Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 241952
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
352 PM EDT Wed May 24 2017

.DISCUSSION...

Some downward adjustments to POPs are worked into the forecast
covering the rest of the afternoon. The exception is in the Detroit
area where numerous coverage is maintained which leaves some room
for upstream activity over Lake Erie to develop westward during late
afternoon and early evening. A late response in coverage due to
surface heating and convergence within the broad cyclonic flow over
lower Michigan is also possible. Hourly mesoanalysis indicates a
respectable lake shadow of lower instability cast by the increasing
northeast low level flow over the region which has kept in check both
coverage and intensity of convection during the afternoon. The
impact of the short wave ridge aloft has also become more apparent
through mid afternoon. This feature is producing some subsidence and
warmer air aloft temporarily before it will be pushed northward as
the upper low begins to move out of the Tennessee valley tonight.

The approach of the low will set the stage for the main portion of
rainfall during this event for SE Michigan. Some initial negative
tilt in the system, and probably some convective influence as well,
will foster surface pressure falls over the Ohio valley that will
sharpen the low level theta-e axis during the night. Expect
afternoon thunderstorms will diminish to showers and remain in that
character through the night. Moisture transport and forcing from
isentropic ascent will generate minimal elevated instability for
thunder but enough to replace coverage of showers during the evening
filling in to near categorical coverage overnight as the
trowal/deformation structure matures. Model QPF at the lower end of
the spectrum, around 0.5 inch, is preferred due to the progressive
nature of the trowal axis and aggressive northward extension of the
dry slot toward sunrise that is already beginning to show up in
satellite imagery today.

The upper low will begin to lift out of the Ohio valley during
Thursday. This will convert the trowal/occlusion into more of a pure
deformation pattern through the mid levels as it pivots back over SE
Michigan through the afternoon. Little or no surface based
instability is advertised in model data through this portion of the
system mainly due to the surface low center passing to our south and
east keeping cool northerly flow in place over lower Michigan. Low
end likely POPs for showers over interior sections are expanded to
categorical through the Thumb region where the deformation axis will
persist longer before diminishing and sliding eastward during
Thursday evening. This will lead to a dry period of weather, at
least for late Thursday night and early Friday. Uncertainty during
this time lies with the relatively fast progression of upper level
features for such an amplified pattern. Lower POPs in the blended
guidance looks good through Friday.

Upper-level ridging is expected to push east across the Great Lakes
region Saturday morning into Saturday afternoon which will act to
keep conditions relatively dry as clouds scour out during this time.
The better chance for rain showers will move in Sunday, with PoP
values holding on into early next week as an upper-level troughing
pattern returns and holds across the Great Lakes. A series of
embedded short-waves will bring on and off rain chances as cloud
cover remains mostly cloudy through the middle of the week. In
addition to the rain chances, thunderstorms will be possible for any
rain that does develop on Sunday as the GFS shows early signs of a
weak warm front protruding through southern Michigan. Otherwise,
potential exists for a drier pattern starting late Wednesday into
Thursday. The GFS pushes a large surface high across the Ohio Valley
which would act to diminish rain chances, however, ECMWF model runs
continue to hold on to shower chances into late next week. Overall,
expect on and off rain chances through the weekend and into early
next week, with late Saturday morning into Saturday afternoon being
the best chance to see diminished rain chances and even some
extended periods of sunshine.

Temperature-wise, daytime highs are expected to peak in the low to
mid-70s through the extended weekend, with a weak cold front capping
temperatures in the upper 60s by Tuesday.

&&

.MARINE...

Strengthening low pressure over the Ohio valley tonight will lift
toward southeast Michigan tonight and remain in the vicinity through
Thursday before exiting to east by Thursday night. This will
generate a period of moderate northeast winds and higher waves into
Saginaw Bay and southern Lake Huron nearshore areas. The Bay
responded strongly to lower stability generated by the warmer water
compared to open lake zones and southern nearshore areas. Small
craft advisories remain in effect through Thursday. Western Lake
Erie will be monitored considering water temps are similarly warm.
The low pressure system will also produce a stretch of unsettled
weather producing episodic showers and a few thunderstorms during
the evening, especially near Lake St Clair and western Lake Erie,
diminishing to showers overnight through Thursday. The eastward exit
of this low will bring improving conditions by Friday.

&&

.HYDROLOGY...

Low pressure developing over the Ohio valley today will lift
slowly northward to southeast Michigan late tonight then slowly move
to the east by Thursday night. Periods of showers from this
afternoon through Thursday evening will produce generally .50 to
1.00 inch of rain with slightly higher amounts possible along the
eastern counties.  This long duration event is not expected to
produce any significant flooding other then some minor ponding on
roadways and low lying areas.

&&

.PREV DISCUSSION...
Issued at 114 PM EDT Wed May 24 2017

AVIATION...

Mid level and Surface low pressure center over western Tennessee will
gradually lift northward to southeast Michigan by Thursday
afternoon. Deep moisture will advect into the area from the
southeast bringing bands of showers, mvfr visibilities and ceilings
starting this afternoon across the southern taf sites spreading
northward to mbs this evening. Ceilings will become ifr tonight at
all locations as surface moisture continues to increase and continue
through taf period. Widely scattered thunderstorms will occur along
the lead edge this afternoon where the best instability though weak
will occur combined with daytime heating. Surface winds across the
area will range from east to northeast.

For DTW...Greatest window for TSRA to lift across the airspace
remains centered 17z-22z.  Movement from southeast to northwest
today.  Existing northeast flow settles to east-southeast by midday,
then back to northeast again tonight.  Ceilings will lower to mvfr
range this afternoon and then continue to ifr range around 700 ft
tonight and Thursday.  Visibilities should should stay in the 3-5
mile range in fog.  Steady east to northeast low through the period
as surface low to south slowly lifts northward.

//DTW THRESHOLD PROBABILITIES...

* mod for cigs aob 5kft this afternoon...high tonight and Thursday.
* Low for tstorms to impact the airspace 18-22z.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

Lake Huron...Small Craft Advisory until 10 PM EDT Thursday for LHZ421-422.

     Small Craft Advisory from 8 PM this evening to 10 PM EDT Thursday
     for LHZ441.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

DISCUSSION...BT/AM
MARINE.......BT
HYDROLOGY....DRC
AVIATION.....DRC


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