311 
fxus63 kdvn 062005 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
305 PM CDT Monday Oct 6 2008 


Synopsis... 
low pressure system analyzed near the Nebraska border in north central Kansas at 
midday...will slowly migrate east-NE over the next 24 hours...spreading 
showers and thunderstorms into the local area. Aloft...middle level 
flow is nearly meridional due to Sharp Ridge axis to the east... 
reaching from fla to north central Canada...and upstream trough reaching 
from Alaska to Texas Panhandle at 12z. This will provide continued slow 
movement of the Kansas low pressure system...keeping the local area in a 
relatively mild airmass tonight into Tuesday morning with current timing 
holding bulk of rain chances off until daylight hours of Tuesday. 


$$ 


Short term...tonight and Tuesday... 
main focus is on timing of rain and thunderstorm chances with the 
advancing upstream trough. 12z NAM appeared to have the best handle 
on both the main upper level circulation and surface pressure 
features at 18z and generally followed. Current area of 
thunderstorms approaching Kansas City north into the MO valley will continue 
to stream north...with potential for widely scattered showers and 
few thunderstorms to reach far western County warning forecast area after midnight tonight 
and have maintained chance probability of precipitation there. On Tuesday...remnants of the Kansas 
upper vorticity and weakening surface Low Pass to the northwest early in the 
morning spreading widespread rain and embedded thunderstorms into 
eastern Iowa. During the day...a secondary vorticity is shown digging into 
the base of the trough and sending a surface low south of the area 
during the afternoon. With decent low level convergence along the 
attendant cold front coinciding with best period of upper 
divergence...will maintain categorical probability of precipitation over most of the area in 
the afternoon. Widespread cloud cover and unimpressive middle level 
lapse rates should limit thunderstorm chances and will keep only 
isolated wording. Have gone close to HPC for quantitative precipitation forecast amounts with most 
areas likely in quarter to half inch range for the event. Any 
embedded thunderstorms could boost local amounts to well over 1 
inch...based on precipitable water values shown to reach 1.5 inches Tuesday afternoon. 


High cloud cover...srly flow...and dewpoints in 50s in the warm 
sector that will envelop most of the County warning forecast area will keep lows well above 
early Oct normals in middle 50s to around 60. Diurnal rise Tuesday will be 
limited by clouds and rain...but most sites should see enough dry 
periods and partial insolation to rise into the 60s to near 70. 
.Sheets.. 




Long term...Tuesday night through next Monday... 
precipitation to continue Tuesday night and end on Wednesday as the 
shortwave energy moves east. Models have a weak front moving through 
Wednesday night. Negatives for precipitation Wednesday night are night time 
frontal passage...high condensation pressure deficits...and a drying 
atmosphere. Two positives are Theta-E advection and some diffluence 
aloft. So...will plan on a dry frontal passage with just a wind shift 
occurring. 


Thursday through Friday night now appear dry as weak high pressure 
moves through the area. Return flow develops Friday night but the 
better low level jet and moisture get no closer than central Iowa. 


Saturday on is more complicated. Next significant piece of energy 
enters the western Continental U.S. And the pattern changes to southwest flow. 
Energy is usually slow to eject from a western trough and normally 
comes out in pieces. Saturday/Saturday night could very well be dry. 
However...given diffluence aloft and warm air advection...will go with silent 20 
probability of precipitation for now. Chances for precipitation should increase Sunday/Monday with 
the better chances Monday/Monday night. ... 
&& 


Aviation... 
conditions will remain VFR with high level clouds streaming 
northeast across the region ahead of a low pressure system in the 
Central Plains. Some diurnal cumulus from mli to brl will dissipate at 
sunset this evening. Scattered thunderstorms along the MO River 
Valley will remain west of the forecast terminals tonight. An 
isolated storm could develop in the vicinity of Cid toward 
sunrise...but the potential is too low to include in forecasts. On 
Tuesday...widespread showers and isolated thunderstorms will begin to 
shift east into the area. For now...will count on widespread MVFR 
and periods of IFR conditions to hold off until afternoon...keeping 
only prob30 chances for MVFR showers from middle to late morning at 
terminal sites due to timing uncertainties ..sheets.. 


&& 


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


&& 


$$ 


Sheets/08