042 
fxus63 kdvn 182000 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
300 PM CDT Friday Jul 18 2008 


Synopsis... 
closed circulation in the low levels showing up nicely on visible 
satellite in extreme northwest MO will track into eastern Iowa by 
Saturday morning. Zonal flow and surface boundary hanging around the 
region into the weekend followed by more of a northwest flow aloft 
into next week. 
&& 


Short term...tonight... 
complicated situation with many features ongoing. First the NAM 
model seems to have a good depiction of closed circulation in 
extreme northwest MO...which will track into eastern Iowa by 
Saturday morning. Good set up for heavy rain with lift provided by 
the circulation...veering profile...increasing low level jet and 
lighter middle level winds depicted by the forecast soundings. This 
will favor rather slow movement of storms increasing threat of flash 
flooding. In addition...precipitable waters increase to 2.3 inches 
which is at least 150 percent of normal. Therefore will continue 
with Flash Flood Watch with low flash flood guidance values where 
heavy rains fell last evening. Surface analysis indicates plenty of 
moisture available to feed northward into system with dewpoints in 
MO around 70...and visible satellite showing cumulus streets in that 
juicy air. Will go with likely probability of precipitation for most of the County Warning Area tonight. 
Several inches of rain not out of the question tonight but exact 
locations yet to be determined. Then we have several subtle 
boundaries left over from yesterdays convection with the main 
synoptic frontal boundary stalled from southwest to northeast Iowa. 
This boundary was beginning to lite up with developing showers. 


.Haase.. 


Long term...Saturday through next Friday... 
Saturday through Sunday night are very busy time frames. Saturday 
morning tonights mesoscale convective system will be slowly exiting the area...affecting 
mainly the morning hours in the east with showers and 
thunderstorms. This mesoscale convective system should be decaying...so no severe or 
significant new heavy rains are expected. A new mesoscale convective system is expected to 
develop Saturday afternoon over northern South Dakota as a new 
shortwave exits The Rockies...and along with a warm air advection 
Wing of precipitation should propagate east across the forecast area 
Saturday night. The GFS is taking the track of this farther south 
than the NAM...but the GFS appears to have some bad feedback 
issues. Have taken a more southerly track than the NAM...but not as 
far as the GFS. Sunday seems more quiet with at least a brief break 
from the thunderstorms. The boundary that pushes through on the 
heels of the overnight convection should push south of the County Warning Area...and 
then it has to wait until Sunday night before another shortwave 
comes along to generate more storms. Have not gone dry for this 
time frame as the timing of this shortwave is problematic...and may 
come in sooner. Thus the slight/low chance probability of precipitation for Sunday. Sunday 
night we are back in the chance probability of precipitation as this next wave moves 
through. Temperatures through the period are to be seasonable...but 
cooler north and warmer south than usual because of the proximity of 
the boundary. Because of the dry conditions Sunday it will be the 
warmest day...with a possibility of 90s down south with southwest 
flow and sunshine. 


Monday through next Friday a very active wet pattern for the 
area...with lots of model differences to contend with. During the 
early part of the week the operational GFS is significantly 
different from the ensembles...which are more similar to the 00z 
European model (ecmwf) and UKMET models that were available at the time. Thus...have 
tried to follow this trend. The solution HPC presents has the front 
positions as indicated by the ensembles...but seems to bring 
precipitation too far north and have trimmed back probability of precipitation and min temperatures 
and raised maximum temperatures for both Monday and Tuesday. Wednesday and 
Thursday the European model (ecmwf) brings a low pressure system across the 
plains...dragging a cold front through the area on Thursday. This 
looks like an outlier...and have gone towards the HPC solution which 
keeps our surface east-west oriented boundary...just rippling some 
weak waves along it through this period. Thus...have some slight or 
low chance probability of precipitation through the remainder of the week with seasonable 
temperatures. ... 


&& 


Aviation... 
scattered-broken MVFR ceilings mainly north of I-80 into this evening due to very 
moist airmass. Area of showers moving across southeast Iowa then 
thunderstorms should begin to increase in coverage and intensity 
this evening and continue overnight. Very heavy rain expected with 
occasional IFR/MVFR ceilings in storms. 


.Haase.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...Flash Flood Watch from 7 PM CDT this evening through Saturday 
morning for Benton-Buchanan-Cedar-Delaware-Iowa-Jefferson- 
Johnson-Jones-Keokuk-Linn-Washington. 
Illinois...none. 
MO...none. 


&& 


$$ 


Haase/le