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Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
628 am CDT Friday Mar 19 2010 


..updated aviation discussion... 


Short term /today/... 
cold front is currently bisecting Iowa northeast to 
southwest...perhaps a little ahead of the NAM/GFS position for this 
time. Colder air will be spilling across the area on brisk northwest 
winds today...but sub-freezing air temperatures will likely only 
make it to a mcw-alo line by late today. Fairly strong qg and 
frontogenetical forcing will spread into the western County Warning Area this morning 
then southwest to northeast through central portions this afternoon. 
Precipitation may have a tough time initially getting into central Iowa as 
latest high resolution rapid refresh model indicates...but when 
precipitation starts as rain or rain/snow mix...the combination of 
evaporative cooling in the lowest layer and cold advection should 
gradually bring surface temperatures down into the 30 to 34 degree range 
all locations where it precipitates with some intensity by late 
today. Cross sections agree pretty well on showing a threat for 
enhanced precipitation rates due to negative epv over area of strongest 
frontogenetical forcing...and some tapping of the dendritic growth 
zone. Will have to keep an eye on this. As far as 
accumulations...have got an inch or two north of Interstate 80 and 
west of Interstate 35. However...with Road temperatures in the low to middle 
40s...and air temperatures near or just above freezing...do not foresee 
much accumulation on roads during the daytime. The caveat would be 
during a brief period of heavy snowfall rates. 


Long term /tonight through Thursday/... 
not too much change in forecast thinking for storm system set to 
affect area through tomorrow. It appears that cold air intrusion 
from the north will be sufficient to change precipitation to all snow 
across the majority of the area by just after sunset...except in our 
southeastern counties where the transition will take place by 
midnight or shortly after. In most areas south of Highway 20 snow 
will likely accumulate to a couple of inches...however ground temperatures 
are very warm and it may take a bit for accumulation to begin even 
after the snow starts to fall. Have shaved just a bit off snowfall 
amounts to account for this...but they essentially unchanged from 
previous issuance. Primary concern remains the potential for heavy 
banded snow...which is more or less confined to southern Iowa south 
of I-80. Strong Theta-E advection and forcing just north of the 
surface front lend support to this threat...however there are the 
mitigating factors of warm surface and ground temperatures that could 
result in some of the initial precipitation from such features falling as 
rain or as snow but not accumulating. In addition...it is very 
difficult to predict precisely where any such heavy bands would 
occur. Have thus not included any heavier snowfall accumulation 
bands in the forecast for now...but this possibility will be closely 
monitored today as any such feature could result in snowfalls of 6" 
or greater. Will also hold off on any advisory at this time as 
advertised/widespread snowfall amounts are marginal...generally 3 to 
4 inches across out south central counties...and there are too many 
factors detracting from a high confidence scenario. 


The storm system should clear all but our far southeastern counties 
by Saturday evening where snow could linger into Sunday as the low 
pressure center solidifies over Arkansas and moves northeastward 
into the Ohio River valley. Thereafter...ridging will dominate for a 
couple of days with a steady warming trend expected into next week. 
Held temperatures down across the south on Sunday due to expected 
fresh snow cover...however even this will not last long as highs are 
expected to rebound into the middle 40s to lower 50s by Monday and the 
melt will be on. In the long range forecast several disturbances 
appear possible in various model solutions from around Tuesday night 
through the end of next week...however there are considerable 
differences between these solutions and have not made any changes at 
this extended range. 


&& 


Aviation... 
19/12z...cold front is now approaching kotm...with gusty north-northwest winds 
settling in behind. Band of rain/snow across northwest Iowa will likely translate 
eastward this morning...but should weaken some. Better chances of 
precipitation develop later this morning and through the afternoon west to 
east...focusing on areas from kdsm southeast beyond 21z. Model 
guidance continues to not hit the ceilings/visibilities too hard...so have 
continued to reflect that in the tafs. Uncertainty regarding 
intensity of snow also a reason not to go too low at this time. 


&& 


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 remainder of the snow pack across our County Warning Area 
eroded yesterday as temperatures reached the lower and middle 50s. 


The most important news...and good news...is that we have lowered 
crest forecasts in most of the Des Moines River basin above 
Saylorville Reservoir. As a result...less inflow is expected into 
the Reservoir which will result in a lower crest of its pool. One 
main reason we lowered our forecast crests in the Des Moines basin 
is that observational data suggests less runoff /traveling on top of 
the ground/ and more interflow /traveling through the ground/ 
occurred in the upper Des Moines basin. It takes mich longer for 
excess water to reach streams via interflow than runoff. 
So...although we believe that we have a good handle on the amount of 
volume coming down the Des Moines River...we believe it will be 
coming down much more gradually. This is beneficial for Saylorville 
Reservoir. 


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. 


&& 


Dmx watches/warnings/advisories...none. 


$$ 


Short term...Moyer 
long term...Lee 
aviation...Moyer 
hydrology...zogg 

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