National Weather Service United States Department of Commerce

Forecast commentary -

Upper level wave/trough sliding through the eastern Great Lakes this evening, with northwest
confluent flow/surface ridging over southeast Michigan. Trapped low level moisture underneath the
subsidence inversion still persists, and low clouds expected to hold mins in the 20s, as low level
warm advection begins to kick in later tonight as well, with 850 mb temps reaching around 0 C by
Wednesday morning.


Dry airmass in place over the Southern Plains today, with 12z raobs indicating pw values generally
running under 50 percent of normal values ( Nebraska/Oklahoma/northern Texas ). Thus, despite would
looks to be good forcing tomorrow, the lack of deeper moisture is expected to limit precip amounts.



Strong upper wave tracking through Nebraska/South Dakota this afternoon will continue to spin/wrap
up over the midwest tonight, and fairly narrow moisture axis ( specific humidity of 3 g/kg from
850-700 mb ) progged to slide through the forecast area late Wednesday/early Wednesday evening, with
good upper level divergence and burst of isentropic ascent ( 284 K ) which should be good enough to
at least generate precipitation ( although bulk of solutions continue to advertise QPF being a tenth
or less ). Mid level lapse rates also look to be steep, in excess of 8 c/km from 700-500 mb. The
warm axis ahead of the low will tend to get pinched off, as the low levels near the surface remain
fairy dry, and wet bulb zero heights under 1000 feet ( per NAM ) suggest bulk of precipitation will
fall as wet snow, but limited duration ( ~3 hr ) at any given location should lead to most locations
staying under 1 inch, as some melting occurs as well with temps mainly just above freezing. It
appears the northern Thumb region/midland/bay counties stand the best chance of seeing an inch or
slightly more with modestly better thermal profiles/forcing, per local probabilistic guidance and
12z hires-arw.


Mid level circulation/cold pool ( -13 C at 700 mb ) to slide east around midnight, and robust post
wave subsidence advertised overnight to shut off precipitation with limited cold air advection, as
925 mb temps fall to perhaps -4 C at 925 mb by 12z Thursday, the coldest solution. Assuming some
sunshine develops on Thursday, should be able to get into the upper 30s, with good warm advection
pattern then kicking in Thursday night, and some fog development is possible if the higher dew pts
come in faster than expected over the frozen ground in place. Either way, increasing chance of
precipitation with warm front lifting through Thursday night, its possible could briefly start out
frozen, especially north of I-69.


A weak upper-level shortwave and right entrance dynamics from a jet streak will provide enough lift
to have the chance for rain Friday morning and afternoon, as an increase in warm air advection
allows temperatures to peak in the lower 40s for a daytime high. Better convergence is now seen
regarding a southern-tracking cut-off low, at least between the ecwmf and GFS, keeping the low well
south of the of the Great Lakes, from Texas eastward. This increases confidence to see dry weather
through the weekend as surface high pressure builds across the state. The GEM continues to suggest
the cut-off low will steer northeast from Louisiana into the Ohio Valley, which would bring the
chances for rain throughout Saturday. As a result, opted to keep the inherited slight chance to
chance of rain showers for Saturday.


Otherwise, higher pressure and lack of meaningful forcing will produce dry weather across se
Michigan on Sunday as milder air holds across the state, keeping daytime highs in the lower 40s. Low
confidence regarding PoP and precipitation type early next week period as long-range model runs
diverge on the passage of a potential cold front. The ECMWF run pushes a weak cold frontal boundary
across the state Monday into Tuesday, but high surface pressure holds off any precipitation C. The
GFS pushes a strong cold front Monday into Tuesday which would introduce lake effect snow chances to
Michigan, with a weaker signal for lake effect seen in the GEM run. Gefs runs trend toward a warmer
solution relative to the operational GFS, thus opted to leave lake effect snow chances out of the
forecast for early next week until further convergence is noted.


. . posted at 315 PM EST Tue Dec 11 2018