061 
fxus63 kilx 080159 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
859 PM CDT sun Sep 7 2008 


Discussion... 
issued 859 PM CDT sun Sep 7 2008 


A narrow band of showers and isolated storms is developing across north MO 
a bit quicker than anticipated. Forcing appears to be a focused 
area of low level warm air advection...which indicates the baroclinic zone is 
tightening up already. Will be added probability of precipitation for tonight in the west 
half of the County Warning Area...with likely probability of precipitation closest to the line...and 
slight chance probability of precipitation as far east as bmi and Dec. 


Temperatures seem on track...especially with cloud cover expected to 
generally increase the rest of the night. Updated grids and zones 
will be available shortly. 


Shimon 
&& 


Aviation... 
issued 643 PM CDT sun Sep 7 2008...for the 00z tafs 


Diurnal cumulus field will continue to dissipate this evening...but a 
middle cloud deck is moving in from the west. A few showers are 
already hitting the ground in southeast Iowa...but they should remain 
very isolated until tomorrow morning. The current band of ac will 
affect the northern County Warning Area...so will leave the vcsh in pia after 
midnight and keep a vcsh for the morning in bmi and spi. The 
bulk of the precipitation should hold off until afternoon for most sites 
besides pia. Instability and shear params point toward some 
thunderstorms for the afternoon...especially in the southeast 2/3rds of 
the County Warning Area...along and south of the baroclinic zone. Will put some 
thunderstorms in the vicinity at all sites in the afternoon. Visible and ceilings should remain 
MVFR for the most part. 


Shimon 
&& 


Previous discussion... 
issued 258 PM CDT sun Sep 7 2008 


18z surface map showed weak high pressure stretching from northern MO east 
through the Ohio Valley. Cold front extended from central Minnesota back 
into central Nebraska. A few storms were developing ahead of the front 
in northern Iowa...while isentropic lift behind the front in Nebraska was 
causing an expanding band of showers. Aloft...a broad longwave 
trough dominated the central Continental U.S....with a shortwave of interest 
dropping through the northern rockies which will be our weather-maker 
for tomorrow. Main forecast concern is system moving through 
Monday/Monday night with associated probability of precipitation and severe weather potential...then 
much below normal temperatures for Tuesday and Wednesday. Pattern could potentially 
turn active again for late-week/weekend but this is dependant on 
tropical activity. 


Short term...tonight through Monday night 
quiet conditions overnight as storm organizes over the plains. 
Upper jet streak of 90+kts and potent middle-level shortwave will 
phase with baroclinic zone stretching across the Midwest to 
develop a surface low from an existing inverted trough. System will be 
intensifying through Monday. Large precipitation shield northwest of trough/low 
expected to spread up from MO and Iowa...into northwest Illinois Monday morning. 
Decent isentropic lift and frontogenetic forcing will affect mainly 
the far northwest with steady rains by midday...with this area slowly 
advancing eastward through the afternoon. Farther south the atmosphere 
will become moderately unstable by mid-afternoon. Forecast soundings 
suggest convective available potential energy approaching 2k j/kg...along with 60 kts of 0-6km 
shear south of the axis of strongest differential heating...mainly 
south of I-74 and west of I-55. With synoptic scale lift 
increasing...the potential exists for some low-end severe 
thunderstorms to develop across northern MO into west-central Illinois by middle 
afternoon and track east through the early evening. Forecast amount of 
wind shear and instability would promote organized multi-cell 
clusters capable of producing near severe downbursts...along with 
some small hail given low wet-bulb zero heights. This will be 
highly dependent on afternoon heating and the development of a surface 
low to increase lift and shear. South of I-70...dry conditions are 
expected until late afternoon or evening when lift increases from 
the northwest. With all these factors in play...expect a large temperature 
gradient with highs ranging from the low/middle 60s far northwest under 
stratiform rain...to middle/upper 80s southeast under partly sunny skies. As 
instability decreases through the evening...general showers and 
thunderstorms associated with the front will track southeast...and 
clear the far southeast counties late Monday night. Areas that clear out in 
the northwest could drop into the middle/upper 40s by Tuesday am. 


Long term...Tuesday through next Sunday 
the Tuesday through Thursday morning period will be dominated by a large surface high 
pressure system crossing the area from west to east. This will 
bring a stretch of nice early Fall weather...with mostly clear 
skies and seasonably cool temperatures. Lows Wednesday am could be the coolest 
of the season so far with widespread middle/upper 40s. As the high 
edges into the eastern Great Lakes by Thursday...southerly flow on the 
back-side of this system will begin to transport warmer and more 
humid air into the area. At the same time...a deep trough is 
expected to form over The Rockies...as shortwave energy forms a 
surface low along baroclinic zone in the plains Thursday. Precipitation associated 
with this system will bring a chance of showers and isolated 
storms mainly Thursday night. By Friday...major disagreement shows up 
between the European model (ecmwf) and GFS. GFS solution favors a drier and warmer 
period into the weekend as weak upper ridging builds across the 
eastern third of the country...while trough re-loads over The 
Rockies. European model (ecmwf) on the other hand would favor a wetter and cooler 
period as the cold front turns stationary across the area into the 
weekend...and impulses riding through the SW flow bring repeated 
rains Friday and Sat. This difference may be caused by the handling 
of Ike...as GFS tracks system similar to Gustav...building ridging 
and subsidence ahead of the feature until remnants potentially 
reach US late in the weekend. Latest 12z European model (ecmwf) run actually takes 
Ike into northern Mexico this weekend allowing the middle-latitude system 
to set up over our region. With little run-to-run or model-to-model 
agreement have made very few changes beyond Friday...with slight chance 
probability of precipitation going Fri-sun. 


Schaffer 


&& 


Ilx watches/warnings/advisories... 
none. 
&& 


$$ 

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