Hail and Damaging Wind in Michigan, September 4, 2017

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
100 PM EDT Mon Sep 4 2017

.AVIATION...

Strong southwest winds early this afternoon, gusting up to 30 knots,
ahead of a strong cold front. This front is expected to trigger a
line of showers and strong to severe thunderstorms as it drops south
through the late afternoon and clears the state around 00z. A short
period of post frontal MVFR Ceilings possible, but good drying
indicated behind the front is expected to lead to mostly clear skies
late this evening into overnight hours. However, Upper level
trough/cold pool arriving toward sunrise may be able to trigger
scattered showers, especially northern taf sites, where prob30 was
introduced.

For DTW...Inherited timing of thunderstorms, 22-00Z still looks good
as front clears shortly after 00z. These storms may be severe,
producing wind gusts of 50 knots. Could be abrupt drying/clearing
just after 00z, and confidence in high MVFR/Low VFR ceilings during
the evening is not high.

//DTW THRESHOLD PROBABILITIES...

* High for thunderstorms between 22-00z.

* Medium confidence in ceilings aob 5000 feet late this afternoon
  and into early this evening, and then again late tomorrow morning
  into the afternoon.

&&

.PREV DISCUSSION...
Issued at 320 AM EDT Mon Sep 4 2017

DISCUSSION...

Weak convection over the northern Great Lakes and NW Ontario is
ongoing along a seasonably strong cold front at the nose of a 45-
50kt low-level jet. This activity will continue to wane as it is
ushered east by shortwave energy embedded within the periphery of
the large polar low over Hudson Bay. Water vapor imagery reveals a
potent trailing shortwave digging into the northern Plains at 06z
this morning. Attendant height falls will sharpen the existing cold
front and ensure continued southeast progress through today.
Momentum fields are progged to ramp up considerably in response.
Surface winds will be sustained at 15 to 20 knots while diurnal
mixing under mostly sunny skies ensures effective downward momentum
transport supporting wind gusts of 30 knots during the peak heating
period with a 50kt low-level jet taking shape immediately atop the
inversion.

Attention for the afternoon will be focused on severe wind potential
as moderately steep lapse rates in excess of 7C/km fill in above the
capping inversion with a well-mixed boundary layer taking shape
below. Strong and largely unidirectional wind field will be in place
through the depth of the column. Considering strongly convergent
forcing along the surface cold front, a higher end severe wind event
featuring wind gusts of 70 mph is not at all out of the question,
particularly where cold fropa will coincide with peak heating
roughly south of an ARB/PTK/PHN line. Concern is particularly
elevated given that individual cells will have ambient wind behind
them potentially reinforcing rear inflow jets.

However, despite capping, there remains concern that the strength of
mid-level warm advection will be sufficient to generate an uncertain
coverage of convection in advance of the front, particularly as
steep lapse rates fill in overhead. The degree to which this occurs
and subsequently exhausts/inhibits instability ahead of the front
seems to be the tipping point in the forecast between a lower
coverage of borderline severe and a potentially more widespread and
higher end event over the SE third of the CWA as described above. In
any case, the environment will probably be favorable even for weak
showers/tstorms to generate at least 40-45kt gusts and suspect that
even in a "short-circuited" scenario, the likelihood of at least a
few severe reports in the area is rather high. Also worth noting is
that any development in the free warm sector will carry a low-end
tornado threat mainly limited by higher LCLs. Nonetheless, rotating
updrafts with 0-1km bulk shear of 35kts will warrant attention.

After reaching a peak in the upper teens today, 850mb temperatures
will fall into the single digits by Tuesday morning and lower single
digits by Wednesday morning. Highs near 70 on Tuesday will mark the
warmest day of the workweek with temps the low to mid 60s through
Friday. Steep boundary layer lapse rates within deep layer cyclonic
flow regime will favor daily development of stratocu with some
nocturnal persistence possible during any periods of lake
enhancement. Depending on cloud cover, overnight lows have the
potential to range from the upper 30s to the upper 40s during this
time. Episodic bouts of light showers as embedded waves propagate
through the trough warrant chc pops for most of the week.

