Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 040441
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1141 PM EST Sun Dec 3 2017


.AVIATION...

Patchy fog possible overnight within light SE flow. However, thick
cirrus and a gradually increasing pressure gradient should limit
development considerably. Period of elevated showers possible by
midday Monday, mainly for KMBS. Better chance of showers to occur in
the 02z-06z time frame with a good chance along cold front after
06z. Cigs will be lowering with the approach of cold front during
this time. Southerly winds will increase on Monday ahead of this
front with a period of LLWS also possible in the morning as 50 knot
low level jet in the 1-2KFT layer spreads in (mainly MBS) before
surface winds increase above 15 knots. Winds increase further into
Monday night, veering to SSW as front encroaches.

For DTW...Thicken mid/upper clouds in the 12-25kft range should
limit widespread fog despite light SE flow from Lake Erie. Still,
will maintain 4sm in the forecast late. Cigs drop during Monday
evening as cold front encroaches with shras possible toward the
end of the forecast period (03z-06z).


//DTW THRESHOLD PROBABILITIES...

* Low for cigs aob 5000 feet late Monday afternoon, medium by mid
  evening, high overnight.

&&

.PREV DISCUSSION...
Issued at 350 PM EST Sun Dec 3 2017

DISCUSSION...

Surface high pressure will begin to break down tonight over the
central Great Lakes in response to 1000-500mb height falls
consolidating in the lee of the Rockies. Nil weather in the near
term as the air mass currently in residence is moisture starved, and
mid to upper level ridging will remain dominant overnight. The
beginning of the warm advection aloft will allow high cloud to
overspread southeastern Michigan and thicken overnight. Item to
watch for will be potential for shallow fog development as model
soundings are adamant in surface based inversion development.
Earlier, HRRR, was suggesting some decent radiational fog potential
for the Thumb with light flow off of Lake Huron, however, latest
hourly runs have backed off a fair amount. Ultimately, feel that
areas of light fog/hz will be possible but increasing southerly
winds with increasing high cloud aloft will be some major limiters.
Did go cool on the cool end of solution space for mins tonight.

Mid-latitude cyclone forecasted to deepen over the central Plains
Monday morning, before lifting northeastward through portions of far
western Lake Superior Monday night. Phasing of various upper level
PV features along with a coupled upper level Jet structure will lead
to a powerful system, forecasted to deepen to around 975mb over
north central portions of Ontario at 18Z Tuesday. As a result, there
is expected to be a strong gradient component for the entirety of
this system over the central Great Lakes. Southerly gradient flow to
ramp up after daybreak Monday with south southeasterly winds of 20
to 30 mph possible by Monday afternoon. Highest end of this range is
expected north of the M 59 corridor, north of the Detroit Metro area.

Initial warm advection surge concurrent with lead CVA and midlevel
stretching deformation will bring a band of showers through the cwa
north of I 69 between 12-18Z Monday. South of I69, a very deep layer
of dry air, sub 10 kft agl, will preclude much in the way of
raindrops reaching the ground. Shortwave ridging and anticyclonic
flow trajectories then forecasted to hold throughout much of the
afternoon. A pre-existing deep low level jet impulse will then lift
out of the mid Mississippi River valley and into the central Great
Lakes for the early evening hours. Time lagged probabilistic
information and various hi-resolution datasets supports
precipitation chances increasing after 00Z Tuesday, peaking for much
of southeastern Michigan between 06-12Z Tuesday. Main forecast
concerns this package center on how the passage of the cold front
will unfold Monday night and Tuesday morning. Latest timing has the
surface cold front pushing into far NW CWA around 08Z Tuesday and
through city of Detroit around 13Z Tuesday. Potential for high
impact wind will fall along this frontal zone as low-topped shallow
convective fine line will be possible. Forecast soundings show high
likelihood of neutral stability to elevated convective instability
which supports the potential for thunderstorms. Latest Swody2 has
the area designated in general Thunderstorms, with the best timing
appearing to be in the 00-08Z timeframe, possibly ahead of
convective fine line. Steepening near surface lapse rates with
initial near surface cold air advection will allow for increased
downward momentum flux. Most recent of probabilistic information
suggests some increasing chances for some windgusts of 45mph tied to
the front. This signals warrants monitoring in later forecast
information. Deepening mixed layer due to both cold air advection
and daytime heating will lead to gusty southwest winds throughout
Tuesday afternoon 20 to 30 mph.

