880 
fxus63 kdvn 061935 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
235 PM CDT Sat Sep 6 2008 


Synopsis... 
18z surface analysis showed high pressure centered over SW Iowa with a 
surface trough over the central Dakota/S and western NE. Another weak 
trough was noted from western lm into west central Illinois. The regional radar 
composite showed an area of light to moderate showers over the SW 
half of MO into eastern Kansas with another area of rain showers/isolated thunderstorms and rain moving 
into southwestern Minnesota. A broad middle level trough was centered over the central 
U.S. With embedded S/W/S rotating through the mean trough. One short wave 
moving through the central and northern plains was driving the rain showers 
activity approaching the MS valley. Another short wave just exiting eastern Montana 
was also producing some scattered rain showers/isolated thunderstorms and rain over the northern plains. 


$$ 
Short term...tonight and Sunday... 
precipitation chances associated with S/W/S currently moving through the 
central and northern plains will be the main focus in the near 
term. Expect rain showers over MO/Kansas to expand northeast into the middle MS 
valley this evening as plains short wave moves through western Iowa. Better 
moisture/forcing with this feature will be across MO...so higher 
chance probability of precipitation were placed over the southern County Warning Area. Did not include any 
mention of thunder at this time. NAM/GFS middle level lapse rates 
remained marginal over Iowa/northern Illinois with steeper lapse rates remaining 
north and south of the County warning forecast area. Montana short wave to cross Minnesota/WI Sunday with some 
weak energy trailing across northern fourth of County warning forecast area. Left some low probability of precipitation 
in Sunday morning over the entire area to account for any linger 
rain showers from overnight...then dropped probability of precipitation central and south during 
the afternoon with low chance probability of precipitation continuing north closer to the 
northern system. Stayed closer to the warmer guidance for low temperatures 
tonight with overcast skies expected. Again...ample clouds north 
should hold temperatures in the 60s Sunday while southern sections should reach 
into the low 70s with more sun expected. 


... 


&& 


Long term...Sunday night through next Saturday... 
Sunday night through Monday night will be quite interesting as a 
significant wave moves through the region with a moderate to heavy 
rain event. Overall...ECMWF/UKMET models are preferred. 


Weak shortwave/vorticity maximum to move through Sunday evening which may be 
enough to generate some rain showers so will continue with schc probability of precipitation. 
Impressive wave in the flow aloft will pull moisture rapidly 
northward late Sunday night/Monday bringing another widespread rain 
event to the area. Initial dry air may slow the start of the precipitation 
but warm air advection and isentropic lift increases significantly during the day 
Monday. Therefore will go with mainly low chance probability of precipitation late Sunday night 
and go with high end likely probability of precipitation for Monday. Probability of precipitation will likely need 
to be raised to categorical with the Sunday morning forecast. Once the 
precipitation moves in on Monday...potential there for temperatures to fall into 
the u50s to l60s for the remainder of the day. Rain to continue 
Monday evening before ending from west to east late Monday night. 


Tuesday/Tuesday night looks to be dry and cool as high pressure builds as 
the thermal trough moves through on Tuesday. Return flow develops in 
ernest on Wednesday but have gone with schc probability of precipitation Wednesday afternoon as 
the best warm air advection remains west of the area during the day. 


Wednesday night through Thursday night a moderately strong wave moves 
through the Midwest with a respectable low level jet. This system will bring 
another round of precipitation to the area but with the surface low passing 
west of the County warning forecast area...warmer temperatures should be seen along with 
thunderstorms and rain. The cold front associated with the system is fairly impressive 
and will drop temperatures behind it for next weekend. Currently 
Friday and Saturday look dry with temperatures below normal. 
... 


&& 


Aviation... 
VFR conditions should prevail at most taf sites through middle day 
Sunday. Rain showers expected to move through the area from middle evening 
through Sunday morning. Best coverage expected south...so couldn/T 
rule out brief MVFR ceilings/visibilities at kbrl mainly between 03z and 14z. 
Also expect cloud cover to limit fog development...but some breaks 
will be possible over eastern Iowa after 09z which could result in 3-5sm 
visibilities in fog to develop in spots through 14z. Winds during the 
period will remain light but should be predominantly out of the SW 
by late tonight through Sunday. ... 


&& 


Dvn watches/warnings/advisories... 
Iowa...none. 
Illinois...none. 
MO...none. 
&& 


$$ 


Dlf/08