MARINE...

Strong southwesterly winds will begin to ramp up through the morning
hours into the afternoon today as a surface pressure gradient
tightens and winds mix down ahead of an approaching cold front. The
southwest winds should approach near gale criteria over the open
waters of Lake Huron, so a Small Craft Advisory will remain in
effect through Monday evening for all marine zones from Saginaw Bay,
around the Thumb and south to Lake St Clair and the western shores
of Lake Erie. Wave heights for Lake Huron are forecast to be around
3-7 feet. There will be a chance for thunderstorms ahead of the cold
front in the afternoon and through the evening. Some of these storms
will have the potential to become severe. An unseasonably cool
airmass will move into Michigan behind the front and reside through
the end of the week. Lighter northwest winds will help keep the
cooler air in place over the warm waters and waterspouts will remain
possible through midweek.

HYDROLOGY...

A cold front is set to move across Michigan this afternoon and
evening and will bring a chance of thunderstorms to the region.
Heavy rainfall could occur as storms initiate and grow upscale
rather quickly as the progress southeastward  through the afternoon.
Rainfall totals within the 0.25 to 0.50 inch range with locally
higher amounts near 1 inch will be possible, which could lead
to minor flooding of prone areas in the urban locations.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement until 10 PM EDT this evening for MIZ049-054-
     055-083.

Lake Huron...Small Craft Advisory until 8 PM EDT this evening for LHZ421-422-
     441>443.

Lake St Clair...Small Craft Advisory until 8 PM EDT this evening for LCZ460.

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

&&

AVIATION.....SF
DISCUSSION...JVC
MARINE.......JVC
HYDROLOGY....AA

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

Hazardous Weather Outlook...UPDATED
National Weather Service Detroit/Pontiac MI
353 AM EDT Mon Sep 4 2017

MIZ047>049-053>055-060>063-068-050800-
Midland-Bay-Huron-Saginaw-Tuscola-Sanilac-Shiawassee-Genesee-Lapeer-
St. Clair-Livingston-
353 AM EDT Mon Sep 4 2017

This hazardous weather outlook is for portions of southeast
Michigan.

.DAY ONE...Today and Tonight

A beach hazards statement for hazardous swimming conditions is in
effect for Tuscola...Huron...and Sanilac counties.

Thunderstorms are likely today. There is a slight risk that these
storms will be severe...mainly along and south of the I-69 corridor.
The primary threat will be damaging wind gusts to 60 mph between
Noon and 5 PM local time. Storm motion will be ESE at 50 mph.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is anticipated.

LCZ460-LEZ444-LHZ361>363-421-422-441>443-462>464-050800-
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-
353 AM EDT Mon Sep 4 2017

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

.DAY ONE...Today and Tonight

Thunderstorms are likely today. There is a slight risk that these
storms will be severe...mainly along and south of the I-69 corridor.
The primary threat will be damaging wind gusts to 60 mph between
Noon and 5 PM local time. Storm motion will be ESE at 50 mph.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is anticipated.

MIZ069-070-075-076-082-083-050800-
Oakland-Macomb-Washtenaw-Wayne-Lenawee-Monroe-
353 AM EDT Mon Sep 4 2017

This hazardous weather outlook is for portions of southeast
Michigan.

.DAY ONE...Today and Tonight

A beach hazards statement for hazardous swimming conditions is in
effect for Monroe County.

Thunderstorms are likely this afternoon. There is an enhanced risk
that these storms will be severe. The primary threat will be wind
gusts to 60 mph with potential for isolated gusts to 70 mph between
4 PM and 9 PM local time. Storm motion will be ESE at 50 mph.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...
Spotter activation is likely. Dispatchers please notify your
emergency managers immediately.

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

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
214 PM EDT MON SEP 04 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0210 PM     HAIL             3 E PINCONNING          43.86N 83.91W
09/04/2017  E0.50 INCH       LHZ422             MI   TRAINED SPOTTER

&&

AM