Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 182342
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
642 PM EST Sat Nov 18 2017

.UPDATE...

There is a region of mid level deformation over Se Mi to the west of
the sfc low now over Lake Erie. Radar suggests some good contraction
of the mid level front ongoing across the area. This along with
cooling due to melting has enhanced a region of snow on the cool side
of the mid level front. Despite some rather good flake size, sfc
temps a couple degrees above freezing and warm pavement temps will
keep accumulations limited to grassy surfaces. The region of snow
should slowly progress across the rest of Se Mi during the evening,
while the back edge is already sliding into the tri cites. Based on
radar and observations, some slushy accumulations around a half inch
seem probable on grassy areas along and north of an Adrian to Port
Huron line. A little warmer boundary layer temps to the southeast of
this line should preclude much accums. Any easing of the precip
intensity is also likely to switch precip type back over to light
rain before ending altogether by late evening.

Despite cold air advection through the night, gusty winds and ample
cloud cover will prevent a sharp drop in temps. In fact, temps are
likely to hold above freezing well into the night. So road surfaces
should just remain wet. Overall the going forecast looks reasonable,
an update will just be issued to reflect current trends.

&&

.PREV DISCUSSION...
Issued at 605 PM EST Sat Nov 18 2017

AVIATION...

A region of enhanced precip rates will affect Se Mi on the back side
of deepening low pressure now tracking across Lake Erie. There has
been enough cooling on the back side of the precip to change the
rain over to snow around MBS. This region of snow will progress
across the rest of Se Mi through 03 or 04Z before exiting east of
the region. Ongoing low level cold air advection through the night
will result in rising inversions and thus lifting cloud bases. There
is likely to be a few lake effect streamers that make their way into
Se Mi overnight. The brevity of any overnight snow showers will
preclude the mention in the TAFs attm. The deepening mixed layer and
increasing gradient will support strengthening winds during the
evening as they back toward the west-northwest. Gusts are still
expected to approach (or briefly exceed) 30 knots.

For DTW...A period of wet snow is expected between 01Z and 04Z.
Relatively warm sfc and pavement temps will preclude accumulation on
anything except grassy surfaces. The wind direction through the
evening will remain a little more northerly, thus inhibiting
crosswind concerns. The wind direction will back westerly overnight.
This may lead to some crosswind considerations by daybreak Sunday.

//DTW THRESHOLD PROBABILITIES...

* High for cigs aob 5kft tonight and Sunday.

* Moderate for snow mixing in with rain 00z-04z.

* Low in crosswind thresholds being exceeded Sunday morning.

PREV DISCUSSION...
Issued at 331 PM EST Sat Nov 18 2017

DISCUSSION...

Categorical rainfall has been spreading across all of southeast
Michigan this afternoon. The center of the surface low is now
located on the central IN/OH border. A cold front also continues its
progression southeastward through central Michigan keeping widespread
rainfall likely through this evening. The current track of the
surface remains further south across northwestern Ohio and through
central Lake Erie. This will focus the highest rainfall totals across
the further southeast counties. It has also kept the more unstable
warm sector south of the MI/OH border. However, some lightning and
rumbles of thunder earlier this afternoon were observed in far
southern portions of the state given mid level lapse rates of around
6 C/km. Do not expect much thunder the rest of the day.

The biggest challenge for this forecast will be this evening with
the timing of the surface low deepening and the arrival of the cold
air resulting in potential snowfall accumulation in the Flint to
Tri-Cities areas. Latest guidance has a stronger push of colder air
in the evening coinciding with precipitation within the area of
strong deformation. Add in the diurnal cooling component and the
column will cool enough to transition from all rain to a rain/snow
mix or mostly snow between 22Z to 00Z. Confidence is increasing for
areas to see some minor accumulations in portions of the northwestern
CWA around the Tri-Cities and Flint area. Enough forcing exists
after the lower levels cool to produce some higher snowfall rates
through the evening. The winds will also pick up substantially
heading into the evening and overnight period. The surface low will
begin to deepen as it reaches the eastern Great Lakes. This will
result in cold and brisk northwest winds as the surface pressure
gradient increases this evening and lasting through tomorrow morning.
Wind gusts up to 35-40 mph still look possible late this evening and
overnight as winds of around 40 knots should be able to mix down.

The bulk of the forcing and precipitation with the system will exit
east of Michigan by 05Z to 06Z leaving Michigan under good northwest
flow. Plenty of clouds are in store tomorrow with the lakes providing
some added moisture. An embedded shortwave within this northwest
flow will progress southeastward across Michigan should provide lift
in conjunction with some lower level instability should support snow
showers tomorrow morning through the early afternoon. Do not expect
much accumulation from this round of light snow showers. High
temperatures will dip below normal topping out in the mid 30s as 850
mb temperatures of -10 C move in by Sunday afternoon.

More dry and quiet weather expected by Monday as height rises drift
through the Great Lakes. Cold temperatures in the mid 20s are
forecast for Monday morning with the deeper cold air still in place.
Winds will shift to the southwest by Monday afternoon ahead of the
next low pressure system to affect the region. This will allow a
brief period warm air advection to bring temperatures back up to the
low to mid 40s, which is more towards normal for this time of year.

Low pressure tracking eastward through Ontario will pull a cold
front through the region on Tuesday. Ahead of the front gusty
southwest winds will develop as highs rise into the upper 40s across
far southeast Michigan. A few showers will be possible with the
frontal passage before colder air quickly filters into the region by
Tuesday night. A calm, dry stretch of weather is then expected
through the rest of the workweek as high pressure remains in place
across the region. Temperatures will remain slightly below average
with highs in the mid to upper 30s/lows in the mid to upper 20s for
the Thanksgiving holiday before moderating slightly by late next
week.

MARINE...

Gale Warnings remain in effect for all marine areas this evening
into Sunday.

Rapidly strengthening low pressure will lift across Lake Erie this
evening on its way east of the Great Lakes by Sunday morning. Winds
will increase substantially during the rest of the day. Northwest
gales are expected to develop near the low from western Lake Erie
into southern Lake Huron early this evening. As winds peak during
the late night hours, gales will expand into northern Lake Huron
with gusts increasing to 40 knots, and perhaps 45 knots at times,
over southern parts of the lake. These wind conditions will produce
waves in the 12 to 16 foot range.

Gales will persist into Sunday, but gradually diminish during the
afternoon and evening. A period of moderate to fresh westerlies will
follow the gales from Sunday night into Monday night. Southwest
gales will then be possible early Tuesday as the next low pressure
system tracks across northern Ontario.

HYDROLOGY...

Light rain will remain mainly focused south of the M59 corridor for
the next several hours. Rain coverage to the west will fill in
throughout the morning as it tracks closer to the area. By this
afternoon, all of SE Michigan will see moderate rain. Additional
rainfall totals will range from around 0.5" in the Saginaw Valley
area to an inch near the Ohio border. The heaviest rain will be
focused in the 1pm to 7pm time frame. Minor urban flooding remains a
possibility.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning until 7 PM EST Sunday for LHZ361>363-462>464.

     Gale Warning until 10 AM EST Sunday for LHZ421-441>443.

     Gale Warning until 7 AM EST Sunday for LHZ422.

Lake St Clair...Gale Warning until 10 AM EST Sunday for LCZ460.

Michigan waters of Lake Erie...Gale Warning until 10 AM EST Sunday for LEZ444.

&&

$$

UPDATE.......SC
AVIATION.....SC
DISCUSSION...AA/JD
MARINE.......DG
HYDROLOGY....DG


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