Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210117
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
917 PM EDT Mon Mar 20 2017

.UPDATE...

Remnant showers have now exited east of Detroit. There are still
returns across much of Central Michigan. These are likely just
sprinkles and are associated with some mid level deformation. This
feature will slide east by late evening, ending the chances for rain.
A weak sfc gradient will hold across Se Mi through much of the
night. Low level dry air, now pushing south of the straits, is not
expected to infiltrate Se Mi until late tonight through daybreak Tues
morning. While there are ample mid/high clouds across the region
attm, they are expected to decrease in coverage during the course of
the night. Locations along and south of the 96/696 corridor currently
have fairly low temp/dewpoint spreads. With some potential thinning
of the clouds, conditions appear prime for some fog development
across metro Detroit/Ann Arbor and points south tonight. This will be
added in the forecast update. Otherwise, the going forecast looks in
good shape.

&&

.PREV DISCUSSION...
Issued at 710 PM EDT Mon Mar 20 2017

AVIATION...

The forcing sustaining the remnant showers will exit east of the
area over the next one to two hours. Drying in the column will then
support a clearing of the sub 10k ft clouds this evening. The lead
edge of very dry low level air has pushed across Upper Mi this
afternoon. The gradient will remain quite weak through the night
across srn Mi. Thus, this dry air is going to be slow to advance to
the southeast. This will open the door for some fog development
during the night. The fog chances look a little less at FNT and MBS,
where daytime heating was able to dry the boundary layer out a bit.
The gradient will also increase a little across the north overnight,
with the lead push of drier air likely arriving into MBS around
daybreak.

For DTW...The extensive cloud cover through the day around metro
Detroit has limited the diurnal drying of the boundary layer. With
the expected clearing of the lower cloud deck this evening and weak
gradient through the night, the boundary layer will be primed for
some fog development. The degree of mid and high clouds, especially
during the first half of the night, does suggest some potential
limitation for dense fog development. This will warrant holding off
on dropping fcst vsby below 1 mile in the TAF attm. Arctic air will
advance into the region Tues afternoon. The increased gradient and
deepening mixed layer will support increasing west-northwest winds
mid to late Tues afternoon.

//DTW THRESHOLD PROBABILITIES...

*  Low in ceilings below 5000 ft this evening.

*  Low in ceilings and/or visibilities dropping below 200 ft/ one
   half mile Tuesday morning.

*  Low in crosswind thresholds being exceeded Tuesday afternoon and
   evening.

PREV DISCUSSION...
Issued at 329 PM EDT Mon Mar 20 2017

DISCUSSION...

Strong PV anomaly tracking through Central Canada this afternoon,
skirting through the northern Great Lakes Tomorrow morning will
help usher in much colder air by Wednesday morning, as 850 mb temps
fall into the negative mid-upper teens (per 12z Euro).

In meantime, a much weaker upper wave (500 mb cold pool of -20 C)
to track through northern Lower Michigan early this evening. Deeper
moisture/instability south of the Michigan border this afternoon,
and convection has already fired up over northern Indiana, and bulk
of activity will continue to track off to the east-southeast, thus
calling into question how much coverage of activity slips north of
the Michigan border. None-the-less, there is good mid level (700-
500 mb) lapse rates with the upper wave and low level convergence
with cold front, thus potential scattered showers, even north of
I-69. Overall, inherited pops for this afternoon/early evening were
in good shape and will be making few if any changes, with most
activity over around or just after 00z.

Modest low level cold advection Tonight into Tomorrow, as surface
ridge builds into lower Michigan. With the cold advection and plenty
of insolation Tomorrow, mixing depths should make a run to 850 MB,
tapping into some stronger winds, with local probabilistic SREF
weighted guidance pointing to gusts of 20 to 25 knots at the
surface. North-South Temp Gradient, with Detroit and points south
likely reaching and slightly exceeding 50 degrees based
on 850 MB temps around -2 C at 21z Tuesday, with upper 40s north.

Strong and deeper surge of cold air arrives Tuesday evening, likely
maintaining some gustiness to the northwest winds. Will have to keep
an eye on Lake Huron for possible Lake Effect activity, as even 925
mb temps lower into the negative low-mid Teens. However,
looks like just enough westerly component from due north to keeps
bands offshore. By the time winds potentially become due
northerly, inversion heights will be very low with the massive
center of high pressure (1036-1040 mb) building over Central Great
Lakes on Wednesday. Regardless, still worthy of low chance pops
along the Eastern Thumb shoreline Tuesday night into Wednesday.

The arctic high pressure over the Great Lakes will continue to keep
dry weather around through Thursday as it moves off towards the
east.  The extended period then turns active as a low pressure
systems looks to move across the region.  A rain/snow mix is
possible as a warm front lifts across the area Thursday evening
before changing over to all rain as warmer air is drawn in on
Friday. Temperatures through the weekend will be near seasonal to
above seasonal.

MARINE...

Window for weak thunderstorm activity remains through late afternoon
across western Lake Erie.  Otherwise, marine conditions to remain
quiet through early Tuesday as winds increase modestly out of the
northwest.

Passage of a strong cold front will then drive a stronger wind
response Tuesday afternoon and evening.  Favorable environment will
emerge under strong cold air advection to support a solid increase
in northwest winds during this time.  This will lead to a period of
gales across a majority of the lake Huron waters.   Small craft
conditions for the remainder of the waters, as waves build over
southern Lake Huron.  Steady decrease in wind speed and gustiness
Wednesday as high pressure then builds back into the region.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 4 PM Tuesday to 4 AM EDT Wednesday for LHZ362-363-
     441-462>464.

     Gale Warning from 8 PM Tuesday to 8 AM EDT Wednesday for LHZ442-443.

     Gale Warning from 4 PM to 10 PM EDT Tuesday for LHZ361.

     Small Craft Advisory from 4 PM Tuesday to 10 AM EDT Wednesday for
     LHZ421.

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

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...SF/SS
MARINE.......MR


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