Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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539
FXUS63 KDTX 042011
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
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.

&&

.PREV DISCUSSION...
Issued at 1145 AM EST Sun Dec 4 2016

AVIATION...

MVFR to lower VFR stratus will be replaced by MVFR/IFR conditions in
the 22z-00z time frame as light snow overspreads southeast Michigan
within zone of isentropic lift in advance of approaching trough of
low pressure. Only minor timing difference remain in model solutions
this morning, so expect peak period of snow in the 23z-03z period.
KMBS/KFNT will most likely see light snow begin an hour or two
before southernmost tafs. Snow will move out of area by 05z-06z, but
MVFR stratus will persist overnight into Monday morning before some
drying in low levels lead to lifting cigs during the remainder of
Monday.

For DTW...The onset of snow still looks like it will be mainly after
00Z this evening at Metro, likely ending in the 05z-06z period. IFR
conditions can be expected during the peak of the evening with MVFR
to lower VFR stratus (and some BR/HZ) predominant for the remainder
of the forecast.

//DTW Threshold Probabilities...

* High in ceilings below 5000 feet.

* Low for CIGS/VSBYS to drop below 200ft and/or 1/2SM tonight.

* Medium in precip type being mostly wet snow this evening.


&&

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

$$

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


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