Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 230142
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
942 PM EDT Thu Jun 22 2017

.UPDATE...

It has been an active evening as cluster of severe thunderstorms
lifted northward through a good portion of the southern Detroit metro
area. Initial cell appeared to get mid to upper level support by
gravity wave forcing to the south of Toledo. The thunderstorm quickly
latched onto the main 850-700mb gradient of tropical moisture and
provided the perturbation to drive additional thunderstorm activity
northward along a mature outflow boundary. Overall, hi-resolution
models have done a poor job resolving this initial thunderstorm
activity within the broad anticyclonic ridging and confidence remains
lower than average. With that stated the signal points to activity
migrating northward throughout the remainder of the evening as the
main equivalent potential temperature gradient lifts northward and
merges with the synoptic scale cold front now over sections of
northern Lower Michigan.

Uncertainty remains high with specifics, but expectations are for
shower and thunderstorm activity to become more organized over the
northern cwa, north of I 69 primarily between 02-09Z tonight.
Activity should then transition south again, south of the I 69
corridor between 09-14Z Friday morning. Severe threat appears to be
waning as model soundings show loss of diurnal heating significantly
compromising lower level lapse rates. One factors in current
activity overturning the air mass, and confidence increases that high
static stability will exist through a deep layer between 2.0 and 9.0
kft agl. Current SPC RAP based shows just a pocket of MLCAPES of
between 1000-1250 J/KG where current thunderstorms reside. Expecting
this instability to lessen considerably during the next hour or two
to around 500 J/KG for the course of the overnight. Deep layer shear,
particularly closer to the front across the north will remain good
enough for storm organization so will continue to monitor. However,
signs point to the severe threat lowering considerably. The main item
to watch for is heavy rain potential with training of elevated
thunderstorm activity. Right now it appears best potential for this
heavy rainfall will be north of I 69.

Inherited grids are in reasonably good shape and little changes are
anticipated.

&&

.PREV DISCUSSION...
Issued at 811 PM EDT Thu Jun 22 2017

AVIATION...

Cluster of severe thunderstorm activity lifting northward through
Detroit is being forced along the lead gradient of 850-700mb
tropical moisture. The thunderstorm activity will continue for the
first hour or two at Detroit terminals before lifting northward with
time tonight. There is a high amount of uncertainty with how tonight
will play out. Hi resolution guidance this afternoon missed
completely on this warm sector thunderstorm development within
overwhelmingly anticyclonic flow trajectories. The idea right now is
that tstorm activity should transition northward to possibly between
the I 69 corridor to Tri Cities sometime after 02-03Z this evening.
This activity should then continue (expected to be elevated and
weaker) throughout much of the night along a slowly sagging cold
front.

Shower and tstorm potential will then pick up towards daybreak again
around the Detroit Metro area before transitioning southward over
portions of Ohio by late morning Friday.

//DTW THRESHOLD PROBABILITIES...

* High for thundestorms early at start of period, low for much of
  tonight and medium for 09-13Z Friday.

* High confidence in ceilings aob 5kft tonight into Friday Morning.

PREV DISCUSSION...
Issued at 353 PM EDT Thu Jun 22 2017

DISCUSSION...

Warm and humid southwest flow will be firmly established over lower
Michigan during late afternoon through tonight. Temperatures will
come down from highs near 90 and heat index in the mid 90s but it
will remain warm and muggy through the night. This will leave
convective trends as the main concern beginning with scattered late
day/early evening development followed by storms arriving from the
Midwest frontal system during the night into Friday morning. Passage
of the front through SE Michigan by early to mid Friday afternoon
will then bring cooler and less humid air into the region for the
weekend.

Afternoon satellite imagery and hourly mesoanalysis/surface
observations indicate warmer and more humid air moving in through
the boundary layer in southwest flow over the region. This is
occuring under a relative theta-e minimum that is a result of both
warmer air moving in aloft that is not quite as moist and possibly
some subsidence trailing morning convective activity. The warmer air
is at least temporarily capping off additional shower/storm
development through peak heating. There is also observational
evidence of deeper moisture moving in from the Ohio valley
associated with the northern fringe of TD Cindy. It is this moisture
surge that is shown to produce late afternoon/early evening
convection over northern OH/IN developing into lower Michigan during
the evening. This activity will occur ahead of the convection
upstream over the Midwest that is being forced along and ahead of
the cold front/outflow combination and which is expected to progress
eastward into SE Michigan during the night. Both rounds of storms
will share a similar near storm environment. Model derived ML CAPE
and shear projections best supported by observations are in the
range of 1000-1500 J/kg along with marginal bulk shear around 30
knots. This combination will be capable of some multicell structures
with otherwise moderate mid level wind speed that could produce some
near severe wind gusts in line with the borderline slight/marginal
SPC outlook that covers SE Michigan. Locally heavy rainfall will
likely turn out to be the main concern as PW ramps up to near 2
inches overnight. Individual convective cells will be moving quickly
enough but there will be a tendency for some training lines that
will have localized potential for flooding of prone areas.

Numerous to widespread showers/storms that spread into the region
overnight will tend to break up as they will be ahead of the surface
front and upper level wave toward Friday morning. As this convection
continues to dissipate during the morning, there will be another
narrow window of opportunity for new development along the cold
front before peak heating and before it boundary exits SE Michigan
during early to mid afternoon. Passage of the front will then open
the door on cooler and less humid air Friday night into Saturday.
Some weak instability showers will be possible Saturday afternoon
and evening but the large scale pattern does not look very
supportive. Neutral southwest flow aloft downstream of the main
upper trough and a featureless westerly low level flow might support
isolated to scattered coverage of light showers during peak heating.

The region will remain in longwave troughing to close out the
weekend and into the beginning of next week.  This will allow for
chances for unsettled weather through this time period before things
start to dry out by the middle of the week.  Longwave ridging starts
to build in on Tuesday as surface high pressure takes control over
the Great Lakes region.  As far as temperatures are concerned, highs
will be noticeably cooler on Sunday and Monday as highs hold around
70 degrees.  Temps then rebound back towards the upper 70s though
the end of the extended forecast period.

MARINE...

Warm and moist southerly flow this afternoon, but topping out around
20 knots for the most part. Scattered showers and thunderstorms
around late today and this evening, with more solid line of showers
and embedded thunderstorms later tonight with cold front, which will
pass through the Central Great Lakes tomorrow. Winds will shift to
the west-northwest behind the front and remain from that general
direction through the weekend with wind speeds under 20 knots. An
unseasonably cold airmass this weekend will bring the chance for
light showers across most of Lake Huron.

HYDROLOGY...

An influx of moisture will trigger scattered showers and
thunderstorms late today and this evening, with more widespread
shower and thunder activity with cold front late tonight into early
tomorrow morning. All told, localized rainfall amounts may reach an
inch or more, especially from around Flint on north into the Saginaw
Valley, where storms expected to arrive earlier and are able to
utilize the waning daytime instability. Urban and small stream
flooding will be possible with the stronger activity.

&&

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

$$

UPDATE.......CB
AVIATION.....CB
DISCUSSION...BT/SP
MARINE.......SF
HYDROLOGY....SF


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at www.weather.gov/detroit.



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