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fxus63 kiwx 231143 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
643 am EST sun Nov 23 2008 


Aviation... 
MVFR visibilities developed overnight at sbn due to strong radiational 
cooling over the snowpack. The fog/moisture there is very shallow 
and patchy and will likely not affect terminal beyond 12z. Just 
some cirrus cloudiness spreading eastward across northern Illinois/WI this morning 
and will likely overspread the terminals today. Low pressure 
approaching tonight will cause deep moisture and an area of 
rain/fzra/sn to move into sbn and FWA from the SW around 12z causing 
ceilings/visibilities to rapidly deteriorate toward the end of the taf period and/or 
just beyond. 


&& 


Previous discussion... /issued 417 am EST sun Nov 23 2008/ 


Short term... 
high pressure centered over the upper Ohio Valley was providing clear 
skies across the County Warning Area this morning. Patchy strato cumulus upstream in warm air advection 
regime across Illinois has been eroding as it moves east. Cs deck ahead of 
next shortwave was topping upper ridge and moving into the upper MS valley 
and should overspread at least northern portions of the area today. Warm air advection 
should raise 850 mb temperatures about 4c today and there will be more 
sunshine than yesterday so expecting highs around 10f warmer than 
yesterday... generally in agreement with NAM MOS. 


Upper air pattern will amplify significantly the next couple of days 
resulting in closed low over northern Saskatchewan this morning dropping southeast and 
phasing with shortwave moving east across central rockies. This results 
in weak surface low currently centered along Wyoming/Colorado Front Range to 
intensify/deepen considerably as it moves east to southern Michigan on Monday 
and then become vertically stacked by 12z Tuesday as it moves to just 
north of Lake Erie. Models continue in good agreement on airmass rapidly 
saturating ahead of this system over our area tonight due to 
combination of incrsg lift and northeastward advection of Gulf moisture. 
Main questions concerning precipitation with this system is when it will 
begin and what type it will be. Based on good agreement that precipitation 
will at least reach SW portion of County Warning Area late tonight...bumped probability of precipitation up 
in this area to likely with tight gradient to schc over far NE. NAM 
is a little slower moving surface low into the area than the GFS Monday 
morning which allows higher Theta-E air in low levels to advect in 
with low level warm nose temperatures on bufr data exceeding 3c as far 
north as southern Michigan. This occurs as surface temperatures hover near 0c 
suggesting period of freezing rain possible. GFS on the other hand is weaker with low 
level Theta-E advection and thus has stronger evaporational 
cooling and saturates low levels near 0c isotherm keeping precipitation 
mainly snow. In both cases strongest uvm remains below dgz so 
expecting snow accums will be minor. Based on NAM/sref felt 
threat of freezing rain sufficiently high to add to forecast NE 1/2 of 
County Warning Area. For now restricted it to just late tonight...though may need to 
stretch this into Monday morning but held off making that change 
at this time. As deepening low moves east of the area on Monday afternoon 
precipitation across the entire County Warning Area will change over to snow as low 
levels cool and wrap around moisture moves into the area. 
Again...bulk of moisture/lift forecast below dgz and surface temperatures so 
expecting little accumulate. 


Moist/cyclonic fetch Tuesday night should bring some lake effect snow 
showers into northern portion of County Warning Area. NAM indicating lake induced cape 
only around 300j/kg however as Delta t's only reach middle teens and 
significant moisture below dgz... so continue to think accums from lake 
effect will be minor. 


Long term... 
Tuesday through Saturday. 


Models coming into agreement on a blocking pattern setting up into 
at least Wednesday and possibly the remainder of the week as the 
strong upper low generally Parks itself somewhere between Ontario 
and Pennsylvania. The NAM is the least aggressive on this setup and 
the warmest with the GFS/gefs/Gem all pointing towards a stronger 
and cooler solution. This will likely slow down any warm up by at 
least a day and also keep a lingering chance for snow showers as 
Delta t's remain right in the 13 to 14 c range. After collaboration 
with GRR and DTX have decided to add in 30 probability of precipitation for downwind of the 
lake for Tuesday night with favorable north to northwest flow still in place. 
Inversion heights will be dropping but still averaging above 5000 feet 
until towards Wednesday morning...more than enough to at least have a 
chance in. Based on these trends have also lowered highs Tuesday and 
Wednesday with 850 mb temperatures remaining in the -7 to -9 c range. 


Looking up stream no significant pattern breakers noted...so quite 
possible that colder trend will continue with only modest warming by 
Saturday. As a result...will not make any changes to the grids past 
Wednesday. 


Aviation... 
VFR conditions expected to continue through this taf period. Skies 
have cleared over the area and will likely remain that way 
overnight. Light southerly winds and overall dry airmass expected 
to prevent fog from developing at the terminals overnight. Strato cumulus 
deck based around 6kft over Illinois moving east and slowly eroding so 
only forecast sct060 moving into the area Sunday morning. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...gale watch from Monday afternoon through Tuesday evening for 
lmz043-046. 


&& 


$$ 


Short term...Taylor 
long term...Fisher 
aviation...Taylor