294 
fxus63 klot 072345 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
645 PM CDT Thursday Aug 7 2008 


Discussion... 
320 PM CDT 


A relatively quiet weather pattern is expected to continue through 
the weekend with main focus on potential for showers Saturday 
afternoon/evening. 


Surface ridge axis across the Dakotas will continue to slowly 
build into the western Great Lakes through tomorrow. A weak upper 
vorticity maximum progressing through Southern Lower Michigan has aided in 
scattered shower/thunderstorm development across Michigan this 
afternoon but not expecting any of these showers to impact 
northern Illinois or Northwest Indiana this evening. Otherwise 
with no significant upstream short waves of note...diurnal cumulus will 
diminish this evening with sunset. Did lower min temperatures a shade for 
tonight and tomorrow given trends in surface dew points today with 
fairly good radiational cooling conditions...with highs in the middle 
to upper 50s except warmer downtown. 


Quiet weather will continue tomorrow with forecast area in non 
divergent upper pattern tomorrow across the forecast area between 
longwave trough across northeast US and southeast Canada and 
longwave ridge across western Continental U.S./Southwest Canada. Perhaps some 
more afternoon cumulus tomorrow but expecting coverage to be less than 
that of today given ridge axis building in and less impressive 
low/middle level lapse rates. Next feature of interest for weather 
across northern Illinois and Northwest Indiana will be vorticity lobe 
currently approaching James Bay. This vorticity lobe is expected to 
drop southward into lower Michigan on Saturday with tail end of 
this short wave likely to clip southern Wisconsin/northern 
Illinois Saturday afternoon. Low/middle level lapse rates will 
steepen in association with this feature as middle level cool pocket 
drops into the Great Lakes. Sref probability of precipitation actually increase into the 
high chance category by late Saturday...but frontal convergence 
will be quite weak and if anything does develop in association 
with the weak surface front associated with this wave...expecting 
only isolated coverage during the afternoon/early evening. Next 
surface high will quickly build into the area behind this weak 
front on Saturday night ending any low precipitation chances. 


Temperatures over the next few days will show little day to day 
variation given weak thermal advection pattern...and despite 
continued weaker north winds tomorrow as surface high builds 
in...relatively Warm Lake temperatures should limit lake cooling to 
middle/upper 70s. With surface high departing early 
Saturday...southwest winds on Saturday should limit potential for 
any lake breeze. Temperatures perhaps a few degrees cooler on Sunday behind 
this frontal boundary with surface winds back to the northeast 
providing a few degrees of lake cooling once again. 


Moisture will slowly creep back into the area later Monday and 
Tuesday as more amplified upper level short wave moves across the 
Pacific northwest and sends a series of weaker short waves east 
dampening upper level ridging a bit. Medium range models are 
showing some significant differences in upper level evolution 
beginning of next week with latest GFS more aggressive in phasing 
another upper low across Hudson Bay with upper level short wave 
tracking eastward from the northwest US. European model (ecmwf) keeps Hudson Bay 
upper low on more eastward track into Quebec. Given large 
uncertainties at this time...did not make many changes to going 
weather grids with low chance probability of precipitation for thunder during the first 
part of next week with at least better chance of deamplifying upper 
ridge and more significant moisture return. 


Marsili 


&& 


Aviation... 
645 PM CDT 


00z tafs...lake breeze boundary has pushed well inland of 
gyy/Ord/mdw turning winds northeasterly. With winds over northern Illinois a little 
stronger than over ecntrl Illinois...the lake breeze is not pushing 
quite as far inland and feel it is unlikely to push through dpa 
before washing out. Expect that the boundary will dissipate by around 
02z and winds will return to more reflective of the large scale 
flow and return northwesterly overnight. With a weak gradient in place as 
the center of the large sprawling high builds southeastward...winds will 
drop off the light/variable overnight...and then return to more northeasterly 
tomorrow. With a dry airmass in place and large scale subsidence 
spreading over the region...expect that visby will remain 
unrestricted through the period and only scattered fair weather cumulus to develop 
tomorrow during the late morning and afternoon hours. 


Krein 


&& 


Marine... 
110 PM CDT 


A large high pressure over southern Saskatchewan will move 
southeastward...settling over the western Great Lakes and Midwest by 
Friday evening. The high will then slowly drift southeastward 
across the Ohio Valley over the weekend...with a ridge remaining 
over the western Great Lakes. This pattern will allow north 
northwest winds as high as 25 knots this afternoon...to diminish 
tonight and become light and variable around 10 knots Friday night. 


Merzlock 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...Indiana nearshore waters until 4 PM Friday. 


&& 


$$