Weather

980 
fxus63 kdvn 280732 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
232 am CDT Thursday Aug 28 2008 


Synopsis... 
convection is move east northeast across Iowa tonight...in response 
to warm moist advection and assent from a short wave at middle levels. 
The low level jet should weaken as this convection moves into the 
area through noon today. However...scattered convection is likely to 
progress across at least the northwest 1/2 of the County Warning Area through the 
early to middle morning hours today. Clouds from this convection are 
widespread...and should remain in place most of the morning. 


Todays forecast will be updated for convective trends prior to 6 am. 


&& 
Short term...today and tonight 
the main question today will be how much heating and instability 
development will occur with the dissipating mesoscale convective system over the region this 
morning. Confidence in this factor is not high given the non linear 
convective modes...and our forecast will represent neither a overly 
pessimistic view of redevelopment...nor one that is aggressive in 
breaking out widespread thunderstorms this afternoon...including 
severe. However...it should be said that wind fields could support 
organized severe storms during the afternoon if we are able to reach 
the middle 80s. The most likely view will be scattered 
development...especially over the eastern 2/3rds during the 
afternoon hours...and continuing into the early evening east as the 
last in a series of embedded vorticity maxes provides lift along the 
progressive cold front. The front...now entering northwest 
Iowa...should reach the Mississippi River by late afternoon...and 
exit the eastern County Warning Area by 7 to 8 PM. If severe weather does occur...it 
should be with storm along the front during this period. Storm Prediction Center 
discusses these threats in the day one outlook...but we generally 
agree that should we destabilize...lower end severe wind gusts and 
hail up to 1 inch in diameter look possible. Otherwise...garden 
variety storms should be the rule. Quantitative precipitation forecast...though somewhat spotty 
should average around 0.25 for the event...with some northern 
sections seeing around 0.50 on average including the current 
activity. After humidity rises today in advance of the 
front...dewpoints should reach 64 to 69...they will fall once again 
tonight behind the front. We will continue to forecast lower 50s 
dewpoints...but looking upstream...these may need to be lowered 
slightly. Lows tonight should drop back to the 50s over the 
northwest 2/3rds...while lower 60s hold on to the southeast. 
... 


Long term...Friday through next Wednesday... 
temperatures are primary focus through weekend as surface high 
pressure brings stretch of dry and mild conditions. High 
pressure...currently over the northern rockies...builds southeast and 
becomes anchored over Iowa and Illinois by Friday night. Will continue to 
undercut guidance by about a category with both highs and lows. Kept 
highs in upper 70s to lower 80s Friday...and lower 80s Sat allowing for 
airmass modification. Ideal nighttime radiational cooling conditions 
and dry airmass should bring similar readings to those earlier this 
week and have lows in 50s. These conditions may again be favorable 
for ground fog...but will leave this for later forecasters to 
consider. Surface high pressure pushes into eastern Great Lakes with 
models in agreement depicting upper ridge over Central Plains 
amplifying and moving east across the area Sat night into sun. 
Resulting height rises as upper ridge shifts east...along with 
increasing low level southerly return flow...will warm highs back 
well into 80s sun and Monday. 


Beyond...00z GFS and European model (ecmwf) have come into better agreement with next 
cold front and associated precipitation chances holding off until 
Wednesday. However...a high degree of uncertainty remains early to middle 
week due potential large scale impacts of Gustav in the Gulf states. 
No changes made to forecast beyond Sunday. 


.Sheets.. 


&& 
aviation... 
scattered thunderstorms will be possible at all sites through 00z 
tonight. Outside of storms...MVFR ceilings are likely this morning 
around 3000ft at Cid and dbq...and possibly mli as a large area of 
showers and storms moves east northeast from central Iowa. Afternoon 
condiions should be VFR outside of storms. By 00z tonight...a cold 
front will usher in drier air...and VFR conditions at all 
sites...occuring first at Cid and dbq...then at mli and brl. 
... 


&& 




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


&& 


$$ 


Ervin/sheets/Ervin