Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 141606
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1206 PM EDT Sat Oct 14 2017

.AVIATION...

Frontal boundary near the southern Michigan border will be slow to
lift north this evening, helping to sustain shower activity,
possibly even across southern TAF sites, with predominantly IFR/MVFR
conditions north. Strengthening low level jet tonight will lift the
front north, allowing surface winds to shift to the south and
increase toward sunrise. However, before the surface winds
sufficiently increase, could be a period of low level wind shear as
2 kft winds increase aoa 50 knots. A low chance of thunderstorms
remains through tomorrow morning, but too low to include at this
time. Strong cold front looks to be sliding through late tomorrow
morning/around noon, with westerly post frontal winds gusting between
35 to 40 knots through the early afternoon hours.

For DTW...Terminal looks to be on the southern fringe of the showers
and low chance of thunderstorm this afternoon, with uncertainty in
how long the low clouds will hold as warm front accelerates off to
the north this evening/tonight. Forecasting VFR conditions for
the majority of TAF period, but confidence is shaky and could see
cigs lower just about anytime. Strong cold front expected to pass
through by Noon Tomorrow, with westerly winds gusting to around 40
knots during early afternoon hours.


//DTW THRESHOLD PROBABILITIES...

* Low in ceilings below 5000 ft into tonight, high confidence late
  tonight into tomorrow morning

* Low in thunderstorms through tomorrow morning.

* High confidence in westerly winds (250 degrees) exceeding 35 knots
  by Noon Sunday, and continuing Sunday afternoon.

&&

.PREV DISCUSSION...
Issued at 442 AM EDT Sat Oct 14 2017

DISCUSSION...

An extremely active forecast period is anticipated during the next
48 hours for southeastern Michigan as a deep, progressive upper
level trough complex digs rapidly from Montana this morning to parts
of far eastern Quebec by mid afternoon Sunday. Forecast data
suggests leeside cyclogenesis commencing over CO/KS by this
afternoon, with a deepening surface low tracking across northern
Lower Michigan Sunday morning. There is a lot to discuss and will
touch on each forecast concern by the day or time period.

This morning...A numerous to widespread coverage of showers have
persisted overnight along the 850-725mb layer of a baroclinic zone
that is becoming increasingly organized over the northern 1/3rd of
the cwa. Not a exceptionally clear system relative isentropic ascent
signal, rather activity has seemed to be a result of the magnitude
increase of midlevel theta e ridge that is now nosing into the
region. The theta e ridge is bleeding to the east off of
anticyclonic trajectories to the initial low level jet surge. System
relative isentropic progs suggests that flow trajectories will
transition more out of the east this morning which should cause this
initial activity to fall apart or merely release eastward in this 09-
12z time window. No significant weather expected this morning.

Around 14Z this morning...Lower Michigan will see a considerable
increase in both synoptic scale and mesoscale forcing, particularly
for areas north of I 69. Temporary increase in synoptic forcing will
come in the form of rear right quadrant dynamics as entrance region
to initial jet streak positions over the northern Great Lakes.
Concurrently, a significant backing of midlevel wind directions will
occur as first impulse of low level jet will lift through the far
southern portions of Lower Michigan. Impressive 850mb front will
sharpen both in response to the nose convergence and the increase in
frontogenesis from jet stream ageostrophic component. Thunderstorm
activity that has developed upstream over Illinois and eastern Iowa
is associated directly to this forcing. Bulk of deterministic
solutions and hi-res explicit convection model runs funnel this
ongoing convection to the northeast while weakening it with time.
Expectations are for an increase in rainfall, remnant convection
north of I69 between 14-18Z. High confidence in weak activity during
that time period with high static stability in the lowest 8 kft agl.

Around 20Z this afternoon...Models are keying off of a secondary
surge of deep moisture lifting out of portions of far southern KS
and MO this morning. This moisture is forecasted to rotate
anticyclonically around the southeastern U.S. ridge and interact
with and override the main baroclinic zone over southern Iowa (same
one that will be organizing over central Lower Michigan). This
interaction is expected to result in additional organized
thunderstorm over portions of IA/MO by 12Z. This thunderstorm
activity will then track along the boundary today, entering portions
of  the northern cwa north of I 69 in the 20-00Z time window. It is
with this particular activity that a heavy rainfall threat will
begin.

