Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 150744
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
344 AM EDT Tue Aug 15 2017

.DISCUSSION...

Overall the coming week seems pretty pleasant but there are a few
periods of potential thunderstorms to note. For the most part, the
pattern changes little from what we`ve been dealing with for weeks
now: a persistent longwave trough positioned over the region with an
upper low centered near Hudson Bay. This keeps SE MI on the cooler
side of the main baroclinic zone with periodic shortwaves passing
over the state in the northwest or westerly flow around the trough.
With only slight modifications to the resident airmass, look for
high temps to remain in the upper 70s- mid 80 range.

First chance of precipitation comes right off the bat this morning
as a mid level shortwave over northern Lake MI swings through
southern MI with a weak cold front in conjunction with the right
entrance region of the jet. IR satellite shows an area of showers
with the shortwave entering northern MI expanding southward while
making its push inland. This should phase with a second area of
showers over southern Lake MI further west along the front. This
should be mostly showers but models do show some thin CAPE so could
see some thunder with the activity. The front will push south of the
area around 18Z this afternoon bringing an end to the showers.

High pressure will then drop in from the north tonight with building
heights aloft as shortwave ridging amplifies in advance of the next
system developing over the plains. The trend has been to strengthen
the trough over the plains resulting in stronger ridging over the
Great Lakes. This will also attempt to steer the system northeast
through WI with the warm front now holding off til closer to
Thursday morning, keeping the area dry through the day and most of
the overnight. This orientation of the arcing front is tough because
convection usually shows up early over the west side of the state but
the northward storm motion keeps it out of SE MI for several more
hours. Models are showing the tight theta e gradient on the lead
edge of a very moist airmass lifting into the area starting around
09Z Thursday morning. We then reside in the warm sector through the
day until the cold front sweeps through after 00Z. The nose of the
upper level jet is forcing this front north with a good signal that
a strong 850mb jet will be right there with it. Good sw flow will
yield bulk shear values around 30 knots through the day with roughly
500-750 J/kg of CAPE. SPC has included us in Marginal Risk for severe
weather Thursday which seems fitting considering the cold frontal
timing, jet forcing, and CAPE/Shear values promotion some organized
storms. Other thing to note with this system is the amount of
moisture. Models are now showing surface dewpoints near 70F with
PWATs around 2 inches. This will lead heavy rainfall with any storms
that do develop.

Friday into the weekend looks pretty decent as surface high pressure
builds back into the region with shortwave ridge aloft. Models are
hinting at a midlevel shortwave now trying to sweep through the
Great Lakes on Saturday. They are also torn as to whether the wave
will be strong enough to beat out the ridge to result in showers, or
will it just pass over as an enhanced cloud layer. Included a slight
chance pop mention just to show it exists and is something to keep
an eye on.

&&

.MARINE...

Weak low pressure tracking across southern Lower Michigan and high
pressure building in from the north will lead to the development of
light northerly winds across Lake Huron today. These winds will veer
to the east-northeast by Wednesday as the high moves to the eastern
Great Lakes. Low pressure is forecast to lift into the northern
Great Lakes on Thursday, sending an associated warm front into the
region. A tightening southeast gradient to the north of this front
will lead to increasing winds on lakes, with gusts possibly topping
20 knots. The approach of this front will also bring a good chance
of thunderstorms to the region. The low is forecast to slowly pass
across Lake Huron on Friday. Gusty west-northwest winds are then
possible in the wake of this system Friday night into Saturday.

&&

.HYDROLOGY...

A warm front will lift across the area on Thursday. This system will
drive tropical moisture into the region and will support showers and
thunderstorms. Widespread rainfall totals between a quarter and
three quarters of an inch are expected. Locally higher amounts are
highly probable in thunderstorms considering the high level of
moisture that will be present over a considerable depth of the
atmosphere.

&&

.PREV DISCUSSION...
Issued at 1203 AM EDT Tue Aug 15 2017

AVIATION...

Surface pressure analysis over the Midwest and western Great Lakes
indicates the next wave of low pressure on the way into Lower
Michigan during the night through late morning. Expect coverage of
showers to increase over the MBS, FNT, and PTK terminals during the
late night through early morning possibly reaching DTW toward
sunrise through mid morning. Thunderstorms are possible but coverage
will be low compared to the showers. The surface low is then
projected to move through the area near the Ohio border presenting
the best chance of lower stratus coverage up to about PTK during the
morning, which is currently expected to be borderline VFR/MVFR
ceiling. A decreasing cloud trend and increasing northeast surface
wind will follow the exit of the system during the afternoon into
Tuesday evening.

For DTW... Showers are expected to redevelop over western Lower
Michigan later in the night. Some of this activity is expected to
brush the terminal toward sunrise through mid morning. VFR ceiling
less than 5000 ft is then likely as low pressure slides by just
south of DTW during the morning through early afternoon.

//DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling below 5000 ft toward sunrise through
  afternoon.

* Low for thunderstorms through late morning.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...DRK
MARINE.......SC
HYDROLOGY....SC
AVIATION.....BT


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at www.weather.gov/detroit.



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