Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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528
FXUS63 KDTX 261710
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 330 AM EDT Mon SEP 26 2016

DISCUSSION...

Large and seasonably strong upper level low taking shape over the
Western Great Lakes/Western Ontario today. Cold front moving through
southeast Michigan this morning. Moisture/instability (negative
showalter values) axis to slide east around 12z Monday, with cold
frontal showers tapering off shortly there after, as good mid level
dry slot (40+ c dew pt depressions at 700 mb) arrives by early
afternoon. Westerly low level flow and sunshine will allow for deep
mixing depths, but flow at 850 mb is only around 30 knots, and thus
expecting surface gusts to top out mainly at or just above 25 knots,
as 6 hr rise/fall pressure couplet also looks rather weak. Mixing
close to 2+ C at 800 mb supports highs in the upper 60s to near 70
degrees, especially with the mild start to the day.

The upper level circulation/cold pool (-23 at 500 mb) looks to be
dropping south through Lake superior on Tuesday, and then through
the Central Great Lakes/Lake Michigan Tuesday Night-Wednesday.
Multiple upper waves rotating around periphery of the low, coupled
with moisture flux/instability generated from the Great Lakes looks
to be conducive for scattered-numerous shower development,
especially Wednesday morning with surface reflection/850 mb
circulation moving through southern Lower Michigan. Low freezing
levels 6500-7000 feet may be sufficient to generate some pea size
hail if capes end up being a bit higher than projected 100-200 J/kg.
With clouds and expected shower activity, maxes should have
no problem holding in 60s both Tuesday and Wednesday.

The 00z gfs indicates the system will be filling late Wednesday, as
circulation becomes sheared out/elongated in a southwest-northeast
fashion as the next upper wave/shortwave trough enters northern
Ontario. Quick modification of the airmass as 850 mb temps rise into
the upper single numbers on Thursday. Still, southeast michigan
appears to be caught into between the outer fringe of the upper
level low and the amplified upper level ridge over the Plains. A
few showers still can`t be ruled out across Detroit area and
points south. In fact, the forecast for Wednesday Night into
Thursday may end up being a too optimistic, as a look at the 00z
Euro indicates the Upper level low remaining intact, and more
symmetric as the center drops into the Ohio Valley, due to the
shortwave tracking through Ontario having little if any influence.
Unfortunately, the strength and position of the 500 mb low will
have an influence right into the weekend, and thus overall
confidence in the extended forecast remains low.

MARINE...

Small craft advisories remain in effect through Tuesday evening.
Fresh southeasterly flow in advance of a low pressure system over
western Ontario will veer to the west after daybreak as a surface
cold front tracks through the region.

Forecast information continues to support that wind gusts over the
open waters of Lake Huron will top out in the lower 30 knot range,
remaining just below gale levels. West winds of 30 knots will
continue through tonight before easing slightly into the 25 to 30
knot range for all of Tuesday. Will continue to forgo a Gale Warning
and will continue to monitor marine observations.

The large upper level low pressure system will track southward
across the central Great Lakes Tuesday and Tuesday night. The low
pressure will maintain very unsettled conditions through the middle
of the upcoming week. The cold airmass associated with this pattern
will be conducive to thunderstorm development and possible
waterspouts as instability increase above the relatively mild waters
of the Great Lakes.

HYDROLOGY...

Low pressure will slowly meander from western Ontario to sections of
Lake Superior today. The low will drive a cold front through
southeastern Michigan this morning. Scattered to numerous showers
and embedded thunderstorms associated with this front will be
possible across southeastern Michigan between 4 am and noon EDT.
Rainfall totals of a quarter to one third of an inch will be
possible. The progressive nature of the front and lack of heavy rain
producing thunderstorms will limit any potential for localized
flooding.

&&

.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.

&&

$$

AVIATION.....JVC
DISCUSSION...SF
MARINE.......CB
HYDROLOGY....CB


You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.



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