197
FXUS63 KDTX 201108
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
708 AM EDT Thu Jul 20 2017

.AVIATION...

A wave of low pressure moving through the northern and central Great
Lakes will make a few showers and an isolated thunderstorm possible
over SE Michigan during the morning along with some stratiform rain
on the east flank of the southeastward moving CHI area MCS. Except
for some light MVFR fog around sunrise, conditions will be otherwise
VFR outside of any rain activity. Satellite indications of clearing
mid clouds over the upper Midwest is a good indication of subsidence
trialing both the short wave aloft and the MCS moving into northern
IL. This subsidence is expected to spread into lower Michigan during
the afternoon as the surface front begins to move back south toward
the Ohio border. The frontal boundary will produce VFR ceiling but
with questionable potential for new thunderstorm development before
moving into Ohio at the end of the peak heating cycle. As the front
exits southward, weak high pressure building from the north is
expected to produce a clearing trend tonight.

For DTW... An isolated shower or thunderstorm and some light
stratiform rain remain possible during the morning as the main
convective complex in CHI area slides west and south. Ceiling around
5000 ft is then expected to develop during the afternoon in the
vicinity of the lingering front. Late timing and limited coverage of
afternoon storms supports holding the mention out of the forecast
for now.

//DTW THRESHOLD PROBABILITIES...

* Moderate for ceiling at or below 5000 ft during afternoon.

* Low for thunderstorms today.

&&

.PREV DISCUSSION...
Issued at 336 AM EDT Thu Jul 20 2017

DISCUSSION...

Potentially active stretch of weather begins this morning, with
periods of showers and thunderstorms possible now through Sunday
night. On a large scale we have fast zonal/westerly flow lined up
along the northern tier of counties with large upper low over
northern Canada and impressive 597dam subtropical high sprawled
across the southern/central plains. On a slightly smaller scale, a
train of mid level waves will race through this westerly flow over
the next few days.  And on an even smaller scale we will have
convective systems complicating the matter, displacing instability
pools and laying out numerous boundaries. Low confidence forecast as
models struggle to initialize well due to the current MCS activity,
thus many models become instantly useless. Will have to rely on
pattern recognition with short fused updates to handle much of the
activity.

Early this morning through this evening: Plenty of activity ongoing
upstream as a train of convective systems have been flaring up
through the night. Most of the activity will stay to the west as it
tracks southeast out of the Midwest following the instability
gradient. Outside of some showers on the northeast flank clipping
our southern counties, this will largely not impact SE MI. What is
in play will be a shortwave tracking across the northern Great
Lakes, phasing with a trough rotating around the Canadian low. In
addition, the strong 850mb low level jet currently forcing much of
the convection upstream is supposed to push eastward through
northern MI today. A combination of the HRRR and 3km NAM (which seem
to have the best handle on the current pattern), would suggest
nothing more than scattered showers developing under the broad mid
level forcing as the lead gradient of the enhanced theta e plume
passes through. Better coverage of storms would lift north of the
area while the train of MCS`s robs the area of better moisture
return and instability.  With this line of thinking, reduced pops
to chance and slight chance thinking drier overall. So we shift
attention the the afternoon where we then have a front pulled south
through the area. less precip early on leads to more heating and
build up of instability thus better chance for storms to develop
later in the day. Best chance will be generally south of M59, maybe
as far north as I69, as the boundary runs into around 1000 J/kg of
surfaced based CAPE and 40 knots of bulk shear (per RAP).  Could see
some organized storms develop. Primary threat with any strong to
severe storms will be wind and hail. Will need to watch cloud debris
from upstream convection to make sure it doesn`t prevent better
destabilization though.

Thursday night and Friday: We`ll dry out for a spell as surface high
slides through the region under a shortwave ridge aloft.
Temperatures will hold in the mid 80s as the airmass changes very
little with the passing cold front.

Friday night through Saturday night: Another active period appears
to be in the works as the boundary draped across the Ohio Valley
gets pulled back north as a warm front ahead of a developing low
over the central Plains. this slow moving east to west oriented
front look to pose a threat for heavy rain and maybe flooding as a
very moist airmass is forced up the front. PWATs of 2+ inches
continue to show up in the models as a connection to the monsoonal
moisture over the SW conus is made. A series of shortwaves could
ride along the front leading to enhanced periods of showers and
storms throughout this stretch.

Sunday through early week: Sunday could feature a few more rounds of
showers as the deformation region of the low wraps around through
the area along with a trailing trough axis. SE MI will dry out
Monday and Tuesday as high pressure builds back into the area with
ridging upstream beginning to nose into the region as well.

MARINE...

A weak center of low pressure will slide through southern Michigan
today along an equally weak front that is stalled over the region.
These features will be able to produce thunderstorms which will be
the primary marine weather impact today. Outside of thunderstorms,
marine wind will be light and have little impact on waves. The low
pressure center will move east of the Great Lakes and the front will
move south of the Ohio border tonight. This will allow high pressure
to temporarily develop over the region. The next low pressure system
is then projected to approach the area Friday night and interact
with the front for another round of stormy conditions during much of
the weekend.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...DRK
MARINE.......BT


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