National Weather Service United States Department of Commerce

Forecast commentary -

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.

. . posted at 423 AM EST Fri Dec 15 2017