Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 171354

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
954 AM EDT Thu Aug 17 2017


Arcing line of elevated convection anchored along the lead push of
mid level theta-e advection will clear the thumb corridor within the
next couple hours. Attention now shifts upstream, as aggressive
expansion of a very moist warm conveyor belt pivots toward the
region. Focused increase in precipitation coverage ongoing as
projected by recent CAMs guidance through this corridor /Lake
Michigan into Indiana/, with a steady downstream propagation into
southeast Michigan anticipated for the early-mid afternoon period.
This timing certainly presents a murky picture in terms of possible
convective vigor moving forward today.

Ambient and inbound enviroment certainly characterized as tropical,
defined by dewpoints in the lower 70s and PW in excess of 2 inches.
This will contain lapse rate potential, while also suggesting that
cloud cover will remain problematic. The aforementioned arrival of
the upstream activity will only contribute to this issue, elevating
concerns that a larger window for better diurnal heating and
subsequent greater destabilization will never materialize. The
underlying forcing - warm frontal, weak shortwave energy and
continued moisture transport - presents higher confidence in
witnessing additional convective development/maintenance through the
afternoon/early evening period, but with low confidence in achieving
a more robust convective response. A solid deep layer wind field and
potential for a slightly backed near surface profile will continue to
give pause for possible organized, rotating updrafts...particularly
if convection can intersect the advancing warm front before the
boundary lifts northeast of the region. In addition, a water loaded
profile could also generate stronger wind gusts given the
corresponding higher cape density.

Update simply to refine precipitation chances going forward based on
latest radar and model trends.


Issued at 655 AM EDT Thu Aug 17 2017


Ongoing moisture advection along/ahead of an approaching warm front
will sustain showers and few thunderstorms across the area this
morning, possibly lasting into early afternoon. The degree of low
level moisture advection will result in steadily lowering ceilings
heights during the course of the morning, with some IFR possible in
some of the heavier showers. Se Mi will be within the warm sector
this afternoon. Ample low level moisture will likely sustain some
MVFR or low end MVFR strato cu. Building daytime instability will
result in convective redevelopment during the afternoon, likely to
impact the terminals either late in the afternoon or evening.

For DTW...Ongoing increasing elevated instability will pose a
continued chance for some thunderstorms this morning. The passage of
the warm front around 15Z will veer the winds toward the south.
Timing for afternoon and evening thunderstorms carries some degree
of uncertainty. Recent hi res guidance suggests convective
development off a sfc trough preceding the actual cold front, then
impacting metro sometime in the 21Z to 01Z time frame.


* Moderate in thunderstorms today and this evening.

* Moderate in ceilings at or below 5000 ft today. Low tonight.

Issued at 344 AM EDT Thu Aug 17 2017


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.


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.


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.


MI...Beach Hazards Statement through this evening for MIZ049.

Lake Huron...Small Craft Advisory from 10 AM to 8 PM EDT Friday for LHZ421-422.

     Small Craft Advisory until 8 PM EDT Friday for LHZ441-442.

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



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