Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 100856
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
356 AM EST Tue Jan 10 2017

.DISCUSSION...

Energy within the upper trough tracking through the Plains will
consolidate as it lifts through the Great Lakes today and this
evening. Surface low center will lift across Wisconsin and the Upper
Peninsula today, with a fairly rapid deepening taking place as it
becomes positioned under a coupled jet structure.

Warm front lifting through Southeast Michigan will provide
precipitation across the area through much of today. Models have
been showing good isentropic ascent for several runs now, lasting
from this morning into the daylight hours. Intense low-level jet has
already begun to surge into the region, and has prompted a nice
response over Lower Michigan in the form of snow. Expect to see
another area of precipitation fill in later this morning over
northern Illinois/Indiana and Southwest Michigan with another low-
level jet surge as speeds increase to around 80kts and the nose
pushes up into Lower Michigan. This should keep precipitation going
through the morning and afternoon. Isentropic ascent before 21Z will
support more steady precipitation character, before slightly drier
and colder air aloft arrives with the mid-level trough and
transitions rainfall to a more cellular and showery pattern. Lapse
rates per model soundings do not look quite strong enough to support
thunder however as we head into the evening and the cold front
approaches. Low-topped convection will be possible, which could
yield concern for strong wind gusts transferring to the surface
given impressive wind field in place over the area.

Precipitation will undergo a transition from rain to snow through
the morning and very early afternoon. Warmer air first arriving
aloft may allow for a brief period of sleet/freezing rain before
surface temperatures rise enough to support all rain (or the melting
layer becomes deep enough). This idea has been supported this
morning by several freezing rain reports over northern Indiana.
Transition to rain will occur from south to north as warmer air
arrives. 80kt low-level jet punching in during the afternoon should
provide enough warm air to change all precipitation over to rain for
all but perhaps the extreme northern thumb by 18Z (1PM). Snow
accumulations have changed little from previous issuance, ranging
from 2 inches near the Ohio border to 4-5 inches across the northern
Saginaw Valley and Thumb. Any icing is still expected to have
minimal impact as amounts will be light and temperatures will
quickly rise into the low 30s. Untreated roads should at least have
some snow to absorb any freezing rain/sleet. Surge of warm air
should help boost temperatures into the 40s today.

Drier air behind the front should put an end to most precipitation
across the area by late evening. The exception may be over the
Saginaw Valley and Thumb as low/mid level moisture wrapping around
the low passing to our north brushes the area. Most of this would
fall as light snow showers as post-frontal cold air rushes in.

After the round of snow this morning, attention focuses sharply on
potential for very strong winds to develop with the passage of the
cold front this evening. Deepening area of low pressure lifting
through the northern Great Lakes will create a tight southwest
gradient over the region, reinforced by ridging building into the
Northern Plains and Upper Midwest. NAM/GFS both indicating strong
isentropic descent (especially 274-284K) along/behind the front,
while cold air advection deepens the mixed layer. Forecast soundings
look solid with 45-55 knots below 2000 feet within this mixed layer,
yielding potential for gusts as high as 60 mph to reach the surface.
Huron County will also see a local increase in speed from off
Saginaw Bay as stronger winds glide easily over the ice there with
lack of friction. High wind watch will be issued for all of
Southeast Michigan with the morning forecast package. It should be
noted that it will be breezy for much of the day but stability from
warmer air aloft will limit the depth of the mixed layer, keeping
gusts closer to 35-40 mph by mid-afternoon.

Next situation to monitor will be potential for ice and light snow
Wednesday night into Thursday. Secondary frontal boundary trailing
the low passing through tonight will slowly work east across
Michigan late Wednesday through Thursday. Warm air on the south side
of the front will support all rain for precipitation type, but cold
sub-freezing surface temperatures on the north side will become
overrun by warmer air aloft creating potential for freezing rain or
light snow until drier air arrives. We could see a band of freezing
rain to develop along and north of M-59 late Wednesday night and
early Thursday morning, then sag slowly southeast behind the frontal
passage into the Detroit-area on Thursday. GFS has been the most
aggressive with this scenario for a few days now. NAM is quite a bit
warmer and supports mostly rain, while the Euro is somewhat in
between but keeps ice amounts really light. Dry air will also spill
back into the area behind the front, so timing of dry air versus
cold air will be a forecast challenge.

