Flash Flood and Flood in Michigan, August 15 to 16, 2020

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
703 PM EDT Sat Aug 15 2020

.AVIATION...

A modestly unstable environment will maintain a low potential for
thunderstorms through the late evening hours. Greater potential will
generally remain west of the Southeast Michigan airspace, with
confidence in occurrence too low to highlight for this particularly
window. Otherwise, VFR conditions with pockets of higher based cloud
cover entering the early portion of the night. Cold front will lift
across the area during the mid-late morning hours. This will be
accompanied by some reduction in ceiling heights and a secondary
window for showers and thunderstorm development. Potential for MVFR
level restrictions at times. Confidence in coverage of thunderstorms
remains quite low given the typically less favorable morning timing.
Gradual clearing from northwest to southeast from late morning into
afternoon Monday.

For DTW...There remains a low potential for thunderstorms late this
evening, but recent radar and satellite trends suggest greater
potential will reside west of the terminal. Main window for possible
thunderstorms centered 12z-15z late Monday morning. Increasing
likelihood for ceiling height to fall below 5000 ft during this
time.

.DTW THRESHOLD PROBABILITIES...

* Medium for cigs aob 5000 ft late tonight, then high after 12z
  Sunday morning into early afternoon.

* Low for thunderstorms impacting the terminal this evening and
  late Sunday morning.

&&

.PREV DISCUSSION...
Issued at 328 PM EDT Sat Aug 15 2020

DISCUSSION...

Inbound mid level height falls associated with an amplified long
wave trough moving across the northern plains/upper midwest are now
slowly advancing across the western Great Lakes. Water vapor imagery
shows an axis of deep layer moisture along the lead edge of the
forcing. An embedded short wave impulse has been lifting more toward
the northwest across Nrn Michigan, leaving a very slow eastward push
to the moist axis across srn Lake Mi. A mid/upper gravity wave
passed through Lower Mi earlier today. The subsidence in its wake has
thus far suppressed convection across Se Mi. Building late day
instability with low level convergence may still support some degree
of convective development over Se Mi this evening, with recent trends
suggesting it will be more widely scattered.

During the night there will be an amplification of the mid level
trough over the western Great Lakes in response to a seasonally
strong upper jet max sliding from the Upper Midwest into the western
Great Lakes. The upper jet dynamics and mid level positive pv
advection will move into Se Mi late tonight through early Sun
afternoon. This forcing will interact with the deep layer moist axis
to the west as it advances across Se Mi, thereby providing a high
chance for showers late tonight into Sunday morning. Mid level lapse
rates around 6C/km will contribute to weak to moderate elevated
instability, with 0-3km MU Cape of 1000 to 1500 J/kg, so a few
thunderstorms will also occur. Cloud cover and showers will limit
sfc based instability Sun morning. Latest hi res guidance suggests
the actual sfc front will not exit east of Port Huron until later
Sun afternoon. This will offer an opportunity for diurnal heating to
build some daytime instability (mainly across the far eastern
portions of the area) prior to a more aggressive push of dryer air
from the west. With deep layer shear near 30 knots, an isolated
strong to severe storm Sun afternoon can not be ruled out. Early day
clouds/showers will temper afternoon highs to the upper 70s/low 80s.

Additional amplification of the long wave trough across nrn Ontario
and the nrn Great Lakes on Monday with another seasonaly strong jet
max diving into the Great Lakes will drive a secondary cold front
into Se Mi Mon afternoon. There have been a few model solutions that
hold sfc dewpoints into the low 60s along this feature, thus
supporting weak instability. Model sounding analysis suggests diurnal
mixing will reduce sfc dewpoints to the low 50s, limiting or
eliminate deep convection. The exception may be across the warm
waters of southern Lake Huron. So nothing more than isolated showers
Mon afternoon seems warranted at this time. Diurnal mixing will also
support highs around 80. There will be a little better push of cold
air in the wake of this secondary front, with 850mb temps forecast
to drop to +8 to +9C by Tuesday. This will result in seasonally cool
conditions lasting into the mid week period.

Model solutions diverge a bit with respect to the amplitude of a
secondary northern stream wave forecast to rotate across the region
Tues night into Wednesday. The GFS is stronger and thus advects a
little better moisture/instability into Se MI on Wednesday along a
associated cold front. While the Euro is much weaker and drier, both
models have shown enough variability within the solution space to
lead to decreased confidence on rain chances mid week.

MARINE...

An inbound cold front is still well off to the west today as weak
easterly composed flow persists across the central Great Lakes.
Primary forecast update relates to the diminishing potential for
afternoon thunderstorms as upper level conditions have inhibited
vertical growth mechanisms, even in the presence of surface
troughing and elevated dewpoints. Still cannot rule out a few
isolated storms, particularly from Lake St Clair to Fort Gratiot.
Broader precipitation coverage arrives late tonight as the front
approaches and the forward speed slows with pre-frontal winds
veering S/SW. It will take most of the day on Sunday before the
boundary makes the transition across the international border
leading to a prolonged period of showers/storms. Conditions will be
particularly favorable for waterspout development midday Sunday
along with the potential for strong gusts and locally higher seas
with any enhanced storms. Northwest gradient flow ensues once the
front clears east ushering in cooler air early next week with
increased wind and wave action, particularly Monday night. A Small
Craft Advisory may be needed. Winds flip southwest and weaken on
Wednesday as a corridor of high pressure expands across the lower
Midwest.

HYDROLOGY...

