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Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
1122 PM CST Thursday Mar 11 2010 


..updated aviation discussion... 


Short term /tonight/... 
surface low directly centered over the County Warning Area and will continue to spin 
across central Iowa tonight into Friday morning. Short term models 
are in decent agreement for tonight...and leaned toward a blend with 
the sref/NAM. 


Low will meander to the northwest through tonight will continue to 
advect warm moist air into the eastern and northern sections of the 
County Warning Area. With alo getting into the lower 50s today...temperatures look 
to stay fairly mild overnight across the east with the warm air advection 
persisting. Soundings remain saturated across the west to northwest 
tonight into early Friday...so kept best chance of rain going in 
this area. Otherwise...more likely to see widespread fog and drizzle 
with the abundant moisture available in the lower levels. 


Long term /Friday through Thursday/... 
low pressure centered over centered northwest Iowa will begin to 
retrograde back to the south by Friday as southern stream energy 
once again takes over. As this occurs...the upper level dry slot 
will move into central Iowa and will keep precipitation chances low over this 
region. Best chances for precipitation will be over the far west and south. 
However no significant precipitation is anticipated. Kept precipitation type as 
rain. Despite 850 mb falling below zero tonight into Friday...low 
level dewpoints remain above freezing and should help keep a 
sufficient warm layer for rain. 


Next significant short wave currently located over central Texas will 
lift northeast into the middle Ohio Valley by late Friday then wrap up 
and retrograde west with some qg forcing into southeast Iowa and 
northern Missouri. Certainly placement issues with this system that 
could have significant impacts on central Iowa flooding. Currently favor 
a more southern track with less quantitative precipitation forecast that would not impact expected flood 
crests. Also...current track would not favor the quantitative precipitation forecast forecast by the 
12z NAM/European model (ecmwf) given activity going on to the southeast and Gulf 
moisture being shut down. 


General trend for Sunday and beyond is for slow warming at the surface 
then thermal ridge beginning to lean into the Midwest by Thursday. A 
little hesitant to raise temperatures to mex guidance given 
potential for cloud cover. Any breaks in the sun though should help 
push readings to near 50. Current forecast is dry with lack of middle 
moisture despite some light returns from long term guidance. 


&& 


Aviation... 
12/06z...surface low will meander over western Iowa overnight before 
moving southeast across Missouri on Friday. Low clouds and pockets 
of MVFR fog will affect the area through the night...with the lowest 
conditions found at kfod closer to the center of the low. Kdsm may 
also experience a few periods of IFR ceilings...otherwise only MVFR 
conditions expected at the remaining terminals. Dz will not have 
much of an effect and other than at kfod MVFR visibilities will be few and 
far between. Conditions will steadily improve after sunrise as the 
low begins to slowly move away...but expect MVFR ceilings to continue 
for most of Friday. 


&& 


Hydrology... 
widespread moderate to major flood event continues to unfold across 
the entire County Warning Area. We/re not looking at another 2008 event however. 
This flood event was expected to occur...though...per our first 
Spring flood outlook from late January which highlighted a high risk 
of significant flooding. The flood threat is migrating from south to 
north with time. Basins of most concern are first the Des 
Moines/Raccoon...followed by the Cedar/Iowa then skunk. Moderate to 
major flooding expected at many locations in the Des Moines/Raccoon 
river basins. 


Ice jams continue to be problematic...with the leading edge of the 
activity migrating from south to north ahead of the overall flood 
threat. The ice jam activity has been rather predictable for this 
event...with the majority of the events occurring within 12 to 24 
hours after streams begin to rise. The ice jam activity has been more 
prolific than normal...which was expected. The ice has become 
thicker than normal...due to our colder than normal winter...with 
thicknesses exceeding 12 to 18 inches in some locations. The ice jam 
hot spots today have included but are not limited to The Fort Dodge 
area...as well as various locations in the Cedar River basin 
upstream of Waterloo. Due to the widespread potential for flash 
flooding from the jams...we elected to issue a Flash Flood Watch for 
counties along and north of U.S. Highway 20. 


We will have to continue monitoring the next weather system to affect our 
County Warning Area Friday night into Saturday. Our present river forecasts do 
include expected rainfall from this system. As the forecasts 
show...we do not expect the rain to add significantly to the crests 
like what happened in 2008 in some locations. 


Regardless of how much rain falls this weekend...however...many 
streams will be slower than normal to recede Post-crest and fall 
below stage...as the runoff from snow melt as well as excess soil 
moisture takes a while to finds their way into the streams. 


&& 


Dmx watches/warnings/advisories... 
Flash Flood Watch through Saturday morning for north central Iowa. 


&& 


$$ 


Short term...klp 
long term...donavon 
aviation...Lee 
hydrology...zogg