Tornado and Hail in Michigan, September 10, 2019

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
348 PM EDT Tue Sep 10 2019

.DISCUSSION...

19Z analysis shows a warm front approaching Saginaw Bay based on the
advance of dewpoints in the upper 60s under S/SW flow. Earlier, an
expansive area of precip including a few thunderstorms overspread
much of the I-69 corridor and northward, but a lull in activity
should hold through the early evening hours. Additional showers with
some strong to severe thunderstorms remains possible early tonight
with the primary threats being marginally severe hail and damaging
winds.

A surface low now over south-central ON will continue to slowly work
east, but much of the focus for storms tonight will be related to
the positioning of the warm front in conjunction with a surface
trough progression over a differential heating boundary. This area
of enhanced forcing aligns with MLCAPE values poised to reach the
1250-1750 J/kg range overspread with H8-H5 lapse rates ranging from
6.0-6.5 C/km supporting a marginally severe hail threat. Storm
motion will start off SW-NE and then become NW-SE with the latter
round. PW values will be unseasonably high /1.75-2.00 in/ allowing
for large volumes of water loading in the strongest updrafts that
develop which is driving the wind threat, along with ample DCAPE
from larger dewpoint depressions in the lower levels. Bulk 0-6 km
shear values close to 40 knots with limited directional variance
should generally yield a multicellular convective mode with possible
cold pool organization and gusty outflow after dark.

Severe threat returns Wednesday afternoon as another line of
convection arrives with the passage of a cold front. HiRes guidance
is not in great agreement with convective mode showing both quasi-
supercellular structures and discontinuous linear convection as
possibilities. Instability forecast looks similar to today with
tall, skinny CAPE profiles, although mid level lapse rates are a bit
weaker as is the case for bulk 0-6 km shear values. Any residual
capping should erode early in the afternoon leading to sufficient
mixing layer depth. A cold front stemming from the northern low
displacement should sag south to about the MI/OH border very late
Wednesday night with additional unsettled weather expected on
Thursday as a series of shortwaves ripple through aloft prior to
brief ridging midday Thursday. Strong upper level cyclonic flow
pattern develops from quick cyclogensis after crossing the Rockies
late Thursday night and will advance into the Great Lakes region
by the end of the short-term period.

Moderately sheared and modestly unstable airmass characterized by 0-
1 MLCAPE under 1000j/kg and bulk shear on the order of 35-40kts
remains in place for Friday and will be supportive of shower and
thunderstorm development in advance of the cold front. Corridor of
stronger forcing will be oriented north and west of the CWA
introducing uncertainty with respect to initiation and coverage.
Pops in the blended grids are quite high given the scenario and
potential for capping. Next round of height falls over the West
Coast has the potential to result in considerable downstream
amplification of the longwave ridge supportive of daytime highs
approaching 90 by Monday. As evidenced by the poor continuity from
the 00z to 12z ECMWF, this outcome is conditional on the degree to
which the western energy detaches from the westerlies - a typical
low-confidence tipping point in the solution space. Temps over the
weekend will hover within a few degrees of seasonal average values
with no precip expected within surface high pressure and confluent
WNW flow aloft. No changes made to the inherited grids.

&&

.MARINE...

A weak frontal boundary will linger across the central Great Lakes
tonight into Thursday, providing a focus for periodic showers and
thunderstorms while the humid air mass remains in place. Southwest
winds this afternoon gradually weaken and veer to the west tonight
as low pressure moves from Lake Superior into southern
Ontario/Quebec and the pressure gradient weakens locally. The
approach of the next wave along the frontal boundary Wednesday will
reorient the winds from the northeast by late in the day, leading to
onshore flow of about 15 to 20 kt. The wave will pass through the
central Great Lakes early Thursday with winds remaining elevated and
out of the east through the day Thursday.

&&

.HYDROLOGY...

