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fxus63 kdmx 200458 aab 
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Area forecast discussion 
National Weather Service Des Moines Iowa 
1157 PM CDT Friday Mar 19 2010 


Short term /tonight/... 
complicated forecast for tonight snow ongoing across the west and north 
most of the day. Isentropic upglide over h700 front continues to 
drop light to moderate snow over the northwest where about 1 to 2 
inches of snow has occurred. Forcing has been slow to translate 
southeast with time...but should arrive into central Iowa by early 
evening hours. Cross sections and qg forcing suggest that best lift 
over south central and southwest will occur between 00 and 06z when 
2 to 4 inches of snow will fall. As area of lift moves east the 
focus will shift to east of i35 after 06z and continue over the 
southeast through 15z Saturday. There has been little in the way of 
a transition zone today. Temperatures aloft area already below freezing and 
the only process left has been to saturate the column. Thus...not 
expecting much of a changeover for as the precipitation begins this 
afternoon and evening. Models have been more consistent today on 
quantitative precipitation forecast values over the south...though placement has once again been 
difficult. GFS seems to have the best handle on the current 
situation with the WRF handling the short term trends better today. 
It is catching onto the slower east southeast transition of 
precipitation at this time. With snow ratios in the teens tonight...am 
looking at amounts of 3 to 5 inches across the south with a broad 
area of 2 to 3 inches over central and east central areas. Cross 
sections show that dendritic layer will be tapped across the south 
and central areas between 03 and 09z...so expecting the worst of the 
conditions to impact south central at this time. Forcing not lining 
up for the best heavy snowfall signature scenario...but certainly 
sustained lift for this period will produce moderate snowfall. Will 
be issuing a Winter Weather Advisory for snow and perhaps some 
blowing snow...though the main concern is for slush and snow covered 
roads tonight. Some drier air will be pulled into the region 
overnight...so it is possible that minor blowing snow is possible. 
Forcing will continue into the morning hours Saturday southeast...so 
will leave the advection going through 18z. 


Long term /Saturday through Friday/... 
snow will linger over south central Iowa on Saturday with some 
additional accumulations up to 2 inches over the far southeast 
possible. Drier air will be pushing south as a large Canadian high 
pressure system drops south through the northern and Western Plains. 
Currently two potent short waves are located over the southwestern 
Continental U.S. With one over Baja California California and the other diving southeast 
through Utah. The upper level system will deepen and close off 
across northern Texas as the two short waves merge. As they merge...the 
moisture will pull even further south by late Saturday and into 
Saturday night. Will need to monitor snow amounts Saturday morning. 
May approach storm totals of 6 inches in a few places with a 
possible narrow upgrade area required. 


High temperatures Saturday will be dependent on snow cover. Have 
coldest temperatures over the south where the deepest snow cover is 
anticipated. Sunday will feature mostly sunny skies central and 
north with some lingering clouds over the south. Once again coolest 
temperatures to the south. Most of snow cover should melt during the 
day Sunday. Thermal ridge begins to lean back into Iowa Monday and 
Tuesday. Tuesday in particular looks quite mild with highs 
approaching the lower 60ss. 


Some uncertainty remains for Wednesday through Friday however better 
continuity amongst long term guidance and ensembles is developing. 
The strong Spring like flow will continue with the next significant 
system expected to move east through the Southern Plains. The 
current forecast track and strength of the system would suggest the 
bulk of the precipitation will be to the south of the state. The 
best chance for rain will be over the south Wednesday and Wednesday night. 


&& 


Aviation... 
20/06z...middle level forcing will continue across southern Iowa through 
18z with snow persisting and widespread IFR conditions. Farther 
north...drier air rushing in at the surface will dissipate the low 
cloudiness down to a koff-kdsm-Kalo line by 15z Saturday with the 
low clouds exiting much of the area by 21z on Saturday. Widespread 
VFR conditions are then expected with winds remaining from the north 
for the entire forecast period. 


&& 


Hydrology... 
widespread flood event nearing its Climax across our County Warning Area 
today...with rivers having either cresting or expected to crest over 
the next several days. The new snowfall that is received over the 
nest 24 hours will melt quickly through Monday. This snow melt however 
is not expected to impact river crests though may briefly delay 
their retreat. 


The most important news...and good news...is that the Des Moines 
River north of Saylorville Reservoir has crested and the inflow into 
Saylorville will continue to diminish. As a result...less inflow is 
expected into the Reservoir which will result in a lower crest of 
its pool. 


Many locations that have not yet seen the river crests will see 
crests lasting longer than normal. In addition...it will take longer 
than normal for the rivers to fall to flood stage. In some 
cases...it may take as much as 2 to 3 weeks for some streams to fall 
back to flood stage. Any heavy rainfall would either slow the falls 
further...or would result in secondary crests. 


We are not out of the Woods yet...however for this Spring. Now that 
the river rises from the snow melt have either peaked or will do so 
within the next several days...our attention will then move toward 
the rest of this Spring season. Soil moisture will likely remain 
high through the end of this Spring season...making US more 
susceptible than normal to flooding from heavy rainfall events. 


&& 


Winter Weather Advisory from 7 PM this evening to 1 PM CDT Saturday 
for Adams-Appanoose-Clarke-Davis-Decatur-Jasper-Lucas-Madison- 
Mahaska-Marion-Marshall-Monroe-Polk-Poweshiek-Ringgold-Tama-Taylor- 
Union-Wapello-Warren-Wayne. 


$$ 


Short term...rev 
long term...donavon 
aviation...cogil 
hydrology...zogg/donavon