775 
fxus63 kiwx 201720 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
1220 PM EST Thursday Nov 20 2008 


Aviation...18z tafs 


Difficult taf forecast with multiple lake effect snow bands this 
afternoon...transitioning into a shore parallel single band tonight. At 
FWA...expect MVFR/low end VFR conditions with numerous shsn through 
03/04z this evening. Partial clearing/VFR conditions can be expected 
by midnight as dry ll air settles in behind a departing secondary 
trough...along with subsidence on east side of a single band that should 
set up well west of the terminal. At sbn...significant visible/ceiling 
restrictions are expected as lake effect shsn activity increases in 
coverage/intensity this afternoon...as approaching secondary wave allows 
for increasing synoptic moisture and inversion heights for deeper 
convective cells. Intense single band likely to set up tonight 
over/near sbn by the way 00z and 03z. Exact location of narrow band of 
heavy snow/LIFR conditions still somewhat uncertain...although prognosticated 
925-850mb flow is favorable for sbn to end up within this band for 
several hours. Will continue with low IFR forecast at sbn tonight and 
will amend as needed. A gradual improvement is expected at sbn 
Friday morning per subsidence behind shortwave trough and backing ll 
flow. 


&& 


Previous discussion... /issued 627 am EST Thursday Nov 20 2008/ 


Short term... 
main concern remains potential for heavy lake effect snow over the 
next 36hrs. Appears surface low moving through the Great Lakes is a bit weaker 
than earlier forecast. This is resulting in less cyclonic flow with 
winds over the lake tending to be slightly more northerly which increases 
the likelihood of a dominant band developing and impacting the 
County Warning Area. NAM indicates main band developing today will extend from SW Michigan 
southeastward into NE Indiana. Land breeze effects will cause stronger 
convergence over eastern lm tonight resulting in a more intense snow 
band which will likely set up farther to the west as steering 
winds veer slightly more northerly. This band expected to affect Berrien...NE 
LaPorte...and St. Joseph counties and extend south through the 
Plymouth and Rochester areas. NAM has been consistent suggesting 
potential for heavy snow amounts with a dominant band tonight with 
strong uvm through the dgz and moist low levels. Difficult to 
reach 80% confidence level with much lead time with lake effect 
events around here... but based on 320-340deg 0-2km mean winds 
combined with intense lake induced instability...high 
inversion/eql heights...and rather moist low levels...pattern 
recognition suggests heavy snow accums over Berrien...NE 
LaPorte...and St. Joe Indiana in this scenario. Thus have lake 
effect snow warnings out in this area...bounded by lake effect 
snow advisories a County or two farther inland. Still potential 
for further upgrades of the advisories counties to warnings. At this time 
most concerned this may be needed for western Cass Michigan and northern Marshall 
Indiana...but given limiting factor of 320-340 fetch for Cass Michigan 
and distance from lake for Marshall...felt advisory criteria snow 
was most likely at this time. 


While some snow showers expected to spread well inland today... 
expect development of strong dominant band over western portion of County Warning Area 
will allow for dry conditions and perhaps some clearing in eastern 
areas tonight. Will be unseasonably cold with 850 mb temperatures falling to 
near -15c. However...expect enough clouds/mixing to keep min temperatures 
in the lower 20s across the area. 


On Friday winds will back to westerly as surface high moves to the middle MS 
valley. This will cause the Strong Lake effect band to become 
detached from the lake and drift eastward across the County Warning Area during the 
day...weakening as it does so. Weak warm air advection will develop...but temperatures will remain 
much below normal. Based on 850 mb temperatures and highs in similar but 
slightly less cold situation on Monday... expect highs in the 
u20s/l30s across the area which is close to coolest of MOS 
guidance... the FWC. 


As surface ridge approaches the area Friday night winds will diminish 
and some clearing expected...especially across southwestern portion of County Warning Area. 
Again sided with lower end of MOS suite... and further downward 
adjustment may be needed depending on cloud cover trends. 


Long term... 
Saturday through Wednesday...the main challenges this period center 
on another upper level system moving southeast out of southern 
Canada into the western Great Lakes region. The European model (ecmwf) and GFS 
operational runs have continued to come into better agreement with 
the timing and track of this system. The GFS ensembles were also 
showing above normal confidence per FSU ensemble analysis comparison 
of the GFS with climatology. This surface low should track to the 
northeast of the area and move through relatively fast. Moisture 
ahead of the low should be limited with the forecast area in the dry 
slot. There is some potential for lake effect snow Tuesday...but Delta T 
values should only be in the 10 to 13 range. So for this 
package...removed the chance for rain Sunday. Kept the chance for a 
mix Sunday night and Monday...followed by a chance for mainly lake 
effect snow Monday night and Tuesday. The GFS mex MOS temperatures 
have trended a little warmer since Tuesday evening. 


&& 


Iwx watches/warnings/advisories... 
in...lake effect snow warning until 1 PM EST /12 PM CST/ Friday for 
inz003-004. 


Lake effect Snow Advisory until 1 PM EST /12 PM CST/ Friday for 
inz005-012-014>016. 


Michigan...Lake effect snow warning until 1 PM EST Friday for miz077. 


Lake effect Snow Advisory until 1 PM EST Friday for miz078. 


Ohio...none. 
Lm...Gale Warning until 11 PM EST this evening for lmz043-046. 


&& 


$$ 


Short term...Taylor 
long term...skipper 
aviation...steinwedel