Seasonably warm and humid conditions will persist through the mid-
week period while a frontal boundary orients itself across the
region, resulting in an active period with multiple chances for
showers and thunderstorms through Friday. Scattered shower and storm
activity this evening will bring up to about 0.50 inches of
rainfall. Another round is likely Wednesday afternoon/evening with
rainfall totals of 0.25 to 0.75 inches more likely for that period.
However, locally higher amounts in excess of one inch are possible
within any persistent areas of heavier thunderstorms. This could
lead to some minor flooding of poor drainage areas. Additional
showers and storms are likely Friday before a cold front ushers in a
cooler, drier air mass for start of the weekend.

&&

.PREV DISCUSSION...
Issued at 121 PM EDT Tue Sep 10 2019

AVIATION...

Modest southwest winds, gusting up to 20 kt, continue this afternoon
as low pressure moves through the northern Great Lakes while showers
and a few thunderstorms continue across areas along and north of I-
69. Additional thunderstorm development remains possible later this
afternoon and evening and will be tied to the boundary left by the
early day activity and a differential heating boundary oriented west-
east across the center of the CWA. Chance for these storms to impact
MBS and FNT remains low but will be higher for the other TAF sites
based on analysis of the boundary's current position and trends in
hi-res guidance. Best window for activity will be from 22z to 04z
and have introduced a mention in the TAFs. Isolated storms may
become strong to severe. Any convection will move out/dissipate late
this evening, though the boundary will linger across the area and
may lead to additional showers/storms on Wednesday. Moist air mass
remains in place tonight, and potential exists for a few hours of
MVFR vsby around sunrise when the gradient relaxes a bit and leads
to some fog/mist development.

For DTW...Recent trends depict an increasing probability for
convection to move through the vicinity 00-04z with the best chance
to impact the terminal currently centered on 01-03z. Cigs look to
remain above 5 kft through tonight though some 5SM haze will be
possible around sunrise - potential remains low and will hold off on
including in the TAF for now.

DTW THRESHOLD PROBABILITIES...

* Low for ceilings at or below 5000 ft today.

* Moderate for thunderstorms late afternoon/early evening.

&&

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

DISCUSSION...KK/JVC
MARINE.......TF
HYDROLOGY....TF
AVIATION.....TF

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

Hazardous Weather Outlook
National Weather Service Detroit/Pontiac MI
355 AM EDT Tue Sep 10 2019

MIZ047>049-053>055-060>063-068>070-075-076-082-083-110830-
Midland-Bay-Huron-Saginaw-Tuscola-Sanilac-Shiawassee-Genesee-Lapeer-
St. Clair-Livingston-Oakland-Macomb-Washtenaw-Wayne-Lenawee-Monroe-
355 AM EDT Tue Sep 10 2019

This hazardous weather outlook is for southeast Michigan.

.DAY ONE...Today and Tonight

There is a chance of thunderstorms north of M-59 today as a frontal
boundary will be in the vicinity, with a slight chance of
thunderstorms tonight spreading south toward the southern Michigan
border. Thunderstorms that develop could be strong, producing wind
gusts to 50 mph and small hail, along with heavy rainfall. Even a
stray severe thunderstorm producing wind gusts to 60 mph and one inch
hail can not be ruled out in the 4 to 10 pm window. Storm motion
looks to be east 25 mph.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

There is a continued chance of thunderstorms Wednesday through Friday.
The best chance for strong thunderstorms appears to focus Wednesday
afternoon and then again on Friday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed today and tonight.

LCZ460-LEZ444-LHZ361>363-421-422-441>443-462>464-110830-
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-
355 AM EDT Tue Sep 10 2019

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

.DAY ONE...Today and Tonight

Scattered thunderstorms over Lake Huron today, with a slight chance
of thunderstorms tonight spreading over Lake St. Clair and Lake Erie.
Thunderstorms that develop could be strong, producing wind gusts to
45 knots and small hail. Storm motion looks to be east at 20 knots.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

There is a continued chance of thunderstorms Wednesday through Friday.
The best chance for strong thunderstorms appears to focus Wednesday
afternoon and then again on Friday.

.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