440 
fxus63 kdtx 042041 
afddtx 


Area forecast discussion 
National Weather Service Detroit/Pontiac Michigan 
341 PM EST Thursday Dec 4 2008 


Short term...this afternoon and tonight 


Numerous snow showers were able to generate earlier in/around the M 
59 corridor as the Lake Michigan snow machine was given a kickstart. 
Visibilities under the heaviest showers were occasionally limited in 
the 1 to 2 mile range which was able to deposit two tenths of an 
inch here at the office. Since then...the convective showers have 
undergone the natural late afternoon maturation/weakening 
process...trending cellular with a few cloud breaks under the more 
intense pockets of cloud scale subsidence. So...the idea is for 
scattered light snow showers for the remainder of the afternoon and 
into the early evening hours. 


A sizable potential vorticity anomaly was unshrouded (versus a 
straight sheared axis) by the initialization of the morning nwp 
guidance. This development does carry support in the latest water 
vapor imagery. The shortwave...now tracking across northern Iowa and 
portions of extreme southwestern Wisconsin...will Cross Lake 
Michigan this evening. The wave will Herald the next round of cold 
air advection as 850-700mb layer temperatures will drop some 5 
degrees with the passage...down to -20c. Therefore...an 
intensification of snow showers can be anticipated this evening with 
equilibrium heights rising to some 9 kft over Lake Michigan. 
Enhanced lower tropospheric convergence should focus farther 
south...generally near/along the I 94 corridor since winds will 
hiccup more northwesterly with the passage of the disturbance. 
Satellite imagery suggests there already is a southern migration of 
the banding now occurring over Lake Michigan. A dusting can be 
expected for most locations...with possibly an inch for the counties 
in and near I 94. Will need to watch if any of the bands are able to 
become stationary. 


Lows tonight will drop into the middle to upper teens above 
zero...which will put apparent temperatures/windchills into the single 
digits above zero. 


&& 


Long term...Friday through Thursday 


The overall flavor of the long term has remained unchanged this 
forecast cycle /below normal temperatures with frequent intervals of 
snow/. An enhanced region of snow showers is expected to be in place 
along roughly the I-94/I-96 corridors Friday morning. Surface high 
building across the Ohio Valley with westerly flow over Lake 
Michigan should set up the necessary low level convergence to focus 
these snow showers across southern portions of the forecast area. 
The low level flow is forecast to back to the southwest Friday 
afternoon...then more southerly overnight Friday in advance of an 
approaching short wave. This will allow the area of convergence to 
lift northward...spreading the more numerous snow showers into the 
Flint and Tri Cities area Friday afternoon. Middle level subsidence will 
lower inversion heights on Friday. This along with the backing wind 
directions suggest keeping any snowfall accumulation to an inch or 
less. The shallow convection should still however occur within the 
good dendritic growth zone. So some prolonged periods of low level 
convergence may lead to some locally higher snowfall totals. 


The aforementioned middle level short wave is still forecast to rotate 
through the Great Lakes on Saturday. The enhanced lift and 
increasing moisture depth associated with this wave will warrant 
categorical probability of precipitation on Saturday. The GFS/ECMWF/UKMET solutions become 
more amplified with this wave in comparison to the NAM. Current 
water vapor lends support to the more amplified solutions. The GFS 
solution has specific humidities of 2 g/kg near 850mb by 18z...with 
quantitative precipitation forecast around 0.15 inch. Given the high liquid to snow ratios...a 
general 1 to 3 inch accumulation seems reasonable at this time. The 
more amplified GFS also brings in a little colder air by 
Sunday...with 850mb temperatures forecast to drop down to -18c. 
North-northwesterly flow is really not a favorable wind direction to 
bring much lake effect snows into Southeast Michigan other than some flurries. 
Given the degree of cold air...maximum temperatures should only reach the lower 
20s. A surface ridge is forecast to advance into the area Sun night. 
Assuming warm air advection clouds hold off until late...min temperatures 
should easily fall into the single digits. 


Low level warm air advection and moist isentropic ascent within 
entrance region of an upper jet should bring another chance of snow 
into Southeast Michigan on Monday and Monday night. Overall...lift does not 
appear to be too terribly strong at this point. This system does 
appear to carry enough moisture to support some minor accumulations. 


Confidence toward the end of the long term is rather low at this 
time due to large spread among the ensemble members and poor model 
run to run continuity. There is some agreement among the medium 
range suite in that a polar vortex will drop south out of northern 
Canada with a fairly strong trough lifting up through the Gulf 
coastal region. However...there is considerable spread with respect 
to the amplitude of this southern stream wave and timing of the 
phasing with the polar vortex. The GFS is quick to drop the polar 
vortex into the Great Lakes by the end of the long term and thereby 
maintains an East Coast storm system. The European model (ecmwf) however is stronger 
with the southern stream wave and is thus slower and slightly 
farther west with the track of the storm system...holding off until 
next Friday before bringing the next surge of Arctic air into Southeast Michigan. At 
this time...preference is toward the European model (ecmwf) in light of the very 
strong upper jet forecast over central North America. This system 
will be one to monitor over the next week. 


&& 


Marine... 


Low pressure is forecast to move into the northern Great Lakes by 
Saturday morning. Increasing south to southwesterly low level winds 
in advance of this system will cause winds and waves to increase 
across Lake Huron. The lingering Arctic airmass will promote deep 
mixing over the lake...which will likely lead to gale force wind 
gusts late Friday night and through the day Saturday. The low will 
pass east of the region Saturday night. This will veer the winds to 
the northwest. A tightening pressure gradient and low level cold air 
advection will cause persistent gusts to gale force Sat night right 
through Sunday afternoon. Weak high pressure will then build into 
the area Sun night...allowing winds and waves to decrease. 


&& 


Previous discussion...issued 117 PM EST Thursday Dec 4 2008 


Aviation... 


Cold air streaming across Lake Michigan has touched off lake 
effect snow showers which have been impacting much of the 
southern two thirds of the County Warning Area. With heaviest activity oscillating 
around the M 59 corridor...will carry MVFR ceilings and limit any 
visibility restrictions to MVFR for snow during the early hours of 
the new taf issuance. Later this evening...forcing appears to 
consolidate near the I 94 to I 96 corridors. Provided the uptick 
in precipitation activity this evening...included a time period 
for IFR visibilities at kdtw/kdet. Chances for snow will drop off 
towards morning with the passage of an upper level disturbance. 
Winds slowly back to the west southwest throughout the period. 


&& 


DTX watches/warnings/advisories... 
Michigan...none. 


Lake Huron... 
Small Craft Advisory...Saginaw Bay...until 7 PM Saturday. 


Small Craft Advisory...nearshore waters from Port Austin to Port 
Huron...from 7 PM Thursday to 7 PM Saturday. 


Lake St Clair...none. 
Michigan waters of Lake Erie...none. 


&& 


$$ 


Short term...cumulonimbus 
long term....SC 
marine.......SC 
aviation.....Cb 




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