Severe Thunderstorm Warning 32, NWS Detroit, MI

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
338 PM EDT Wed Jun 10 2020

.DISCUSSION...

Numerous reports of wind damage, some significant, noted with the
lead convection along the deeper moisture lifting from Indiana into
central Michigan early this afternoon. Among other reports, there
was an impressive sustained (2-min avg) wind of 54 mph at Saginaw
Bay Lighthouse, though the gust apparently succumbed to automated
QC.

Confidence is high that a widespread damaging/significant severe
thunderstorm episode is imminent for the remainder of the forecast
area, including Metro Detroit. Bulk shear with severe convection
thus far today has averaged a respectable 35-40+ kts per RAP
analyses. However, as noted in the morning update, momentum fields
can now be expected to ramp up considerably as height falls increase
and dynamic forcing rapidly intensifies from south to north during
the next several hours. A key element to the rapidly evolving
environment that cannot be understated is the shedding of existing
anvil debris to the north-northwest. This has allowed uninhibited
destabilization to occur over the southeastern half of the CWA.
Latest RAP analysis indicates a broadening corridor of 2200-2500
j/kg MLCAPE over the CWA with at least another 1-2 hours of
potential quality destabilization remaining. Meteorology remains
largely unchanged - ever increasing bulk shear of 50+ kts
(especially eastern zones/Detroit area) will support some potential
for supercell modes. Given the high degree of instability, hail of
the 1 to 2 inch variety certainly possible but greater any greater
potential will be limited by rather pedestrian lapse rates on the
order of 6-7C/Km and the warm overall thermal profile. Extremely
limited availability of streamwise vorticity given deep/veered SSW
flow regime suggests the greatest tornado threat, if any, will lie
with bottom-up type processes unless an updraft can grab an outflow
and and use that as a means to deviate. The greatest threat by far
will be straight line winds of 70 mph, locally higher, and the
dominant convective mode will be individual downdraft modulated
clusters or bows. The potential magnitude of wind will increase with
both time and eastward extent. Finally, PWATs around 2 inches will
support torrential rainfall and the potential for localized
flooding. The progressive nature of overall convective line will
prevent widespread flooding impacts, though some training in the
Saginaw Valley has reportedly resulted in flooding on I-75 where
dual pol indicates 1 to 1.5" has fallen. Similar urban and small
stream impacts should be expected elsewhere.

Previous...Broad upper level trough on Thursday with sharper trough
axis diving south through the Great Lakes on Friday, delivering a
good shot of cold air to start the weekend, as 850 mb temps bottom
out around zero over Lower Michigan, Per 00z Euro. The cold front
coming through Friday looks moisture starved and LI's look to stay
just positive. Nonetheless, would not be surprised if shower
activity has better coverage than outgoing forecast (slight chance)
Friday afternoon into Friday evening.

The forecast for the weekend and early next week will hinge on where
cutoff 500 MB low center develops, as spokes of energy and diurnal
instability will provide for periodic chances of showers. However,
the GFS and Canadian solutions are much farther east than the Euro
solution and suggest dry conditions and seasonably warm
conditions, which looks to be the direction of the forecast for
now.

&&

.MARINE...

Severe thunderstorms containing hail of 1 to locally 2 inches in
diameter and wind gusts of 70 mph (locally higher) will affect the
nearshore waters of Lake Huron and the lower waters of Lakes St.
Clair and Erie during the remainder of the evening. A severe
thunderstorm watch is in effect accordingly. The gale warning has
been cancelled as the background wind field and overlake instability
remains largely insufficient to generate gusts to gales. Gusty
southwest flow becomes westerly behind the cold front on Thursday.
Elevated offshore wind warrant small craft advisory with some
elevated sig waves of 3 to 4 feet likely near the tip of the Thumb
by midday Thurs. High pressure slowly builds in on Friday with flow
gradually becoming northwesterly. This will maintain favorable
boating conditions into the beginning of next week.

&&

.HYDROLOGY...

