584 
fxus63 kdvn 210815 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
310 am CDT Thursday Aug 21 2008 


Synopsis... 
cut-off low over OK opening up and lifting north ahead of deep shortwave 
trough diving into the Pacific northwest. Subtropical moist plume 
can be seen on water vapor advancing north through MO with precipitable water/S 1.5-2 
inches. Regional radar composite shows large area of rain also advancing 
northward within this plume. Couple of vorticity lobes seen on satellite 
and in radar imagery aiding in more robust lift resulting in 
heavier rains... with one lifting north/northwest through northwest MO near Chillicothe 
(kcdj) and the other more significant vorticity lifting north/NE into southwest MO 
near Springfield (ksgf). Showers mainly south of Highway 34 attim but 
fairly light as they lift north away from better lift and deeper saturation. 
Middle level dry air per 00z dvn radiosonde observation also chewing away some at the precipitation 
as it lifts north. No lightning being detected as soundings depict stable 
vertical column with meager middle level lapse rates. Temperatures are running 
5-10 degrees warmer than yesterday at this time in many areas central and 
south due to thicker cloud cover. Meanwhile... Tropical Storm Fay which 
snuck out for a dip in the Atlantic... about to come onshore yet again 
over Florida this time north of Daytona Beach. ..05.. 


&& 


Short term...today and tonight... 
pretty straight-forward forecast in the near term... with considerable clouds 
and showers on the way for most...if not all of US. Cut-off low 
over OK will shear and weaken/fill as it slowly lifts north toward the 
middle Mississippi Valley next 24 hours. Subtropical moist plume and showers 
will continue advancing northward overspreading the area today. Rain 
to pick up in intensity by middle morning into the afternoon as more robust lift 
from approaching vorticity maximum over southern MO leads to deep saturation through 
the column. Heaviest rains (0.5 to 0.75 inch with some higher amounts 
possible) likely to focus across the SW 1/3-1/2 where low level flow more 
cyclonic and stronger isentropic ascent prognosticated in advance of 850 mb warm front. 
Not optimistic we/ll see any thunder given such saturated vertical 
profiles... but left isolated mention in with vorticity maximum approaching this afternoon. 
As for temperatures...clouds and rain should result in maxes nearly 10 degrees or so 
below climatology. Sided near or just below cool side of guidance most areas... 
and could still be optimistic especially portions of central and south where 
rain more persistent. Temperatures in the rain likely to hold steady in the 
u60s to l70s... similar to what was observed yesterday afternoon over the Ozarks. 
Given this have gone with a non-diurnal trend on temperatures. Tonight... 
clouds and showers to limit diurnal range... and sided at or just above 
warm side of guidance. Would not be surprised to see some areas around 70 
on lows. Best coverage of showers looks to be early evening with vorticity maximum moving 
through. Some showers and possibly areas of drizzle overnight with 
deep moisture axis lingering. ..05.. 


Long term...Friday through next Wednesday... 
internal signals from the models indicate that rain showers will linger into Friday 
morning as the first slug of moisture moves away from the area. Have 
lowered probability of precipitation somewhat Friday afternoon/evening but it is possible that very 
little precipitation will occur. Models really do not show any forcing to produce 
new precipitation. However...given that the lower atmosphere is saturated...what 
daytime heating that is available may result in diurnal convection. 


Cold front approaches the County warning forecast area late Friday night. Synoptic lift will be on 
the increase and any weak shortwave/vorticity maximum should be sufficient to produce 
more precipitation. Therefore...have upped probability of precipitation slightly with the highest in the 
far northwest. Cold front to move through the County warning forecast area Saturday/Saturday evening. 
Should see some strong convection due to the strength of the front and what 
daytime heating occurs over the County warning forecast area before frontal passage. Convection should linger 
into the evening hours before rapidly ending late Saturday night. 


Sunday on...no changes to current forecast. Very quiet and pleasant weather expected 
across the Midwest with slightly below normal temperatures. ..08.. 


&& 


Aviation... 
deep moisture plume surging north over area with middle level disturbance 
approaching from MO will lead to rain showers overspreading the terminals 
today. VFR ceilings/visibilities transition to MVFR ceilings/visibilities with pockets of 
IFR possible in heavier bursts of rain. Cant rule out isolated thunder late 
afternoon into early evening as middle level disturbance passes... but given such 
saturated vertical profiles believe mostly just rain or showers and left 
out mention. Deep moisture plume lingers tonight along with some showers. 
Ceilings/visibilities generally MVFR range with areas of IFR and LIFR with drizzle 
possible. ..05.. 


&& 


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


$$ 


05/08