An upper level wave and sfc cold front will slowly advance across Se
Mi late tonight into Sunday afternoon. The system will produce
fairly widespread showers with some embedded thunderstorms. Total
basin average rainfall of just under a tenth to up to three quarters
of an inch is expected. The highest rainfall totals are forecast
across the thumb and Port Huron regions. Locations east of a Detroit
to Bad Axe line stand the better chances for thunderstorms and
locally heavy rainfall on Sunday.

&&

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

AVIATION.....MR
DISCUSSION...SC
MARINE.......KK
HYDROLOGY....SC

You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.

Hazardous Weather Outlook
National Weather Service Detroit/Pontiac MI
333 AM EDT Sat Aug 15 2020

MIZ047>049-053>055-060>063-068>070-075-076-082-083-160845-
Midland-Bay-Huron-Saginaw-Tuscola-Sanilac-Shiawassee-Genesee-Lapeer-
St. Clair-Livingston-Oakland-Macomb-Washtenaw-Wayne-Lenawee-Monroe-
333 AM EDT Sat Aug 15 2020

This hazardous weather outlook is for southeast Michigan.

.DAY ONE...Today and Tonight

Widely scattered thunderstorm activity is expected to develop this
afternoon with the heating of the day and last into the evening
hours. A cold front is then expected to bring additional
thunderstorms overnight. Isolated severe thunderstorms are possible
with the afternoon and early evening activity. Damaging winds to 60
mph, hail to around 1 inch, and localized flooding are the main
hazards. Storm motion looks to be variable around 10 mph.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

There is a chance of lingering thunderstorms Sunday morning.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today and tonight.

LCZ460-LEZ444-LHZ361>363-421-422-441>443-462>464-160845-
Lake St Clair-
Michigan Waters of Lake Erie from Detroit River to North Cape MI-
Lake Huron from 5NM east of Mackinac Bridge to Presque Isle Lt
beyond 5 NM off shore-
Lake Huron from Presque Isle Light to Sturgeon Point MI beyond 5NM
off shore-
Lake Huron from Sturgeon Point to Alabaster MI beyond 5NM off shore-
Outer Saginaw Bay SW of Alabaster to Port Austin MI to Inner Saginaw
Bay-Inner Saginaw Bay SW of Point Au Gres to Bay Port MI-
Port Austin to Harbor Beach MI-Harbor Beach to Port Sanilac MI-
Port Sanilac to Port Huron MI-
Lake Huron from Port Austin to Harbor Beach beyond 5NM Off Shore-
Lake Huron from Harbor Beach to Port Sanilac beyond 5NM Off Shore-
Lake Huron from Port Sanilac to Port Huron beyond 5NM Off Shore-
333 AM EDT Sat Aug 15 2020

This hazardous weather outlook is for portions of Lake Erie, Lake
Huron and Lake St Clair.

.DAY ONE...Today and Tonight

Isolated thunderstorm activity is expected to develop this afternoon
with the heating of the day and last into the evening hours. A cold
front is then expected to bring additional thunderstorms overnight. A
severe thunderstorm can not be ruled out, with wind gusts to 50 knots
and hail to around 1 inch being the main hazards. Storm motion looks
to be variable around 10 knots.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

There is a chance of thunderstorms on Saturday and Sunday. Isolated
severe thunderstorms are possible Saturday afternoon.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today and tonight.

You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.

SF

Special Weather Statement
National Weather Service Detroit/Pontiac MI
722 PM EDT Sat Aug 15 2020

MIZ068-160015-
Livingston-
722 PM EDT Sat Aug 15 2020

...AN AREA OF STRONG THUNDERSTORMS WILL AFFECT LIVINGSTON COUNTY...

At 722 PM EDT, strong thunderstorms were clustered over Oak Grove, or
7 miles north of Howell, moving northwest at 5 mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

This storm will be near...
  Oak Grove around 735 PM EDT.
  Cohoctah around 810 PM EDT.

Other locations impacted by this storm include Parkers Corners,
Lakeland, Plainfield, Island Lake State Recreation Area, Chilson,
Unadilla, Pinckney State Recreation Area, Conway Township, Hell and
Tyrone Township.

Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.

LAT...LON 4278 8369 4243 8366 4242 8414 4277 8416
TIME...MOT...LOC 2322Z 137DEG 5KT 4271 8388

KK

Special Weather Statement
National Weather Service Detroit/Pontiac MI
751 PM EDT Sat Aug 15 2020

MIZ063-160045-
St. Clair-
751 PM EDT Sat Aug 15 2020

...A STRONG THUNDERSTORM WILL AFFECT SOUTHEASTERN ST. CLAIR COUNTY...

At 750 PM EDT, a strong thunderstorm was located near Marine City, or
near St. Clair, moving west at 10 mph.

Winds in excess of 30 mph and pea size hail are possible with this
storm.

This storm will be near...
  Richmond around 845 PM EDT.

Other locations impacted by this storm include Anchorville, Columbus,
Rattle Run, Pearl Beach and Adair.

If outdoors, consider seeking shelter inside a building.

Torrential rainfall is also occurring with this storm, and may cause
localized flooding. Do not drive your vehicle through flooded
roadways.

LAT...LON 4266 8252 4263 8252 4261 8254 4263 8264
      4266 8261 4269 8265 4269 8271 4271 8273
      4290 8274 4286 8247 4281 8249 4277 8247
TIME...MOT...LOC 2350Z 082DEG 8KT 4276 8252

KK