Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 201618
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1218 PM EDT Tue Aug 20 2019

.UPDATE...

Warm and muggy day underway with dew points now in the lower-70s from
the southern Metro region up to Flint with associated temperatures
ranging between the low to mid-80s. Thunderstorm complex currently
over Illinois will bring upper-level cloud debris to Michigan for all
of the afternoon and evening, but will have minimal impact on daytime
highs as warm front amplification downstream of approaching shortwave
aids in producing highs in the mid to upper-80s.

For this afternoon... Isentropic ascent and best moisture transport
will set up from southwest to northeast across the Tri-Cities into
the Thumb which will be the focal point for initial showers and
convection starting this afternoon. Less confidence to see showers
and thunderstorms in the early afternoon across the Metro region given
stronger capping between 900 - 800 mb, with the RAP suggesting a
secondary cap at 700 mb. However, given very moist airmass in place
(PW at 1.50, 850 dp at aoa 12C), cannot rule out spurious shower or
storm. As a result, have increased PoPs slightly along and north of
I-69 and decreased slightly south to account for above. Approaching
PV anomaly and surge of low-level theta-e will then bring additional
chances for showers and thunderstorms across all of SE MI for the
late afternoon and evening. MLCAPE values between 1500 - 2000 J/kg
(SPC RAP already advertising 3000 J/kg SBCAPE over Metro) with bulk
shear around 25 knots will bring the potential to see organized
convection with the marginal risk for severe weather possible across
all of SE MI. Given strong low-level lapse rates building to or in
excess of 7 C/km along with high LCLs and supportive theta-e index
profiles, isolated gusts to 60 mph will be possible with any
convection. Severe hail with any storms cannot be ruled out given
moderate mid- level lapse rates. The severe weather threat will
diminish after sunset as PV anomaly travels east of the cwa post 06Z.

&&

.PREV DISCUSSION...
Issued at 636 AM EDT Tue Aug 20 2019

AVIATION...

There is a weak sfc boundary draped across roughly the I-69
corridor. Ongoing low level moisture advection into this frontal
boundary has supported some isolated showers. The main push of low
level moisture and the lead edge of some steeper mid level lapse
rates into far SW Lower Mi early this AM has resulted in a few
thunderstorms. The potential for thunderstorms impacting the
terminals through at least early afternoon is too low to include in
the TAFs. An upper level short wave feature emerging out of the
convective complex over Iowa will track across northern Indiana this
afternoon. This system will drive a little better instability push
into Se Mi this afternoon and will support at least a scattered
coverage of thunderstorms. The most probable time period for TSRA
will be between 21Z and 02Z.

For DTW....Low level moisture will develop into a sct to bkn
afternoon cu field, based between 4k and 5k ft per model soundings,
with the onset of daytime heating. There has been increasing
probabilities for thunderstorms around metro Detroit late this
afternoon/evening. Enough confidence to introduce thunderstorms in
the TAF.

DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 feet today.

* Moderate in thunderstorms late this afternoon and evening.

PREV DISCUSSION...
Issued at 342 AM EDT Tue Aug 20 2019

DISCUSSION...

Abundant low-level moisture is present along and south of I-69 with
dew points in the mid to upper 60s, resulting in a muggy night and
morning. Just enough moisture advection into the stalled frontal
boundary across the region has been able to touch off a few showers
overnight, while radiational cooling and light to calm winds has
allowed for patches of fog to develop across the region. Today will
be a hot and humid day as the moisture continues advecting northward
along with higher temperatures. Upstream obs suggest highs in the
mid to upper 80s are in store. Heat indices in the lower 90s can be
expected along and south of the I-69 corridor.

A well-advertised MCS is taking shape over northwest Iowa this
morning and will act to enhance a shortwave in the westerly flow.
The 00z NAM, WRF-ARW, ECMWF, and RGEM have moved toward the GFS
solution and trended southward with this wave/MCV, tracking it
either along the MI/OH/IN border or just south by this evening. A
modest 10-20 kt LLJ ahead of this will introduce a surge of unstable
air into SE Michigan by the early afternoon. Appears there will be
enough lift via PVA and isentropic ascent during the afternoon to
result in isolated to scattered showers and storms. Model soundings
depict weak midlevel subsidence producing a modest cap around 700mb
which will limit some of the instability early in the day, but
impressive midlevel lapse rates of around 7 C/km will contribute to
CAPE of 1250-2000 J/kg by the late afternoon. Bulk shear on the
order of 20-25 kt and this amount of instability will allow for some
isolated stronger to possibly severe storms in a 2-9pm window. The
best chance for any stronger storms will be generally south of
I-96/696 where the greatest amount of instability resides, but the
threat will spread north if the instability can march farther north.

The instability will linger across the area with a slight chance of
showers and storms continuing tonight. A potent upper trough will
swing through the northern Great Lakes tomorrow afternoon/evening,
pushing a cold front across the state during the afternoon and early
evening. This could provide a focus for more scattered showers and
storms, but soundings look relatively dry and the front may not have
much to work with. Additionally, forcing from the upper wave and jet
streak will be delayed/displaced over the upper peninsula. The front
will however be very effective at delivering a much cooler and drier
air mass, with 850 mb temps falling from the mid teens to the mid
single digits C by Thursday morning. Strong subsidence will ensue
with a surface high drifting across the region during the late week
and weekend. Dry weather, highs in the 70s, and lows in the 50s can
be expected. A warming trend will commence by Sunday with return
flow bringing warmer air back north. The next chance for rain holds
off until the early week when longwave troughing begins to expand
across the central CONUS.

MARINE...

High pressure will depart the region to the east today. The gradient
will still remain weak across the Great Lakes, so south to southeast
winds in the wake of the high will be light. A cold front will drop
south across the region on Wednesday. The gradient along the front
will remain weak, so overall winds will be light. The passage of the
front will be marked by a veering of the winds to the northwest. A
push of cooler post frontal air will then arrive across Lake Huron
late Wednesday and will result in an increase in the northwest
winds. A strengthening north-northwest gradient within a push of
colder low level air will affect all of the lakes Wed night. Winds
and waves will decrease slightly late Thursday into Friday as high
pressure nudges into the region from the northwest.

&&

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

$$

UPDATE.......AM
AVIATION.....SC
DISCUSSION...TF
MARINE.......SC


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