After 03Z tonight...Some signal exists in the models that a short
lull in convection could occur between 00-03Z this evening
(confidence in this is shaky) before another round of eye raising
700-500mb deformation centers on the Tri Cities region. Overall,
depth of moisture and magnitude of equivalent potential temperature
content is very high. A second round of midlevel deformation that
will reside within a very broad, and impressive triple point region
attendant to the approaching strengthening mid latitude cyclone.
Needs to be highlighted that depth and amount of deep moisture will
be impressive with PWATS climbing above 1.60 inches this evening.
Heavy rainfall is a concern with some models suggesting amounts
reaching and exceeding 2 inches for some areas. As a result, have
issued a Flood Watch for Bay, Midland and Saginaw Counties. The
concern with the setup this afternoon through tonight will be the
possibility of training of thunderstorms. Subsequent shifts can
monitor the amount of precipitation that will fall today and makes
changes to headlines as necessary. While confidence is high that
large scale forcing will increasingly center on the area, there is
some uncertainty that exists. The primary reason for uncertainty
the overwhelming dominance of anticyclonic flow trajectories and
shortwave component that could hold on over southeastern Michigan.
Latest Swody1 outlook has all of southeastern Michigan in a marginal
risk for severe weather with Tri Cities in a Slight Risk. The
potential for severe weather appears conditional to the positioning
of the front. A northern solution of the front (north of the Tri
Cities) brings a greater strong to severe thunderstorm risk
primarily in the 04-08Z timeframe. The severe threat appears
contingent on whether or not the activity remains purely elevated in
nature. Shear will be extremely high with storm relative helicity
values exceeding 400 m2/s2, making embedded mesocylone structures
possible. There are a couple of significant limiting factors,
marginal CAPE perhaps up to 250 J/KG, and stability that will remain
too high in the near surface environment. Consensus suggests high
static stability persisting in the lowest 3500 ft agl.

Important to stress here that confidence in precipitation occurrence
is lower south of I 69 today and tonight, particularly for the
Detroit Metro area. Certainly possible the southern cwa measures
very little precipitation, given the degree of shortwave ridging.

Sunday...Main weather narrative for Sunday will be strong wind
potential that will develop during the morning and persist into
afternoon hours. Explicit convection models are all consistent with
developing a broken line of low topped convection in the 13-16Z
timeframe for all of southeastern Michigan which would carry a
potential for stronger wind pop. The strong wind potential would
then transition to deeper mixing, downward momentum transport during
the afternoon with increase in cold air advection and steepening low
level lapse rates. Still a bit of uncertainty with exactly how this
will play out, what wind gusts will be as probabilistic guidance is
lowering probs for +40 knot gusts across southern portion of
forecast area. Important to note that ECMWF was slight weaker with
surface low here in lower Michigan and slightly farther north with
surface track. Still time yet to fill in the details on higher wind
potential Sunday. Forecast will read wind gusts of 40 to 45 mph
possible Sunday.

MARINE...

A slow-moving frontal boundary will stall today and tonight near
Saginaw Bay and central Lake Huron, as an area of low pressure
develops along it. This area of low pressure will move northeast
along the stalled frontal boundary into Sunday across central Lake
Huron, allowing the frontal boundary to move east of the region into
Monday. Showers and thunderstorms will be possible through Sunday
morning, with heavy rain possible especially tonight.

South of the boundary, south to southwest winds will prevail, but
veer to the northwest north of the boundary and in the wake of the
boundary as it moves east on Sunday. Wind gusts will generally
remain 20 knots or less into this evening, but increase late tonight
into Sunday as the area of low pressure tracks across the area.

For Sunday, there is uncertainty with the onset timing of peak winds
and gusts, with the 06Z-15Z timeframe currently the favored period
for gusts to Gale force beginning over Lake Huron and further south
to Lake St. Clair and the western shore of Lake Erie. Peak wind
gusts to 45 knots will be possible especially over the open waters
of Lake Huron, with gusts around 35 knots possible across Lake St.
Clair and the western shore of Lake Erie. These winds will result in
wave heights well in excess of 10 feet in the open Lake Huron
waters. Along the nearshore waters, 3-5 foot waves will be possible,
except along the Thumb where waves of 4-8 feet will be possible.

Winds and waves will then improve markedly Sunday night as a ridge
of high pressure begins to builds into the region.

HYDROLOGY...

A slow-moving frontal boundary will stall across Saginaw Bay and
central Lake Huron today and tonight, with developing low pressure
helping to focus multiple rounds of rain and embedded thunderstorms
across portions of the region. Rain will gradually become more
widespread across the region throughout the day, with rounds of
heavier rain possible mainly north of the I-69 corridor late this
afternoon and again tonight.

Forecast models continue to be in good agreement with the heaviest
rain axis being located north of the I-69 corridor, where a
widespread 1-2 inches of rain will be possible, with localized
higher amounts in excess of 2 inches. With these forecast rainfall
amounts, a Flood Watch has been issued for the Tri-Cities region for
late this afternoon and through much of tonight. Rainfall will end
across the region Sunday as the area of low pressure and frontal
boundary exit east of the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Flood Watch from 4 PM EDT this afternoon through late tonight for
     MIZ047-048-053.

Lake Huron...Gale Warning from 8 AM Sunday to 5 AM EDT Monday for LHZ361>363-421-
     422-441>443-462>464.

Lake St Clair...Gale Warning from 8 AM to 8 PM EDT Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 8 AM to 8 PM EDT Sunday for LEZ444.

&&

$$

AVIATION.....SF
DISCUSSION...CB
MARINE.......IRL
HYDROLOGY....IRL


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