NWS Forecast Discussion

000
FXUS63 KDTX 192247
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
647 PM EDT Fri Jul 19 2019


.AVIATION...

Busy forecast period with several rounds of convection quite
possible. Ongoing convection will fester this evening and probably
not dissipate until after 03z. Will carry some mention of thunder
KPTK south to cover this period. A strong MCS will organized over
the upper midwest and encroach on the area late tonight. Hard to say
how far southeast it will make it into the area, but will at least
maintain thunder mention KMBS. Another round of storms is expected,
mainly Saturday afternoon/evening. The timing/location will largely
be dictated by evolution of overnight MCS.

For DTW...At least VCTS will be needed for several hours early this
evening as next batch of storms work southeast into the area. VFR
conditions are then expected overnight into Saturday with another
round of significant convection quite possible again late Saturday.

.DTW THRESHOLD PROBABILITIES...

* Moderate for thunderstorms this evening (thru ~02z)

* Low for ceilings aob 5kft at times overnight.

* Low for timing/coverage of tsras late in the forecast (~21z+)

&&

.PREV DISCUSSION...
Issued at 401 PM EDT Fri Jul 19 2019

DISCUSSION...

NEAR TERM / REST OF TODAY THROUGH TONIGHT

As of 400 PM EDT...Heat Advisories/Excessive Heat Warnings continue
as southeast Michigan continues to reside in the hottest and most
humid air of the year, with widespread heat indices peaking 100-110
degrees. These heat indices have been a byproduct of high
temperatures reaching into the lower 90s for most areas, and
dewpoints surging into the mid/upper 70s with arguably an 80 degree
reading not out of the question in the farmland areas of
Washtenaw/Lenawee/Monroe counties. The very hot and humid airmass has
also resulted in impressive instability building across the region,
with MLCAPEs of 2000-3000 J/kg coinciding with PWAT values around 2
inches.

Earlier morning convection that affected the Thumb has raced east,
spawning additional thunderstorms across southern Ontario and
eventually into the eastern Lake Erie/western Lake Ontario region by
this evening. Closer to home, this morning`s convection left an
outflow boundary oriented west/east that has slowly been drifting
southward towards the M-59 corridor with time. Additionally, remnant
convective debris from the morning activity that has slowly been
eroding and giving way to a bubbling cumulus congestus field has
allowed for a quasi-differential heating boundary to also develop,
providing just enough moisture convergence for thunderstorm
development across portions of the Detroit metro/Ann Arbor over the
past few hours. This activity has exhibited vigorous storm
development in the loaded instability environment, with a few cores
reaching over 50 kft. Lack of more organized deep layer shear has
prevented more coherent storm organization thus far, but the strong
thermodynamic environment coupled with precip loading in updrafts
(freezing level around 15 kft) will enable the potential for a few
wet microbursts producing brief wind gusts as high as 40-60 mph.
Small hail will be tough to come by with the strongest storms though
given the very warm thermal profile in place through the column.
Acknowledging the impressive instability in place, SPC has now
placed almost all of southeast Michigan in a Slight Risk for severe
thunderstorms for this afternoon/evening.

The other big threat from thunderstorm activity this afternoon will
be the potential for locally heavy rain. Rainfall rates of 1-2
inches per hour will be possible with the strongest activity, and
may produce rain totals easily exceeding 2 inches in spots leading
to significant poor drainage flooding concerns, and possibly
localized flash flooding should storms hit more susceptible
urbanized areas.

Additional thunderstorm development is already underway across
Ingham/Livingston/western Oakland counties in the aforementioned
cumulus congestus field, and will likely continue further
development east/southeast towards the Detroit metro heading into
the evening hours. This second round of activity will also pose the
risk for brief wet microbursts, but will have to watch closely for
any better cold pool organization that could result not only in a
better wind threat, but an increased risk for excessive rainfall
over a larger area. Isolated thunderstorms will also be possible
further north across the Thumb as a weak midlevel impulse rotates
through.

Heading into the late evening hours, any lingering thunderstorms
should wane/depart east giving way to drier conditions for at least
the first half of the night. Attention will then shift to a robust
MCS/possible derecho that will be racing east across eastern
Minnesota into northern Wisconsin and eventually northern Lower
Michigan late tonight. After this afternoons/evening convection, all
indications point to the cap (12 C at 700 hPa) becoming
reestablished across lower Michigan, and pushing the nose of the
theta-e ridge further north outside of the immediate region. There
remains a mixed signal in model guidance regarding the track that
the MCS will take, but given the prevailing zonal flow aloft and
current trends in water vapor imagery across the upper Midwest,
forecast thinking is that the bulk of the remnants should remain
north of southeast Michigan. Will continue to play somewhat bearish
with the forecast north of I-69 and leave chance PoPs in the
forecast, with a current dry remainder of the night forecast further
south.

Outside of MCS watching tonight, it will continue to remain very
warm and muggy through the night as low temperatures only fall into
the mid/upper 70s for most areas, and possibly not even out of the
lower 80s for portions of the Detroit metro. These high minimum
temperatures will flirt with records for tonight and Saturday night
as well, and will bring little relief from the oppressive heat from
this afternoon.

