584 
fxus63 klot 200545 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1245 am CDT Wednesday Aug 20 2008 


Discussion... 
154 PM CDT 


B9 weather likely to continue for the next couple days with no big changes to 
going forecast. Short range models remain in good agreement on 
Southern Plains upper low opening up and shearing out as it lifts 
NE into the middle MS valley late this week. Not impossible that some 
isolated ts could develop as early as Thursday PM over western County Warning Area...but 
more organized (tho still probably just sct) ts will occur Friday. 
Temperatures by Friday a bit tricky...as any sunshine could allow temperatures 
to reach into M/u 80s. 


Over the weekend still appears as though northern stream shortwave 
trough will translate eastward across southern Canada/far northern 
Continental U.S.. associated surface low prognosticated to stay well north of Canadian 
border with trailing cold front expected to sweep across area 
Sat/Sat night time frame. Given relatively modest height 
falls/forcing this far south and some lingering doubt on how 
quickly front will move through the area...decided to keep probability of precipitation 
somewhat modest for now. Does appear Canadian high will build 
southeast into the Great Lakes states early next week bringing 
another spell of temperate August weather. 


Eni 


&& 


Aviation...discussion regarding 06z tafs... 
1245 am CDT 


Main player in weather next 24 hours will be large hi pressure area now over 
Northern Lake Huron. This high will slowly move southeastward across the lower Great Lakes 
by 06z. As a result we're looking a generally VFR day once again. 
Minor exception will be Erly this morning due to formation of radiation 
fog/ground fog. Surface analysis shows dew points in lower 60s across 
much of northern Illinois/northwestern in. Expect areas away from chi/gyy heat islands 
will have some tempo lowering of visible around sunrise...perhaps to hi 
range IFR Cat. Expect some flat diurnal cumulus to form this afternoon based 
on forecast soundings that show steep low level lapse rates once again...but 
substantial middle level thermal cap...remaining in place as 500 mb ridge axis 
passes ovhd during peak surface heating...will preclude any deep 
convection. By 00z...some hi level ceilings will be invading from S as 
cloud shield assd with old cutoff low finally begins getting picked 
up in westerlies. 


Winds will be light through period...generally at or below 10 kts. Direction will 
veer from east-northeast into southeast during today as hi center slides off to southeast. 
Exception will be in vicinity of Lake Michigan where lake breeze is forecast to initiate 
around 18z. 


Merzlock 


&& 


Marine...discussion regarding morning forecast issuance... 
1245 am CDT 


High pressure centered over Northern Lake Huron will continue 
southeastward across the lower lakes today...then on to the middle 
altlantic coast by Thursday. Meanwhile...a low pressure center is 
forecast to develop over the northern High Plains on 
Thursday...helping to draw another low northeastward out of the 
Southern Plains and over the upper Mississippi Valley by Thursday 
evening. Winds across Lake Michigan will remain light through 
Wednesday...veering from east to southeast today...and remaining 
southeast through Wednesday. On Thursday...southeast winds will 
begin to increase due to the gathering low pressure off to the west. 


Merzlock 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...none. 
&& 


$$