121 
fxus63 kdmx 161729 cca 
afddmx 


Area forecast discussion 
National Weather Service Des Moines Iowa 
1230 PM CDT Tuesday Mar 16 2010 


Short term /today/... 
shortwave developing from northern Minnesota into western South Dakota will drop southward across Iowa today. The 
system has little to work with and as a result do not expect more 
than scattered -rn out of this. Upward vertical velocity is minimal and it will take some time 
to saturate the sounding enough to get precipitation going. Quantitative precipitation forecast values on the 
order of a few hundredths. Initially low clouds will prevail this 
morning. There is little chance of much breakup as the hi clouds will 
develop by middle to late morning. Allowd for temperature rises today on the order of 
10 degree from morning lows due to the clouds and scattered -rn. 


Long term /tonight through Monday/... 
a few areas of light rain may linger into tonight as a 500 mb 
wave/vorticity maximum departs to the south and southwest. Maintained slight 
chance probability of precipitation overnight as qpfs are very light. Thereafter...more mild 
temperatures in store for Wednesday and especially Thursday as deep layer 
ridge moves overhead and winds come around to west southwest on 
Wednesday night into Thursday. 


The primary impact event of the next seven days will be the storm 
system set to affect the area from Friday through Saturday. Models 
have come into much better agreement tonight on the evolution of 
this system. At 500 mb a northern stream wave will sag into southern 
Manitoba on Thursday while a shortwave further southwest moves 
rapidly across the northern rockies and digs into The Four Corners 
region by Friday evening. This will place US in southwesterly flow 
aloft and promote low level cyclogenesis in the Lee of The Rockies. 
It appears that an inverted trough will set up extending from a 
surface low around southeast Colorado/west Kansas/OK and Texas panhandles northeastward 
across the southeastern half or so of our County Warning Area by 00z Saturday. 
Strong forcing combined with impressive moisture advection above the 
surface trough will support widespread precipitation developing across much 
of our area by late Friday and persisting into Saturday as the low 
to the south becomes more stacked and moves across Missouri and into 
the Ohio River valley...bringing precipitation in our County Warning Area to an end in the 
southeast by late Saturday night. There is a strong signal for some 
significant quantitative precipitation forecast across our area north of the surface 
front...essentially our northwestern half on Friday 
afternoon...central counties Friday night...and southeastern half on 
Saturday. 


Precipitation type is somewhat difficult to forecast with this 
system as it appears the precipitation will begin as all rain on Friday 
before gradually transitioning from northwest to southeast 
throughout the event. With bands of moderate to heavy precipitation likely 
at times as the low approaches to the south...it will be tricky to 
determine rain/snow accumulations as it will depend on the precise 
timing of the transition of p-type. A band of moderate snowfall is 
at least possible in this scenario...however at this time it is 
impossible to determine where such a band might set up. This will be 
a major forecast challenge as this week progresses. 


&& 


Aviation... 
16/18z...IFR ceilings continues across the area...and are expected to 
remain throughout the taf period. An upper level disturbance will 
spread isolated to scattered showers across the County Warning Area from north to 
south throughout the period. These showers are currently impacting 
kmcw...and will move into Kalo and kfod this afternoon...and kotm 
and kdsm by this evening. Visibilities range from MVFR to VFR...with a few 
pockets of IFR. Visibilities may drop due to fog again after the showers 
have moved through the area tonight. Skies are expected to begin 
clearing from north to south through the day Wednesday...but for now 
it appears to be just beyond the forecast period. 


&& 


Hydrology... 
widespread moderate to major flood event continues. 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 jam problems continue to 
diminish. 


Some streams are near crest now...with others expected through early 
next week. There is a possibility of moderate to heavy rainfall late 
this work week. Looking on the positive side...if this rain does 
materialize then it will have fallen after the crests occur...which 
means it would happen during the hydrograph recessions. Once the 
hydrographs go into recession...it takes a lot of extra water to 
turn them around. We will have to keep an eye on things. 


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 find its way into the streams. 


&& 


Dmx watches/warnings/advisories...Flash Flood Watch until 7 am CDT 
Wednesday for Emmet...Humboldt...Kossuth...Palo Alto...Pocahontas. 


$$ 


Short term...MS Mar 10 
aviation...beerends 
long term...Lee 
hydrology...zogg