Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 150923
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
423 AM EST Fri Dec 15 2017

.DISCUSSION...

Some perpetuation of very light snow showers/flurries ongoing early
this morning, the result of renewed eastward penetration of the lake
Michigan moisture plume. This process a response to a period of
backed west/southwest low level flow occurring in advance of an
inbound shortwave now translating across northeast Wisconsin.
Attendant surface trough leading in the main height falls with this
system, with an extensive region of light snow ongoing tied to both
an increase in mid level ascent and low level convergence. This
forcing on track to lift across southeast Michigan during the
daylight period. A noted increase in moisture quality as respectable
synoptic moisture taps the resident lake Michigan moisture plume
will yield a period of deep layer saturation with respect to ice.
This improvement in microphysics under the background of increasing
large scale forcing will subsequently support a fairly widespread
region of light snow/flurries locally. Highest coverage centered
late morning and early afternoon. Accumulation of less than an inch.

Late day transition toward potential lake effect snow showers/
squalls, as large scale support sweeps east and westerly low level
flow deepens. The overlake thermodynamic profile certainly
supportive, yielding a convective depth near 10 kft and upwards of
100 J/KG of cape within the outward projecting lake bands. The
underlying mean low level wind field will favor some degree of
downstream penetration of this lake convection. This setup favors
multi-bands or segments focusing somewhere between the I-69 and I-94
corridors, but with no clear convergence signal to suggest one
particularly will be more susceptible in this case.  Inbound lake
bands likely be quite squally, given expected gusts to 30 mph under
modest late day mixing.  This will lead to brief, localized high
intensity snowfall rates, with potential for a quick dusting to an
inch of additional accumulation.

Weak frontal boundary will remain in the vicinity of southeast
Michigan tonight and Saturday. The combination of weak fgen forcing
and subtle mid level warm air advection will maintain a narrow
ribbon of ascent over or near the region during this time. Mixed
signal across the model spectrum in terms of placement and magnitude
of this forcing, but recent probability guidance suggests at least
modest potential for another period of light snow to develop locally
during this time. Existing boundary will provide a slightly greater
thermal gradient on Saturday, as the resident air mass begins the
slow moderation process under increasing mean thicknesses. Highs
ranging from lower 20s north to lower 30s south.

Pattern of warm air advection will strengthen Saturday night and
Sunday, in advance of a southern stream wave ejecting out of the
plains.  High degree of uncertainty yet in terms of the possible
track and strength of this system, which will ultimately determine
potential coverage/timing and ptype of any associated precipitation.
Forecast will continue to conservatively call for a rain/snow mix at
this stage.  Potential for all locations to inch above the freezing
mark on Sunday.

&&

.MARINE...

High pressure is in place across the southern lakes while the next
clipper begins to track across Lake Superior. This clipper will
bring gusty southwesterly winds and elevated waves back to Lake
Huron this afternoon. Though gusts should stay below gale force they
may reach 30 knots this afternoon before turning northwest behind
the departing system this evening. The winds will result in elevated
waves around the tip of the Thumb this afternoon through tonight
necessitating a small craft advisory for a portion of the nearshore
waters. Winds decrease and turn out of the east this weekend as a
high pressure system drifts just north of the Great Lakes.

&&

.PREV DISCUSSION...
Issued at 1158 PM EST Thu Dec 14 2017

AVIATION...

Low level thermal plume off of Lake Michigan is currently spreading
across southeastern Michigan this evening. Temperatures of -10 to -
13C reside within the could bearing layer and is generating a
relatively good amount light returns on radar. Very transient
activity could result in a quick tenth or two this morning. Moisture
and shortwave energy arrives over the central Great Lakes after 12Z
Friday. Very good synoptic scale support for ascent with a
dampening, but direct CVA, some left exit region dynamics, and
midlevel theta e advection. Steepening lapse rates Friday afternoon
will then lead to a period of snow squall over the area. The
outstanding question remains how widespread and/or transient the
snow squalls will be.

For DTW...Expecting cigs heights below 5000ft by 04Z this evening,
continuing throughout the remainder of the forecast period. Will
watch very light returns to drift southward towards the airfield.
Latest indications are bulk of these flurries/light snow showers
will remain north of DTW. Question is on how widespread in coverage
snow squall will be 18-00Z Friday.

//DTW THRESHOLD PROBABILITIES...

* High confidence in cigs aob 5000 ft agl around 04z tonight.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
Lake Huron...Small Craft Advisory from 4 PM this afternoon to 4 AM EST Saturday
     for LHZ421-441.

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

$$

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


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