Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 151733
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
133 PM EDT Sun Oct 15 2017


.AVIATION...

The cold front and chances of convection/thunderstorms has exited
east, with cold advection and mainly MVFR ceilings expected into the
evening hours. Still could be a few more light showers around to
start the period, before strong westerly winds further dry out the
low levels. Looking at westerly winds in the 15 to 20 knot range,
with gusts in the 30 to 35 knot range for another couple of hours,
with winds shifting to the northwest and decreasing this evening.
Northwest flow with the cold airmass coming off Lake Superior-
Michigan may be able to sustain low cloud (2500-4000 kft) streamers
(SCT coverage) overnight into Monday morning. Diurnal VFR CU up
expected tomorrow, as light winds also back to the west.

For DTW...Winds have shifted due west, but have come down
dramatically, and should remain below 35 knots before decreasing
further and shifting to the northwest.

Upstream observations support carrying MVFR/Low VFR ceilings late
this afternoon into evening hours, with the timing of the clearing a
bit more uncertain, but somewhere near 3Z appears reasonable.

//DTW THRESHOLD PROBABILITIES...

* High in ceilings below 5000 ft through early evening then low.


* Low in crosswind thresholds being exceeded this afternoon.

&&

.PREV DISCUSSION...
Issued at 426 AM EDT Sun Oct 15 2017

DISCUSSION...

Prototypical, autumn mid-latitude cyclone set to bring yet another
15 hours of active weather to southeastern Michigan. 06Z MSAS
analysis centers a 1001mb surface low over Manitowoc, Wisconsin.
This supports a bulk of the deterministic solutions which depicts a
surface low track through the Straits at 12Z this morning. Main
forecast concerns today are trying to gain a handle on heavy
rainfall potential that may still exist this morning, as well as
providing discussion on the strong west winds set to develop during
the late morning and early afternoon.

With the surface low in place over eastern Wisconsin, the main warm
advection sector south and east of the low has lifted into place
over most of Lower Michigan. Convective activity from Grand Traverse
Bay down to Lansing is within this very impressive 850-650mb
equivalent potential temperature maximum. Per observational trends
from the overnight, precipitation rates as high as one half inch per
hour would seem possible yet with this area of precipitation
especially with the environment of current PWATS of 1.60 to 1.70
inches, some 250 percent of normal. However, some serious
reservations exist as models suggest that ongoing activity will
weaken while shearing out rapidly to the northeast as a secondary
jet streak emerges out of the subgeostrophic region of the deep
midlevel trough axis. A loss in precipitation coverage is being
advertised after 09Z in a large subset hi-res solutions including
the namnest, ARW, NMM and regional GEM. Given this expected lull,
and the current fading of reflectivity in KDTX 0.5Z, expectations
are for the heavy rainfall potential to break down into an isolated
coverage. Will note that trajectories and kinematics remain
favorable for activity over MO/IL to streamline out ahead of the
front later this morning in the 13-17Z timeframe. Therefore, it does
remain possible that Detroit, which has largely avoided much of the
precipitation to this point, will still get in excess of a quarter
inch of rainfall today.

In response to the center of surface low pressure lifting northeast
of the Straits late this morning, the main surface cold front will
push across southeastern Michigan. Model sounding suggest a fair
amount of higher static stability in place through as much as the
lowest 6.0 to 7.0 kft agl. It will take streamlining, and pooling of
aforementioned moisture from the southwest directly in front of the
frontal boundary to erode the lower tropospheric capping. Certainly
an interesting mix of forecast soundings, particularly both from
early iterations of RAP and the 15.00Z ARW which suggests that
SBCAPE as high as 400 J/KG will be possible between the 14-17Z time
window. Relatively lower confidence exists that everything will be
able to come together soon enough to result in thunderstorm activity
over southeastern Michigan. Trends in explicit model solutions has
been to show greater potential for surface based convection is to
the east over OH/PA. Given the impressive environmental wind field
with 50 knots possible down to 2500 ft agl, any thunderstorm or deep
convective activity will be capable of producing damaging wind gusts
to 60 mph. Area of interest for southeastern Michigan is extreme
southeastern portions, including the city of Detroit. While current
thinking is the convective potential is not likely, it will be
nonzero.

A synoptic wind threat will then develop for the afternoon hours as
shallow cold air advection steepens the lower tropospheric lapse
rates. Main message to communicate is model data seems to have
arrived with slightly weaker wind potential as the front wave
response is set to be more flatter. The less amplified solutions
appears to have an impact on system relative flow trajectories,
impacting the efficiency of subsidence in the wake of the system.
Straight mixing potential from soundings suggests that in many areas
a best case would be to mix into 40 knots off of the deck, slight
better up north. Never a 1:1 given eddy dispersion principles. With
this in mind, no changes will be made to the Wind Advisory for the
northern half of the cwa, wind gusts to 45 mph. Will need to watch
for shoreline areas adjacent to Saginaw Bay, where onshore winds of
50 mph will certainly be possible.  For areas south of the advisory,
winds will veer from the southwest to the northwest this afternoon
with wind gusts of 35 to 40 mph. High confidence exists that winds
will weaken rapidly by evening.

Weather will then quiet substantially for next week as a sprawling
surface ridge build into the majority of the eastern contiguous U.S.
First item to note is very cool temperatures tonight. Readings are
expected to drop into the middle to upper 30s for most areas away
from local waterways. No frost headlines will be issued. Second item
to note is that a trailing shortwave will dig through the central
Great Lakes on Monday. Quality of midlevel moisture is progged to be
very low. Will not include any shower mention in the forecast, but
expectations are it will illicit some sort of lake effect response.
Much cooler conditions Monday with temperatures limited in the
middle 50s.

MARINE...

A deepening area of low pressure will track across northern Lake
Huron today, swinging a cold front through the rest of the local
waters this afternoon. Ahead of the frontal passage today, southwest
winds will quickly ramp up to gale force across Lake Huron, Lake
St. Clair, and the western shores of Lake Erie this morning before
swinging to the northwest this afternoon. Sustained gale force winds
will be possible over the open waters of northern Lake Huron, with
frequent high-end gale gusts possible across the rest of the waters.
Slightly more stable air should limit any potential for a rogue
storm gust over the Lake Huron open waters. In response to the
fresh, strong winds today, waves will rapidly build to 15 feet on
Lake Huron with peak wave heights as high as 20-25 feet. On Lake St.
Clair and the western shores of Lake Erie, waves will build to
around 5 feet. The strong northwest winds will also lead to low
water levels today for the western shores of Lake Erie.

In the wake of the departing low pressure, lingering low-level
moisture and unstable conditions will promote the potential for
waterspouts over the Lake Huron waters tonight and into Monday.

Winds and waves will rapidly diminish briefly on Monday as high
pressure builds across the region. Winds will shift to the southwest
in response to the high, but briefly ramp up once again due to a
tightening pressure gradient as low pressure passes by well to the
north, with gusts in the 25 to 30 knots range over Lake Huron Monday
night into Tuesday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory until 8 PM EDT this evening for MIZ047>049-053>055-
     060>063.

Lake Huron...Gale Warning until 5 AM EDT Monday for LHZ362-363-441>443-462>464.

     Gale Warning until 8 PM EDT this evening for LHZ361-421-422.

Lake St Clair...Gale Warning until 8 PM EDT this evening for LCZ460.

Michigan waters of Lake Erie...Low Water Advisory until 11 PM EDT this evening for LEZ444.

     Gale Warning until 8 PM EDT this evening for LEZ444.

&&

$$

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


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