Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 260121
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
921 PM EDT SUN SEP 25 2016

.UPDATE...

The 00Z DTX sounding showed a respectable mid level warm and dry
layer, a result of the persistent mid level subsidence which was
overhead today. It is therefore no surprise that the
showers/thunderstorms moving into wrn Lower Mi this evening have
been diminishing as they advance to the east. The mid level ridge
will be forced east of the region tonight with the approach of the
upper level trough to the northwest. This will invoke some mid
level height falls across Se Mi overnight (helping to erode the
mid level warm layer), which will make showers more probable
later in the night as the deep layer moist axis (now rotating into
wrn Lower Mi) slides into Se Mi. More importantly, a mid level PV
filament now sliding into wrn Minnesota will move across Wisconsin
early Mon morning, causing the upper trough to take on a negative
tilt. This will invoke a deep layer frontogenetical response
along the back edge of the departing deep layer moist axis,
thereby sustaining some showers behind the sfc cold front
(tracking through between 09z and 12Z). While there is some
concern that the larger scale forcing will be too far removed from
the deep moist axis, most locals still seem to have a high
likelihood of showers sometime between 2 AM and 11 AM Monday
morning.

The main adjustments to the forecast will be to nudge back the
timing of the showers a couple hours. Very weak elevated
instability will support a slight chance for a thunderstorm, but
nothing more.

&&

.PREV DISCUSSION...
Issued at 702 PM EDT Sun SEP 25 2016

AVIATION...

The high pressure across the eastern Great Lakes will maintain its
influence across Se Mi through the rest of the evening, sustaining
VFR conditions with Southeast winds generally at 6 to 10 knots. The
high will exit the region to the east overnight, allowing an axis of
moisture and instability to traverse the area associated with a cold
front. The cold front will move across Se Mi between 09Z and 12Z.
The ascent ahead of the front will lead to numerous showers late
tonight, while the influx of low level moisture supports steadily
lowering ceiling heights. The amount of instability along the front
will be considerably less overnight as it crosses the area, so
despite the amount of convection upstream, overnight thunderstorm
chances remain quite low at the terminals. The passage of the front
will be marked be a veering to the wind to the southwest, a gradual
rise in ceiling heights and a few hours of post frontal showers. A
strengthening west-southwest gradient and deepening mixed layer will
support wind gusts up to or in excess of 25 knots Mon afternoon.

For DTW...Fropa is expected to occur around 12Z at metro. Again
weakening instability aloft will reduce the chances of overnight
TSRA at metro. Showers, both ahead of the front and behind should
however be fairly widespread. Ceiling heights will likely show
considerable variability throughout Mon morning. In the 16 to 17Z
time frame, there is expected to be a slight veering of the winds
toward the W-SW (generally around 260 deg), with a notable uptick in
gusts as the boundary layer deepens.

//DTW Threshold Probabilities...

* High in ceilings below 5000 ft overnight through Monday morning.

* Low in thunderstorms overnight.

* Low in wind gusts exceeding crosswind thresholds Monday afternoon.

PREV DISCUSSION...
Issued at 328 PM EDT Sun SEP 25 2016

DISCUSSION...

Hi-res and NWP Models in better agreement with tonight`s and
Monday morning`s rain event. Increase in coverage across WI and IL
noted this afternoon, right on schedule. This is in response to a
strong frontal system with good right rear entrance region jet
support. There is also good and deep deformation and Fgen forcing.
Instability is elevated and marginal, around 500 J/kg, by the time
it moves over Southeast MI. Will keep the flavor of the current
forecast with showers moving in after midnight with a chance of
thunderstorms.

The surface front passes through the forecast area by 12z. However
the 925 and 850 mb front take a little longer.  These frontal
systems are expected to be somewhat active hanging onto some sort of
decent and deep Fgen and deformation. Instability is gone by then so
only expect showers for the morning hours. Things dry out
considerable in the dry slot of the synoptic system during the
afternoon with some peaks of sunshine.  The dry slot, cold air
advection and that sunshine will help to mix down gusty winds up to
30 mph. Not much more in the low level wind profile than around 30
kts so do not expect the winds to get too much higher than that.

Upper low starts to migrate southward across the western Great Lakes
as the cool air continues to filter over the region.  By late Monday
afternoon the Lakes will be activated with 850 mb delta t`s in
excess of 15C and up to 20C.  The combination of the upper low,
surface low and lake effect will keep the region unsettled from late
Monday afternoon through Tuesday.  Showers will be first limited to
the Saginaw Valley and Thumb for Monday night with southwest flow
off of Lake Michigan and closer to the upper low forcing.  Then for
Tuesday, will bring the chance of showers as far south as the M 59
corridor.

With plenty of clouds around, went toward the higher end of guidance
for the overnight lows each night.  MOS guidance for highs were
close and did not stray from that.

That upper-level low will bring the chance for rain showers on
Wednesday, with a continued chance for precipitation as we head
into the weekend. The low will move from Michigan into the Ohio
Valley Wednesday into Thursday, taking on a south-southeast track.
Uncertainty lies within the tracking of this low as we head into
the weekend. GFS and GEM models show decent convergence, placing
the low over the Carolinas on Thursday and veering it
northeastward along the east coast. Central Atlantic ridging over
the east coast exhibited in the ECMWF model gives the low a more
northward track into Michigan throughout the weekend. As a result,
cloud coverage has been upped for the later part of the week. Due
to low uncertainty, chance of precipitation will remain on the
lower side, but will have to be increased if future model runs
exhibit convergence and show a more northern path with the upper-
level low.

MARINE...

Small craft advisories remain in effect from tonight into Tuesday
evening as southeast flow increases in advance of approaching cold
front and leads to steadily building waves into tonight with strong
southwest flow then becoming established in the wake of this front
from Monday into Tuesday.

Average wind gusts over the open waters of Lake Huron appear to top
out around 30 to 32 knots, both in southeast flow over northern Lake
Huron tonight and west to southwest flow from Monday into Tuesday.
While gusts may approach gale force at times, will forgo a Gale
Warning given the expectation of most gusts remaining closer to 30
knots.

A large upper level low pressure system will gradually build into
the central Great Lakes in the wake of the vigorous lead shortwave
driving the cold front. This 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 deepen as it shift along the north shore of
Lake Superior. This low will force a cold front through the region
Monday morning. Showers are likely late tonight in advance of this
front with activity continuing into Monday morning as the cold front
crosses lower Michigan. Rainfall totals of a quarter to half 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 from 2 PM Monday to 10 PM EDT Tuesday FOR
     LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 2 PM Monday to 10 PM EDT Tuesday FOR
     LEZ444.

&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...RBP/AM
MARINE.......DG
HYDROLOGY....DG


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