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fxus63 kgrr 042140 cca 
afdgrr 


Area forecast discussion 
National Weather Service Grand Rapids Michigan 
416 PM EST Thursday Dec 4 2008 


Latest update...all but aviation... 


Synopsis...(416 PM EST Thursday Dec 4 2008) 
cold polar air continues to stream over the western Great Lakes as a 
polar high pressure system continues to slide south of the Great 
Lakes Friday. An upper air disturbance will enhance the snow shower 
activity tonight into Friday morning resulting in enhanced lake 
effect snow showers south of Muskegon and west of Highway 131. 
An Alberta clipper type storm will cross lower Michigan Saturday 
bringing snow across the area. The heaviest snow though will be near 
and west of Highway 131. That will be followed by another polar high 
pressure system for Sunday. 


&& 


Short term...(416 PM EST Thursday Dec 4 2008) 
(tonight through Saturday night) 


I have keep the lake effect Snow Advisory as is for tonight and 
Friday morning. There is a strong shortwave as seen on the water 
vapor image as of 330 PM over southern Wisconsin heading toward 
lower Michigan. Once that Cross Lake Michigan early this evening 
that will put southwest Michigan in the deeper polar air and in turn 
increase the inversion heights to near 8000 feet. Given the 850 mb 
temperatures near -16c...the dgz being saturated and the best lift being in 
the dgz it seems to ME getting advisory criteria snowfall should not 
be that hard to do. Actually there are two periods for enhanced 
snowfall or lift... the first is from 03z till 09z... the second is 
from around 12z until 18z. Boundary layer winds will be from 290 
degrees around 20 knots during tonight then become more westerly 
during the day on Friday. All of this clearly supports advisory 
criteria snowfall during this time period. 


During Friday afternoon the surface convergence near the Lake Shore 
decreases significantly and day time heating will tend to break up 
any organized snow bands anyway. So while the snow showers will 
continue I do not see the snow showers accumulating much then. 


As the Alberta clipper comes in Friday night into Saturday morning a 
southwest flow enhanced event sets up. I would have issued a Winter 
Storm Watch for this event but the boundary layer winds continue to 
shift in time as the system moves through so no one location will 
get a concentration of heavy snowfall. Even so... areas located 
west of US-131 and north of Muskegon will see the most snow from 
this event. Most of that will fall between 06z and 18z on Saturday. 
There is enough isentropic lift with this system the entire County Warning Area will 
see snow from this but areas west of US-131 and north of mkg will 
see the most snow. 


Saturday night boundary layer winds turn north northwest and that 
will favor lake enhanced snows over extreme western Van Buren and 
Allegan counties. 


&& 


Long term...(416 PM EST Thursday Dec 4 2008) 
(sunday through thursday) 
the extended period will start out with Arctic air over the state 
behind the departing clipper. This will help produce More Lake 
effect snow showers...however ridging will move in by late day... 
diminishing this activity. As the ridge slides by to the east we 
will experience somewhat warmer temperatures by Monday...however the snow 
will return on the leading edge of the warm advection. The models 
are in better agreement today...keeping a developing low pressure 
system to our south Monday night and Tuesday. This system still 
bears watching though as the heavier precipitation shield is still shown to 
almost reach the Michigan/in border. As the low heads up the East Coast 
it will pull down another surge of Arctic air over the Great Lakes 
into middle week. The European model (ecmwf) is slower with the cold air...but it does 
bring it in by late Thursday. Therefore given this regime we have 
continued the chance of snow just about every period through 
Thursday...however no major snowfall is expected as long as the 
Monday night/Tuesday storm remains to our south. 


&& 


Marine...(416 PM EST Thursday Dec 4 2008) 
I once again extended the Small Craft Advisory. Now it runs until 00z Sunday since 
the winds remain above 15 knots and waves above 4 feet. The Alberta 
clipper system that comes across the central Great Lakes Saturday 
may need an upgrade to gale warnings...however at this time this 
seems to be a marginal issue. Freezing spray potential will increase 
at that time with colder air moving in. 


&& 


Aviation...(1243 PM EST Thursday Dec 4 2008) 
lake effect snow showers persist early this afternoon. Temperature 
inversion heights climb toward evening and lift increases as an 
upper level disturbance will further enhance the snow showers. It 
appears the lake effect should get fairly intense between 22z and 
06z with LIFR likely within the heavier snow showers. Beyond 06z 
the short wave will be through and the lake effect is expected to 
diminish slightly. There could be a diurnal component to this 
activity also with some intensification possible toward sunrise on 
Friday. At this point it appears the snow showers should diminish 
then by middle/late Friday morning. Wind flow through the next 24 
hours will be west-northwest which will favor azo and GRR getting more frequent 
snow showers...however the remaining taf sites will also see 
occasional snow showers with IFR conditions. 


&& 


Hydrology...(416 PM EST Thursday Dec 4 2008) 
no significant Hydro issues in the next few days as temperatures 
expected to generally be below freezing through the next week. 
Little to no runoff can be expected into area streams and rivers. It 
does appear that we will build a snowpack with lake effect snow 
showers expected most days and a few systems moving through. 


&& 


GRR watches/warnings/advisories... 
Michigan...Lake effect Snow Advisory for Ottawa...Kent...Allegan... 
Barry...Van Buren and Kalamazoo counties from 5 PM this evening 
until noon on Friday. 
Lm...Small Craft Advisory in effect through 00z Sunday. 


&& 


$$ 


Synopsis: wdm 
short term: wdm 
long term: jk 
marine: wdm 
aviation: jk 
hydrology: wdm 

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