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afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
343 PM EST Thursday Dec 4 2008 


Short term... 
cold air advection/lake effect cloudiness continues over the Great Lakes region...including 
NE 1/2 of our County Warning Area this afternoon. Some lake effect snow has occurred much 
of the day across much of southern Michigan and adjacent northern Indiana 
counties...however inversion which tamdar soundings indicate is based 
around 3-5kft...along with short west-northwest fetch has limited intensity 
of snow showers in our area. Colder air will continue to advect 
into the region tonight. Shortwave seen on water vapor over the upper 
MS valley with a surface reflection seen in surface observation moving into northwest Illinois 
accompanied by a few flurries is expected to move east across our 
County Warning Area tonight possibly causing some flurries outside of lake effect 
areas. Also inversion will rise overnight as the trough moves 
through which should increase intensity of lake effect snow 
showers... though continued short fetch will still be limiting factor 
to significant accums...went with 1-2" over southern Michigan with <1" in adjacent 
in/Ohio counties. 850 mb temperatures expected to fall to around -15c in our 
County Warning Area overnight. Upstream temperatures this morning in this airmass in 
single digits/teens. With less snow cover in our area than Iowa/northern 
Illinois and partly to mostly cloudy skies expected overnight...kept lows 
in the teens. 


With thermal trough moving across the area on Friday...temperatures only 
expected to rise into the l-m20s. Surface ridge will move across the 
area during the day causing winds to back to SW bringing lake effect 
to an end across our County Warning Area by evening with the possible exception of northern 
Berrien County. Amount of clouds questionable. Both NAM and GFS 
indicate h925 will be saturated...but this layer was too moist in 
models today...so optimistically went with partly sunny skies 
again across southern portion of the County Warning Area. 


Another strong cold front will drop southeast into the western Great Lakes Friday night. Any 
remaining low clouds expected to advect NE out of the area in the 
evening as southwesterly low level flow increases. High clouds will be spreading 
into the area ahead of the upper trough so expecting mostly cloudy skies 
with little temperature drop. Continued with small chance of snow for late Friday 
night as front approaches...but still appears best chance of snow 
with the front will be Saturday. 


&& 


Long term...(saturday through thursday)... 


Continued cold and active pattern through the extended period with 
quick clipper and lake effect Sat night/sun followed by another potential 
strong system moving through the eastern US midweek. 


Extended begins with clipper system impacting northwest County Warning Area as surface 
low moves through the upper Great Lakes. Significant timing 
differences between slower NAM/faster GFS bringing precipitation into the 
County Warning Area on Saturday and have sided with the timing of 09z sref as a 
compromise. However as an addnl nod to model performance with the previous 
system have maintained earlier probability of precipitation indicated by the GFS and 
undercut the model consensus quantitative precipitation forecast by a marginal amount as the 12z GFS 
decreased quantitative precipitation forecast from 00z/06z runs possibly foreshadowing similar 
outcome of realizing lesser amounts. Net result is light/moderate snow 
settling in Saturday morning and expanding across the County Warning Area with 
relatively minor synoptic quantitative precipitation forecast amounts. Zone-Omega technique indicates 
snow ratios on the order of 15:1 translating to 2-3 inch snow 
totals tapering off from northwest towards the southeast. As synoptic forcing 
wanes in wake of surface low winds turning northwest produce lake effect 
setup beginning around 00z Sat and continuing into Sunday. Bufr 
soundings show Delta-t's peaking at 20 degree with inversion located 
in low end of growth zone and unidirectional 20kt winds through 
the layer. Have remained conservative with snow amounts given 
location of eql levels however potential exits for more significant 
snow with small change in thermal profile favoring better snow 
growth. 


Upper trough shifts eastward with flow returning to southerly by Monday 
shutting down any leftover lake effect mechanism and warming temperatures 
back to upper 20s and into low 30s for Tuesday. Relief is 
potentially short lived however as models continue to show strong middle 
week storm taking shape as surface low ejects NE through the eastern 
US while second deeper low wraps up near base of middle level trough 
along Gomex while a push of Arctic air into the US follows. 
Considerable spread among operational models and ensembles as one 
would expect this far out so have not attempted to go beyond the 
big picture which paints probability of precipitation Monday-Wednesday and colder temperatures behind 
front on Wednesday. Have kept late week temperatures above guidance as 
GFS tends to consistently overdo cold air intrusions several days 
out and European model (ecmwf) is not as aggressive with advancement of cold air 
into the upper Mississippi Valley. Stay tuned. 


&& 


Previous discussion... /issued 1259 PM EST Thursday Dec 4 2008/ 


Aviation... 
lake effect/Post frontal cloudiness has been diminishing from west-east across 
northern Indiana this morning with west edge extending from mcy to 
just west of FWA at 17z. Appears there is sufficient northwest component 
to the winds to keep the clearing line from progressing much 
farther east so left both terminals with predominantly MVFR ceilings. A 
weak trough moving east across the MS river at this time should move across northern 
Indiana this evening causing -shsn with MVFR visibility at both terminals. 
Although 280-300deg flow not favorable for significant snow showers at 
sbn... there may be a few periods of MVFR visibilities...especially overnight 
as winds veer slightly behind trough passage this evening. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...Small Craft Advisory until 7 PM EST Saturday for lmz043-046. 


&& 


$$ 


Short term...Taylor 
long term...Ludington 
aviation...Taylor 

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