Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 180915
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
415 AM EST Sat Nov 18 2017


.AVIATION...

Next several hours marked by intermittent -RA/-DZ in the Detroit
area with accompanying IFR. Moderate rain lifts across the area
after 18z with potential for +SHRA with any embedded convection.
Late aftn changeover to snow at KMBS/KFNT possible with minimal
accumulation, though a full changeover during a heavier burst could
feasibly coat the runways. High likelihood for IFR/LIFR throughout
this time while light winds back from S to NE. Wind shift to NW will
coincide with onset of cold advection and developing wind gusts to
30 kts or more...highest and longest duration in the Detroit area.
CIG/VSBY improvement to at least MVFR likely as CAA increases
mixing. Stout NNW wind gusting to 20-25 kts settles in and gradually
backs to NW through the nighttime hours.

For DTW...Noise abatement issues become increasingly likely as wind
backs to NE/N after 18z. This will be followed by developing
crosswind concerns from 340-310 degrees for much of the evening and
potentially the overnight hours. Rain may taper off as a few
snowflakes, but scouring out of deeper moisture, a warm column, and
warm/wet ground suggest virtually nil threat for impacts.

//DTW THRESHOLD PROBABILITIES...

* High for cig aob 5kft.

* Low for cig aob 200ft approx 13-17z.

* Moderate for exceeding crosswind threshold from 340 to 310 degrees

* Moderate in crosswind thresholds being exceeded Saturday evening
  after 22z.

&&

.PREV DISCUSSION...
Issued at 310 AM EST Sat Nov 18 2017

DISCUSSION...

Lead shortwave and associated rainfall was pushing east of the
forecast area early this morning.  Attention now turns to the main
wave over the high plains of CO/KS/NB as it moves into the Lakes
region by this evening. Associated with this wave is the deepening
surface low. Models are still struggling with the location, the
strengthening and the timing of this surface low.  Models continue
to be slower with the timing of the deepening of this feature with
the slightly flatter nature of the upper wave. This keeps the surface
low south of the state and moves it from northern IN to central Lake
Erie this afternoon and evening while deepening it into the mid
980s mbs.

The bulk of the forcing with the lead wave has pushed east of the
forecast area. However a ribbon of 700-500 mb FGen across far
southern Lower MI with modest isentropic lift/moisture advection
will keep numerous showers in the forecast for the areas south of M
59 during the morning.  Just a chance of showers north of that
during the morning hours. Between 15z and 18z, very good mid level
Fgen and deformation works back over lower MI and continues through
00z. Also associated with that will be a little couple jet dynamic
from the right entrance region of the jet across the northern lakes
and the left front exit region of the 140kt jet cutting into the
Ohio valley. Precipitable water will be just under an inch for this
event while mid level lapse rates may be just steep enough with the
dynamics to have a rumble of thunder for far southeast Lower MI.
Without a doubt, the best instability and moisture will lie across
northern IN and northern OH near the warm front, but think there
will be enough moisture and forcing between 700 and 300 mbs to bring
widespread half inch rainfall from 18z to 00z for southeast Lower
MI. Will lean toward the cooler end of guidance temps with the
surface low staying south of the state with all the clouds and rain.

NAM is aggressive with the column cooling for the Tri Cities and FNT
areas from 22z to 00z early this evening. Local ensemble output
suggest a non-zero chance of accumulating snow, but more like 20 to
30 percent chance. Will certainly have snow mixing with the rain as
it ends during the evening across much of the forecast area, but
will hold off on any minor accumulations because of how quickly the
forcing pulls out of the region.

As the low deepens winds will increase this evening. Ensemble output
is suggestive of 35 to 40 mph peak wind gusts. This looks good given
not much more than 40 kts in the low level mixed layer.

Expect Sunday to be cloudy with plenty of flurries or even full
fledged snow showers through early afternoon. A good secondary
shortwave passes through over the forecast area and model soundings
indicate good shallow instability up to 8 to 10k feet. That should
be plenty good for scattered snow showers.  Will lean toward a
cooler forecast highs mostly in the mid 30s on Sunday rather than
around 40 degrees of guidance. Hourly model guidance suggest that it
will be a struggle to get up into the mid 30s let alone 40.

After this weekend, weather will be a little quieter as far as
precipitation. Warmer, but seasonable conditions expected on Monday
and Tuesday. A cold front passing through the state on Tuesday will
be accompanied with wind both ahead and behind it. Temperatures will
again fall back to seasonably cool conditions for Wednesday and
Thanksgiving.

MARINE...

Rapidly strengthening low pressure will lift from central Indiana
across Lake Erie this afternoon. As it does so, NNW wind will
strengthen bringing widespread gales to the waters. Portions of the
southern Lake Huron basin may see periods of sustained gales
overnight. Significant wave heights of 12 feet or more will be
common with maximum wave heights potentially eclipsing 20 feet in
the open waters. Gale warnings remain in effect for all marine
zones. After a period of moderate to fresh westerlies Sunday night
through Monday night...potential for SW gales develops early Tuesday
as low pressure tracks across northern Ontario.

HYDROLOGY...

Light rain will remain mainly focused south of the M59 corridor for
the next several hours. Rain coverage to the west will fill in
throughout the morning as it tracks closer to the area. By this
afternoon, all of SE Michigan will see moderate rain. Additional
rainfall totals will range from around 0.5" in the Saginaw Valley
area to an inch near the Ohio border. The heaviest rain will be
focused in the 1pm to 7pm time frame. Minor urban flooding remains a
possibility.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 3 PM this afternoon to 7 PM EST Sunday for
     LHZ361>363-462>464.

     Gale Warning from 7 PM this evening to 10 AM EST Sunday for LHZ421-
     441>443.

     Gale Warning from 7 PM this evening to 7 AM EST Sunday for LHZ422.

Lake St Clair...Gale Warning from 7 PM this evening to 10 AM EST Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning from 7 PM this evening to 10 AM EST Sunday for LEZ444.

&&

$$

AVIATION.....JVC
DISCUSSION...RBP
MARINE.......JVC
HYDROLOGY....JVC


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