Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
655 PM CDT Sat may 18 2013 


Synopsis... 
issued at 208 PM CDT Sat may 18 2013 


An elongated area of low pressure extended from the Southern 
Plains into the the western Dakotas this afternoon. An ill defined 
warm front arced southeast from the low over the Dakotas through 
the middle Mississippi Valley. The region was rain free this 
afternoon with the closed upstream precipitation located over Kansas and 
Oklahoma. This area of showers and isolated thunderstorms was 
occurring ahead of short wave moving northeast through western Kansas per 
recent water vapor imagery. 


&& 


Short term...(this evening through sunday) 
issued at 208 PM CDT Sat may 18 2013 
the overall synoptic pattern remains unchanged through Sunday 
morning with the upper trough remain over The Rockies and the plains 
surface low moving very little. During the day Sunday a short wave ejects 
out of the base of the trough and becomes negatively tilted as lifts 
into MO by late Sunday afternoon. 


For tonight the main concern will be thunderstorm chances late 
tonight as the SW Kansas short wave lifts northeast through western Iowa. 
Expect storms to develop late this afternoon or early this evening 
over the central and Southern Plains as the western Kansas short wave interacts 
with the developing low level jet and elevated convective available potential energy around 2000j/kg. As the 
short wave moves through western Iowa late this evening/early Sunday 
morning the storms should move northeast reaching east central 
Iowa early Sunday morning. Models show convective available potential energy waning by this time as 
well as a weakening low level jet suggesting any storms will be weakening as 
they reach east central Iowa. Since the storms are expected to be 
dissipating as The Reach eastern Iowa...have kept late night probability of precipitation 
limited to the western third and in the low chance or slight 
chance category. Given the moist air mass already in place and clouds 
blowing off of convection approaching from the west we should see 
lows tonight in the 60s. 


For Sunday have gone with broad brush chance probability of precipitation during the 
afternoon. The low levels will destabilize with low to middle 60 
dewpoints in place and temperatures in the 80s. However with the 
main upper trough well to the southwest and no coherent surface 
boundary in place...the threat of organized storms will be 
low...making it difficult to Pin Point any one area for higher 
probability of precipitation. The lack of an obvious trigger and weak deep layer shear will 
also mean the threat for severe storms during the afternoon will 
be low. 


Long term...(sunday night through saturday) 
issued at 208 PM CDT Sat may 18 2013 


Main focus is on thunderstorm chances and potential for severe 
storms Sunday night through Monday. Slow moving upper low over the 
plains will keep the potential for showers and thunderstorms...along 
with above normal temperatures...going until it finally exits middle to 
late week. High pressure at the surface should bring dry 
condition and temperatures closer to normal late week with rain 
chances increasing over the weekend. 


Sunday night...developing upper level difluence and a middle level vorticity 
maximum ahead of the upper level jet diving into the western U.S. 
Trough...is shown by model consensus rounding the upper trough and 
lifting northeast...overspreading especially the western half of the 
forecast area in the evening. Even compensating for model dewpoints 
likely trending a few degrees too high...surface based convective available potential energy will 
likely be in excess of 2000 j/kg in the early evening...dependent on 
where any afternoon convection fires. At the same time...a middle level 
50 knots plus speed maximum will provide at least moderate levels of deep 
shear. Uncertainties for convective development include the lack of 
a strong surface boundary and the strength of the capping elevated 
mixed layer. Overall...signals point toward a complex developing in 
the far west or just northwest of the forecast area in the 
evening...spreading NE...then east-NE following corfidi vectors across 
at least the northern half of the forecast area overnight. There may 
also be additional development away from any complex as the 
nocturnal jet...shown around 40 knots kicks in...feeding Gulf moisture 
into the region while steepening middle level thermal and thetae 
lapse rates. As mentioned in the previous discussion...this setup 
supports large hail...damaging wind and locally heavy rainfall. Storm Prediction Center 
has a slight risk over the entire area in the current day 2 outlook 
which runs through 12z Monday. Have increased probability of precipitation to categorical in 
the far northwest with likelies elsewhere. High dewpoints will keep 
temperatures in the 60s through the night. 


Monday and Monday night...subsequent shortwaves in the southwest flow 
and low level boundaries lingering from overnight convection will 
trigger additional thunderstorms. Depending on the forecast 
model...surface based convective available potential energy may again reach well above 2000 and 
possibly higher than 4000 j/kg. This will be greatly dependent how 
the overnight convection dissipates and the coverage of associated 
cloud cover. Latest models continue to suggest deep layer shear will 
be stronger as the upper jet approaches from the southwest. At the 
same time...the deepening upper low over Nebraska will cool temperatures 
aloft...providing a weaker cap than Sunday...and even steeper middle 
level lapse rates. This will be more supportive of supercell 
thunderstorms especially in the afternoon and early evening hours. 
These supercells will continue the severe weather threats of large 
hail and damaging wind...and introduce an increased risk for 
tornadoes. Storm Prediction Center maintains a slight risk encompassing the entire 
forecast area for day 3...which covers Monday through Monday night. 
Will keep probability of precipitation in the high chance range in the daytime due to more 
limited coverage...then increase to likely probability of precipitation for overnight when 
more widespread coverage of showers and thunderstorms is expected. 
Temperatures remain close to persistence in a largely unmodified 
environment with highs in the 80s and lows in the 60s. 


The upper low gradually weakens and sweeps east of the area by 
Thursday. This will keep chances for showers and at least isolated 
thunderstorms going through Thursday with temperatures trending back 
closer to late may normals along with lower humidity. High pressure 
that follows will provide dry weather through for at least Friday. 
Saturday...low confidence slight chances for rain are introduced 
based on the more progressive European model (ecmwf) that brings a warm front into 
the area. The GFS maintains a dry forecast with the area under the 
influence of high pressure retreating slowly toward the Great 
Lakes. 


&& 


Aviation...(for the 00z tafs through 00z Sunday evening) 
issued at 639 PM CDT Sat may 18 2013 


VFR conditions expected to prevail for most if not all of the 
taf cycle. Could see some patchy fog with MVFR visibilities late 
tonight...but negative factors include increasing high clouds 
from plains convection and increasing pressure gradient 
resulting in S/southeast winds sustained around 10 kts with some 
local gusts possible. Due to the uncertainty have only hint of 
light fog with visibilities 6sm attim. Have maintained mention of vcsh 
at kcid and kbrl overnight until middle am Sunday for potential 
of weakening convection attendant to diminishing low level jet. 
Should see showers and storms redevelop with heating by middle to 
late Sunday afternoon in broad southerly flow ahead of an upper 
level disturbance emerging into the plains. More widespread 
showers and storms suggested Sunday evening. Winds Sunday will 
be gusty at times from the S/southeast at 10-20 kts. 


&& 


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


$$ 


Synopsis...dlf 
short term...dlf 
long term...sheets 
aviation...05