Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 090503
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1203 AM EST Fri Dec 9 2016

.AVIATION...

MBS to possibly as far south as FNT will be the area where limited
lake effect snow showers will be possible during the night through
late morning. Evening trends suggest it will be a struggle to make
it down from northern Lower and limited to a stray occurrence of
borderline MVFR/IFR restriction. This will occur within clear sky
otherwise which will spread southward through the region during the
night interspersed with filaments of MVFR stratocu through the
morning. Predictability on sky condition will then be limited to
near term trends until the low level winds back toward the west by
afternoon. A return to MVFR in clouds and snow showers is then
expected into Friday evening.

FOR DTW... The bulk of snow shower activity will settle south of DTW
during the early overnight while MVFR ceiling lingers a little
longer. Broken VFR ceiling later in the night is expected to give
way to a clearing trend by sunrise with just a few pockets of MVFR
stratocu during the morning.

//DTW Threshold Probabilities...

* Moderate for ceiling 5000 ft or less through the night.

* High for snow as precipitation type/low for coverage by afternoon.

&&

.PREV DISCUSSION...
Issued at 905 PM EST Thu Dec 8 2016

UPDATE...

There are a few lake effect flare-ups to monitor for the rest of
tonight through Friday morning. The first is off central Lake
Michigan which is getting low level support inland along and ahead
of the larger scale surface trough preceded by weaker convective
elements over central sections of our area during mid evening. The
stronger part of the band has a chance to brush the western
suburbs of Detroit to the Ohio border before northwest flow sets
in overnight. The wind shift will open the door for clusters of
lake effect snow showers to make it down from northern Lower
Michigan overnight through Friday morning. As mentioned in the
afternoon forecast discussion, equilibrium level will remain in
the 7-8 kft range through the night which will make it possible
for elements from a Lake Superior to Lake Michigan band to reach
the Saginaw Valley. Surface analysis indicates a lake aggregate
pressure trough setting up favorably to support that trajectory.
About the only negative factors are the downslope from higher
terrain in the north and 0-1 km theta-e lapse rate dropping to
about 1 K/km by western Saginaw Bay. This actually may act to
limit activity from reaching Flint more than the Tri Cities, so
plan to maintain high chance/low likely POPs in that area. The
other trouble spot is central Lake Huron where lake effect has
been strong enough for a few lightning strikes near Goderich.
Expect mid lake bands to remain nearby but offshore of the Thumb
while the Lake Superior/Lake Michigan contribution to aggregate
troughing remains strong. As soon as it weakens, then the mid Lake
Huron surface trough will become more dominant and bands will
migrate farther from the Thumb into mid lake. Interestingly, not
much change in equilibrium level/convective depth is shown in
model soundings suggesting mid level subsidence will be weak and
not very effective at suppressing lake effect as the short wave
ridge builds overhead by morning. About the only forecast
adjustment being considered is an upward bump in POPs to
likely/numerous in the Saginaw Valley depending on how strong the
snow showers remain as the Lake Superior/Michigan band matures
during the rest of the evening.

PREV DISCUSSION...
Issued at 325 PM EST Thu Dec 8 2016

DISCUSSION...

Snow squall activity around this afternoon will be
diminishing/decreasing in coverage as sharp shear axis/trough swings
south through the Central Great Lakes this evening, allowing for
wind shift to the northwest and general subsidence and a drying out
process (some breaks in cloud cover). However, there looks to be a
good chance for a dominate/narrow Lake Superior-Lake Michigan band
to set up overnight into Friday, as average 925-850 mb temps looks
to be lowering to favorable -10 to -12 C range. 12z Regional GEM
most aggressive with the band Impacting Tri-Cities into Flint
Vicinity, while the HRRR is farther west. 12z nam appears to be the
farthest east, with northern Thumb region being impacted on Friday.
Inversion heights look to be at or above 7 kft, which favors the
case for band(s) reaching southeast Michigan. Narrow nature of the
band and uncertainty with movement makes for difficult call on
amounts, but potential for localized amounts greater than 1 inch is
there as we get into the tail of the upper level pv lifting
northeast, providing some support during Friday, with near saturated
and cold 700 mb temps of -20 to -23 C. Backing flow in the afternoon
will also likely bring some snow shower activity back into the I-94
corridor as well.

Upper level northwest confluent flow Friday Night/Saturday, but
westerly flow and 850 mb temps still hovering in the negative mid
teens will continue to support flurries or light snow showers.

Warm advection/Fgen Saturday Night will bring increasing chance of
light snow by Sunday Morning. Favorable thermal profiles and 850 mb
specific humidity approaching 2 G/KG suggest a few inches possible.

A brief lull from snow will be possible throughout the late morning
to early afternoon hours. Any periods or little to no snow will be
short-lived as low pressure centered over northern Texas/Oklahoma
early Sunday will begin to push northeast into the Ohio Valley into
early Monday. The chance for snow will once again increase
throughout the late afternoon to early evening hours on Sunday, with
the chance for snow continuing into Monday morning. While we are
starting to narrow down on timing of the system as models converge,
uncertainty still exists regarding placement of the low. The GFS and
ECMWF models have flip-flopped over the past 24 hours, with the GFS
model bringing the low more north, clipping SE MI Monday morning and
afternoon. This allows SE winds to pool in warmer air into the
region, allowing 850 mb temperatures to straddle the 0 degree
isotherm during the morning, which has the potential to influence
precipitation type.  Both the ECMWF and GEM models keep the low just
south, tracking across Ohio throughout Monday morning, keeping
conditions much cooler and allowing precipitation to remain as snow.
We will likely be a day off before issuing snowfall estimates for
Monday`s snow, as models continue to converge, however, it`s safe to
assume accumulation totals will impact travel Sunday through Monday.
Snow from this system will taper off late Monday, however, a shift
in winds from northeast to east will bring additional chances for
lake effect snow throughout the early part of the week. The second
main story for the extended will be the advent of polar air, which
looks to take daytime high from the lower 30s to upper 20s during
the early part of the week, to the upper 20s to lower teens by mid-
week.

MARINE...

Existing near gale conditions across the southern Lake Huron waters
will gradually ease early tonight as the become more northwesterly
with a frontal passage.  This will maintain small craft advisory
conditions for all near shore waters into tonight.  This will shift
to northwesterly will introduce a greater potential for snow squall
development tonight into Friday.   Building high pressure south of
the Great Lakes will then allow moderate winds to take hold for the
end of the week before the arrival of the next low Sunday into
Monday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.

Lake Huron...Small Craft Advisory until 10 AM EST Friday FOR LHZ441-442.

     Small Craft Advisory until 6 AM EST Friday FOR LHZ421.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
UPDATE.......BT
DISCUSSION...SF/AM
MARINE.......MR


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