Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 201740
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1240 PM EST Sat Jan 20 2018

.AVIATION...

Observations so far today indicate VFR over Lower Michigan that is
expected to hold through afternoon. A plume of MVFR stratus is shown
developing from the central Plains on pace to reach Lower Michigan
during the evening. The clouds will be the first stage of
deterioration in aviation conditions through tonight as mild air
moves into the region. A downward trend into IFR in stratus and fog
is expected as this air moves in over the snow cover that extends
down through at least the Ohio River valley. These conditions will
be augmented during Sunday by even milder air already producing IFR
over east Texas and Arkansas today which could lead to LIFR over SE
Michigan. The bulk of this moisture and associated drizzle/rain
potential will arrive after sunrise as temperatures warm steadily
above freezing from the Ohio border northward, if not already above
freezing by then. This will be monitored but the potential for icing
remains low at this time. LIFR/IFR will however continue through
Sunday along with increasing chance of rain showers through the day.

For DTW... VFR will lower to MVFR in stratus during the evening, and
then down to IFR/LIFR in stratus and fog overnight. Temperatures are
expected to be above freezing before the greatest potential for
drizzle or light rain develops Sunday morning.

/DTW THRESHOLD PROBABILITIES...

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

* Moderate for rain as precipitation type Sunday morning.

&&

.PREV DISCUSSION...
Issued at 340 AM EST Sat Jan 20 2018

DISCUSSION...

Nearly zonal lower amplitude westerlies will define conditions
through the upcoming weekend, as recent model guidance continue to
suggest that a slow pattern amplification will not impinge on the
region until early next week. This pattern will favor the
maintenance of a milder and increasingly moist low level profile,
driven by persistent low level southwest flow. Thermal advection
remains relatively neutral today, as a northern stream wave passing
by well to the north does veer the wind field below 850 mb just
enough to apply a limiting outcome. This may allow some pockets of
stratus to creep in from the north late today. However, increasing
evidence within the near term/hi res model suite that greater
stratus development will await the northeast expansion of 925 mb
moisture currently anchored over AR/TN, as this moisture lodges
within a broadening region of weak warm air advection across the 850-
925 mb layer. This is in opposition to the typically aggressive
NAM/ARW moisture schemes under this pattern, which would suggest
stratus becomes an issue during the daylight period. Will side with
the more optimistic RAP/HRRR, shading the cloud forecast more
optimistic with cloud coverage dictated simply by periodic thicker
cirrus. Subsequently, this will allow for a slightly greater diurnal
thermal response. Highs near or just above 40 degrees.

Deeper low level moisture /mainly below 925 mb/ will start to expand
across the region tonight. Dewpoints will be slower to respond,
suggesting that fog potential may prove limited given the additional
contribution of a persistent 5kt southerly gradient. Anticipate an
extensive low stratus taking hold, with gradual lowering of ceiling
with time perhaps allowing for some pockets of very light
rain/drizzle to lift north of the border toward daybreak. Latest
model guidance arriving a touch warmer, leaving surface temperatures
at the diurnal minimum at or just above freezing. This minimizes the
risk of any light precip arriving as freezing rain/drizzle Sunday
morning.

Cloudy and damp conditions on Sunday, the gradual increase in
boundary layer moisture sustaining an extensive stratus deck and
possible fog as snow continues to melt. Rainfall potential will
increase to some degree by afternoon under sustained yet weak
isentropic ascent. Still difficult to discern the validity of the
qpf output for those models prone to carry a moist bias in this
environment, but the underlying setup with a warm frontal structure
in the vicinity would suggest at least a chance for development
through Sunday night. Depending on nocturnal thermal trends, this
may position the tri-cities and northern thumb for a short window of
light freezing rain/drizzle Monday morning. Fog potential may
increase Sunday night given the elevated dewpoints and possibility
for low level flow to become easterly north of the surface warm
front.

Mature dynamic low-mid level cyclone will eject out of the plains
early next week. Strengthening deep layer southerly flow along the
eastern flank will direct the moist warm conveyor belt into the
region on Monday. The high magnitude of theta-e advection within the
background of strong upper diffluence and low level jet forcing will
support widespread rainfall throughout the day. Given the dynamic
forcing and strong advection, certainly could see a few rumbles of
thunder embed within these showers. Precip chances will briefly
diminish as the mid level dry slot punches in late Monday into early
Monday night.

Lead cold front frontal boundary anchoring the edge of the inbound
upper height fall gradient will bring a secondary shot of precip
Monday night. A corresponding downturn in temperatures will
eventually introduce the potential for rain to mix with and/or
changeover to snow during this time. Unsettled conditions Tuesday,
as the deep upper low pivots over the region and sustained cold air
advection ensues. This will maintain the potential for rain/snow
showers augmented by diurnal processes.  The upper low trajectory
may also provide a window for an axis of deformation snowfall to
setup somewhere across central lower MI.  An uncertain outcome at
this stage, but certainly worth monitoring going forward for the tri-
cities and thumb.

MARINE...

High pressure centered over the southeastern US will largely remain
in control of the weather over the Great Lakes through Sunday. A
weakening cold front has dropped down into the central Great Lakes
and will stall out today. This will result in mainly just a slight
veering of winds north of the boundary and overall weakening of the
flow. Flow will primarily remain out of the southwest today as we
remain on the northwestern edge of the high pressure but will back
to the south/southeast by Sunday afternoon and evening as the next
low pressure system approaches. This low will brings increased
chances for rain Sunday and Sunday night with the best chances
coming on Monday. Winds will become elevated as the center of low
pressure system enters the Great Lakes region but should be somewhat
tempered by the warm air advection with this system through the
weekend. Expect winds to pick up on Tuesday as cold air returns to
the region on the backside of the low.

HYDROLOGY...

A large low pressure system will bring very mild air into the region
this weekend. This will result in the melting of the remaining
snowpack before rain showers become widespread by Monday. The
combination of rainfall on top of frozen ground could lead to
ponding of water on roads and other prone areas. Creeks and streams
around the region could also become elevated due to runoff. Total
rainfall is expected to be around 1 inch between Sunday night and
Monday night.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...

MI...NONE.
Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.

&&

$$

AVIATION.....BT
DISCUSSION...MR
MARINE.......DRK
HYDROLOGY....DRK


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