Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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817
FXUS63 KDTX 211157
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
757 AM EDT THU JUL 21 2016


.AVIATION...

High pressure will maintain its grip over southeastern Michigan
throughout much of the day. Cirrus debris from upstream convection
is the expected to overspread the region today along with some
spotty cumulus development. The main item that is being monitored
are convective trends and what sort of potential will exist for an
MCS to impact lower Michigan later tonight. As an introduction,
included a prob30 group for thunderstorms the latter half of
tonight.

For DTW... Southwest winds with high confidence in wind gusts
reaching between 15 and 20 knots. Introduced thunderstorm mention
in tafs with a Prob30 group at DTW for 8-11Z.

//DTW Threshold Probabilities...

* Low for ceiling 5000 ft or less during 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 from 1 PM this afternoon to 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.....CB
DISCUSSION...RBP
MARINE.......CB
HYDROLOGY....CB


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