Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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387
FXUS63 KDTX 210357
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1157 PM EDT Sun May 20 2018

.AVIATION...

A convincing late evening fog signal in observations matches up with
shallow saturation in the latest model soundings. The earlier
updates added a trend down to IFR for a few hours during the late
night and early morning. High clouds will be increasing coverage and
thickening during the night, first in the DTW area after losing the
low clouds, and then later toward FNT and MBS. These clouds will
prevent dense fog while the shallow nature of the moisture profile
allows quick dissipation around sunrise, especially as easterly wind
increases into the 5 to 10 knot range. Clouds thicken and lower but
VFR holds through early afternoon despite showers moving in from the
west. Borderline VFR/MVFR ceiling and visibility is then expected to
return mid afternoon into Monday evening as the rain battles dry
easterly flow in the low levels.

For DTW... MVFR ceiling gets nudged south and west of DTW during the
night and stays there for much of the day. The wind remains light
and keeps a northeast to east direction through the day, likely
requiring continued northeast traffic flow. Ceiling may dip below
5000 ft once again late in the day as rain moves in from the west.

.DTW THRESHOLD PROBABILITIES...

* Medium for ceiling 5 kft or less during late afternoon.

&&

.PREV DISCUSSION...
Issued at 325 PM EDT Sun May 20 2018

DISCUSSION...

NEAR TERM / REST OF TODAY THROUGH TONIGHT

As of 325 PM EDT...A dreary late May day continues across southeast
Michigan as low stratus continues to linger across much of the lower
part of the state. These clouds have kept it on the cooler side with
temperatures struggling to get into the 60s across inland areas, with
50s along the Lake Huron shoreline. Remnant midlevel frontal forcing
and theta-e gradient coupled with a weak midlevel wave has allowed
for scattered light showers/drizzle to propagate eastward into
portions of southeast Michigan, further adding to the dreary
conditions.

High pressure continues to attempt to build in at the surface, but
has struggled with southwesterly flow pushing into
Illinois/Indiana/Ohio. The shower activity is expected to wane into
the evening hours, however, as northward moisture transport is
shunted to the south with a MCS that will likely develop over the
mid-Mississippi River Valley later tonight. Drier air with the high
will then be able to better filter in from north to south, allowing
for a brief clearing trend in cloud cover especially late this
evening and into the early overnight. Low temperatures will
subsequently plummet, especially across the northern Thumb where
upper 30s readings are in play as clearing skies and decoupling
boundary layer winds allow for possible patchy frost to develop.
Across the rest of southeast Michigan, low temperatures will fall
into the 40s except near the Ohio border, where cloud cover will
hold on through the night and keep lows from falling below the low
50s.

SHORT TERM / MONDAY THROUGH TUESDAY NIGHT

Active stretch of weather becoming increasingly likely Monday
through Monday night...

The evolution of the aforementioned MCS continues to come into
better focus as it enters a wider envelope of the hi-res guidance
window. Much of the hi-res guidance (HRRR, NAMNest, WRF-ARW) is in
pretty good agreement with the MCS emerging over central Illinois
and tracking northeastward towards lower Lake Michigan by the early
morning hours Monday. As the MCS tracks northeastward, it will run
into an increasingly unfavorable airmass in terms of moisture and
instability availability, and is expected to weaken with time as it
enters lower Michigan during the morning hours. Convective debris
clouds will preclude the arrival of any precipitation, with cloud
cover increasing rapidly from southwest to northeast during the
morning. Remnant showers will be possible mainly west of the I-75
corridor, and cannot rule out a few embedded rumbles of thunder as
some elevated instability riding the nose of a low-level theta-e
gradient attempts to nudge its way across the Ohio/Indiana border.

As the parent MCS continues to weaken, shortwave energy currently
located over central Oklahoma will eject into the fast confluent
flow aloft and induce cyclogenesis as it races northeastwards
towards lower Michigan. Although the outgoing forecast calls for
likely PoPs Monday afternoon, there will likely be a lull in
activity late morning/early afternoon as we transition in between
forcing mechanisms. As the developing surface low tracks towards
lower Michigan, low-level southwesterly flow will strengthen and
advect a plume of deeper moisture into the region as PWATs surge
back over 1.50 inches by the evening hours.

Abundant cloud cover during the day will limit solar insolation, and
have gone below guidance for high temperatures Monday, with only mid
60s expected for most areas, although these readings may still be
too high. One area to keep an eye on, however, is across Washtenaw,
Wayne, Lenawee, and Monroe counties, as there are some weak signals
in the model guidance in a dry punch accompanying the southeast
flank of the decaying MCS. If this occurs, breaks in cloud cover
could occur and allow for temperatures to spike into the 70s, and
with dewpoints rising into the 50s, surface-based instability could
near 1000 J/kg.

Regardless, expecting the second round of showers and thunderstorms
to occur beginning mid/late afternoon and persisting through at
least midnight. Global guidance has now converged on this solution,
as the surface low/midlevel shortwave and tightening thermal
gradient to the northwest provide plenty of dynamics for this
activity. Much of the instability will be elevated in nature, as
forecast soundings depict a sharp low-level inversion that will make
it hard for most updrafts to become rooted in the boundary layer.
Heavy rainfall will be the main threat with thunderstorms, and at
this time not expecting any severe weather. The potential exists for
areas that receive the heaviest amounts of rainfall to exceed an
inch, and poor drainage flooding will be possible.

Another flooding concern will be the potential for renewed lakeshore
flooding across the shores of western Lake Erie and Saginaw Bay
Monday afternoon into Monday night. Easterly flow (Erie) and
northeasterly flow (Saginaw Bay) will ramp up as the pressure
gradient tightens along with any mesoscale pressure rise/fall
couplets associated with the MCS/later thunderstorms. With water
levels running high, a favorable wind direction and only a 10-15
knot increase could make the difference in vulnerable areas seeing
flooding.

Shower and thunderstorm activity will end from west to east late
Monday night, and will favor the more progressive of solutions with
the confluent flow aloft. Drying conditions will ensue Tuesday as
high pressure begins to build in from the upper Great Lakes.

LONG TERM / WEDNESDAY THROUGH SATURDAY

High pressure will continue to build across the Great Lakes for the
midweek period, along with rising upper-level heights. Anticyclonic
flow trajectories through the column and drier air filtering in from
the north will lead to a stable and quiet period of weather through
at least early Friday. During this period, temperatures will
gradually moderate as thicknesses increase, with 70s readings
getting into the lower 80s by Friday. Northern stream energy diving
out of the northern Plains will lead to an increasingly unsettled
pattern for the weekend with increasing chances for showers and
possibly thunderstorms.

MARINE...

Winds will vary in direction dramatically across the central Great
Lakes through Tuesday. A compact low pressure system will cross
southern Lake Huron on Monday. Speeds and accompanying waves are
expected to increase late Monday into Tuesday as the low exits the
region. Showers are expected Monday afternoon with this low pressure
system. Thunderstorms are possible Monday evening, mainly confined
to Lake St. Clair and west Lake Erie.

HYDROLOGY...

Showers are expected Monday - especially during the midday time
frame. Amounts should be less than a quarter inch with this
activity. During the late afternoon and evenings hours, additional
thunderstorm development is possible. Localized rainfall amounts of
around an inch are possible with these storms. The greatest chances
for thunderstorms will be along and south of Interstate 69.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

AVIATION.....BT
DISCUSSION...IRL
MARINE.......Mann
HYDROLOGY....Mann


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