Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 261918
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
318 PM EDT MON SEP 26 2016

.DISCUSSION...

Forecast issues will center around the slow migration of a large
upper low pressure system as it shifts southward from its current
position over the northern Great Lakes this afternoon to the Ohio
Valley by late Wednesday. While model differences remain, suite has
more or less settled into solution best captured by the ECMWF over
the past 2 or 3 days. Namely, having the closed low pivot sharply
south/southeast through the area and then cut off to the south and
southeast of the area through the middle of the week.

Lead shortwave pivoting around the base of this upper low will exit
the forecast area late this afternoon, with mid level dry slot more
or less shutting down shower chances for the rest of the day and
actually all/most of the night for a large portion of the forecast
area.

Will have to back off on precipitation chances as wraparound
moisture associated with this low pressure center will be delayed
until at least Tuesday afternoon and most likely Tuesday night for
the southeastern portion of the forecast area. This is largely due
to the fact that the center of this system wobbles back to the
southwest and then drops south along the length of Lake Michigan
Tuesday and Tuesday night before shifting into far lower Michigan
Wednesday.

Will maintain relatively high precipitation chances overnight
Tuesday night through Wednesday given this scenario and then taper
back somewhat overnight Wednesday night as the low settles south of
the area. Greater uncertainty is already creeping back into the rain
forecast by this time frame as it is certainly possible that a lobe
of vorticity may pivot back southwest into area around this low and
maintain a good chance of showers for at least parts of southeast
lower Michigan into Wednesday night.

In addition to better shower chances into Wednesday, instability
also increases as beginning of warm conveyor pivots back west into
the general area around the upper low as it settles just south of
the area. This will promote embedded thunderstorm activity, first
over the relatively warm Great Lake waters, but most likely across
the whole area during peak instability Wednesday afternoon. NAM12
produces pocket of SBCAPE near 1000 J/kg by that time with HiRes
NMM/ARW suggesting a very similar progression with instability on
the increase from the east/southeast by 12z Wednesday.

As for temperatures, expect a very fall-like period as temperatures
hover near late September averages. Generally, this will mean highs
ranging from the lower 60s north to mid/upper 60s south with low
temperatures large in the 40s with 50 degree readings confined to
the inner Metro Detroit urban heat island and also areas adjacent
the relatively mild waters of the Great Lakes.

Cooler temps and unsettled weather will continue to dominate the
extended  forecast period.  An upper level closed low will be over
the central Great Lakes region through the weekend.  Changes will
still be in play concerning the track of the low as models have it
sliding to the south Thursday then back northward over the area this
weekend. This will lead to a continual chance for unsettled weather
with highs in the upper 60s. The upper level low finally starts to
shift away from the area by the beginning of next week.

&&

.MARINE...

Small craft advisories remain in effect through Tuesday on account
of persistent fresh offshore flow from the southwest. Wave action
will be somewhat suppressed in the nearshore zones but maximum wave
heights will likely exceed 10 feet at times over portions of central
Lake Huron. Winds and waves will begin to diminish late Tuesday. Low
pressure will then stall south of the Great Lakes for the remainder
of the week, setting up several days of moderate to occasionally
fresh wind from the northeast and east.

&&

.PREV DISCUSSION...
Issued at 110 PM EDT Mon SEP 26 2016

AVIATION...

VFR expected through the period with minimal concerns aside from
wind. Cool airmass overspreading the area will feature a steep lapse
rate environment supportive of gusty WSW wind and low VFR cu. After
waning and backing with nocturnal cooling tonight, the gradient will
increase tomorrow allowing sustained winds to increase as low
pressure approaches from the NW.

For DTW...Low VFR cu expected this afternoon after passage of high
cloud. Dissipation likely this evening, but confidence in total
clearing below 5kft is low due to potential for moisture trapping
beneath the inversion. Latest data suggest some difficulty in better
moisture rotating into Detroit on Wednesday, calling into question
cu coverage. The potential for the resultant increased mixing depth
warrants a slightly more aggressive SW wind forecast.

//DTW Threshold Probabilities...

* Medium for cig aob 5kft today. Low for tonight and Wednesday.


&&

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

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

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

&&

$$

DISCUSSION...DG/SS
MARINE.......JVC
AVIATION.....JVC


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