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fxus63 kiwx 232349 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
750 PM EDT Wednesday Jul 23 2008 


Aviation... 


Isolated cumulus field will decrease rapidly during the next two hours. 
This sets the stage for VFR conditions through the entire 24 hour 
forecast period. 


Previous discussion... /issued 323 PM EDT Wednesday Jul 23 2008/ 


Short term...tonight into Thursday 
closed upper low in far southeast Ontario will lift slowly to the 
north along the Quebec/Ontario border with a broad surface anticyclone 
forecast to build southeastward from the western Great Lakes this 
evening to northern Indiana by 12z Thursday. Shortwave rotating 
around backside of upper low just east of DTX and its associated 
cold pool has allowed a scattered cumulus deck near 5kft to develop across the 
eastern two thirds of the County Warning Area this afternoon. Dry conditions will prevail 
tonight with synoptic scale subsidence behind departing shortwave. 
Should decouple nicely tonight with surface high near/overhead...which 
will lead to comfortable lows in the low to middle 50s. High pressure 
will be in control on Thursday with highs near 80f and no more than 
some scattered high based cumulus by late morning/afternoon. 


Long term...Thursday night through Wednesday 
medium range models continue to agree on a pattern involving upper 
troughing across eastern Canada...ridging across the central 
Continental U.S....and therefore northwest flow aloft across the forecast area through 
the middle of next week. This should lead to slightly below normal 
temperatures and low humidities for this time of year. Main forecast issues 
include possible mesoscale convective system Thursday night into Friday morning...chance of precipitation with 
frontal passage Sat...and precipitation chances with numerous shortwaves likely to 
eject from the northern Pacific trough into southern Canada and 
eventually into the Great Lakes by day 6/7. 


Thursday night into Friday morning...a southeast propagating mesoscale convective system seems 
likely...with initiation expected near the Iowa/MO border into wc Illinois 
at the nose of a 40-50kt ll jet in advance of a weak shortwave in northwest 
flow. This feature will follow along Theta-E ridge/instability close 
to 850-300mb thickness lines and prognosticated corfidi vectors...which may 
allow it to clip southwestern portions of the forecast area Friday 
morning. Continued with chance probability of precipitation...highest in the SW...with exact 
track/timing of shower/storm complex still uncertain. Will continue 
with low chance probability of precipitation elsewhere Friday morning with models hinting at 
some weak fgen forcing in the 85-7h layer and isentropic ascent. 


Friday afternoon through Saturday...convective cloud debris from the 
morning mesoscale convective system may limit destabilization for Friday afternoon. And with no 
obvious trigger other than outflow boundaries thought about 
omitting precipitation chances Friday afternoon. For collaboration/persistence 
sake decided to continue with low chance probability of precipitation for now. 12z model 
guidance remains consistent in having a stronger shortwave track 
from the northern plains into the northern Great Lakes late Friday 
night into Saturday. Introduced chance probability of precipitation for showers/storms on 
Saturday as lift from a 60kt plus middle-level speed maxima and surface 
cold front should interact with instability in place to produce a 
quick round of showers/storms. 


Sunday through Wednesday...cooler/drier air on tap for Sunday and 
Monday as a surface front will settle to the south...with surface high 
pressure spreading into the Great Lakes. Pattern will flatten out 
a bit by the middle of next week...with several bundles of 
shortwave energy forecast to track from the northern plains 
southeast into either the Great Lakes or Ohio Valley. Track/timing 
of these waves with eventual Washington/Theta-E advection and placement of 
Theta-E ridge uncertain so will continue with a dry forecast Tuesday 
and Wednesday. This matches up with the 12z GFS ensemble mean in 
keeping surface high entrenched across the Great Lakes with dry ll NE 
feed. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...none. 
&& 


$$ 


Aviation...Lewis/weisser 
short term/long term...steinwedel