Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 010358
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1158 PM EDT FRI SEP 30 2016

.AVIATION...

The surface-based occluded front moved a little more northward
toward the Ohio border during the evening and IFR has filled in over
lower Michigan. There were a few pockets of VFR around Toledo at
press time but expect these to fill in and for ceiling to continue a
downward trend into LIFR during the night. Pockets of drizzle and
rain showers will then just modulate visibility mostly around the
IFR/MVFR borderline through morning before some improvement occurs
during afternoon, mainly in the DTW area as the low pressure system
and surface occlusion continues to move northward.

For DTW...LIFR ceiling will be the main concern as northeast surface
wind 10 knots or less veers toward the east during the night and
then toward the southeast during the morning. Ceiling will make
steady improvement during the morning and especially by afternoon
while showers become less numerous.

//DTW Threshold Probabilities...

* High for cig aob 5kft tonight into Saturday evening.

* Low for 200 ft ceiling or 1/2 mi visibility during the night.

* Low for tstorm impacting KDTW airspace tonight.

&&

.PREV DISCUSSION...
Issued at 1018 PM EDT Fri SEP 30 2016

UPDATE...

A quadrant of subsidence within the upper level low is moving over
southern lower Michigan during the late evening and will spread
over the remainder of the area into the early overnight hours.
Associated isentropic descent will replace the ongoing pattern of
showers with drizzle until the next batch moves in from Ohio
later on in the night. This next wave does not look as well
organized as the evening pattern and is expected to move through
in benign fashion outside of a rumble of thunder.

A slight uptick in gradient wind with more of a southeast
direction over western Lake Erie will continue to produce
conditions favorable for lakeshore flooding. The surface low and
occluded front over central Ohio will continue to support the wind
field until both move closer to the border Saturday morning.

PREV DISCUSSION...
Issued at 342 PM EDT Fri SEP 30 2016

Active weather pattern continues across the Great Lakes as the
expansive upper low continues to wobble over the southern Ohio River
valley. The low is forecast to begin lifting northward overnight,
eventually centering over SE MI Sunday morning. Earlier guidance did
pretty well with afternoon development starting to expand around
18Z, with current radar showing several bands of showers spreading
across SE MI. Forcing this shower activity is a combination of
increasing 850mb easterly jet, left exit region of cyclonically
curved upper level jet nosing into the area, with increasing
diffluence aloft as the northeastern flank of the upper level begins
to open as it begins the slow merger with the northern stream of the
jet.

For tonight into Saturday we`ll continue to advertise a high pop
forecast with QPF remaining in question. Nearly all models advertise
an additional wave or two to lift through southern MI tonight and
again early Saturday morning. Question is what QPF will we receive
from this?  Radar is mimicking water vapor with several different
bands of showers firing up. This due to a complex trowel structure
advertising several different levels of forcing to work with.
earlier thinking was the forcing would converge toward one level
with the upper level forcing all coming together. As we get deeper
into the event, doubt is creeping in as the mid/upper levels may
just be too chaotic to converge like that. Still, several bands of
showers are occuring and will continue into the evening hours. There
is a chance of convection with these bands but as of now, all the
convection is to our south in Ohio and Indiana. We tempered down QPF
slightly with the potential loss of a dominate band of showers and
the potential loss in convectively enhanced rainfall rates. For this
reason, will not adjust the current timing of the Flood Watch going
until 00Z this evening. If we can`t get high enough totals out of
these initial bands, the progressive nature of the overnight showers
will have a hard time resulting in my flooding across the area. So
will give the evening shift time to assess afternoon rainfall
amounts before possibly extending the Watch through tonight.
Overall, expect up to an inch of additional accumulation for most
locations by Saturday morning. If convection can move into the area
then some higher totals will be possible.

Indications for Saturday are that the deeper moisture will be forced
north and east by morning with the upper level jet shifting off to
the east. Mid level dry slot could easy in here early Saturday as
well before the low forces that east as well. With upper level low
and broad cyclonic flow overhead, and any shortwaves or vorticity
lobes rotating around the low, chances are good to see addition
showers through the day. Should be more cellular in nature than
we`ve seen the last few days with diurnal support and steepening
low/mid level lapse rates with the cool pool aloft.

The low will be pulling away to the east on Sunday with only a
lingering trough laid out across the region. This could result in
some light showers through the early part of the day, most likely
for the eastern and northern Thumb. Otherwise most locations will
see improving conditions through the day as surface high pressure
starts to build through the western Lakes.

Early to mid week will feature drier conditions as upper level ridge
builds into the region. Return of southwesterly flow will allow high
temps to slowly climb back into the low 70s with lows remaining
around the low 50s. The next chance of precip will not arrive til
later in the week when a cold front passes through the region.
Current timing looks to be Thursday night into Friday morning but
adjustments are likely as we see how Hurricane Matthew affects the
longwave pattern.

MARINE...

Low pressure that had been stalled over the Ohio Valley will finally
lift northward over the weekend. Fresh easterly/northeasterly flow
will continue tonight along the northern periphery of this low,
prompting a continuation of Small Craft Advisory headlines into
Saturday morning. Winds will diminish from south to north Saturday
into Sunday as the low pressure center moves overhead and the
pressure gradient slackens. Showers and a few thunderstorms will
continue to be likely over the weekend in conjunction with this
system. Conditions will become slightly unstable over the waters as
the upper low moves overhead, so waterspouts will be possible late
Saturday into Sunday. A moderate northeasterly wind will set up on
Monday as the low exits to the east and a drying trend will ensue.

HYDROLOGY...

Periods of rainfall will continue this afternoon into the overnight
hours as a stalled low pressure system remains over the Ohio Valley,
transporting a good deal of moisture north and west from the Mid-
Atlantic region. A half inch to an inch of additional rainfall is
possible through the overnight period. Additional rainfall will
exacerbate urban and small stream flooding concerns, especially over
portions of Metro Detroit where 2-5 inches of rain fell in the past
48 hours. Expect showers to continue Saturday and Sunday as the low
pressure system migrates northward, though rainfall totals are only
expected to be in the 0.2 to 0.6 inch range during the weekend as
the region gets cut off from the better moisture feed.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Lakeshore Flood Advisory until 5 AM EDT Saturday FOR MIZ076-083.

Lake Huron...Small Craft Advisory until 5 AM EDT Saturday FOR LHZ422.

     Small Craft Advisory until 10 AM EDT Saturday FOR LHZ421-441>443.

Lake St Clair...Small Craft Advisory until midnight EDT tonight FOR LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 5 AM EDT Saturday FOR LEZ444.

&&

$$

AVIATION.....BT
UPDATE.......BT
DISCUSSION...DRK
MARINE.......DRK
HYDROLOGY....DRK


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