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Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
643 am EST Thursday Dec 4 2008 


Aviation... 


MVFR ceiling will have a firm hold on Southeast Michigan today and 
tonight. The exception will be intervals of VFR during the middle to 
late morning hours and then again late this evening and overnight. 
Otherwise, cold air moving into the region will combine with 
daytime heating and the lakes to generate MVFR stratocu with wind 
gusts near 20 knots. 


Flurries or brief snow showers are possible throughout the day but 
coverage will not be great enough to include at any one site. 
However, later this evening and overnight there is potential for 
lake effect snow to become more organized off the south end of 
Lake Michigan. The westerly wind pattern then favors the Detroit 
area to be impacted by additional MVFR ceiling and visibility 
toward sunrise Friday morning. 


&& 


Previous discussion...issued 418 am EST Thursday Dec 4 2008 


Short term...today 


What is left of frontal/Post-frontal forcing will exit Southeast Michigan 
before sunrise with steady light snow leaving less than an inch of 
accumulation. The door will then be open to the cold air that will 
take temperatures below freezing with little or no recovery during 
the rest of the day. Snow showers will be scattered around the area 
as well with the highest concentration south of Interstate 69. 


Precipitation struggled to get going with this system with any 
intensity, despite reasonable upper jet and short wave support. 
Moisture was also plentiful judging from 00z soundings and specific 
humidity in model analysis fields ranging from 3 to 4 g/kg. Its 
turning out that we can expect enough accumulation to make things 
slick but not much more as the frontal circulation is too shallow 
and stable for much vertical motion and greater amounts of 
precipitation. Surface analysis, however, leaves little doubt about 
the cold air moving into the region behind this system. Expect 
readings to drop into the upper 20s all areas and struggle to reach 
30 again during the daytime hours, accompanied by a 10 to 20 miles per hour 
wind. 


The cold air has already activated lake effect from Lake Michigan 
which will drive most of the snow showers in our area today, along 
with attempts at daytime heating. Convective depth holding around 
850 mb will enable lake contributions to reach our area in a slowly 
backing wind field that will tend to focus activity over the south 
half of our area through the afternoon. 


Long term... 


And so it begins...the persistent cold northwest flow has been 
established from central Canada...with a reinforcing shot of cold 
air to impact the Great Lakes region over the weekend in the form of 
a clipper...setting up what looks to be the coldest night (sunday 
night) since middle February as a full latitude upper level trough 
develops east of the Mississippi. 


00z radiosonde observations show 850 mb temperatures of -12 c at Green Bay...-17 c at 
International Falls...and -20 c at the pas in Manitoba...all along 
the northwest trajectory. Of course we will have some airmass 
modification from Lake Superior and especially Lake Michigan. With 
those lakes comes added moisture and low level instability...which 
will result in scattered snow showers tonight and Friday...with 
models still implying some type of band/convergence near I-94/I-96. 


Not planning on making many changes to current zones...with 
widespread light accumulations expected on Saturday as moisture 
starved surface low tracks through the central Great Lakes. Better 
moisture and dynamics will be well south of the region as left exit 
region of main jet core moves into Kentucky/Tennessee. So...an inch 
or two of snow is probably going to do it for Saturday with the 
burst of 280-275 k isentropic ascent. With 925 mb temperatures forecasted 
to be -11 c south to around -13 c north on Sunday...did lower 
maxes into the middle 20s. With the sprawling surface high settling 
over the area Sunday night and inversion heights 
crashing...reasonable confidence temperatures will be falling into 
the single numbers as skies clear out...with the good radiators 
possibly bottoming out around zero. 


Marine... 


Northwest to west winds will be near 20 knots today and then begin 
gusting to around 25 knots tonight on Saginaw Bay and Lake Huron. 
Small craft advisories are in effect tonight through Saturday as an 
extended period of rough marine conditions get underway. Winds will 
slowly back toward the southwest on Friday and Saturday and remain 
strong enough to be a small craft hazard. The next low pressure 
system and cold front may even be capable of gales by late Saturday 
into Sunday. 


&& 


DTX watches/warnings/advisories... 


Michigan...none. 


Lake Huron... 
Small Craft Advisory...nearshore waters from Port Austin to Port 
Huron including Saginaw Bay...from 7 PM Thursday to 7 PM 
Saturday. 


Lake St Clair...none. 


Michigan waters of Lake Erie...none. 


&& 


$$ 


Aviation.....Bt 
short term...bt 
long term....sf 
marine.......bt 




You can obtain your latest National Weather Service forecasts online 
at www.Weather.Gov/Detroit (all lower case).