Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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354
FXUS63 KDTX 211713
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
113 PM EDT THU JUL 21 2016


.AVIATION...

While MCS has attempted to rejuvenate this morning/early afternoon,
recent trends suggest this system is becoming less organized with
time, especially within the southern portions. Northern parts of the
complex remain most formidable, so still expect a decent band of
storms to move through MBS and probably FNT early in the forecast,
18z-20z. PTK south is a tougher call, but still feel slightly weaker
and more scattered convection may move through in the 20z-23z time
frame. Otherwise, southwest flow will gust into the 20-25 knot area
through 22z-00z today. Overnight convective potential is highly in
question and will not include in this forecast. Will have to further
evaluate once this initial MCS progresses SE of the area.

For DTW...Most organized portion of ongoing MCS should remain north
of terminal, but expect some convective potential by 20z-21z or so
as, at the very least, additional storms induced by outflow from the
MCS percolate through the area for several hours. Meanwhile, wind
gusts should push 20 knots with southwest flow into early evening.

//DTW Threshold Probabilities...

* Medium in thunderstorm potential mid to late afternoon.

* Low for ceilings aob 5000 ft during the afternoon.

&&

.PREV DISCUSSION...
Issued at 1027 AM EDT Thu Jul 21 2016

UPDATE...

12z GRB sounding indicating plentiful elevated instability above
850mb that is maintaining the ongoing MCS currently crossing Lake
Michigan. Moisture transport attendant to the low-level jet has had
no problem sustaining elevated convection over WI and now over Lake
Michigan so far. Meanwhile, the 40 to 45kt forward speed of the
convective complex given the relative lack of supporting background
flow is strongly indicative of a mature coherent cold pool. With
destabilization underway over the Lower Pensinula, expect that
convection is likely to become surface-based with time and increase
in intensity as it crosses Northern and Western Lower. Subsequent
reinforcing of existing cold pool will support a threat for wind
gusts to 60 mph entering the area from west to east after noon. Lack
of stronger background flow precludes a well-defined threat for any
higher end gusts.

Supporting LLJ will not be particularly progressive, which is
forecast to confine the severe threat to areas north and west of
the Detroit Metro Area through peak heating. Uncertainty remains
rather high here though. Thus, the threat south and east of the
current watch area will be monitored through the day and will be
conditional on timing/instability as well as MCS evolution.
Likewise, a widespread atmospheric overturning across the Lower
Peninsula today would spell trouble for previous forecasts for
convection tonight. Failure to resolve the current MCS has already
rendered the NAM12 AND 4-km NAM useless for tonight. Suspect
tonight`s threat may already be in the process of diminishing
though. Will reevaluate later this afternoon.

PREV DISCUSSION...
Issued at 302 AM EDT Thu Jul 21 2016

DISCUSSION...

00z model suite has thrown an wrinkle into the forecast for the
next 24 to 48 hours. The handling of the convection leaves a lot
to be desired this morning. The ARW and NMM had the best handle of
the current convection over ND and northern MN...and dive this to
southern WI and northern IL by late this afternoon. This path of
the MCS would running out of the best inflow, moisture advection
and instability eventually as that looks be be over northern WI
and the U.P. by this afternoon. Convection hasn`t been able to
maintain itself very well either on the downstream side of the big
ridge. These solutions may develop a few storms on the northeast
arm of the outflow between 21z and 03z this evening over Lower
MI...and that would be it! Meanwhile, the GFS which has no clue
about the MCS keeps everything moving to the east with convection
across the UP and never really develops or drops it farther south
until 12z Friday when convection propagates southwestward from
southwest Ontario into Lower Michigan. 00z 12km and 4km NAM offer
some semblance of continuity with current forecast with the
Canadian and ECMWF not too far behind, but they all struggle with
the current MCS. All this leads to a focus on the larger scale
players which is the shortwave kicking off the ND and MN
convection at it`s associated height falls. The shortwave brushes
the forecast area this evening with 500 mb height falls maximized
overnight before departing by Friday. Overnight is when the best
wind fields move over the forecast area and the right rear
entrance region of the jet is over Lower MI. Will keep a similar
feel to the forecast regarding thunderstorms the chance pops for
after 20z this afternoon for areas north of M 59...best pops
overnight...and then chance pops mainly in the south half of the
forecast area mainly Friday morning.

Severe threat still looks good for this evening into the overnight
hours. Have the previously mention jet structure and height falls.
500 mb winds increase to 35 to nearly 50 kts while 850 mb winds are
30 to 40 kts. Bulk shear from 0-6 km will be around 30 in the south,
but 40 to 45 kts in the north half. ML CAPEs should be around 2000
J/kg.

The heat is the other impactful aspect of the forecast. Given the
above forecast for thunderstorms...expect mostly sunny skies for a
good chunk of today before debris clouds move in and some cumulus
develop on increasing 850 mb moisture.  Will continue to see warming
at 925 mb and 850 mb. Surface dew points in the upper 60s were just
on our doorstep.  Will continue to lean toward the warmer NAM/MET
MOS which will get heat indices approaching 100 today in the
advisory area.  Same issues for Friday with the forecast
thunderstorm coverage mainly along the Ohio border, expect at least
partly cloudy conditions and eventually become mostly sunny skies.
Will keep the low to mid 90s in the forecast with dew points solidly
in the lower 70s. Heat indices will range from 100 to 105 for the
entire forecast area.  Will expand to include the thumb given their
heat indices are 96 to 99 this afternoon and then around 100 on
Friday, which is not too different from other areas within the heat
advisory.

Saturday continues to look dry just behind a weak dew point front
and under a shortwave ridge as the big ridge splits in two. While
temperatures will again reach the lower 90s, dew points fall back
into the 60s. Some question about whether the heat advisory would
need to be extended.  Thinking we have some time to see not only how
the thunderstorms affect Thursday and Friday`s heat advisory, but
also to see how potent that dew point front will be.

Sunday could be another potential round of severe weather with
plenty of instability as temps rise to near 90 with dew points in
the lower 70s. Wind fields again increase as a decent jet moves
towards the Great Lakes. After the cold frontal passage Sunday
night, drier and cooler conditions will persist into the middle of
next week.

MARINE...

Light southerly winds will transition to moderate southwesterly
winds today and tonight in advance of a weak surface trough
advancing across the northern Great Lakes. The potential will exist,
particularly tonight, for a thunderstorm complex to develop over the
northern Great Lakes and track southward through lower Michigan
possibly including sections of Lake Michigan and Lake Huron. A weak
pressure gradient will then sustain light winds through the
remainder of the weekend. The next chance for organized thunderstorm
activity will be Sunday and Sunday night.

HYDROLOGY...

A complex of thunderstorms is forecasted to develop over the
Northern Great Lakes this afternoon. The storms may dive south
during the evening and potentially impact Southeast Michigan later
tonight. The potential exists for a basin wide average of
25 to .30 inches or greater.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until 10 PM EDT Friday FOR MIZ047>049-053>055-060>063-
     068>070-075-076-082-083.

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....DG
UPDATE.......JVC
DISCUSSION...RBP
MARINE.......CB
HYDROLOGY....CB


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