Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150132
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
932 PM EDT Sat Oct 14 2017

.UPDATE...

There was a significant waning in rainfall coverage and intensity
this evening, with rain ending altogether across portions of the tri
cities. Attention now turns to the large region of rain and embedded
thunderstorms pushing into SW Lower Mi. The upscale growth of
convection across nrn Illinois this evening has developed a mid level
short wave impulse which will track across cntl Mi tonight. Out
ahead of this feature, there has been some enhanced forcing along the
low level baroclinic zone. These factors will lead to an expansion of
the rainfall across the area, primarily north of an Ann Arbor to
Detroit line. The degree of elevated instability remains weak. As
such, convection has been weakening as it advances across Lower Mi. A
little better influx of instability is expected during the early
morning hours on Sunday preceding the sfc cold front.

The highest rainfall amounts thus far have along the I-69 and M 59
corridors where between 1 and 2 inches has been reported, with the
highest totals across Livingston County. Moisture quality through the
night will remain high (pwats around 1.6 inches), so some additional
heavy rainfall is not out of the question. Based on the rainfall that
has already occurred and given recent radar trends, flooding looks
more of a concern along the M 59 corridor (possibly up into
Shiawassee and Genesee Counties), generally south of the current
flood watch. Given the more progressive nature of the forcing
overnight, the flood watch will not be expanded south. In fact, given
that rainfall across most of the flood watch region has been below
an inch today, the flood watch will be cancelled. If additional heavy
rain occurs across Livingston and Oakland Counties, it would not be
out of the question for a flood advisory to be issued (again this is
south of the current flood watch area). This will be guided by near-
term radar trends.

A forecast update will be issued to remove the flood watch and make
some minor adjustment to precip trends. The wind advisory in effect
for the northern half of the forecast area on Sunday will remain
unchanged.

&&

.PREV DISCUSSION...
Issued at 713 PM EDT Sat Oct 14 2017

AVIATION...

Sfc low pressure over ern Iowa will lift into northern Lower
Michigan overnight, driving a warm front (now over Toledo) northward
across Se Mi late this evening into the first half of the overnight.
Frontal forcing along with upper level short wave impulses lifting
into the region from the southwest will support periodic showers. An
influx of weak instability during the overnight will also support a
chance of thunderstorms. Highly variable cigs and vsby will trend
toward predominate MVFR late this evening as the warm sector lifts
into the area. Some heavier showers are however likely to result in
brief reductions in cigs/vsby. The increase in southerly flow
overnight will lead to a steady increase in sfc winds and low level
mechanical turbulence. A significant increase in sfc wind gusts will
occur with the passage of a strong cold front (14 to 15Z time frame)
with gusts possibly reaching 40 knots.

For DTW...The influx of higher instability will take some time to
reach metro detroit. In the meantime periodic showers will remain
prevalent through the evening, and possibly well into the overnight.
The better instability is expected between the 09Z and 14Z period
prior to the passage of the sfc cold front. Strong winds will occur
with the passage of the front. The winds will veer toward the west
during the afternoon, possibly supporting some cross wind concerns.

//DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings below 5000 ft Sunday morning and early
  afternoon.

* Moderate in thunderstorms Sunday morning.

* Moderate in crosswind thresholds being exceeded Sunday afternoon.

PREV DISCUSSION...
Issued at 300 PM EDT Sat Oct 14 2017

Discussion...

Frontal boundary has remain anchored west to east across region near
the I-94 corridor as near surface based convective have continued to
fire and train into the region along this front. While much of this
activity has weakened on its way in the forecast area, several waves
riding along the front have afforded better lift/support and allowed
narrow bands of moderate to locally heavy showers to traverse areas
between I-94-I-69 (centered mainly M-59 and just south). One such
wave is moving through the area early this afternoon.

Upstream radar mosaic suggests that the influence of lower based
convection will also continue to some extent with the next shortwave
which will track into the area from mid/late afternoon into the
evening. However, this wave is much stronger than the previous two
and also is pulling in the main high theta-e airmass with PWATs of
1.50"-1.75" into area. So, while convective activity will probably
linger near M-59/I-94 late today, there will also be a notable
expansion of rain to the north (both as this increasingly moist
airmass lifts up the frontal surface and as the front slowly begins
to edge north in response to forcing from this strong shortwave. An
embedded storm or two will also become increasingly likely as better
instability is pulled north into the face of this front during the
rest of the afternoon/evening.

