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fxus63 kind 091112 
afdind 


Area forecast discussion 
National Weather Service Indianapolis in 
612 am EST Friday Jan 9 2009 


Aviation...discussion for the 12z tafs 




Early morning radar returns across the northwest are due to frontogenetical 
forcing which is forecast to shift north-northeast away from laf. Do not anticipate 
more than a brief period of light snow at laf if that through 14z. 
The remainder of the sites should stay dry today. Conds should be 
VFR today through 00z. Upward motion will increase after 03z tonight 
leading to precipitation and decreasing ceilings/visible conds at all sites. IFR or 
LIFR conds may also develop at all sites in the 9z to 12z Sat time 
frame as low pressure tracks across southern Indiana. First guess on 
precipitation type suggests that huf/ind have the greatest risk for 
sleet/pl with some risk for freezing rain/fzra. Bmg looks to be 
mainly rain while laf may be mainly snow with some risk for sleet/pl 
as well. The strongest forcing appears to occur in the 9z to 15z Sat 
time frame when a change from freezing/mixed precipitation to all snow 
could occur. Visible/ceilings may be worst at this time with LIFR conds 
possible at most if not all sites. 


&& 


Discussion... 


..Winter Weather Advisory late tonight through Saturday... 


Weather map shows high pressure across the Appalachians with a low 
across Nebraska. Temperatures were in the teens with dewpoints in the low teens 
and light to calm winds. Infrared loop shows some 6kft clouds moving in 
from Illinois. 


Forecast focus is timing/precipitation type for event tonight/Saturday. 
Model temperature guidance very similar so used a blend for the short 
term...leaning toward FWC where minor differences occur as this is 
matching up well with current temperatures. 


Isentropic lift will be across the area all day today...however best 
lift/humidity/cond pressure deficits indicate potential for snow across 
the north this morning. By midday forcing is all north of the area and 
thus kept only a slt chance snow in the NE. Blended guidance for highs 
in the 40s S to low 30s north. 


Tonight isentropic lift gets going in the SW and frontogenetical 
forcing kicks in across the north right around 6z. Ramped probability of precipitation up to 
categorical at 5z considering and this is right in line with mav 
numbers. Cross sections of frontogenesis with divergence q and epv indicate 
best fog forcing across the area from 6-12z with best instability 
around then as well and some upper support arriving around 12z. Epv 
pretty negative indicating the possibility for some banding to occur 
during this time. 


Now for precipitation type...fortunately NAM and GFS are very similar in 
their thermal profiles. Both show enough saturation in the -10 to 
-20 area to introduce some ice crystals. Sounding analysis as well 
as thicknesses indicated northwest should be mostly snow with a chance for 
some sleet. Mie snding shows more potential for sleet with the snow 
as well as perhaps some freezing rain depending on the depth of the 
surface cold layer and the advance of the warm layer aloft. For ind top 
down method indicates sleet most likely ptype...but this is because 
warm layer never climbs above 3c. While this is true the warm layer 
is over 200 mb thick and the surface cold layer is rather shallow so 
prefer the energy method conclusion of mostly freezing rain with some sleet 
mixed in. Huf looks like mainly rain. Perhaps a brief period of freezing rain 
so will include a chance for that for a small buffer zone against the 
freezing rain in the central counties. For the southern 2 tiers soundings and 
surface temperatures look too warm for any winter precipitation and thus kept rain 
there. 


Sat more cold air advects in from the northwest. Sndings indicate 
changeover to all snow for the northern counties. Central still indicates 
some potential for freezing rain through 15z so included it from 12-18z with 
snow. In the S used a rain snow mix as we should see a changeover 
during that time but cold air aloft should be arriving quickly enough 
that freezing precipitation should not be an issue. After 18z it appears 
everywhere will change over to snow. Models are moving slower with 
the system and thus probability of precipitation for Sat increased. 


For snow amounts...looking at possibly 3-5 across the north...depending on 
how much sleet mixes in. 1-3 across the central and most of that would 
be on Sat...again dependent on changeover from freezing rain. At this point 
not expecting more than a tenth of an inch of ice anywhere but this 
will need to be reevaluated with next issuance. 


Given all of the above will issue a Winter Weather Advisory for 
northern two thirds of forecast area from 5z Sat to 0z sun. 


Sat nt the system moves out and everything is dry through sun nt. Temperatures 
near normal. 


No changes made to extended. 


&& 






Ind watches/warnings/advisories... 
Winter Weather Advisory around/north of Interstate 70. 


&& 


$$ 


Public...cp 
aviation...Colorado