Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 180741
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
341 AM EDT Wed Apr 18 2018

.DISCUSSION...

Much attention remains on the strong and compact upper level low
that will race across the northern Ohio Valley today and tonight,
spreading a mix of rain and snow across southern MI. After this low
exits Thursday morning dry weather moves in with a much anticipated
warming trend.

The wave is currently cutting off the base of the trough over the
western conus lifting up into Canada. It will track from the central
Plains to near southern Lake MI by 00Z this evening before getting
absorbed by a trough rotating down through the eastern Great Lakes
tonight. This will kick the wave east of the area by Thursday
morning. The lead leaf of isentropic ascent north of the occluding
front will move in this afternoon, likely around 21Z, producing rain
as temps this afternoon rise into the low to mid 40s. Northerly flow
will increase overnight as the wave and trough phase creating a
northerly gradient. Cold air advection sweeping southward will
change ptype from rain to snow soon after dark across the north,
with a slower transition occuring farther south as warm air rotating
up around the low is slower to give way. Forcing will be decent at
times showing fgen lighting up at both 850mb across far southern MI
tight to the center of the low, and a 700mb front up across the
Saginaw Valley and Thumb. The 700mb front could be enhances by
compaction of the elevated front as the northern trough sinks down
into it. One trend noted in model runs is the stripping out of
moisture. Most models offer up less QPF and more front end loaded
with dry air quickly scouring out the mid levels on the backside of
the system. Model soundings don`t really support a strong moderate
to heavy snow look to them. Though the profile becomes saturated up
to 8-10kft, it`s nearly isothermal around -5C most of the column.
Some hints there could be a couple hour period where the top of the
saturated layer dips below -10C though. Overall not looking for good
snowfall rates once the changeover does occur. Duration of snowfall
is also going against higher totals. Looks like only 3-6 hours for
any location as the system sweeps through. Taking into account poor
temperature profiles, transient nature of the forcing and lack of
duration, forecast snowfall amounts will be around 1 inch for most
locations. Lower amounts likely across the far north due to lowest
QPF and also across the far south due to increased duration with
warm air locked in place. If these fgen bands can line up with the
precip and cold air, not out of the question to get a burst of
higher snow rates but confidence is too low to try to pin point.

Upper level ridge building over the western Plains and Rockies will
push quickly into the western Great Lakes on Thursday. Meanwhile, an
expansive surface high will build across much of the central and
eastern conus by Friday. These features will hold over the region
through the weekend and into next week providing a long duration of
dry weather. Not a good warm air advection pattern as a cutoff low
will drift across the southern conus, blocking the potential influx
of warm air, but will see a warming trend regardless as heights
slowly rise and the mid/late April sun is able to help moderate the
airmass. Highs will into the mid 50s this weekend with 60s coming
next week.

&&

.MARINE...

Northwest winds across Lake Huron will weaken this morning as high
pressure moves over the eastern Great Lakes. Northerly winds will
then develop this afternoon and strengthen tonight into Thursday.
This will be the result of low pressure traversing the Ohio Valley
this afternoon and evening, with a tightening northerly gradient
developing in the wake of this low as it heads to the east coast on
Thursday. The resulting cold air advection will deepen the over lake
mixing depths. Probabilities are high that wind gusts will range
from 25 to 30 knots across much of Lake Huron overnight tonight
through Thursday afternoon. The chances of gale force wind gusts are
quite low, so no gale headlines appear warranted. Conditions across
the southern half of the lake will however be hazardous to small
craft. Large high pressure will expand across the region on Friday
and will persist across the region through the weekend, the result
will be generally light winds.

&&

.PREV DISCUSSION...
Issued at 107 AM EDT Wed Apr 18 2018

AVIATION...

Ceilings have remained solidly overcast this evening and have begun
lowering with development of low level inversion. Expect mvfr cigs
into Tuesday morning with perhaps several hours break in the lower
clouds before light rain and lowering cigs again move in by late in
the afternoon/early in the evening. Precipitation will change to
light snow with time late evening and focus south of KFNT as low
pressure passes over the northern Ohio Valley. Cigs will lower to
borderline mvfr/ifr late in the forecast period.

For DTW...Mvfr cigs will persist into Tuesday morning and reform
again by late afternoon after perhaps a brief scatting out during
the day. Light rain will overspread the terminal by around 22z and a
gradually mix with or change to snow will occur in the 04z-06z time
frame. By this time, cigs may dip to ifr. Winds will east/northeast
late in the forecast period as low pressure passes to the south.

/DTW THRESHOLD PROBABILITIES...

* High for ceilings aob 5000 ft through Wednesday morning, low
  during the day Wednesday, high again after 22z Wednesday.

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


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 2 AM Thursday to 4 PM EDT Friday for
     LHZ421-441>443.

     Small Craft Advisory from 2 AM to 4 PM EDT Thursday for LHZ422.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

DISCUSSION...DRK
MARINE.......SC
AVIATION.....DG


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at www.weather.gov/detroit.



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