Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 220357
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1157 PM EDT THU JUL 21 2016

.AVIATION...

A stray shower or thunderstorm remains possible during the night
into the morning hours. So far, radar is quiet over lower Michigan
and this will be monitored before adding any short term activity
into the TAF. Conditions will also be monitored for MVFR fog and
ceiling overnight through Friday morning. Widespread ground moisture
and recovering surface dewpoint will favor fog development toward
sunrise and there is loose evidence in model data for stratus
development along and ahead of the weak front settling through lower
Michigan. Should this develop, then it will follow a typical diurnal
upward trend through afternoon. This front will be weak but capable
of supporting a few thunderstorms by mid to late afternoon into
Friday evening, however timing and coverage still need refinement.

For DTW... VFR will be in place through late tonight with only
cirrostratus debris from the CHI area MCS. Confidence is moderate
for MVFR fog around sunrise through mid morning after earlier
rainfall and recovering humidity. There could also be some stratus
but southward extent from northern lower is in question at this
forecast issuance.

//DTW Threshold Probabilities...

* Low for ceiling 5000 ft or less during the morning. Moderate
  during afternoon.

* Low for thunderstorm potential during afternoon into Friday
  evening.

&&

.PREV DISCUSSION...
Issued at 943 PM EDT Thu Jul 21 2016

UPDATE...

Mid level subsidence and meso-high pressure settled over SE
Michigan in the wake of the earlier convective line and is
expected to hold at least through the evening and likely well into the
overnight hours. The DTX evening sounding exhibits evidence of
this with a pronounced cap around 850 mb but also steeper lapse
rates 850-700 mb which suggests some elevated convective
potential will linger through the night, especially as mid level
moisture leaks back into lower Michigan around the north flank of
the WI/IL MCS and ahead of the weak front settling through the
northern Great Lakes. Plan to maintain a chance of ordinary
thunderstorms overnight into Friday morning as nocturnal/elevated
moisture transport could release the pre-existing potential instability
aloft.

The other element of note is the heat advisory. Current conditions
are below advisory criteria in the wake of the earlier storms.
Expect dewpoint will recover on steady southwest wind through the
night and set the stage for a run at heat index touching 100
Friday afternoon. The only adjustment here will be to freshen the
wording and focus the headline on the Friday afternoon conditions.

PREV DISCUSSION...
Issued at 342 PM EDT Thu Jul 21 2016

DISCUSSION...

Line of thunderstorms will bring brief widespread wind gusts of 30
to 40 kt this evening as they propagate east along the lead edge of
the elevated instability gradient. Surface-based component exists as
evidenced by consistent wind gust production, but 850-700mb cap
evident on 18z DTX sounding is really causing the line to struggle.
Nonetheless, it continues to march east and will likely bring a
round of tstorms of moderate wind gusts to nearly all locations.
Marginally severe gusts still cannot be ruled out as the maximally
destabilized portion of the CWA still lies ahead.

Attention then turns to convective potential tonight, the forecast
for which has been upended a bit by today`s MCS-that-could. The next
round of thunderstorms has begun to develop over western Wisconsin
along the north edge of the instability gradient. These storms will
try to build east as the gradient allows them to slowly sag south
this afternoon, but the further east they build they closer they
will become in a temporal sense to the recently overturned
environment over Lower MI. The forecast plays this strongly and
really attempts to limit eastward potential with highest chc pops
west of US-23. Steeper lapse rates and LLJ feed will nonetheless
fold into the area warranting chc pops over most of the area through
tonight. Diminished instability and potential for remnant subsidence
wake/diabatic heating bubble cast doubt on severe potential.
Potential for upscale organization diving south along the deep layer
thermal gradient still exists, but it appears it will be largely
confined to upstream locations within the corridor of optimal
surface based instability. Not out of the question that a severe
complex impinges on western areas, but high end/widespread severe
threat is now diminished.

Hot/humid airmass remains in place through Friday. Additional
convectively enhanced ripples in the flow will provide a potential
focus for forcing conditional on tonight`s convective evolution. The
preferred scenario is that sufficient convection develops tonight to
shunt the bulk of instability south for Friday. In the event that
convection severely underachieves over WI/Western Lower, a marginal
severe threat will persist along the instability gradient through
Friday. Otherwise, expect the main character of the day to be
similarly humid but noticeably hotter given an expected lack of
convection. Will carry heat advisory onward given heat index
forecasts of around 100 degrees. Instability gradient and associated
moisture axis sag south Saturday. Deeper mixing that results may
provide the hottest day temperature-wise, but diminished humidity
will confine heat indices closer to the ambient temperature.

SUNDAY THROUGH FRIDAY: Late Monday/early Sunday, an area of low
pressure centered over western Ontario, will generate another round
of hot and muggy conditions and a potential for showers and
thunderstorms, as the associated weather front moves through
southeast Michigan on Sunday/Monday. High pressure builds over the
region by Monday afternoon and persists through Wednesday. Another
wave of low pressure is expected for Thursday/Friday.

MARINE...

Moderate southwest flow will persist into tonight in advance of
surface trough. Scattered to broken convection will also expand
across Lake Huron this afternoon and eventually into southernmost
waters late afternoon and early this evening as an MCS crosses the
central Great Lakes. There may be a few showers or storms along
aforementioned surface trough overnight as well with some of this
activity perhaps lingering into Friday, generally south of Lake
Huron.

A weak pressure gradient will then maintain light winds through the
remainder of the weekend. The next chance for organized thunderstorm
activity will come Sunday into Sunday night as a low pressure system
crosses through the northern Great Lakes and pushes a cold front
east through the area.

HYDROLOGY...

A complex of thunderstorms will bring locally heavy rainfall to the
area from late afternoon into early this evening. Basin wide average
rainfall of one quarter to locally one half an inch can be expected
over much of the area before these storms weaken and progress east
and southeast.

While additional isolated to scattered thunderstorms will be
possible later tonight into Friday, the next chance of widespread
thunderstorms will come Sunday into Sunday night as a low pressure
system and associated cold front cross the region. Average rainfall
in the one quarter to one half inch range seems reasonable with this
system as well.

&&

.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.....BT
UPDATE.......BT
DISCUSSION...JVC/DE
MARINE.......Mann
HYDROLOGY....Mann


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