&&

.MARINE...

Southerly winds will continue to increase as the gradient
strengthens downstream of an organizing winter storm system. Wind
gusts to gales will develop over much of the Lake Huron basin during
the mid morning hours today. Wind gusts to gales are then expected
for all area waters by the late afternoon or early evening hours
continuing into the overnight. The strongest period of winds is
expected between 23-09z tonight as a cold front blasts through the
region. A high end gale event is likely with gusts to 45 knot gales
for much of the area waters. Winds are expected to diminish rapidly
by the mid morning hours on Wednesday.

A very warm air mass is then forecasted to build into the Central
Great Lakes for Wednesday and Thursday. This will likely lead to
very stable conditions over the adjacent waters. Uncertainty remains
high with regards to the exact positioning of a very strong
stationary front. Adjustments in future forecasts both with respect
to wind direction and wind speeds for the middle and end of the week
forecast are likely with any changes to the positioning of the front.

&&

.HYDROLOGY...

Low pressure will lift through the upper midwest into the northern
Great Lakes and Ontario on Tuesday. An area of snow will expand into
the region overnight before changing to all rain on Tuesday. The
transition will occur from south to north, beginning near the Ohio
and Michigan state line around sunrise, then finally reaching the
Thumb during the afternoon. Snow amounts will range from 2 inches
near the Ohio border to  nearly 5 inches over parts of the northern
Thumb and Saginaw Valley. Total rain and rain equivalent is expected
to range between one half and three quarters of an inch between
midnight Tuesday to midnight Wednesday.

&&

.PREV DISCUSSION...
Issued at 1149 PM EST Mon Jan 9 2017

AVIATION...

The initial band of snow is advancing toward the terminals as
expected. This band will be fairly brief in duration. However,
strong moisture transport overnight will lead to the development
of more widespread snow across srn Mi around or shortly after 09Z.
This will be a heavy wet snow, with rates of a half inch to one
inch per hour. Strong warming aloft will then support a rapid
transition to rain from south to north in the 12 to 14Z time
frame. Model soundings are suggesting some warming aloft prior to
the change over, suggesting a brief period of sleet possible.

For DTW....A quick change over to rain (with some sleet at times) is
expected around 12Z. Accumulations of an inch or two of wet snow is
expected in the 09Z to 12Z period. Temperatures have already warmed
above freezing across cntl Indiana. The expectation is that by the
time precip changes over to rain, sfc temps will be just above
freezing and rising. So freezing rain will remain left out of the
terminal.

//DTW Threshold Probabilities...

* High in ceilings below 5000 ft through the TAF period.

* High probability of all snow prior to 12Z. Medium probability of
  sleet in the 12 to 15Z time period. Low in freezing rain between
  12Z and 15Z. High in ptype being all rain after 15Z.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 10 AM EST this morning FOR MIZ075-076-
     082-083.

     High Wind Watch from this evening through late tonight FOR
     MIZ047>049-053>055-060>063-068>070-075-076-082-083.

     Winter Weather Advisory until 1 PM EST this afternoon FOR
     MIZ047>049-053>055-060>063-068>070.

Lake Huron...Gale Warning until 7 AM EST Wednesday FOR LHZ362-363-462>464.

     Gale Warning from 4 PM this afternoon to 4 AM EST Wednesday FOR
     LHZ421-422-441>443.

     Gale Warning from 7 PM this evening to 7 AM EST Wednesday FOR
     LHZ361.

Lake St Clair...Gale Warning from 1 PM this afternoon to 1 AM EST Wednesday FOR
     LCZ460.

Michigan waters of Lake Erie...Gale Warning from 1 PM this afternoon to 1 AM EST Wednesday FOR
     LEZ444.

&&

$$

DISCUSSION...HLO
MARINE.......CB
HYDROLOGY....CB
AVIATION.....SC


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