432 
fxus63 kdvn 230850 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
250 am CST Monday Nov 23 2009 


Synopsis... 
band of light rain in central Iowa and just grazing the far western 
dvn County Warning Area early this morning...moving northward. Weak wave moving 
through the region with stronger trough pushing across the northern 
rockies. Another even stronger wave approaching the British Columbia 
coast. Mild night again across the County Warning Area with most locations currently 
in the 40s. 


&& 


Short term...today and tonight... 
band of light rain associated with 850 mb convergence and 2-d 
frontogenetical forcing occurring across central Iowa...and just grazing 
the far western tier of the dvn County Warning Area. Drier southeast feed seen on 
00z dvn sounding continues to erode the rain east of this rain band. 
Operational models similar in weakening the convergence and lifting 
it northward early this morning. Will continue with likely probability of precipitation far 
west early this morning with sharp edge to rain/vs no rain right 
next to it...then dry everywhere from middle morning through this 
evening. Storm Prediction Center 4.0km WRF-nmm model depicts this scenario rather well. 
Should be plenty of middle/high level clouds today in the diffluent 
flow aloft as northern rockies trough dives into the Central Plains. 
This is expected to keep maximum temperatures mainly in the 50s to near 60. Not 
quite as warm as yesterday but still well above normal for late 
November. 


As system digs into the Central Plains and begins to deepen tonight 
and head east...rain will be increasing. This will be especially 
true west of the dvn County Warning Area based on the consensus of the models. Will 
keep the evening dry and have chance probability of precipitation in the west after 
midnight...with the bulk of the rain impacting the County Warning Area on Tuesday. 
See long term discussion below. With southeast winds and cloud cover 
min temperatures will once again be in the mild 40s. 


.Haase.. 


Long term...Tuesday through Sunday... 
a phenomenally dynamic upper air pattern looks to take aim on the 
County Warning Area through Thursday...as two major upper waves phase into a broad 
upper low. Atmospheric moisture is expected to be typical for a 
November storm system...the result of a only partially open Gulf. 
Precipitable water values during the event should range from 75 to 
125 percent of normal...but due to the strong dynamics we should see 
seasonally moderate to heavy amounts in the region. The first 
significant wave will be a closed low moving directly across the 
area Tuesday...associated with surface low pressure tracking from 
Kansas through the southeast or central portion of the County Warning Area during 
evening Tuesday. The second phase of this storm will occur as 
another very strong upper wave drops into the northern plains 
Tuesday night. This will serve to slow the first low pressure...and 
spread moisture back across the County Warning Area through Wednesday night and 
Thursday. In addition...colder air aloft will establish 
itself...shifting the form of precipitation from rain to snow. The GFS and 
European model (ecmwf) are now in good agreement with the track and strength of the 
systems and high probability of precipitation will be issued in this forecast for Tuesday 
through Wednesday night. Rainfall Tuesday and Tuesday night should 
total between a quarter inch in the far southeast...to 0.75 to 1.25 
inch in the north 1/2 or so. Additional precipitation amounts Wednesday and 
Thursday should be light...under 0.25. This should occur with the 
second upper low crossing over the County Warning Area from northwest to southeast. 
A persistent band of light to moderate rain changing to snow is 
expected over the northern 2/3rds of the County Warning Area. Temperatures should 
remain above freezing during the period of precipitation. 


Looking at snow potential...though the transition to snow is nearly a 
guarantee...the very mild ground temperatures combined with only light to 
moderate intensity snow occurring in a 31 to 34 degree air 
temperature suggest that this potential accumulation will be of 
little if any travel hazard. Grassy accumulation of 1 to possibly 2 
inches are likely during the overnight hours Wednesday night...melting 
off during the day Thursday despite additional snow showers 
occurring. Since this is a major travel period...I will highlight 
the snow potential in the severe weather potential statement...but more as a headsup that it should 
not be a significant travel hazard. 


No changes made beyond Thursday. 
... 


&& 


Aviation... 
local 4-6sm br until 15z otherwise VFR conditions remainder of today 
and this evening...with ceilings mainly at or above 8k feet. Later this evening 
and overnight MVFR ceilings will spread into eastern Iowa as a storm 
system deepens in the Central Plains and heads in our direction. 
Widespread rain and MVFR/occasional IFR conds expected on Tuesday. 


.Haase.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...none. 
Illinois...none. 
MO...none. 


&& 


$$ 


Haase/Ervin