Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 110858
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
358 AM EST Sun Feb 11 2018

.DISCUSSION...

A winter weather advisory remains in effect for all of SE Michigan
as snow continues to develop over the region through sunrise. The
snow will be heavy at times, possibly reaching 1 inch per hour rates
early to mid morning, in the Flint to Tri Cities region and across
the Thumb. Total accumulation is bumped into the 3-6 inch range in
that area. There is also a chance for heavy snow mixed with sleet
during the morning as far north as a line roughly from Ann Arbor to
Pontiac to Port Huron. A mix of snow, sleet, freezing rain and/or
freezing drizzle is expected farther south from that line to the Ohio
border. After a late morning peak, a diminishing trend will begin
during early afternoon with precipitation ending mid to late
afternoon from west to east across the area. Some activity could
linger shortly past the 4 PM advisory expiration but it will be light
and brief.

The upper level trough digging into the Plains and the entrance
region of the strong upper jet over the Great Lakes has this event
off to fast start during the early morning. Upstream radar composite
and surface observations indicate a swath of snow and mixed precip
over Iowa and Illinois with snow extending into Lower Michigan.
Several reports of 1/2 mile moderate snow have come in from the
Chicago area with a few reports of freezing rain and/or unknown
precip through central sections of Illinois. This data supports
model trends that have nudged warmer air farther to the west leading
to elevated instability west of the surface low track and to much
stronger vertical velocity in the primary area of snow. This initial
phase of forcing is strong isentropic ascent into the entrance region
of the upper jet that will move quickly over SE Michigan around
sunrise. Model cross sections indicate outstanding parameters for
strong vertical motion within the northward moving frontal zone
aloft. Theta-e profiles indicate very low stability over central
Lower Michigan allowing a strong ageostrophic response within the
frontal zone and under the entrance region of the jet. Moisture is
very plentiful judging from specific humidity averaging 3 g/kg at 700
mb and model QPF is now pushing 0.4 to 0.5 for 12 hours ending at
18Z. The main question is whether snow ratio will verify at less than
10:1 in the below freezing but deep isothermal layer hovering around
-5 C below 700 mb. Expect accumulation to straddle the 10:1 value,
slightly higher near Saginaw Bay and lower toward Flint, and be
mainly in the 3-5 inch range and locally near 6 inches where fgen is
maximized for a couple extra hours during early afternoon.

Farther south, the unstable profile will support a showery,
convective mix of snow and sleet during the morning. Mixed precip is
not expected to make it much farther north of the Ann Arbor to
Pontiac to Port Huron dividing line as the thermal profile will be
rate dominated on the colder side that far north. The question then
becomes how far north the channel of mid level dry air will reach
while cutting off the snow and transitioning to freezing drizzle.
This dry air is materializing over Missouri and southern Illinois
before sunrise and is expected to make it over the Detroit area
around noon. There is a fair amount of agreement among the higher
resolution models showing the mid level dry air punching at least to
the M-59 corridor as forcing transitions to a deforming fgen pattern
farther north. Expect this to bring freezing drizzle into the
picture for the afternoon before the event diminishes late in the
day. Total accumulation here will be cut into by the mixed bag and
is expected to range from around 1 inch at the Ohio border into the
2-4 inch range north of M-59 to the north and west of Pontiac.

The exiting Ohio valley surface low will draw the frontal zone south
and east toward the Atlantic coast tonight followed by a dry
northern stream front over the Great Lakes. This will usher in high
pressure and a clearing trend overnight into Monday. A welcome
period of dry conditions is then expected through mid week. This air
mass will be a continental/maritime hybrid and is expected to modify
into a warming trend during mid week. Highs are trending upward
to the lower 40s Wednesday and Thursday.

&&

.MARINE...

Light to moderate wind across the waters through the weekend
becoming fresh northwesterly over portions of the open waters of
Lake Huron in the wake of a cold front late Sunday night into
Monday. Ice cover will limit instability, but a corridor of gusts
approaching near-gales will be possible in the large ice free area
in the central portion of Lake Huron.

&&

.PREV DISCUSSION...
Issued at 1154 PM EST Sat Feb 10 2018

AVIATION...

Precipitation will expand north across area overnight as shortwave
approaches from the west and instigates FGEN forcing along boundary
draped over/near area. KMBS/KFNT will remain all snow with LIFR
vsbys at times Sunday morning. KPTK will be mainly snow, but ice
pellets may very well mix in at times. I-94 terminals look to see a
mix of light snow, ice pellets and eventually freezing drizzle/rain
Sunday morning. Conditions will improve in the 18z-21z time frame as
this system passes east with some clearing expected by evening.

For DTW...Widespread precipitation will redevelop by 08Z. This will
start as light snow, but ice pellets look to mix in pretty quickly.
Eventually, air aloft looks to warm enough to support a period of
freezing rain (or at least a wintry mix including fzra). The best
potential for icing looks to occur in the 12z-18z window.

/DTW THRESHOLD PROBABILITIES...

* High of cigs aob 5kft by 08z-09z tonight.

* High confidence in a mix of light snow and sleet overnight with
  medium confidence of freezing rain/drizzle mixing in 12z-18z.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...Winter Weather Advisory until 4 PM EST this afternoon for
     MIZ047>049-053>055-060>063-068>070-075-076-082-083.

Lake Huron...NONE.

Lake St Clair...NONE.

Michigan waters of Lake Erie...NONE.

&&

$$

DISCUSSION...BT
MARINE.......JVC
AVIATION.....DG


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