Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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605
FXUS63 KDTX 050317
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1017 PM EST Sun Dec 4 2016

.UPDATE...

The snow is still on target to exit northeast of the forecast area
between 11 PM and 1 AM with large scale subsidence and its
associated mid level drying in its wake. The southeasterly winds
along Lakes Erie, St Clair and southern Lake Huron held sfc temps
up in the mid to upper 30s a good 6 to 10 miles inland. This
caused the precip to never fully change over to snow in these
locations, leading to little to no accumulations across eastern
Monroe, ern Wayne, se Oakland, and much of Macomb and St Clair
counties. Farther north and west, precip fell entirely as snow.
There were however a couple instances where some pockets of mid
level dry air punched into the eastern sections of the forecast
area and cut down rates. Locations mainly northwest of a Howell to
Sandusky line (where forecing and moisture depth was a little more
persistent) will likely end up with two to possibly as much as
three inches of snow. To the southeast, accums will likely be an
inch or less.

Despite the large scale subsidence, low stratus will linger
well into the day Monday. There have been some drizzle reports
upstream. Sfc temps will hold at or slightly above freezing, so
any drizzle should not cause issues. A forecast update will be
issued once the precip ends across the area.

&&

.PREV DISCUSSION...
Issued at 545 PM EST Sun Dec 4 2016

AVIATION...

Snow will overspread the terminals by 00Z. There should be an abrupt
end to the snow around 04Z. The peak intensity will occur between
01Z and 03Z. Surface temps will remain at or above freezing tonight.
So snow will struggle to stick to runways. During peak intensity
(between 01Z and 03Z) there may however be some slush able to
accumulate. During peak snow, sfc visibilities and cigs will drop to
LIFR and VLIFR. Observations across wrn Illinois and Wisconsin
suggest low stratus will persist through the night (and likely well
into the day Mon). Gradually lifting inversion heights and dry air
entrainment should support a gradual rise in ceiling heights from
VLIFR late this evening to MVFR by late Mon morning.

For DTW...

Expect the snow to move into metro around 00Z, with the peak
intensity between 01 and 03Z. Once the snow begins, sfc temps will
drop a good 3 to 5 degrees, settling around or just above the
freezing mark by 02Z. Once the snow ends (again around 04Z) temps
will hold steady just above freezing if not slowly rise during the
overnight.

//DTW Threshold Probabilities...

* High in ceilings below 5000 feet tonight and Monday.

* Moderate for CIGS/VSBYS to drop below 200ft and/or 1/2SM between
  01Z and 03Z.

* High in precip type being all wet snow this evening.

PREV DISCUSSION...
Issued at 515 PM EST Sun Dec 4 2016

UPDATE...

Observations indicate that precip is starting out as strait snow.
With sfc dewpoints in the upper 20s over Se Mi, expect this will
also be the case across the forecast area over the next couple of
hours given just enough wet bulb cooling. While there may be a
brief period of drizzle between 10 PM and Midnight as the snow
lifts off to the northeast, this will be negligible. A forecast
update will be issued to remove the mention of rain. Based on
upstream reports thus far, a 1-2/1-3 inch type forecast area wide
looks in order. While these accumulations will be confined to
grassy and elevated surfaces, there have been some reports of
slush on secondary roads across wrn Lower Mi and NW Indiana. This
is certainly possible across Se Mi, particularly on bridges and
overpasses.

PREV DISCUSSION...
Issued at 311 PM EST Sun Dec 4 2016

DISCUSSION...

Negatively tilted upper trough extending through MN/IA will swing
through Lower Michigan this evening and overnight. This will provide
a mix of snow and rain to the area as slug of moisture currently
seen on satellite and radar over Illinois, Indiana, and west
Michigan slides northward ahead of the surface trough axis. So far,
drier air has been eating into returns showing up on radar allowing
for only virga as isentropic ascent increases in broad warm air and
moisture advection pattern ahead of the trough. Current surface
temp/dewpoints are mostly in the mid 30s/mid 20s, with slightly
higher readings of both T/Td from the City of Detroit southward.
This far southeast corner of the state looks to see less of a chance
for wet-bulb effects to change precip over to all snow until at
least mid-evening, cutting into snow totals there (a slushy inch or
slightly less, mostly on grass and elevated surfaces). Upstream
observations favor snow for the rest of the area, and given current
status of T/Td over Southeast Michigan, nudged the forecast in a
snowier direction with a general 1-3 inches expected across the area
before 1 AM. Bulk of precipitation is expected to fall as snow north
of a Dexter to Pontiac line. Highest totals should occur north of
Saginaw and over the northern Thumb, where cooler thermal profiles
will favor better snow ratios and a total of 2-3 inches. Most of the
precipitation should fall between 5-11PM, then taper off behind the
surface/upper trough axis. There had been some uncertainty regarding
precip type changing back to rain as dewpoints rise close to
freezing this evening. Upstream observations over Northern Illinois
have so far remained snow as this occurred, lending more confidence
to slightly higher snowfall totals this forecast cycle.

High pressure will bring quiet weather tomorrow. Flow off the lake
and strengthening subsidence inversion will however keep a stratus
deck intact during the day. We should start to see some breaks
during the very late afternoon and evening as flow over Lake
Michigan turns more southerly on the backside of the surface high.
Southerly flow will usher in warmer air tomorrow, at least above
925mb. Inversion at about this level will limit mixing into the
bubble of warm air, while clouds limit insolation. Despite warmer
start to the day in the low 30s, will keep temperatures towards the
lower end to slightly below guidance, with forecast max temps in the
low 40s.

Shortwave ridge will exit east Monday night as the first area of low
pressure embedded within the longwave trough over the western U.S.
lifts through the Upper Midwest. Models still differ with the amount
of phasing that occurs between this low and another upper low (more
tropical) now over Northern Mexico as it races northeast. Consensus
takes this wave through the extreme southern Ohio Valley on Tuesday,
with precipitation shield only clipping portions of southeast
Michigan. We could still see some light snow at precipitation onset
early in the day before warm air advection and daytime heating
likely allows precipitation to change over to rain. Most of the
event should then fall as a cold rain, and remain limited to south
of I-69. Precipitation looks to exit during the evening as a strong
cold front pushes through the area. Drier air behind the front will
limit duration and amounts of light snow on the backside of the
system.

Wednesday is expected to stay more on the quiet side as low pressure
settles over the Hudson Bay, keeping precipitation mainly north of
the Lower Peninsula. The main story for Wednesday and the theme
throughout the remainder of the week will be the cool arctic air
that will enter the region. A shift in winds from the east to the
northwest will allow colder air the pool into Michigan late Tuesday
into Wednesday. Winds are expected to remain westerly into the
weekend, keeping the cold air in place. As a result, expect
temperatures to reach the mid-30s for a daytime high on Wednesday,
with temperatures struggling to reach the 30s until Sunday.
The chance for snow showers will be on and off. Slight chance for
snow exists through Sunday, with no significant accumulations
expected with any one event at this time.

MARINE...

Southeast winds will increase somewhat into this evening in advance
of an approaching trough. Winds will then veer to the southwest in
the wake of this surface trough overnight. The lack of a strong
pressure gradient will keep wind gusts below 20 knots through the
event. Southwesterly flow will continue into Monday, but decrease
as high pressure builds back into the region from the south.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...HLO/AM
MARINE.......DG


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