Tropical moisture resulting in torrential downpours/efficient
rainfall production with some urban flooding reported in the Saginaw
area. Despite the overall progressive nature of the convective line,
individual convective elements are moving in a NE direction which has
resulted in some degree of training. Therefore, urban and small
stream flooding should be expected elsewhere as this airmass will
easily produce 1 to 2 inches of rain in a short time.

&&

.PREV DISCUSSION...
Issued at 114 PM EDT Wed Jun 10 2020

AVIATION...

Thunderstorm organization and coverage has increased over the last
hour from Central Lower Mi into central Indiana. Daytime instability
combined with forcing ahead of a strong upper level wave and surface
cold front will sustain fairly widespread coverage of convection
during the afternoon. The cold front will advance across the Se Mi
terminals between 23Z and 00Z. There will be an abrupt drying trend
in the wake of the cold front, thus ending the risk of additional
thunderstorms. The boundary layer will remain very well mixed both
ahead of and in the wake of the cold front. South winds this
afternoon will veer west-southwest with the passage of the front.
Post frontal mixing depths and a strong low level wind field will
support wind gusts around 30 knots late this evening. Some semblance
of low level moisture circulating around departing low pressure to
the northeast will lead to some MVFR bases stratus overnight.

For DTW...Convection will increase in coverage across the western
portions of the metro airspace through 20Z. By 20Z or shortly after,
widespread convection is expected to become centered over metro
Detroit. While the risk of storms will end at metro by 00Z, the
eastern portions of the airspace may see thunderstorms linger
through 01Z.

DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings at or below 5000 feet this afternoon and
  tonight.

* High in thunderstorms affecting the area this afternoon and
  evening.

* Low in crosswind thresholds being exceeded tonight.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Lakeshore Flood Warning until 5 AM EDT Thursday for MIZ063-070.

Lake Huron...NONE.
Lake St Clair...Small Craft Advisory until 8 PM EDT Thursday for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory until 8 PM EDT Thursday for LEZ444.

&&

DISCUSSION...JVC
MARINE.......JVC
HYDROLOGY....JVC
AVIATION.....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
412 AM EDT Wed Jun 10 2020

MIZ047>049-053>055-060>063-068>070-075-076-082-083-110900-
Midland-Bay-Huron-Saginaw-Tuscola-Sanilac-Shiawassee-Genesee-Lapeer-
St. Clair-Livingston-Oakland-Macomb-Washtenaw-Wayne-Lenawee-Monroe-
412 AM EDT Wed Jun 10 2020

This hazardous weather outlook is for southeast Michigan.

.DAY ONE...Today and Tonight

There is a moderate risk of severe thunderstorms between 3-9 PM today,
as numerous severe thunderstorms are expected to develop.

The main hazard will be damaging wind gusts to around 70 MPH, but a
few tornadoes and large hail in excess of 1 inch are also possible.
The storms will also produce torrential rainfall, which will lead to
minor urban and small stream type flooding.

Storms will track northeast between 50-55 mph.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...
Dispatchers please notify your emergency managers immediately...
Spotter activation is likely this afternoon and early this evening.

LCZ460-LEZ444-LHZ361>363-421-422-441>443-462>464-110900-
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-
412 AM EDT Wed Jun 10 2020

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

.DAY ONE...Today and Tonight

Numerous severe thunderstorms are expected to develop this afternoon
and evening producing damaging wind gusts of 50 knots or greater.
Large hail and waterspouts are also not out of the question.
Torrential rainfall will occur with these storms, significantly
reducing visibility. Storm motion will be northeast at 45 knots.

Gale Warnings and Small Craft Advisories are in effect. Refer to the
latest marine forecasts for details.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Small craft advisories continued Thursday for Lake St. Clair and
Lake Erie.

.SPOTTER INFORMATION STATEMENT...

Dispatchers please notify your emergency managers immediately...
Spotter activation is likely this afternoon and early this evening.

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

SF

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