Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI
FXUS63 KDTX 200152
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
952 PM EDT WED OCT 19 2016
The beginning stage is underway for what will eventually be a
textbook midlevel fgen precipitation event for southeastern
Michigan tonight and Thursday. All things considered this will
likely end up as a relatively longer duration event that may take
until 00Z Fri to get the last of the stratiform rain shield out of
the eastern cwa. Radar to this point looks ominous with
considerable virga, but some major headwinds persist for the
remainder of tonight. Namely, deep dry air in the lowest 12 kft
agl on 00Z KDTX raob and residual upper level ridging and
anticyclonic flow trajectories aloft over far semich. Difficult to
move away from chance pops this evening far south and likely
between 06-12Z tonight. Main motivation on the update was to increase
PoPs to very high categorical for a good portion of the southern
CWA for the day Thursday. High confidence in these high
categoricals. The one forecast item that needs to be ironed out is
the northern gradient to the PoPs. Guidance remains at odds with
amount of precipitation that will fall over the Saginaw Valley.
Will allow the midnight shift the full 00Z NWP suite and make this
Issued at 759 PM EDT Wed OCT 19 2016
Regional Z mosaic and satellite trends supports the latest NAM
timing with regards to midlevel forcing. Saturation has become
increasingly organized from central Lower Michigan southwest to
portions of southern Wisconsin and northern Illinois and also
across far northern sections of Indiana and Ohio. This largely
matches up with increasing system relative confluence and
deformation that is occuring between 700-600mb. It is going to
take a fair amount of time to saturate the lowest 9000 ft agl...so
many areas will be observing virga. Better convergence and low to
midlevel fgen is expected to lift northward over the far southern
terminals after 04Z. Frontal structure and lift will be better in
this corridor and will likely see the beginning of the numerous to
categorical rain chances after 4z for the Detroit terminals.
Midlevel convergence, fgen, deformation will only contract and
increase in magnitude after 12Z Thursday. The inherited TAFs
remain on point with MVFR cloud and moderate rain after 12Z. There
is some evidence that a narrow mesoscale band of rain may
develop. uncertainty would then exist on where the band will
reside, over the Detroit terminals or across the northern Detroit
suburbs and PTK.
For DTW...Developing N wind component may result in noise
abatement concerns by Thursday morning. There appears to be
moderate confidence in a forcing to northeasterly flow operations
as crosswing threshold may be met. Surface winds speeds are
expected to remain relatively muted for this event.
//DTW Threshold Probabilities...
* Low confidence in cig below 5kft through 12z. High confidence
Issued at 339 PM EDT Wed OCT 19 2016
The next system to target southeast MI is coming together as planned
thus far. Upper level trough is digging through the Central Plains
this afternoon and will continue to deepen and amplify through
Thursday night as it travels further east, when the axis aligns with
the Appalachians. Cyclogenesis ahead of the trough is well underway
with a strung out surface low developing over the Mid Mississippi
Valley. Models have been very consistent with the track of this low
NE through the Ohio Valley tonight through Thursday, with the
deformation zone and elevated fgen band passing through southern MI.
Enhancing this region of fgen will be a cold front currently
dropping through WI which will reach mid MI early Thursday morning.
This will encourage some compaction of the elevated front around
700mb over Saginaw Valley. High res models are advertising this but
questions remain as to whether it will be too far removed from the
lower end of the front with the deeper moisture to actually result
in much precip. Also working against the north end of the CWA will
be persistent northerly flow bringing in drier air, reinforcing the
already dry boundary layer. Soundings show the dry layer to be
around 10kft deep which matches well with the ceilings reported from
current METARS across mid MI. Top down moistening will drop ceilings
toward 5-6kft so expectations are for light rainfall tonight,
tapering off later Thursday. Further south around Metro Detroit, the
fgen will be stronger and lower on the frontal surface with more
moisture to work with as the theta e plume slides across far SE MI.
Light rain should begin after 03Z this evening with the steadier,
moderate rain mainly falling between 12-18Z. This will be when the
right entrance region of the approach jet slides through lower MI.
Overall looks to be a longer duration rainfall for SE MI starting
tonight and finally tapering off early Friday morning. Rainfall
amounts will approach 0.75 inches across the south, tapering off as
you head north. Could be a tight gradient around PTK to FNT where
the qpf drops off quick.
The cold front will usher in a colder airmass for Thursday, and
combined with ample cloud cover will result in highs only in the
mid/upper 50s. A backdoor cold front could then brush SE MI on
Friday bringing even cooler air into the Great Lakes. 925mb temps
will fall to around 2C through this time keeping temps in the low
50s. High pressure building into the region with the thermal trough
overhead will allow temps to fall into the mid 30s Friday night.
Another chilly but dry day Saturday with the thermal trough
overhead. Back to seasonable temperatures on Sunday with rising
heights. Medium-range models depict a quick-moving shortwave in
northwest flow Sun/Sun night, however current consensus favors the
best forcing remaining north of our CWA. High pressure builds back
in for early next week, with seasonable temperatures continuing.
Light southwest winds will veer to the north and strengthen through
Thursday. Moderate to fresh winds will continue out of the north to
northwest Thursday evening through Saturday as cooler air spreads
over area waters. The persistent N to NW fetch will build waves to
hazardous levels in the nearshore zones of Lake Huron as early as
Thursday night. An extended period of Small Craft Advisories for
both winds and waves is likely between Thursday night and Saturday.
A frontal zone will slowly move through the area as low pressure
lifts from Paducah to Buffalo. Light to occasionally moderate rain
will develop on the northwest side of this system bringing up to one
half inch of rain to areas along and south of I-69 between late
tonight and Thursday evening. The heaviest rain is forecast to be
the heaviest between approximately 8am and 2pm Thursday during which
time at least half of the forecast amount of rain is expected to
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
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