Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 170811 CCA
AFDDTX

Area Forecast Discussion...UPDATED WATCHES/WARNINGS/ADVISORIES
National Weather Service Detroit/Pontiac MI
344 AM EDT Thu Aug 17 2017

.DISCUSSION...

A mature and stacked low pressure system over the upper MS Valley
will lift northeast into the northern Great Lakes tonight and into
Quebec by Friday night. This system will provide the focus for
showers and thunderstorms over the next two days as well as the
chance of severe weather this afternoon. A warm front will lift
north through lower MI this morning putting SE MI solidly in the
warm sector for the bulk of the afternoon and evening hours. A
prefrontal trough will push through this afternoon/evening before
the cold front sweeps through early tonight. Best chance of severe
storms will occur from about 17-02Z.

Current radar shows the eastern flank of the 850mb front lifting
into southern MI at press time. This early convection should remain
elevated but could produce some brief heavy downpours. Best frontal
forcing this morning will be well to the NW closer to the low and
the current 850mb jet is surging northward through WI and western
portions of Lake MI. This leading front will usher in the first wave
of deeper moisture with PWATs rising to nearly 2 inches. Models
advertise a second jump as the surface warm front lifts through
sending PWATs over 2.25 inches.

The setup for the afternoon will garner a bit more attention as
ingredients continue to come together for potential severe weather.
As the upper low slides north and east, region of stronger upper
level diffluence will slide overhead.  This occuring as the left
exit region of the jet noses into lower MI. Looking lower in the
column, a resurgence of the low level jet will work into lower MI
with speeds of 30-40 knots. MUCAPE is set to rise toward 1500 J/kg
while ongoing convection tries to limit surfaced based CAPES more
between 500-1000 J/kg. HIRES wind profile is keeping winds backed a
bit more to the SE increasing the chances of rotating storms and
perhaps a tornado. Low level helicity and model hodographs look
pretty good so potential is there, especially taking into account
such a moist environment and lower LCLs. Otherwise, the instability
and shear present should be enough to produce strong to severe
storms. Wind is the primary threat with heavy rainfall producing
localized flooding a possibility as well. SPC has included SE MI in
Slight Risk for severe weather this afternoon.

Chances of showers will continue into Friday as a shortwave trough
rotates around the low and through SE MI. Lingering low level
moisture and increase low level lapse rates with cooler air moving
in aloft behind the cold front should produce a scattered coverage
of showers. Broad area of surface high pressure will begin building
back north into the region Friday and looks to hold at least into
Monday as it drifts toward the Atlantic coast. Before ridging
follows suite aloft, a shortwave and strengthening vort max will
dive down through the backside of the trough on Saturday. This will
bring a chance of showers and thunderstorms during the day. The
trend is for a stronger system which will dig deeper southward into
the Ohio Valley. The result would be for decreasing precip chances
across Mid MI, possibly becoming a dry forecast. As for
temperatures, the surface ridge sliding to our southeast will open
the door for southerly return flow and a warming trend early in the
week. This will end Tuesday when the next cold front is slated to
sweep through the region.

&&

.MARINE...

The combination of low pressure lifting into Upper Michigan today
and departing high pressure to the east of the region will support
increasing southeast winds during the course of the day. An
associated warm front will lift into Lake Huron by evening, which
will only act to enhance the S-Se pressure gradient. Gusts across
the open waters of Lake Huron will top 25 knot at times today, while
the warm front provides showers and thunderstorms. Rainfall from
this convection will be quite heavy and will significantly reduce
visibility. Winds will veer to the southwest tonight as the surface
low passing across the northern Great Lakes drives an associated
cold front across the lakes. Post frontal cold air advection will
boost the over-lake instability profile, thus causing gusty winds
and hazardous small craft conditions that will persist through the
day Friday. Winds and waves will then decrease over the weekend as
the gradient relaxes.

&&

.HYDROLOGY...

A low pressure system approaching the northern Great Lakes will
drive a moisture rich atmosphere into Se Mi this morning. The
initial arrival of this air mass will be marked by the passage of a
warm front this morning, triggering showers and thunderstorms. A
moist and unstable air mass will then persist across the area
through late evening when the passage of an associated cold front.
Multiple rounds of showers and thunderstorms will be possible
between these two frontal features. With a tropical airmass in
place, thunderstorms will be capable of producing very heavy
rainfall. There is high probabilities that 24 hour rain totals,
ending Friday morning, will range from one half inch to one inch.
Locally higher amounts are expected, possibly greater than two
inches. This raises the concerns for some localized flooding today.

&&

.PREV DISCUSSION...
Issued at 1152 PM EDT Wed Aug 16 2017

AVIATION...

VFR conditions should hold much of the night. However, increasing
low level moisture in advance of a warm front will bring some light
fog potential and eventual MVFR cigs with showers/thundestorms on
Thursday morning. This stratus/strato-cu should hold much of the day
as the front slowly lifts through lower Michigan and another late
day round of scattered showers/storms possible as a trough axis
pivots into area around low pressure to the north that is forcing
this front north through the area today. East/southeast flow will
become southwest with the passage of the front with wind gusts on
the order of 20 knots or so.

For DTW...Low level moisture increases substantially late tonight
into Thursday morning with ceilings below 5kft on and off through
the balance of the forecast. Two possible waves of thunderstorms are
anticipated - one in the 12-16z window and the other during the mid-
late afternoon on Thursday. These window still seem reasonable and
first wave probably warrant an "upgrade" to TEMPO now that forecast
is inside of 9 hours from onset.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceilings at or below 5000 ft after 12z.

* Medium for thunderstorms affecting the airspace Thursday morning,
  but lower confidence for late afternoon activity at this time.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 10 AM to 8 PM EDT Friday for
LHZ421-422.

     Small Craft Advisory from 11 AM this morning to 8 PM EDT Friday for
     LHZ441-442.

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

$$

DISCUSSION...DRK
MARINE.......SC
HYDROLOGY....SC
AVIATION.....DG


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at www.weather.gov/detroit.



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