Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 251928
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 1255 PM EDT Sun SEP 25 2016

AVIATION...

VFR conditions will transition to MVFR late tonight into Monday as
shras/stratus spread into area with cold front. Pulled the onset of
shras forward to 08z for I-94 terminals given morning guidance. Will
not mention thunder ATTM given limited instability and expectation
of isolated tsra at best. Gusty WSW flow (gusts near 25 knots during
best afternoon mixing) after FROPA on Monday as area of subsidence
overspreads area. While this subsidence may also bring period of SCT
clouds, generally expect BKN strato-cu bordering on MVFR-VFR.

For DTW...Rough timing for shras in advance/along cold front will be
~08z-14z with at least periods of MVFR conditions. Post-frontal flow
on Monday should be gusty after 18z, perhaps touching 25 knots.

//DTW Threshold Probabilities...

* Low confidence in ceilings below 5000 ft this morning, High Monday
  morning (12-15 Z).

* Low confidence in thunderstorms Monday morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 10 PM this evening to 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.

&&

$$

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


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