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fxus63 kdvn 192025 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
325 PM CDT Friday Mar 19 2010 


Synopsis... 
complex upper trough continues to dig over the western U.S. One vorticity was 
dropping into northern mex with another pushing southeast through Utah/western Colorado. 
The cold front was over the far eastern County warning forecast area this afternoon 
extending from southern lm through west-central MO. The front should slow up 
some as middle/upper flow continues to back ahead of digging western 
U.S. Trough. -Sn and -ra/-sn continue to expand along strong Post 
frontal baroclinic zone from central WI SW through the northwest half of Iowa 
into NE. 


&& 


Short term...tonight and Saturday 
models are in fairly good agreement handling early Spring storm 
through the next 24 hours. Have chosen to use the GFS as the basis 
for the forecast since it was handling the developing -sn over NE 
and northwest Iowa this morning/early this afternoon. 


Expect -sn/-ra over southwestern Iowa to continue to expand NE this evening 
as strengthening thermal gradient/fgen increase behind the front. The 
precipitation at the onset over the northwest half of the County Warning Area may start as a mix 
of -ra/-sn since temperatures over part of the area warmed well into the 
50s. However with precipitation beginning after sunset and evaporative 
cooling occurring...would expect low levels to cool quickly 
changing the precipitation to snow. Over the southeast half of the County warning forecast area precipitation 
should be a mix or all -ra at the onset late tonight then 
changing to all snow. There is some uncertainty on the timing of 
the change over with the tight thermal gradient over the area 
tonight...but played the earlier change over due to the strong 
forcing expected late tonight. 


Expect the snow to taper off during the day over the northwest half...but 
the southeast should see snow through the day Saturday as upper diffluence 
increases ahead of upper low moving through Texas. 


Snow fall amounts are expected to be in the 3 to 4 inch range 
over east central/southeast Iowa/NE MO and wcentral Illinois with around 2 inches 
expected elsewhere by late Saturday afternoon. Have opted to go with a 
Winter Weather Advisory from late this evening through Saturday 
noon roughly along i80 in eastern Iowa and from late tonight 
through midnight Saturday along the Highway 34 corridor. The 
impacts are expected to be marginal due to warm soil/Road surface 
temperatures going into the event. However there could be some 
accumulations on roads with the heavier bands of snow. ..dlf.. 




Long term...Saturday night through next Friday... 
initial forecast concern is deformation zone precipitation from exiting 
system Saturday night into Sunday. New 12z European model (ecmwf) remains consistent 
with the upper low digging into the arklatex by 12z Sunday...which 
would suggest diminishing chances of precipitation lingering into Sunday 
this far north. Have trimmed northern edge of precipitation chances for 
Sunday slightly. Also...system will be very wound-up which suggests 
warm air should wrap well to the north and result in a precipitation 
mix rather than all snow before it ends. 


Deep ridging Monday will make for a nice day...then return flow and 
gradient increasing Monday night should keep US from bottoming out 
on temperatures. 


Next forecast concern is the system middle to late next week. Hi-res 
European model (ecmwf) shows a cold front crossing the area Wednesday. Upper support 
is fast moving...plus limited moisture ahead of the front will 
result in a Post-frontal precipitation event...ending Thursday from north 
to south as the wave opens up and lifts out. ..33.. 






&& 


Aviation... 
conditions to deteriorate to IFR from northwest to southeast this evening as area 
of snow or a rn/snow spreads NE over the area. Lowest conditions 
expected late evening into the early morning hours at kcid/kdbq/kmli 
with the heavier bands of snow. The IFR conditions are expected 
to reach kbrl late tonight. Kcid/kdbq should see conditions 
improve to MVFR Saturday morning as snow tapers off. ..dlf.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...Winter Weather Advisory from 1 am Saturday to midnight CDT 
Saturday night for Des Moines-Henry Iowa-Jefferson-Lee-Louisa- 
Van Buren. 


Winter Weather Advisory from 10 PM this evening to noon CDT 
Saturday for Cedar-Clinton-Iowa-Johnson-Keokuk-Muscatine- 
Scott-Washington. 


Illinois...Winter Weather Advisory from 1 am Saturday to midnight CDT 
Saturday night for Hancock-Henderson-McDonough-Mercer- 
Warren. 


Winter Weather Advisory from 10 PM this evening to noon CDT 
Saturday for Rock Island. 


MO...Winter Weather Advisory from 1 am Saturday to midnight CDT 
Saturday night for Clark-Scotland. 


&& 


$$ 


Dlf/33