Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210451
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1151 PM EST Sat Jan 20 2018


.AVIATION...

Warm front will become increasingly organized as it lifts northward
into the forecast area later tonight. Outstanding, synoptic scale
forcing will aid in the development of the boundary and provide what
should be a fair amount of overrunning and isentropic ascent with
time. MVFR stratus deck largely arrived on time. Question now
becomes whether or not the cloud line remains progressive and moves
northward. Model data is suggesting that it may have trouble now
that its in place over metro Detroit. Regardless, first push of 300K
isentropic ascent will arrive over the Detroit terminals after 09Z
which should allow for onset of IFR. A secondary cloud line has
emerged on satellite upstream and times very well to 09Z IFR onset.
Given the lack of deep saturation in the forecast soundings,
especially in ARW and RAP soundings, continue to favor a -dz
mention.  High confidence in surface air temperatures above
freezing. Widespread rain is expected to hold off until around
daybreak Monday.

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 develops Sunday morning.

/DTW THRESHOLD PROBABILITIES...

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

* Low confidence in visibilities less than 1/2 mile.

* Low confidence in ceiling heights at or less than 200 feet.

* High for drizzle as precipitation type Sunday morning.

&&

.PREV DISCUSSION...
Issued at 329 PM EST Sat Jan 20 2018

Weak surface gradient flow under nearly zonal flow aloft will lead
to pleasant conditions with light winds and a few high clouds
streaming over Michigan today. High pressure across the southeastern
US will keep the warmer southwesterly flow across southeast
Michigan, though not much of a change compared to yesterday from a
thermal advection standpoint. This will only allow the diurnal
response to increase high temperatures slightly from yesterday into
the low 40s for most of the area with a few locations remaining in
the upper 30s.

Low stratus clouds associated with a plume of moisture pushing
northward in the 925-850 mb layer has been slow to reach Michigan,
but anticipate that it will eventually enter the area by tonight.
The eastern edge of this moisture can be see on satellite moving
over IL/IN this afternoon. This push of moisture will bring with it
some fog potential. How dense the fog becomes remains uncertain, at
least for the early part of tonight, with the higher dewpoints
lagging a bit. Though, conditions will be favorable for some patchy
fog to develop. This warm air advection and surge of theta-e may
allow for some areas of light rain/drizzle to develop by tomorrow
morning across the lower half of Michigan. The warmer guidance with
temperatures remaining at or slightly above freezing overnight into
the early morning hours. This suggests only a small window of
opportunity with little impact expected if any light precipitation
were to occur at locations hovering around the freezing mark before
the diurnal response squashes any lasting frozen precipitation
chances.

Tomorrow will likely be a gloomy day with cloudy and moist
conditions and the potential for fog to stick around as snow melt
continues throughout the day. An increase in rain chances for Sunday
into Sunday night look possible given the weak isentropic ascent and
presence of a frontal boundary stretching eastward from the
developing surface low in the Central Plains. Will need to monitor
precip and temperature trends heading into tomorrow night, mainly in
the northern Thumb and northern parts of the Tri-Cities. Current
forecast lows overnight will be below freezing in the low 30s and
upper 20s. With increased precipitation chances, a small window of
light freezing rain/drizzle will be possible.

A dynamic system will bring categorical PoPs for Monday as a low-mid
level cyclone ejects out of the Central Plains and into the western
Great Lakes by Monday afternoon. The zonal flow will shift to strong
south/southwesterly flow with southeast Michigan residing on the
eastern end of the surface low and in the warm sector. Uncertainty
with the position and timing of the surface cyclone remain with the
GFS a little faster and the ECMWF a little slower. However, rain
chances still remain good Monday afternoon and into the Monday
evening time frame. Transition for rain to a rain/snow mix enters
the forecast late Monday night into Tuesday morning as temps drop
overnight.

Low pressure is expected to be positioned across Lake Huron by 12Z
Tuesday, and will continue to push northeast into Ontario throughout
the afternoon, bringing lingering precipitation chances to southeast
Michigan. The chance for light scattered showers will hold on
throughout the morning hours before cold Canadian air wraps around
the northwest side of the low, bringing the slight chance for a
wintry mix to snow transition throughout the afternoon. Minimal snow
accumulations are expected at this time for the rain that does
transition to snow.

Otherwise, an amplified long-wave trough and associated broad
surface high pressure system will fill in across the eastern U.S.
through the mid to late-week period, bringing an extended period of
dry conditions Wednesday - Friday. Cloud cover is expected to start
to scour out throughout Thursday, bringing periods of sunshine for
both Thursday and Friday as temperatures remain seasonal to slightly
above seasonal. WAA is expected to continue to ramp up into
Saturday, allowing temperatures to potentially peak in the mid to
upper-40s for a high. Saturday, however, will also mark next chance
for likely widespread rain chances as southwesterly brings a warm
front and a surge of moist air to the region, providing enough
forcing and moisture for widespread rain chances.

MARINE...

A weak cold front will settle over the north third of Lake Huron
tonight and stall there during Sunday. This will allow a warm front
to bring mild air in from the Ohio valley over all other marine
areas. The boundaries will merge into a stronger warm front by
Monday as low pressure strengthens over the central Plains and
Midwest. This system will produce widespread coverage of
precipitation, primarily rain from Saginaw Bay southward and a
wintry mix to the north. Increased easterly gradient flow will
result over the north half of Lake Huron, which will be enhanced by
cold air moving in from high pressure centered over Hudson Bay.
Easterly gales will be possible Monday afternoon through Monday
evening and a gale watch is now in effect for this time period. The
wind will weaken and then shift northwest as the low moves across
the lake Tuesday. Northwest wind trailing the system could approach
gales briefly Tuesday night into Wednesday.

HYDROLOGY...

A large low pressure system will bring very mild air into the region
tonight through Monday. This will result in additional melting snow
followed by increasing coverage of rain showers Sunday and then
becoming widespread Monday. The snow will melt but the ground will
not thaw fast enough to prevent runoff from both the snow and
rainfall. This could result in 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 with most falling during
Monday.

PREV DISCUSSION...
Issued at 327 PM EST Sat Jan 20 2018

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Gale Watch from Monday morning through Monday evening for
     LHZ361>363.

Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
&&

$$

AVIATION.....CB
DISCUSSION...AA/AM
MARINE.......BT
HYDROLOGY....BT


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