Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 171019
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
519 AM EST Sun Dec 17 2017


.AVIATION...

A weak front will lift north through the area today. Cool low level
easterly flow and saturated boundary layer will result in IFR
conditions through most of the forecast period. Some obs upstream
are coming in LIFR which will need to be watched through the morning
hours. There is a chance of some light precip falling out of this
low saturated deck, but chances and confidence are too low to
include in the tafs. With temperatures quickly rising above 32,
would likely be drizzle or light rain. HRRR guidance shows the area
of VFR south of the front will flirt with the Detroit area
terminals, but most of the guidance advertises staying socked in
which is where we`ll lean with the 12Z tafs. A second period of
light precip could come later tonight as a low lifts along the
front, but again models are not showing much coverage with this.

For DTW...Expanding area of IFR stratus is affecting the terminal
this morning. There is potential for drizzle and fog with the
frontal boundary in vicinity of the airfield but at this point will
highlight the CIGs with some lower VSBYs until we start seeing obs
come in nearby. Better potential appears later today and tonight.

//DTW THRESHOLD PROBABILITIES...

* High for ceiling 5000 ft or less tonight through Sunday.

* Very Low for freezing drizzle precip type tonight through Sunday
  morning.

&&

.PREV DISCUSSION...
Issued at 355 AM EST Sun Dec 17 2017

DISCUSSION...

Nearly stationary mid level frontal boundary lodged within slightly
confluent west-northwest flow currently draped across the OH/MI
border early this morning. Low level easterly flow still tucked
beneath this benign frontal slope, sustained south of high pressure
anchored over central Ontario. This pattern presents a challenge in
defining low level moisture trends /below 925 mb/ for today, as the
easterly component likely draws the lake Erie moisture plume into
the broad sub-inversion layer residing beneath the mid level
boundary. Evidence of this low level saturation process already
underway with an expanding IFR level stratus along the Ohio border.
Caution is given yet to the degree of saturation noted within the
model solution space, with a notorious moist bias often presented
under fresh snowpack /particularly the NAM/. With that said, a
gradually veering low level wind with time as the pressure gradient
responds to an inbound shearing low-mid level impulse will favor
some form of northward stratus through the day. No real signal
within the model output or ensemble probabilistic guidance to
suggest the potential for fzdz/dz is high enough to warrant a
mention. Not out of the question that a smaller scale response does
emerge should weak convergence develop along the advancing surface
warm front, but will allow near term trends dictate an inclusion.
Weak warm advection today remains off the surface, leaving the
existing near surface air mass virtually unchanged with minimal
potential for mixing given the easterly flow. General moderation
underway as weak shortwave ridging pivots through, so perhaps a
slight upward adjustment relative to yesterday. Highs range from
around 30 degrees north to upper 30s near the Ohio border.

Fraying mid level dynamics tied to the aforementioned wave will
largely pass by to the south late today into early tonight. This
process will draw a weak surface reflection across southern lake
Michigan and the west-central lower MI tonight. Renewed low level
moisture advection/isentropic ascent as an attendant surface-925 mb
thermal ridge arcs into the region will increase the potential for
some very light precipitation development, particularly during the
latter half of the night through early Monday. A warming thermal
profile coincident with this process gives little opportunity to
introduce ice into the column, favoring an all liquid ptype. Best
shot for measurable liquid focused south of I-69, where model
consensus suggests that the window for sub-freezing surface
temperatures will be closing just as conditions become more
favorable for precip production during early morning period. This
will limit the potential of witnessing light freezing rain or
drizzle overnight.

Moist and milder conditions Monday under moderate low level
southwest flow. Highs will make a run toward 40 degrees across
southern sections. Some perpetuation of very light rain or drizzle
possible through the day, sustained by some combination of weak warm
air advection and brief cva tied to the passage of low amplitude
wave.

Cold front remains projected to lift across the region early
Tuesday, attendant to a northern stream wave shearing across
southern Canada. Limited moisture quality and lack of larger scale
dynamics or frontal convergence still suggest a dry frontal passage.
Resident time of the existing milder environment likely great enough
to allow readings to lift into the lower/middle 40s, before ensuing
quick shot of cold air advection takes over late in the day. This
will send the temperature profile back to below average values for
Wednesday, under passing high pressure.

A more complex pattern evolution for the late week period will bring
the greatest potential for meaningful precipitation over the next
week, followed by what looks to be an extended arctic plunge next
weekend lasting through the Christmas holiday. Strong height falls
across the western conus by midweek will set the stage for likely
surface cyclogenesis somewhere over the plains, with an eventual
northeastward ejection of this system along the main height fall
gradient. Potential for a quick shot of warm air advection driven
snowfall Thursday depending on where the main moisture axis aligns
with eastward extent, but otherwise recent model trends would
suggest southeast Michigan largely lands within the dry downstream
`warm` sector for a time before the main system dynamics arrive
Friday. Forecast uncertainty derived from the usual sensitivity to
changes in the governing track, strength and interaction of the pv
filaments, meaning additional model adjustments remain likely within
this period until the key features attain better sampling later this
week. Some timing differences aside, there is a clear signal for an
extended stretch of below normal temperatures to emerge in the wake
of this system, as high amplitude troughing reestablishes control.

MARINE...

Quiet conditions across the eastern Great Lakes today as high
pressure tracks east through Ontario into Quebec this morning. A
weak front just south of the Michigan border will lift north as a
low pressure system moves through the central Great Lakes today. The
system may bring some light rain or snow to the lakes but winds will
remain below 20 knots. Quiet conditions continue through Monday
before a larger and stronger low pressure system approaches early
next week. The potential for westerly gales is beginning to show
with this system by Tuesday.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....DRK
DISCUSSION...MR
MARINE.......DRK


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