Main upper level system then progress quickly from the northern
plains into the Great Lakes overnight with strong cyclogenesis
expected as H3 upper jet on the order of 125-135 knots draws the
trough east. This will lead to very strong forcing on the existing
frontal boundary late this evening/overnight with the warm front
lifting north of the forecast area with time. While an isolated
severe storm or two may traverse this frontal boundary near the
triple point as surface low develops into Wisconsin, the main issue
late today into tonight continues to be heavy rainfall as higher
theta-e air continues to funnel north/northeast into area. Early
convection has set up the southern bound for heavier rainfall on the
order of 1 to locally 2 inches of rain as far south as M-59/I-696.
However, best surge of moisture, helped by strong forcing with the
final two (and strongest waves), still focus best potential for
widespread heavy rain across the northern forecast area (especially
the Saginaw/Genesee Valley region). Will maintain the existing Flood
Watch and extend it to several more counties to better envelope the
expected area where 2 inches (locally more) of rainfall will be more
common at least based on trends in ongoing precipitation and the
most recent model solutions for late today into tonight.

As frontal boundary lifts north overnight, rainfall will become more
scattered in nature with time from south to north, but chances will
continue into Sunday as strong cold front wraps into area around the
deepening low pressure as it lifts through northern Lower Michigan.
At this point, the attention turns to wind potential with the strong
system. HiRes models suggest that there will be a scattered/broken
thin line of convection along the immediate front as it crosses the
area Sunday morning. This would support a short burst of 40-50 mph
wind gusts along front given atmospheric wind profile in place with
this system.

A combination of tightening pressure gradient around the south side
of deepening low pressure and eventual cold air advection will then
lead to generally gusty conditions throughout the day as winds
gradually veer from southwest to northwest. It appears southern
area will generally receive gusts from mid 30s to lower 40s mph.
Further north, roughly I-69 north, better gradient nearer the low
pressure will lead to periods of 45 mph wind gusts with locations
over the Thumb immediately downwind of Saginaw Bay probably reaching
50 mph wind gusts at times by early/mid afternoon. A Wind Advisory
will be issue in anticipation of these conditions.

Cooler/calmer/drier weather then sets up Sunday night into Monday
night as a ridge of high pressure settles east southeast into the
immediately area.

Surface high pressure centered over the Mid-Atlantic by Tuesday and
ridging aloft will dominate the extended forecast. Mostly sunny
skies and dry conditions are in store through next week. Persistent
southwest flow into the area will also result in a gradual warming
trend. A weak shortwave is progged to pass over Wednesday evening,
though models are suggesting no more than a slight increase in cloud
cover with this feature. Temperatures Tuesday will be seasonable
with highs in the mid 60s and overnight lows falling to the mid 40s.
By the end of the week, highs will reach the lower 70s for most
areas with dry conditions expected to persist.

MARINE...

Warm front will be lifting north through the Central Great Lakes
tonight, as deepening low pressure tracks across northern Lake Huron
tomorrow. Gales remain likely tomorrow as a associated cold front
passes through during the mid day, with a brief gust to storm force
not out of the question over the open waters of lake Huron. Any
thunderstorm that develops will also be capable of producing wind
gusts in excess of 50 knots.  Northwest gales Sunday evening will
diminish Sunday night into Monday, with winds dropping into the 5 to
15 knot range by Monday afternoon as a ridge of high pressure
arrives. However, Southwest winds will quickly ramp up once again,
with gusts in the 25 to 30 knots range over Lake Huron Monday night
into Tuesday.

HYDROLOGY...

Rainfall has been focused mainly along the M-59 and I-69 corridors
during the day today, with amounts in excess of half an inch. The
band of heavier rain is expected to slowly lift northward this
evening, and 1-2 inches of rain remains possible along and north of
the I-69 corridor tonight, with localized higher amounts in excess
of 2 inches possible if thunderstorms develop. The flood watch
remains in effect for the Tri-Cities region, with the Flint Vicinity
also added. Rain showers will taper off during the day on Sunday as
strong cold front tracks through.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory from 8 AM to 8 PM EDT Sunday for
      MIZ047>049-053>055- 060>063.

Lake Huron...Gale Warning from 8 AM Sunday to 5 AM EDT Monday for LHZ361>363-421-
     422-441>443-462>464.

Lake St Clair...Gale Warning from 8 AM to 8 PM EDT Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 8 AM to 8 PM EDT Sunday for LEZ444.

&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...DG/TF
MARINE.......SF
HYDROLOGY....SF


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