upward slightly from FNT northward tonight as a
pattern of showers slides mainly through the Tri Cities.
Isentropic ascent ahead of the height fall surge over the Midwest
is the primary forcing mechanism but it is a fragile setup. The
upstream height falls will induce surface cyclogenesis over
Indiana during the night and cause the leading isentropic ascent
to dissipate over central Lower Michigan. This process is expected
to cause the rainfall pattern to remain north of the I-69 corridor
and be variable an coverage and intensity before shifting back to
the west and southwest toward sunrise. Expect dry conditions over
the region through mid Wednesday morning before showers reorganize
and move up from the south late morning through Wednesday
afternoon.

PREV DISCUSSION...
Issued at 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.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
UPDATE.......BT
DISCUSSION...DG/SS
MARINE.......DG


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