SHORT TERM / SATURDAY THROUGH SUNDAY NIGHT

Heat Advisories and Excessive Heat Warnings will remain in effect
through Saturday as one more oppressively hot and humid day will
affect the region. Should the aforementioned nighttime MCS up north
actually stay to the north, and its associated convective debris
clouds are less opaque and allow for greater insolation, the
potential easily exists for Saturday to be even hotter than today.
Will leave the current heat headlines as is though, as there is a
bit of uncertainty again with cloud cover and possible remnant MCS
activity in the Heat Advisory counties to upgrade to a warning at
this time. However, those in the Excessive Heat Warning will be in
for the summer bake as high temperatures soar easily into the mid
90s, and possibly upper 90s in spots. With dewpoints still lingering
in the mid/upper 70s, several locations south of I-69 look to see
heat indices exceed 110 degrees at times, possibly peaking as high
as 115 degrees.

The continued heat and humidity will allow for another day of more
than plentiful instability to be in place as MLCAPEs look to
potentially exceed 3000 J/kg. More widespread coverage of
thunderstorms is expected by Saturday afternoon as weak midlevel
impulses ride through the zonal flow and interact with a slowly
southward moving cold front boundary. Coverage will initially be
greatest north of M-59 during the second half of the day, with
coverage eventually drifting southward with time heading into the
night. Isolated severe thunderstorms will be again be possible
Saturday with slightly better shear (30-35 knots) moving in from the
north and the continued ambient high instability environment.
Damaging wind gusts would be the primary threat. Locally heavy
rainfall and additional flooding concerns will also be possible,
especially for thunderstorms that move over areas that see high rain
totals Friday.

Saturday night will be another muggy night, although not quite as
muggy as Friday night, as the cold front slowly moving south across
the region allows low temperatures to range from the upper 60s/lower
70s north to mid 70s south. Dewpoints during this time will also
begin to slowly decrease into the upper 60s/lower 70s as well vs
mid/upper 70s.

Guidance has slowed considerably with the passage of the cold front,
and it now looks like it will get hung up at least for the southern
half of the region on Sunday. Additional scattered thunderstorms
will be possible as a result, with coverage slowly waning by late
Sunday night. High temperatures Sunday will be in the 80s with lows
Sunday night much cooler in the upper 50s/lower 60s. Heat indices
Sunday will also be notably cooler, and only reach into the lower
90s closer to the Ohio border.

For more on the record heat today and Saturday, refer to the Climate
section below.

LONG TERM / MONDAY THROUGH THURSDAY

Seasonably cool conditions expected Monday as continued cold/dry
advection from the north allows 850mb temps to settle around 10C.
This will translate to highs in the mid to upper 70s. Dew points in
the 50s will bring a noticeably less humid feel to the air compared
to this weekend.

The Great Lakes will be positioned beneath a region of negative
vorticity advection for the early week as a longwave trough slowly
advances to the eastern seaboard. The result will be dry conditions
and high pressure building in at the surface. High temperatures will
increase slightly each day, eventually reaching near normal values
in the lower 80s by midweek. There is a very low chance of showers
and storms on Thursday as height falls move overhead with an
approaching trough, otherwise dry weather looks to prevail into the
late week.

MARINE...

The warm and humid theme will continue both today and tomorrow for
the open waters which will result in continued chances for heavy
showers and strong thunderstorms. Primary concern for any
thunderstorm development will be strong winds that could produce
variable wind conditions and erratic wave development. Otherwise,
non-thunderstorm wind direction will remain from the southwest. A
cold front remains on track to push through the Great Lakes late
Saturday into Sunday which will veer wind direction from the
southwest to the northeast.

HYDROLOGY...

Hot and humid air mass remains parked over the Great Lakes both
today and tomorrow which will bring the continued chance to see
showers and storms, some strong. The source of showers and
thunderstorm development will be residual outflow boundaries from
previous storms and differential heating boundaries from prior cloud
cover. Strong thunderstorms will be capable of producing a quick 1-2
inches of rainfall accumulation in a short period of time, as has
been observed via radar estimates this afternoon, around the Ann
Arbor/Ypsilanti region. The most likely chance for thunderstorm
development this afternoon and evening will be around the greater
Metro Detroit area, but a possible storm complex, currently over MN,
may move downstream towards and across MI overnight which would be
the source of additional heavy rainfall. Additional strong showers
and storms are expected again tomorrow, but confidence is low at
this time regarding spatial extent and timing of thunderstorm
development. Overall, expect the continued chance for heavy showers
and strong storms both today and tomorrow. Rainfall totals will be
highly variable across SE MI, but localized flooding, including
flash flooding, remains possible.

CLIMATE...

July 19/20th                     July 19/20th
      Record Highs                     High Minimum

DTW  100 (1946) / 97 (1977)        78 (1986) / 77 (1932)

FNT   99 (1946) / 102 (1930)       78 (2013) / 75 (1977)

MBS   99 (1935) / 102 (1930)       74 (2013) / 73 (2011)

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Heat Advisory until 8 PM EDT Saturday for MIZ047>049-053>055.

     Excessive Heat Warning until 8 PM EDT Saturday for MIZ060>063-
     068>070-075-076-082-083.

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

$$

AVIATION.....DG
DISCUSSION...IRL/TF
MARINE.......AM
HYDROLOGY....AM
CLIMATE......BT


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NWS DTX Office Area Forecast Discussion