Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 161459
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1059 AM EDT Sun Jul 16 2017

.UPDATE...

Forecast in good shape. No update planned.

Upper level low circulation/cold pool (-16 C at 500 MB) reaching the
Ontario/Quebec border this afternoon, with little extension of the
colder 500 mb temps extending southwest (-12 C per 12z DTX raob).
Still just looking at an isolated/marginal severe threat as cold
front continues to track through southeast Michigan, but by the time
MLCAPES build toward 1500 J/kg toward Ohio border, the front should
be just about clearing the CWA (20-21Z), with good drying indicating
in the 850-700 MB Theta-E/PW fields pushing in early this evening.
Thus, more than likely, if an isolated severe storm develops it will
be on Canadian or Ohio side to our southeast.

&&

.PREV DISCUSSION...
Issued at 617 AM EDT Sun Jul 16 2017

AVIATION...

Cold front will progress south through the area today. Current
observations are more pessimistic than previous guidance suggested
and current guidance seems to have latched onto that a bit. Will
trend CIGS down at least through the today period. Still looks like
skies will attempt to clear out from north to south tonight but may
hit a road block around PTK.  So gave FNT and MBS improved CIGS
tonight, with locations south holding onto MVFR possibly through the
overnight. Other issue will be possible convection as unstable air
advects in ahead of the front. Best chance will be around 18-22Z for
the Detroit area taf site, but initiation near PTK could lead to
some close activity there.

For DTW...VFR conditions this morning will give way to possible MVFR
cigs through the afternoon. TSRA chances continue ahead of the cold
front. Looking more favorable for a period of MVFR along/behind
front well into the overnight hours.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceilings aob 5kft starting early this afternoon through
  tonight.

* Low confidence thunderstorms will effect terminal this afternoon
  and evening.

PREV DISCUSSION...
Issued at 348 AM EDT Sun Jul 16 2017

DISCUSSION...

Isolated showers and thunderstorms in progress over central lower
Michigan at press time are in response to warmer and more humid air
moving into the region. Early morning surface analysis indicates
dewpoint in the mid 60s moving into the western part of the state in
southwest low level flow as a sign of the low to mid level moisture
axis leaking eastward ahead of the cold front. The parent upper level
trough will also become more supportive of shower/storm coverage as
it deepens and digs southward across the southern Great Lakes. The
added dynamic support to the vertical motion field is expected to
sustain nocturnal forcing longer through the morning. This,
combined with the weak process of moisture transport, will lead to a
more generous scattered coverage of showers with isolated
thunderstorms of ordinary intensity, as well as increasing coverage
of lower clouds along and ahead of the front through the morning.

For this afternoon, the concern is about the development and
intensity of surface based convection and severe weather potential.
The morning clouds and residual showers lasting into early afternoon
are expected to be an obstacle for development of surface based
instability which, combined with a slightly faster southward pace of
the frontal wind shift, is expected to limit the potential for
strong to severe storms in our area. Assuming the morning activity
is not continuously supported as weaker convection through the day
by the upper wave, then a cautious approach to the scenario suggests
surface based storm initiation in the 18-21Z time period this
afternoon. By then, model consensus offers a solid projection of the
frontal wind shift positioned well south of the I-69 corridor if not
to the I-96/696 corridor. Expectations then remain similar on the
bulk parameters discussed over the last few forecast cycles as a
range of mesoscale and synoptic scale models offer MLCAPE in the
neighborhood of 1700 J/kg with a unidirectional 0-6km shear profile
around 25 knots. This is plenty adequate for discrete storms at
initiation strong enough to produce near 1 inch hail. Convective
growth into multicell clusters capable of marginal damaging wind will
then be possible during late afternoon. The time it will take to
organize will likely limit coverage to near the Ohio border before
exiting south with the front toward evening.

Some post frontal showers may linger over the area near the Ohio
border during the early evening before the exit of the upper level
trough axis. After that, some boundary layer moisture is shown to
hold through the night in the cooler northerly post frontal flow.
This will result in mixed coverage of stratocu through Monday
morning before giving way to mostly sunny sky during the afternoon.
Surface high pressure will settle in through the day under strong
subsidence aloft as the upper ridge builds into the western Great
Lakes. Highs generally in the 70s will then quickly rebound back
into the 80s by Tuesday which will mark the beginning of a warming
trend that will last through mid week. The chance of rain will be
dependent on how far south the next front can reach from the
stronger westerlies over the northern Great Lakes into the broad
upper level ridge over the southern states, the fringes of which
will be over the southern Great Lakes during Wednesday. Model trends
show some erosion of heights over lower Michigan compared to the
last few forecast cycles and a better chance of the front reaching
the Ohio border by Thursday.

MARINE...

A cold front dropping through the region will cause winds to shift
from westerly to northeasterly from north to south through the day.
Models continue to advertise winds gusting to around 20 knots across
the Huron basin, with potential for funneling down Saginaw Bay to
increase these gusts closer to 25 knots. Combine that with the long
fetch of northern flow leading to wave heights in excess of 3ft
along the nearshore waters of Lake Huron, and decided to issue a
Small Craft Advisory this afternoon and evening. High pressure will
rebuild for Monday and Monday night, yielding a return to dry
weather and light winds.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this evening for MIZ049.

Lake Huron...Small Craft Advisory until midnight EDT tonight for LHZ421-422-441-
     442.

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

$$

UPDATE.......SF
AVIATION.....DRK
DISCUSSION...BT
MARINE.......DRK


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