Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 221337

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
937 AM EDT Tue Aug 22 2017


A forecast update was issued simply to adjust for recent precip
trends. Widespread rain has overspread almost the entire forecast
area this morning as good upper level divergence within the entrance
region of an upper jet has interacted with the deep moist plume over
the area. This region of widespread light rain will decrease over the
next few hours as this upper support wanes, with forcing then
transitioning to boundary layer convergence associated with the main
surface cold front now moving into central Lower Mi and a prefrontal
trough already igniting some convective cells across the Saginaw
Valley. The lack of instability will remain a large limiting factor
for the coverage of thunderstorms and their severity. As the morning
showers and cloud cover undergoes some degree of diminishing
coverage this afternoon, there is a potential for diurnal heating to
boost temps a bit. This will offer some weak sfc based instability
across the far eastern portions of the area (Port Huron down through
metro Detroit and Monroe), giving at least a good chance for some
afternoon thunderstorms. A significant push of dry air will follow
the passage of the cold front, ending rain chances from northwest to
southeast across the forecast area during the course of the


Issued at 705 AM EDT Tue Aug 22 2017


Showers and thunderstorms will continue to expand over SE Michigan
during the morning as moisture surges northward ahead of a strong
cold front. MVFR ceiling will fill in through the warm sector and be
mixed with IFR visibility restriction in heavier showers and storms
which will persist into the afternoon along and ahead of the front.
The latest timing on the frontal passage is about 17Z at MBS to 22Z
at DTW. Southwest wind gusting near 25 knots will turn northwest
behind the front followed by a rapid clearing trend during the
evening. The incoming air will be chilly enough for some lake
induced stratocu to form overnight but the northwest wind direction
will keep coverage scattered compared to upstream areas through
sunrise Wednesday.

For DTW... Borderline MVFR/IFR restriction in showers and storms
will hamper operations during the morning through mid afternoon.
Southwest wind will be gusty near 25 knots but will at least permit
favorable southwest traffic flow operations.


* Medium for thunderstorms today.

* High for ceiling 5000 feet or less today.

Issued at 342 AM EDT Tue Aug 22 2017


Overall picture currently shows the placement of the surface low
over the northern Great Lakes as it continues to trek eastward.  The
cold front associated with this system was laid out across WI and
back into IA. This front will continue to slide east and will be the
focus for rain/storms to Southeast Michigan this morning into the
afternoon. A line of storms over southwest Michigan at issuance time
will continue to push east/northeast through the morning hours and
will provide another round of storms to the area around sunrise.
Behind this initial line, a more expansive coverage area of
rain/storms moves in.  Area continues to be outlooked in general
thunder for the day.  A weakening trend is currently be seen with
the initial line of storms as they push across lower Michigan.

Working against the severe threat will be the timing of the front.
The bulk of the activity will be during the morning/early afternoon
hours, thus cutting into any increasing instability associated with
diurnal heating as skies remain overcast.  Better instability will
stay to the south and east of the state which coincides with SPC`s
outlook of general thunder over much of the CWA and the slight risk
off to the southeast. Dynamics are still there given the front
itself and the area will continue to see rain and thunderstorms
during this time with the main threats being heavy rainfall and
gusty winds in the moisture rich environment.

Rain and isolated storms will move into portions of the CWA in the
next couple of hours.  Current radar trend shows this line with a
break in the rain before activity increases again just ahead of the
front.  Main frontal passage will take place later this afternoon
with activity pushing off towards the east by 00Z Wednesday.  CAA
behind the front will bring in cooler and drier conditions this
evening, bringing an end to the humidity that has been around the
past few days.  Temperatures will be more pleasant for the remainder
of the week as highs stay in the lower to mid 70s with lower
dewpoints in the upper 40 to low 50 degree range.  As the gradient
weakens and troughing pushes east towards the East Coast, high
pressure starts building in.  This high will set up across the Great
Lakes area bringing quiet weather conditions to the area through the


Moderate southwest wind will continue over marine areas during the
morning ahead of a strengthening low pressure system and cold front.
Early morning observations over the southern Great Lakes indicate
wind conditions holding below SCA thresholds leaving widespread
showers and thunderstorms along and ahead of the front as the
primary marine weather impact before the frontal passage. Saginaw
Bay will be an area to watch for funneling of enhanced southwest
flow that could produce gusts around 25 knots at times.

Low pressure near the Straits during the morning will move toward
James Bay and deepen considerably during the afternoon based on the
latest model depictions. The stronger trend on this system now makes
northwest gales likely over the open waters of northern Lake Huron
beginning late afternoon through tonight and a gale warning is in
effect during this time. Waves exceeding SCA threshold will brush
the northern Thumb tonight where a SCA is in effect. Elsewhere, the
wind pattern will be from the northwest enough to keep the highest
waves offshore of the southern Lake Huron nearshore zones and
generally in the 1 to 3 ft range on Saginaw Bay before the wind
weakens Wednesday and then veers toward the north Thursday. As the
wind diminishes, the incoming air mass will be cold enough to
produce instability over the water adequate for marginal waterspout


A low pressure system near the Straits during the morning will move
into Canada while pulling a strong cold front through Lower Michigan
today. Warm and humid air in place ahead of the front will fuel
showers and thunderstorms beginning during the morning and lasting
into early evening. Basin average rainfall totals remain projected
in the 0.25 to 0.50 inch range today. Locally higher amounts of 1 to
2 inches are possible where lines of storms or multiple rounds of
thunderstorms occur which will make localized flooding possible in
prone areas. Otherwise, flooding potential will be limited by the
fast movement of the associated cold front which is timed to exit
eastward by early evening.


Lake Huron...Gale Warning from 5 PM this afternoon to 5 AM EDT Wednesday for

     Small Craft Advisory until 5 PM EDT this afternoon for LHZ421-422.

     Small Craft Advisory until 5 AM EDT Wednesday for LHZ441.

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



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