Critique on the forecast - Today and Tonight

Amplified upper level ridge axis will slide over the central Great Lakes this afternoon/evening.
Inversion heights will be crashing down this morning, sub 4000 feet, effectively ending any threat
of additional lake effect flurries/snow showers by around noon, with warm advection pattern kicking
in this afternoon, as 850 mb temps rise into the negative low single numbers toward 00z Saturday.
Still, with the cold start, late November insolation and snow cover across much of southeast
Michigan, max temps will probably fall short of 30 degrees ( based on the 925 mb temps forecast of
-9 C ). Clear skies and winds attempting to decouple early this evening should allow most locations
to dip into upper teens to around 20 degrees before clouds and increasing warm advection pattern
takes over. Strung out upper level PV advection/good isentropic ascent ( 285-290-295 K ) very late
tonight ( 9-12 z ) will support a chance of some type of frozen precipitation, with borderline 3 C
warm layer ( 925-850 mb ) indicated. Any freezing rain/sleet/snow will ultimately be dependent on
precipitation rate/wet bulbing effects, as 850-700 mb wet bulb temp remains at or below freezing
through 12z Saturday ( per 00z NAM ).

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Remaining Forecast discussion -

Saturday morning, extensive pattern of warm air advection working into the exiting arctic airmass
will be well under way by early Saturday. Sliver of stronger 285-290K isentropic ascent lodged along
the southern periphery of a weak PV filament will translate through Southeast Michigan during the
latter half of the morning period. This will maintain a chance for light precipitation production
during this time. Steady warming through the column at the same time will leave a firm warm
nose/melting layer at 900-800 mb, recent soundings on the NAM and GFS showing a mean temperature of
3-4C within this layer. Some lag to the response in the near surface layer will present a narrow 3-4
hour window /12-16z/ for light freezing rain as this layer works to climb above freezing. A rapidly
warming thermal profle and the limited duration/intensity and corresponding low QPF /a few
hundredths/ will contain the potential for ice accretion... Perhaps generating a few slick spots on
elevated surfaces.

Saturday afternoon and evening, persistent but weak ascent within a strongly sheared west-southwest
flow/advective process will sustain a saturated profile in the lowest 5 to 6K ft during the latter
half of the day and into the evening period. Se Michigan settles along the northern periphery of an
anticylonic upper jet core during this time, some degree of weaker frontal forcing or convergence
holding in place. Sufficient depth into the top of the inversion layer to suggest some pockets of
light rain or drizzle will emerge at times in this environment... Worthy of a low PoP mention. Late
afternoon temperatures in the vicinity of 40 degrees. Rest of the weekend, dynamic southern stream
wave will eject northeast out of Texas Saturday night, reaching the Great Lakes by late Sunday. This
process will provide a renewed increase in moist isentropic ascent downstream anchored on a
secondary warm frontal boundary. This forcing will work across Southeast Michigan early Sunday,
yielding an uptick in rain chances through this time. The extensive cloud cover and a slight
easterly component to the low level wind field will cap highs Sunday in the middle and upper 40s. A
deeper moisture plume will then accompany the arrival of the main mid level ascent/vorticity axis
during the Sunday evening/night period. High confidence for a widespread coverage of showers given
the degree of forcing and moisture quality during this time, worthy of an upgrade to categorical
pops. Early next week, initial period of weak cold air advection ensues on Monday as the associated
deep surface low exits northward through northern Lake Huron. A period of stronger convergence along
the passing cold front may provide an additional round of showers during the daylight period.
Trailing northern stream trough will then provide a stronger downward spiral in temperatures by
Tuesday... 850 mb temperature projected to fall below -10C once again. This moist cyclonic flow and
cold air advection will provide the next opportunity for snow showers within Monday night and
Tuesday timeframe.

. . posted at 343 AM EST Friday November 21 2014


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