721 
fxus63 klot 110858 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
358 am CDT Sat Oct 11 2008 


Discussion... 
351 am CDT 


Warm weather will continue today with the high temperature in the 
lower 80s. 


Today...a large high was over the Quebec and Ontario border this 
morning. There was a weak low over eastern South Dakota... 
southeast Colorado and southern Utah this morning. The combine 
flow around these surface features will give a south wind over 
northern Illinois and Northwest Indiana today. The 850 mb analysis 
at 00 UTC shows 16 degree celsius air over Lincoln Illinois. There 
is a 30 knot wind from the southwest over central and western 
Illinois at 08 UTC...as seen on the VAD wind profiles from nearby 
radars. The forecast sounding shows a nearly dry adiabatic lapse 
rate below 900 mb by 3 PM this afternoon. A forecast of rather 
light surface wind and dewpoints in the 50s will keep the fire 
weather forecast out of the red flag area. But with that high of a 
temperature at 850 and with mixing in the boundary layer we would 
expect a high around 81. 


Sunday...the temperature will reach the lower 80s again on Sunday. 
The lows in the Rocky Mountain area will consolidate into one low 
over eastern Colorado. A large 500 mb wave trough will move east 
from Idaho and Nevada to Colorado and Utah. The center closed low 
will move northeast into Montana. This will occur during the day 
Sunday. Both the European model (ecmwf) and GFS models show this. As the low 
rapidly intensify in the Colorado...the south wind will increase 
in speed over Illinois. We expect more warm air advection. 


Monday through Friday...the 500 mb wave trough will move into 
Minnesota by Tuesday morning and then to central Ontario by 
Wednesday night. We will use GFS model forecast. There will not 
be any significant changes to the going forecast. A surface low 
will move across Ontario Monday night. This will bring a cold 
front into Illinois Tuesday morning. We will keep the forecast 
showers and thunderstorms Monday night through Tuesday night. For 
Wednesday and the rest of the week...we expect dry weather with 
high temperatures in the 60s and low temperatures in the 40s. A 
large high will move into the Ohio Valley and upper Mississippi 
Valley by Thursday morning. 




&& 


Aviation... 
1255 am CDT 


00z tafors... 


With minimal eastward progression of highly amplified upper 
pattern the center of the large anticyclone over the lower Great 
Lakes only drifts east to southern Quebec/the upper St. Lawrence 
Valley by 12/06z while storm over the Desert Southwest gradually 
lifts to the central rockies. 


This maintains the south to south-southeast low level flow across 
the local area through the taf valid period. By the time the 
surface heats up to raise the mixed layer depth up to 1km this 
afternoon the low level is prognosticated to have already backed a bit to 
the southeast so will keep prevailing direction of 150-160 degrees 
during daylight with speeds of 10kt or so as models indicate 
speed of only around 15kt at top of mixed layer this afternoon. 
Typical diurnal decrease and slight backing at dusk. 


A short wave lifting northeast across the upper and middle MS valley 
during the late night and early morning will bring scattered to broken cirrus 
across the terminals. Another short wave trough currently lifting 
from Mexico into southern Arizona and nm will spread more high level 
moisture spun off from Hurricane Norbert...southwest of the 
southern tip of the Baja California peninsula...across the plains and 
eventually to the middle and upper MS valley tonight. 


Trs 


&& 


Marine... 
333 am CDT 


With minimal eastward progression of highly amplified upper 
pattern over North America the center of the large anticyclone 
over the lower Great Lakes only drifts east to southern Quebec/the 
upper St. Lawrence Valley/New England by 12z Sunday while the storm 
over the Desert Southwest gradually lifts across The Rockies to 
the Central High plains. This maintains a moderate breeze from the 
southeast and south through Sunday. The low eventually lifts 
across the northern plains and into Manitoba by Monday morning. A 
cold front trailing southwestward from this low will push east 
through the upper Great Lakes Monday night and Tuesday as the low 
continues moving northeastward across southern Hudson Bay. 


Operational 00z run of the GFS discounted as it appears to be an 
outlier in the significant deepening of the low as it moves across 
Ontario on Tuesday...along with resulting strong cold air advection 
and surface winds following the cold frontal passage...when compared 
to GFS ensemble members and other model solutions. 


Trs 


&& 


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


$$