Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 160136
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
836 PM EST Fri Dec 15 2017

.UPDATE...

Latest Hi-res models and their ensembles along with the 18z models
are much more bullish with the FGEN/Isentropic forcing later tonight
into Saturday morning. Nice response of this forcing is starting to
show on the radars from central WI to SW Lower MI. Local ensembles
suggest categorical pops from I-94 to I-69, especially west of M-53,
and then likely pops as far northwest as Midland and Saginaw. Also
coincident with this increase in pops will be more precipitation with
a max just over a tenth (0.10). With snow ratios around
climatological norms of 12/13 to 1, an inch of snow is becoming
likely along the I-69 and I-96 corridors with a half inch possible
down to I-94 and then north to Midland and Saginaw. Latest update
incorporates all of those changes.

Have also adjusted low temperatures a few degrees higher with a trend
of midnight lows and then steady or slightly rising from there.


&&

.PREV DISCUSSION...
Issued at 637 PM EST Fri Dec 15 2017

AVIATION...

Snow squalls were winding down early this evening with only PTK and
DET sites getting IFR visibilities possible before 01z. WSW Winds
will continue to decrease as early this evening as well.  Should be
a small lull in precipitation until about 04z when the
isentropic/frontal snow starts over southern Lower MI. First around
04z to 06z start times for the Detroit and Flint area TAFs. That
snow should continue just past 12z. Snow and light fog will reduce
visibilities down to low MVFR/higher IFR conditions for the most
part. The exception will be for FNT and PTK which will have a good
shot of visibilities being reduce to 1 to 2 miles in the snow from
09z to around 12z. After the morning snowfall ends, just MVFR
ceilings as winds become northeast with the passage of a weak
frontal boundary during the mid day.

For DTW...On the edge of the frontal type of steady snow for the
late night and early morning hours. Looks like a two to three hour
window around 05z and then again at 11z will be the best chance to
touch IFR visibilities with the light snow.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs aob 5000 ft agl through the period.

* Low for visibilities less than 3SM after 04z with light snow.

PREV DISCUSSION...
Issued at 332 PM EST Fri Dec 15 2017

DISCUSSION...

NEAR TERM / REST OF TODAY THROUGH TONIGHT

As of 330 PM EST...Midlevel impulse that brought a period of light
snowfall to the region continues to track across southeast Michigan
this afternoon, with near-surface flow becoming westerly in its wake.
Snowfall from the morning through the early afternoon was generally
light, with most areas seeing under an inch of accumulation, although
a few spots, like here at the NWS office, did pick up exactly 1 inch
of snow. Winds have picked up behind the impulse, with occasional
gusts of 20-25 mph possible the remainder of the afternoon which may
lead to some areas of blowing/drifting snow.

Snow squall potential is still there through about 6-7pm, but not as
impressive in nature as the hi-res guidance keeps rolling in. The
main culprit seems to be a lack of a deep enough convective
layer/low EL coupled with not as steep low-level lapse rates as
anticipated (worked over from earlier morning snow) to limit the
potential punch of highly favorable microphysics.

Still, a few widely scattered snow squalls will be possible mainly
through 5pm, especially west of Detroit metro up north through I-75.
Flow trajectories favor bands along either the I-94 or I-96
corridors, with a current snow squall (around 30 dBZ on radar)
moving along I-94 at the time of this discussion. Will continue to
monitor and issue Special Weather Statements as appropriate for
possible brief periods of localized heavier snow, treacherous
travel, and rapid reductions in visibility. Any additional snow
accumulations through this evening will generally be under an inch,
although slightly higher amounts are possible within any squall that
lingers over an area long enough.

This evening, any lingering snow showers/snow squalls look to shut
down briefly especially across the northern two-thirds of southeast
Michigan, with southern activity becoming absorbed into a lingering
frontal boundary associated with a weak passing clipper well north
of the region. This frontal boundary will wring out the remaining
Lake Michigan moisture plume along a gradually weakening FGEN
forcing field tonight, with additional light snow showers possible
mainly from Flint and points south. Accumulations tonight are
expected to be under an inch as remnant dynamic forcing weakens with
time. After seeing high temperatures top out into the 20s for most
areas, low temperatures tonight have the potential to quickly fall
yet again with a still fresh snowpack, but the nearby frontal
boundary should limit lows from being as cold as the past 2 nights,
generally in the teens for most.

SHORT TERM / SATURDAY THROUGH MONDAY NIGHT

Aforementioned frontal boundary will continue to provide the focus
for some scattered light snow showers into early Saturday, before
the front washes out/surface high pressure ridges in from the north
to end activity. Moisture trapped in the boundary layer under a
strengthening capping inversion will still yield plentiful cloud
cover despite dry weather for the second half of Saturday and into
the first half of Sunday. Highs on Saturday will be slightly warmer
than today as the dying front slowly lifts north allowing for some
weak warm air advection to take place. Highs Saturday will range
from the mid 20s north to lower 30s south. Lows Saturday night will
be in the lower 20s to around 20 with highs Sunday looking to warm
above the freezing mark for most locations into the mid 30s.

Forecast becomes a bit more complex Sunday night and into Monday, as
there are mixed signals of a potential light precip event as
shearing southern stream energy ejects out of the lower Plains. For
now, will go with chance PoPs mainly for southern areas, with the
potential for a light overrunning event producing some wintry mix
and a slippery morning commute Monday. Still time to monitor this
system and see how the thermal profile evolves over the next 48
hours.

LONG TERM / TUESDAY THROUGH THURSDAY

A surface high pressure system will build in across the Ohio Valley
late Tuesday and upper-level ridge building in across the Midwest on
Wednesday will bring several days of dry weather as temperatures
remain seasonal to slightly above seasonal for the midweek period.
The next likely chance for widespread precipitation will then move
in late Thursday into Friday as a low pressure system develops and
pushes northeast from OK/MO. Slightly better convergence is now seen
across the long range models regarding the placement of the low,
with it settling just south of Michigan, over northern Ohio/Lake
Erie by Friday afternoon. A warm front pushing through the state
prior to the low, will bring the chance to see precipitation across
the Central part of the state and across the thumb, throughout
Thursday. A wintry mix to snow solutions will be the most likely
solution for any precipitation seen on Thursday as surface
temperatures sit at or slightly above freezing, with temperatures at
925 and 850 mb ranging between -2 to -5 C. The more likely chance
for precipitation will enter late Thursday into Friday morning as
the low pressure system approaches the state. With new model runs
trending cooler, a snow solution is looking more likely for this
event, however, additional models runs and convergence will be
needed to increase confidence regarding the timing, placement, and
strength of the low.

MARINE...

Gusty W-SW winds will continue through the evening as a low pressure
system move through the northern Great Lakes. Anticipate winds to
remain below gale force, but they could reach around 30 knots before
turning to the NW behind a front tonight. Small Craft Advisories for
the Outer Saginaw Bay and portions of the Thumb along the nearshore
waters will remain in effect for increase wave heights and rough
waters. Winds will diminish by tomorrow afternoon and remain light
out of the east this weekend as a high pressure passes to the north
of the Great Lakes.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory until 4 AM EST Saturday for LHZ421-441.

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

$$

UPDATE.......RBP
AVIATION.....RBP
LONG TERM...AM
MARINE.......AA


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