Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 172302
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
702 PM EDT Tue Apr 17 2018


.AVIATION...

Difficult call on the amount of clearing overnight into Wednesday
morning, but given the lowering/strengthening low level inversion,
will maintain a rather pessimistic cig forecast with some breaking
out between 12z-16z. Any vfr period will be pretty brief though as
next approaching low pressure spreads light rain into area by 21z or
so with a return to mvfr and eventual ifr late in the forecast
period (22z-02z). The light rain will also change to light snow just
beyond the 24 hour forecast period.

For DTW...Mvfr to lower vfr cigs will lower back to solid mvfr
overnight. Will maintain some clearing after 12z, but confidence is
low on the actual amount of clearing. Light rain will spread into
terminal around 21z Wednesday and transition to -rasn or -sn in the
04z-06z time frame. At the same time, any vfr cigs will lower back
to mvfr or perhaps ifr.

/DTW THRESHOLD PROBABILITIES...

* High for ceilings aob 5000 ft through tonight into Wednesday, low
  much of the day Wednesday, medium for return of these cigs 21z-00z
  Wednesday.

* High for rain as precipitation type Wednesday afternoon, low in
  timing of transition to light snow Wednesday night.

&&

.PREV DISCUSSION...
Issued at 335 PM EDT Tue Apr 17 2018

DISCUSSION...

More snow on the way for Wednesday night.

Massive/near record cold upper level low encompassing the easterly
half of the U.S. this afternoon with the center over Eastern Great
Lakes/New England.

Upper level northwest confluent flow/subsidence tonight into
tomorrow. Tough call on how much moisture is trapped underneath the
lowering inversion, and if low clouds scatter out/clear resulting in
mins falling into the 20 to 25 degree range, otherwise around 30
degrees if solid low clouds hold tonight. Plenty of mixed signals,
with NAM/RAP indicating saturation at 925 MB, while regional
gem/euro drier and more optimistic. With the soaked ground and low
level winds coming around to the north, skewed the forecast toward
the cloudier side, with the best chance of clearing toward the
southern Michigan border.

Strong upper wave coming out of the Four Corners region this
afternoon, with good cyclogensis/spin up occuring over the Central
Plains this evening. As this tight/compact circulation approaches
southern Lower Michigan tomorrow, its going to merge with the
streaking upper wave coming out of Hudson Bay, and thus shunted off
to the east, into the Mid Atlantic states Wednesday night. Southeast
Michigan will reside on the cold side as surface low tracks through
the northern Ohio Valley, but boundary layer will need to cool to
support accumulating snow, and how quickly this occurs will determine
amounts. The other complicating factor is the the unstable airmass
over the Ohio Valley potentially producing convection, disrupting the
moisture transport or displacing the forcing south of the state. In
any event, it appears the good 700 MB FGEN/trowal will reach the I-69
corridor/Thumb region, which appears to be the prime area for snow
accumulations, as thermal profiles look better as well, with 850 mb
temps lowering to at least -6 C in the evening, to -8 to -10 C by
Thursday morning as the northern stream wave arrives.
Deformation/cold air then sweeping south late Wednesday night into
early Thursday morning will also likely result in a changeover to
snow for locations farther south as well, and could have issues for
the Thursday morning commute. However, surface temps should be more
in the 30-32 degree range vs the upper 20s we had this morning. Thus,
would not expect roads to be as icy, but would not rule out a narrow
swath of 3+ inches of snow, but in general, half an inch to 2 inches
should cover the majority of southeast Michigan (with those
accumulations being mainly on grassy/colder surfaces).

An upper ridge will begin to slide into the Great Lakes on Friday
along with a stable surface high pressure. This will bring dry
conditions and will also help promote some warmer temperatures.
Temperatures at 850mb will hover around the -1 to -3 C, but the
April sun angle and fewer clouds will help daytime highs begin the
climb closer to normal temperatures for this time of year. The only
exception will be in the Thumb where air coming off the relatively
cooler waters of Lake Huron under northeasterly winds will keep
temperatures cooler than the rest of southeast Michigan. A low
pressure swinging across the southern US will cut off any good
warm/moist air advection through the weekend. As this low moves east
through the Gulf states, southeast Michigan will be able to see some
southwesterly flow as the ridge slides to the east. This will allow
a subtle push of some warmer air into the region. This will result
in temperatures in the 40s and low 50s during the weekend increasing
slightly into the 50s for Monday and potentially the 60s for much of
the area by Tuesday.

MARINE...

Low pressure departing to the east in conjunction with deep mixing
depths from cold air aloft will maintain northwest winds with gusts
to 25 knots through this evening. High pressure building in from the
west late tonight will allow northwest winds to gradually decrease
before becoming light through much of Wednesday. Another quick-
moving low pressure system will then bring increasing northerly
winds Wednesday night. Colder air in the wake of this system will
again allow increased mixing depths with gusts approaching 30 knots
throughout the day on Thursday. High pressure then returns for the
end of the week bringing calmer conditions.

HYDROLOGY...

A quick-moving low pressure system tracking through the northern
Ohio Valley will bring another round of precipitation to Southeast
Michigan Wednesday night. This precipitation is expected to begin as
rain Wednesday evening before transitioning to snow by late
Wednesday night. A widespread 0.2 to 0.4 inch of precipitation is
expected to fall by early Thursday morning. While precipitation will
be generally light, it will fall on top of saturated ground from
recent rounds of heavy precipitation that has led to rises in area
rivers and streams. A flood warning will continue for the Cass River
at Frankenmuth downstream to Bridgeport where the river is expected
to crest late Tuesday night. A flood warning will also continue for
the Saginaw River at Saginaw where the river is expected to crest on
Wednesday afternoon.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 10 PM EDT this evening for LHZ421-422-
     441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.

&&

$$

AVIATION.....DG
DISCUSSION...SF/AA
MARINE.......JD
HYDROLOGY....JD


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