A high amount of uncertainty with exactly how the lake effect snow
potential will unfold Tuesday night. Went with a blanket chance pop
for snow showers with cyclonic flow and some shallow saturation
through the -10C to -15C layer. Amount of midlevel subsidence and
associated dry air is forecasted to be high end, so potential for
significant amounts will be low Tuesday night.

Wednesday will mark an extended stretch of below-normal temperatures
and will bring several chances to see snow showers through the
weekend. To start, a weakening low pressure system well off into the
Hudson Bay will continue to usher in a Canadian airmass over the
Great Lakes region, with -13C 850mb temperatures in place overhead
by Weds 12Z. Conditions will remain breezy throughout the bulk of
the day as mixing fueled by CAA helps bring stronger winds aloft
down to the surface, which will allow for sustain wind gusts between
20-25 mph. Additionally, low-level westerly flow over a relatively
warm Lake Michigan will bring lake effect snow chances to the CWA
throughout the bulk of Wednesday. Breezy conditions will diminish by
Thursday as winds weaken aloft, however, lake effect snow chances
will continue throughout the day as low-level westerly flow holds
steady. Opted to put chance/likely snow chances mainly between the I-
94 to I-69 corridor Thursday morning and afternoon as the GFS/GEM,
and ECMWF model runs all pick up a band of lake effect moving across
the area during that timeframe.

Upper-level troughing will dig across the Midwest starting Friday
and will end persistent westerly flow which will put an end to lake-
effect chances, however, troughing aloft and embedded shortwaves
will still bring possible snow chances through the weekend. Models
fail to converge on a general solution regarding both snow chances
and potential timing, thus PoP values will remain sub-50 percent at
this time for Friday onwards. Overall message...Cold temperatures
and lake effect snow showers will arrive starting Wednesday. Expect
daytime highs to max out in the lower 30s Wednesday until at least
Sunday. For reference, average highs for the first half of December
reside in the mid to upper-30s.

MARINE...

Increasing southerly winds tonight and tomorrow, as deepening low
pressure tracks northeast out of the Central Plains and reaches
western Lake Superior by Monday Evening. Gale force winds are
expected across the majority of Lake Huron on Monday, with high end
gales over Northern Lake Huron, persisting into Monday Night. The
strong cold front will cross the Central Great Lakes early Tuesday
morning, shifting the winds to the southwest-west, with sufficient
cold air and mixing to sustain Gales. In fact, westerly gales are
expected to continue right through Tuesday night over the open
waters of Lake Huron, as the deeper cold air arrives, which will be
sufficient to generate snow squalls for much of the mid week period.

Any gales gusts, if any, that occur Lake St. Clair and Lake Erie
should be brief in nature, and peak gusts around 30 knots should be
more representative Monday evening through Tuesday Night.  Small
craft advisories have been issued.

HYDROLOGY...

Low pressure developing over the northern Plains Monday will rapidly
deepen into a powerful storm system as it tracks northeast across
Lake Superior and into central Ontario Tuesday. A strong cold front
accompanying the system will cross the region Monday night and into
early Tuesday, bringing widespread showers and a chance of
thunderstorms. Most areas look to see rainfall amounts around a
quarter of an inch, with up to a half of an inch possible in the
Saginaw Valley. Locally higher amounts will be possible in
thunderstorms. No flooding concerns are anticipated due to the
progressive nature of the system.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Warning from 4 PM Monday to 4 AM EST Wednesday for LHZ441>443-
     462>464.

     Gale Warning from 10 AM Monday to 4 AM EST Wednesday for LHZ361>363.

     Gale Warning from 4 AM Tuesday to 4 AM EST Wednesday for LHZ421-422.

Lake St Clair...Small Craft Advisory from 4 PM Monday to 4 AM EST Wednesday for
     LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 4 PM Monday to 4 AM EST Wednesday for
     LEZ444.

&&

$$

AVIATION.....DG
DISCUSSION...CB/AM
MARINE.......SF
HYDROLOGY....SF


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