Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
709 PM CDT Friday may 24 2013 


Synopsis... 
issued at 242 PM CDT Friday may 24 2013 


Latest surface analysis was indicating large cool core ridge starting 
to push eastward across the western Great Lakes......while return flow 
and pressure falls increase across the plains into western Iowa. 12z 
upper air analysis also indicating elevated moisture feed at 850 mb 
mb and warm moist conveyor also becoming more pronounced up the 
Western Plains. Diffluent southwesterlies with an embedded vorticity 
across the northwestern High Plains pushing into weakening Omega 
ridge and inducing and increasing elevated convective region across 
the eastern Dakotas. 


&& 


Short term...(this evening through saturday) 
issued at 242 PM CDT Friday may 24 2013 


Tonight and Saturday...various short range hi-res models continue to pick 
up on the eastern Dakotas activity...but still most are 
lagging/trying to catch up in speed...timing and areal coverage of 
what is occurring. Feel the lead warm air advection Wing off this ongoing process 
will induce high based showers and possibly a few thunderstorms 
into the western County Warning Area after 02z this evening...better chance of 
even spread eastward to the MS river through 05z-06z if even in a 
weakening Mode. But low level jet with thta-east feed from Central Plains 
source region will be increasing...nothing very strong but still 
enough to converge on slower flow at the same level and enhance of 
probably mesoscale convective system development generally from northwestern Iowa 
southeastward into the west central to southwestern dvn County Warning Area after 
midnight and into Sat morning. The 12z run UKMET and nam80 mesoscale convective system 
forcing tools both support this train of thought and after the 
intial Wing of elevated showers/storms...will Blossom probability of precipitation from 
west to east after midnight and through at least 15z Sat morning. 
Precipitable water feed of 1-1.3 inches and convergence suggest a few locations 
that get hit by the main complex could get at least an inch of 
rain by middle Sat morning. Before southeasterly flow increases 
overnight...temperatures to drop to low values of lower 40s in the far 
east...to the upper 40s or near 50 in the west closer to faster increase 
in surface/return flow. After morning mesoscale convective system decreases...mesoscale convective system spawning 
parameters start to refocus back on northwestern Iowa Sat 
afternoon...thus hopefully some lull in the activity for midday 
and into the afternoon especially north of i80. But will keep chance 
probability of precipitation for any renewed convection along out flow boundaries or warm 
front trying to retreat out of MO into the southern County Warning Area. Will play 
highs on the cool side...but if sun breaks out and more dry 
periods occur Sat afternoon than currently expected...advertised 
temperatures will be too cold in many spots. ..12.. 


Long term...(saturday night through friday) 
issued at 242 PM CDT Friday may 24 2013 


Prolific rainfall is expected through Tuesday...as a series of warm 
advection driven showers/thunderstorms...possible full mesoscale convective system activity 
moves across Iowa and Illinois. There is very high probability of 
this occurring...as strong low level jet convergence is indicated overhead on 
every model...and the past 2 days have seen quite a bit of elevated 
activity...in Oklahoma yesterday...and in South Dakota today. This 
elevated boundary looks to remain over the County Warning Area through 
Tuesday...thus a growing threat for significant rainfall is 
expected. A Hydro section will be added to this afd. 


It is hard to argue against the model output probability of precipitation for Saturday night 
through Monday...which solidly indicated likely or higher. Since 
this stretches over many period...I will not go categorical on any 
period...as there should be some breaks in the rain/mesoscale convective system activity. 
Thus...a prolonged period of likely probability of precipitation should cover the 
uncertainty on timing rounds of rainfall. In generally it appears a 
maximum in coverage in rainfall should occur in the overnight and 
morning hours...which is typical of mesoscale convective system patterns. 


Temperatures will continue to cool through Sunday...with highs in 
the 60s north...lower 70s south...but by Monday we should see higher 
dewpoint air surge north...thus highs in the upper 60s to upper 70s 
should be more widespread. The remainder of the week...we should be 
in the warm sector...and should see highs in the upper 70s to middle 
80s. Lows in the 60s can be anticipated much of the extended after 
Saturday night...which will bottom out in the upper 40s to middle 50s 
from northeast to southwest. 
Ervin 


&& 


Aviation...(for the 00z tafs through 00z Saturday evening) 
issued at 702 PM CDT Friday may 24 2013 


VFR through this evening and possibly 03-04z with a high pressure 
ridge and dry air still over the area. A swath of warm air 
advection will move across the area late this evening will bring 
elevated showers with VFR conditions. Then...a stronger wave of 
forcing and warm air advection will spread an area of showers and 
embedded thunderstorms into the area...bringing MVFR ceilings and 
visibilities to most terminals beginning around 12z...through at least 
17z with some light rain and MVFR conditions lingering into early 
afternoon. Through the period winds will be southeast 8-12kts. 
Le 


&& 


Hydrology... 
issued at 242 PM CDT Friday may 24 2013 


Widespread rainfall of 1 to 4 inches is possible through early this 
upcoming week. In some cases where training takes place...amounts 
over 4 inches are certainly possible. While the heaviest axis of 
rainfall is far from certain...it does appear that this rainfall 
will support significant new rises on most rivers...and may cause 
new flooding or prolonged flooding. This is especially true in 
eastern Iowa...where the Cedar...Iowa...and wapsipinicon rivers are 
already running high from rainfall upstream in north central Iowa 
last week. 
Ervin 


&& 


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


$$ 


Synopsis...12 
short term...12 
long term...Ervin 
aviation...le 
hydrology...Ervin