Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 162308
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
708 PM EDT Wed Aug 16 2017


.AVIATION...

VFR conditions expected into tonight as high pressure shifts off to
the east. An approaching warm front will bring MVFR cigs/vsby
potential late tonight with scattered convection associated with
this front on the day Thursday. It appears MVFR stratus will hold in
a rather grungy setup along/north of the front most of the day.
Winds will veer from southeast to southwest with its passage late
with gusts of 20 knots quite possible late in the day.

For DTW...Low level moisture increases substantially overnight
through Thursday with ceilings below 5kft on and off through the
balance of the forecast. Two possible waves of thunderstorms are
anticipated - one in the 12-16z window and the other during the mid-
late afternoon on Thursday. Will not adjust going forecast at this
time as these windows remain reasonable. South winds will become
gusty Thursday afternoon.

//DTW THRESHOLD PROBABILITIES...

* Medium for ceilings at or below 5000 ft after 09z-11z.

* Medium for thunderstorms affecting the airspace Thursday morning
  and late afternoon.

&&

.PREV DISCUSSION...
Issued at 341 PM EDT Wed Aug 16 2017

DISCUSSION...

Surface ridging tucked beneath weakly confluent mid level flow will
sustain deep layer dry and stable conditions heading into tonight.
Attention shifts to elongated shortwave energy now noted on water
vapor ejecting out of the Rockies. This energy will consolidate into
a deeper wave within the next 24 hours as it settles into the upper
MS valley. The immediate kinematic response downstream will result a
substantial northward surge of moisture lodging within a deepening
southwest flow. The effective system relative isentropic ascent tied
to a burgeoning warm conveyor along the eastern flank will then
shift this moisture directly toward the Great Lakes on Thursday.
CAMS model solutions are now indicating a progressive early response
locally along the advancing 850 mb warm frontal slope, with the lead
mid level theta-e gradient working to saturate the column and
potentially generating a scattered coverage of showers by the 08z-
11z window. Steady deep layer moisture flux and diminishing mid
level stability thereafter as the warm front lifts through will then
bring a higher probability for elevated convective development from
mid morning through the early afternoon hours. High moisture content
- PW over 2 inches - could certainly lead to some localized
regions of heavy rainfall.

A very moist warm sector will briefly intrude southeast Michigan
late Thursday. Model guidance resoundingly push dewpoints of lower
70s up into the region within this narrow pre-cold frontal window.
Destabilization potential governed by the degree of removal and
overall exit timing of the early day cloud debris/precip. Given the
moisture content, plausible this environment simply freely convects
with persistent pockets of weakly organized convection prior to the
arrival of the pre-frontal trough. This in turn would short circuit
the prospects of realizing better airmass recovery. In addition,
lapse rates will remain paltry given the moist profile, so there
remains a ceiling for potential instability despite the high
dewpoints. However, a more progressive exit of warm frontal debris
could still allow for a moderate level of instability to emerge -
MLCAPE perhaps reaching 2000 J/KG. Should this materialize, then
bulk shear in excess of 30 knots would support deep organized
updrafts. General signal for a multi-cell mode, with the prospects
for wet microbursts elevated given high cape density within heavily
precip loaded updrafts. In addition, heavy rainfall a definitive
threat. Greatest window for possible severe convection centered 21z-
01z.

Frontal passage Thursday night will sweep the deep moisture plume
eastward. Simply a subtle airmass change in terms of temperatures,
primarily noteworthy for the reduction in moisture content. A
moderately mixed profile up through 850 mb will support highs
ranging from mid 70s north to lower 80s south. A firm west-southwest
gradient contained within the southern bounds of a surface low
lifting north of lake huron will provide gusty conditions during the
daylight period. Just enough lingering moisture working through the
backside of this system to perhaps generate some isolated showers
across the north.

A broad surface high pressure system building east from Iowa into
the Ohio Valley will look to keep conditions relatively dry
throughout the weekend, with the exception of Saturday morning and
afternoon, where troughing aloft with an embedded upper-level
disturbance will bring the chance for showers and thunderstorms.
Otherwise, both Sunday and Monday will look to stay on the dry side,
as south to southwesterly flow provides ample heat to push daytime
highs in the mid 80s, and possibly even the upper 80s across the
Metro Detroit area. The next chance for showers and thunderstorms
will return on Tuesday as a cold front pushes across the state. Dry
air will filter back in behind the front for the mid-week period, as
temperatures stay capped in the mid to upper 70s for a high.

MARINE...

Southeast to south flow will increase substantially Thursday as a
storm system moves across the Great Lakes region. Showers and
thunderstorms are expected with this system on Thursday. Behind the
cold front, Thursday night, moderate southwest to west flow will
setup. The stronger winds will encourage higher waves to initially
setup in the southeast flow along the Lake Huron shoreline, then
transition to a more typical offshore flow setup with higher waves
in Saginaw Bay on Friday.

HYDROLOGY...

A very moist airmass will advance into Southeast Michigan during the
day Thursday. Scattered showers and thunderstorms are expected along
the incoming warm front. Additional showers and thunderstorms are
expected to develop in the moisture rich environment during the
afternoon and evening hours Thursday. Rainfall amounts in excess of
a half inch are possible with localized amounts exceeding one inch
with the strongest thunderstorms.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 11 AM Thursday to 8 PM EDT Friday for
     LHZ441-442.

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

$$

AVIATION.....DG
DISCUSSION...MR/AM
MARINE.......Mann
HYDROLOGY....Mann


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