Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 110040
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
840 PM EDT Thu Aug 10 2017

.UPDATE...

Convection entering Lower MI in response to nice shortwave over
southwest Upper and northeast WI, but also along the leading edge of
good moisture advection in the 850-700mb layer. This convection is
more surface based with the diurnal cycle and will diminish if not
dissipate around midnight just west of the forecast area as it moves
into a more stable and drier air mass. But then nocturnal features
take over. That moisture advection will continue to advance as
elevated instability develops and moves over the forecast area. Lapse
rates lifting from around 950 to 900 mbs could be as steep as 7C/km
for a decent depth. Expect scattered showers and storms to redevelop
on this moisture surge first around 07-08z in the Tri Cities and by
11-12z for metro Detroit. Therefore, added thunder to tonight`s
forecast and increased the POPs into a high chance category.

These showers and storms should only last for a couple of hours at
any one location. Subsidence in the wake of the early morning
convection should also dampen out most of the chances after about
13z until the diurnal cycle can get going again during Friday
afternoon. Nearly all of the last few hi-res models and ensembles
are moving strongly to this scenario giving additional confidence.

&&

.PREV DISCUSSION...
Issued at 712 PM EDT Thu Aug 10 2017

AVIATION...

Convection over WI and IL was moving into Lake Michigan and will
continue to weaken and diminish before reaching southeast Lower MI.
This convection is in response to the upper wave over the U.P. and
WI along with decent moisture advection in the low and mid levels of
the atmosphere.  Through 06-08z expect just debris clouds above 12k
feet. Then with that moisture advection working over southeast
Lower, expect broken ceilings from 4000 to 6000 feet. Hi-res model
runs all suggest that this moisture surge will bring a couple of
hours of showers around the area from 08z to 12z and thus added that
to the TAFs.

Should be quiet during the Friday morning hours as a broken cumulus
build for the day around 5000 feet. Thunderstorms will develop after
18z in response to that upper wave working through Lower Michigan
and the associated cold front that will very slowly work through the
region. At this time coverage looks scattered and will continue the
PROB30 in the TAF.

//DTW THRESHOLD PROBABILITIES...

* Low for ceilings at or below 5000 feet late tonight through early
  Friday morning. Medium after 15Z.

* Low for thunderstorms after 18Z Friday.

PREV DISCUSSION...
Issued at 321 PM EDT Thu Aug 10 2017

DISCUSSION...
Mostly sunny skies today and for the rest of the afternoon due in
part to very dry low and mid levels as seen on 12z dtx sounding with
pws 0.75 and dewpoint spreads over 20 degrees at surface.  This and
800mb cap limiting much cu development.  There is a bit more thin
high clouds crossing the northern counties from dissipated showers
earlier to the west.  Temperatures most locations in the 80s and
will have pretty much a repeat of last evening.

Tonight and Friday, weak mid level wave and surface low centered
over northern Wisconsin and an attendant cold front will drift
eastward across Michigan and exit to the east early Friday night.
Moisture, decent instability and shear is all lacking with this whole
system. Showers and thunderstorms are finally developing this
afternoon aided by diurnal heating across Wisconsin. As this activity
moves eastward it will weaken/dissipate by the time it reaches the
far western portion of the forecast area around or after midnight.
Will slowly spread small chance for showers tonight across the entire
area by morning as a few showers may continue with increasing theta-e
and weak isentropic forcing.

The front and wave axis will take the whole day Friday and maybe
early Friday night to cross and exit to the east.  As diurnal
heating commences, shower coverage will gradually increase
throughout the day with enough instability achieved in the afternoon
to over come weak mid level cap and produce some thunderstorms. Any
storms will not be severe as mentioned earlier, instability and shear
are very weak and moisture limited with pws around 1.0 inch. Any
shower activity Friday night will occur along the far eastern
forecast area and end by midnight followed by plenty of cloud cover.

Saturday low level moisture will lead to plenty of diurnal cu.  A
secondary cold front will drop southward in the afternoon triggering
some scattered showers down to about the Detroit Metro Area before
daytime heating wanes. Highs Saturday will only make it to the ;ow
70s in the Thumb with earlier passage of the front to nr 80 along
the Ohio border. High pressure building in will bring clear skies
Saturday night and cool temperatures again in the 50s.

High pressure is projected to be more firmly in control of weather
conditions around SE Michigan during Sunday. This will ensure a dry
finish to the weekend with temperatures near normal values in the
mid 70s to around 80. Otherwise, global model solutions remain
locked in on the large scale blocking pattern in the upper level
flow over North America for the first half of next week. A short
wave system embedded in the large scale upper trough over eastern
Canada is shown to move through the Great Lakes during Monday. The
feature could generate some high based light rain showers over Lower
Michigan while the bulk of significant rainfall will be to our south
where the primary surface low pressure reflection and frontal zone
will reside. Model agreement is better on the location, strength,
and timing of this system in the latest NWP cycle moving it east of
the region by Tuesday. The upper level ridge within the omega block
is then expected to drift into the Midwest and support a prolonged
period of dry weather with temperatures on the warm side of normal
for the mid week period.

MARINE...

Weak low pressure will slowly track across northern Michigan tonight
and Friday. Showers and a few thunderstorms will be possible
tonight, mostly over Lake Huron, with scattered to numerous showers
and thunderstorms then developing on Friday as a cold front works
across the region. Winds will be southerly ahead of the system
tonight into Friday, then flip to the northwest behind the cold
front late during the evening and overnight. Sustained wind speeds
should remain at 15 knots or less through Friday night.

&&

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

$$

UPDATE.......RBP
AVIATION.....RBP
DISCUSSION...DRC/BT
MARINE.......HLO


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