Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 141646
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
1146 AM EST Sun Jan 14 2018


.AVIATION...

Remnant early morning stratus across metro Detroit and points south
has since developed into a sct-bkn strato cu field, aided by the
onset of daytime mixing. There was actually enough low level
moisture near the Ohio border to produce some lower end MVFR based
clouds. The low level flow will remain southerly this afternoon,
keeping the Lake Michigan moisture plume well west of Se Mi.
However, the southerly winds are ducting the remnants of the early
morning stratus under the low level inversion, leading to a
northward expansion of the strato cu field, with bases generally just
above 3k feet. These clouds are likely to persist through a good
portion of the afternoon before the loss in the diurnal heating
component and a push of drier air arrives from northern Indiana.
Light snow is forecast to overspread the terminals in the 09Z to 12Z
time frame with the approach of an upper wave from the west.

For DTW...In addition to the early morning stratus, the onset of
diurnal mixing resulting in the development of a low end MVFR cu
field and even led to some flurries. Expect these lower clouds to
persist over the next couple hours before mixing into some drier air
erodes the clouds. The latest guidance suggest light snow beginning
around or shortly after 11Z Mon morning.

//DTW THRESHOLD PROBABILITIES...

* Moderate in ceilings below 5000 ft this afternoon. Low tonight.
  High on Monday.

&&

.PREV DISCUSSION...
Issued at 241 AM EST Sun Jan 14 2018

Discussion...

While a band of lake effect stratus has maintained "relatively mild"
temperatures early this morning south-southwest of metro Detroit,
clear skies and calm conditions over the remainder of the forecast
area have allowed temperatures to fall below zero, particularly over
the interior Thumb region where readings are approaching 10 below.
This will lead to a very cold start to the day, but southerly flow
will increase between exiting arctic high pressure and approaching
clipper low and bring temperatures to near 20 degrees this afternoon.

By that time, sunny skies will be replaced by gradually thickening
upper level clouds as a weak wave over the central plains is pulled
east into the area in advance of the aforementioned clipper system
now dropping south through the Canadian Prairie Provinces. Clouds
will continue to lower/thickening tonight likely resulting in lows
occurring in/around midnight with steady to slowly rising
temperatures thereafter.

A broad area of isentropic ascent will spread over the area as these
two shortwaves consolidate and encroach on the area. Light snow is
expected to spread eastward into the area late tonight, most likely
after 06z-08z. A theta-e ridge associated with the southernmost wave
will also aid in overall lift and bring a ribbon of mixing ratio
values of 1.50 g/kg. Most of the lift will initially exist above the
-10c/-20c layer, so snowflake size should remain rather small and
limit accumulation potential. This may change slightly late in the
day as the surface reflection of this system moves into lower MI.

This will increase forcing/lift below 10kft (-20C level) which will
bring some potential for larger snowflakes. All in all though, this
system should only be able to squeeze out enough moisture to bring a
few inches of snow to the area given the lack of good moisture and
only weak to occasionally modest lift. This accumulation will occur
over about an 18 hour period from early Monday morning through the
evening hours.

A notable lake aggregate response over the Great Lake then looks to
"latch on" to the surface low pressure associated with the system
itself and maintain some degree of low level forcing/convergence
overnight Monday night into Tuesday before the upper level feature
finally passes east of the region. Moisture quality, already iffy to
start with, will continue to diminish into Tuesday, so accumulation
potential will lessen dramatically even as scattered to numerous
snow showers persist in some locations. Overall, snowfall character
will transition from widespread light snow affecting all/most of the
area on Monday to more localized snow shower concentrations along
best low level forcing on Tuesday

Very cold air filters back into the area behind this system from
Tuesday into Thursday, maintaining well below average temperatures.
However, this shot of cold air will be more transient as the upper
level pattern evolves into a more progressive character as this
system pivots into the Canadian Maritime and focuses largely pool of
arctic air from Hudson Bay eastward. Quasi-zonal upper flow will then
bring a moderating trend late next week as Pacific maritime area
shifts back into the vicinity. Generally, expect temperatures to
climb back well into the 20s Thursday, into the 30s Friday and most
likely into the lower/mid 40s Saturday.

By next weekend, the upper level pattern will likely be undergoing a
major transformation as dominant upper troughing sets up over the
western CONUS as ridging translates eastward over the remainder of
the CONUS. In the immediate term, this would suggest a continued
moderation into next Sunday as a significant storm system ejects from
the western trough into the area. There is some suggestion that this
may become the dominant upper pattern for the rest of the month
which would bring a milder (and stormier) pattern to the region as
compared to our recent bouts of relatively persistent arctic air.

MARINE...

High pressure will continue to build eastward across the region
today, with light southwesterly flow increasing with time heading
into tonight. Wind gusts will generally remain at 20 knots or less
through the daytime hours, before increasing to 25-30 knots,
particularly Lake Huron, tonight ahead of an approaching clipper low
pressure system. Peak gusts look to stay below gale force, strongest
in the northern and central Lake Huron basin. As the clipper low
moves across southern Michigan Monday, winds will back to the
southeast and then become northerly Tuesday as the system exits to
the east.

&&

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

$$

AVIATION.....SC
DISCUSSION...DG
MARINE.......IL


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