National Weather Service United States Department of Commerce


000
FXUS63 KDTX 180841
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
441 AM EDT Fri Jun 18 2021

.DISCUSSION...

Complex forecast in play for today given several moving parts in
regards to the ongoing MCS complex in place across southern
Wisconsin and western Michigan, progression and duration of the
system as it travels over Southeast Michigan this morning, and
resulting destabilization and mechanisms of forcing that will take
place following the passage of this system this afternoon. To start,
leading edge of the system continues to lose steam as precipitation
has outran axis of better instability, with latest mesoscale
analysis depicting little to no MUCAPE in place across SE MI. Modest
mid-level lapse rates will still support isolated thunderstorm
activity through the mid-morning hours with any activity, but
expecting mainly scattered showers with initial activity given the
lack of instability. LLJ to then strengthen and re-orient across SE
MI through 09-15Z. This will advect a very moisture-rich theta-e
plume across Southeast Michigan on the leading edge of a weaker
instability axis, resulting in increasing shower coverage and the
continued chance for thunderstorm activity through the late morning
hours.

Current thinking is that passage of the MCS system will result in
stable conditions across SE MI through the late morning to early
afternoon hours which will limit the chances afternoon activity,
however, destabilization of the atmosphere will begin to take place
through the later afternoon hours in the wake of a warm front as dew
points rise into the upper 60s to possibly the lower 70s, while h850
temperatures rise to a warn 16-18C and h700 temperatures rise to or
above 8C. Concurrently, southwest flow at the surface with zonal
westerly flow in the mid-level noted by h700 wind speeds around 40
knots will support effective bulk shear values to or above 40 knots
in the afternoon with internal probabilistic guidance showing MLCAPE
values rising between 1500 - 2000 J/kg across the southern Metro
Region. Outflow boundaries from this morning`s activity will likely
hold around or south of the MI/OH border, but surface trough in the
wake of an advancing cold front will aid in thunderstorm potential
with the forecasted destabilization. HRRR/NSSL and to a lesser
extent the ARW is on board with CI mainly across the Metro region
between the 18 - 23Z time frame, tied the better instability axis.
However, the FV3 and NAM nest holds the bulk (or in FV3s case, all)
of the convection south of the MI/OH border, owing to a lesser
degree of destabilization, and even with surface flow more westerly
in the FV3. Thus the main message for this morning: thunderstorm
potential is conditional on the ongoing activity and CI mechanisms
and will require continued analysis after the MCS passes. Being
said, have hedged towards high-end chances for redevelopment (50-60
percent) along and south of M5 given the favorable synoptic set-up
in place.

Given the strong shear and building instability in place, any storms
that do develop will have the potential to become organized & strong
to severe, with all hazard types in place. Latest SWODY1 places
locations south of the Tri-Cities in a marginal risk for severe
weather, with Monroe/Lenawee under a Slight Risk. All severe hazards
will be in play, as mesocyclone development/supercells will be
possible with veering profiles with height and the increased length
of the hodograph in the lower-levels with the strong winds at h700.
HREF mean STP values approach 1 closer to the MI/OH border where low-
level SRH values will be maximized thus there is a non-zero chance
for tornado development. Otherwise, steep mid-level lapse rates and
high TEI values with organized convection suggest hail to or in
excess of an inch and wet microbursts are possible. But again, any
severe weather is conditioned on chance that storms will develop
north of the MI/OH border.

SWODY2 places locations south of the Tri-Cities to northern Thumb
again in a marginal risk as sufficient bulk shear remains in place
with building instability likely on Saturday afternoon , following
the early morning passage of an embedded strong short wave within
the mean zonal flow. Convective activity will again be dependent on
destabilization across the southern Great Lakes and Ohio Valley as
overnight activity will be ongoing along a diffused frontal boundary
across Ohio, where the best theta-e gradient is set-up. Have added a
chance mention (40-50 percent) for precipitation for this forecast
package between 18Z - 00Z Saturday. Otherwise, The day is expected
to be warm and muggy with temperatures in the 80s and dew points in
the 60s. Additional chances for widespread rain with some
thunderstorms is looking likely late Sunday into Monday as a
seasonally strong low pressure system travels somewhere along the
Midwest into the Great Lakes, with uncertainty still surrounding the
exact location of the cyclone center. Overall, a very active pattern
will hold through the weekend into early next week.

&&

.MARINE...

Shower and thunderstorm activity currently tracking across southeast
Michigan to progress east of the Great Lakes by mid-morning.
Embedded storms may produce lightning and brief periods of gusty
winds/enhanced wave heights. Development of a wake low behind
morning convection will lead to enhanced wind gusts along the
nearshore waters, peaking to 20-25 kts by late morning. As a result
Small Craft Advisories are in effect through mid-afternoon. A second
round of shower/thunderstorm activity is possible late this
afternoon along an approaching cold front, mainly across the
southern marine zones. As the front passes through southeast MI,
expect winds to gradually weaken and rotate to the northwest by
Saturday. High pressure then returns for the weekend, with overall
dry conditions expected.

&&

.PREV DISCUSSION...
Issued at 1154 PM EDT Thu Jun 17 2021

AVIATION...

Convection west of the region late this evening will work east,
southeast into the area overnight into early Friday morning as
strong elevated warm air advection regime spreads into the lower
Great Lakes. A wedge of relatively unstable air will precede this
convection so will maintain tsras in the forecast with a TEMPO
around dawn. Activity later in the forecast in advance of a cold
front is in question given the expected influence of this early day
convective system. Given 00z HiRes model trends, it appears there
will be a window for activity to fire before shift southeast so will
include another PROB30 group for the I-94 terminals. Main period of
showers with embedded thunder will be from 08z-09z through 13z-14z
with the next round of scattered convection somewhere in the 18z-22z
time frame. Southwest winds will persist through the forecast,
veering to a more west direction late Friday with the cold front.

For DTW...00z guidance still suggests showers with some embedded
thunder seem likely in the 10z-14z time frame. This activity is
already underway over the upper midwest and expect convection to
track into the area late tonight within zone of warm air advection.
Will include PROB30 group for additional convection in the mid
afternoon, but confidence in this period is much lower.

DTW THRESHOLD PROBABILITIES...

* Low to medium in thunderstorms with cigs aob 5000 feet impacting
the terminal early Friday morning. Low Friday afternoon as new
development may fire near/south of the terminal.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 PM EDT this afternoon for LHZ421-422-
441>443.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...AM
MARINE.......MV
AVIATION.....DG


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