Weather

551 
fxus63 kdvn 112004 aaa 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
304 PM CDT Sat Oct 11 2008 


Synopsis... 
large upper trough continues to dig/develop over the western U.S. 
While downstream...strong ridge remains in place from the western 
Gulf through the middle Atlantic coast. As result...little change in 
the surface pattern has occurred during the past 24 hours over the 
middle-Mississippi Valley. Area remains between strong...but nearly 
stationary front from western Kansas through central Minnesota and northern 
WI and high pressure ridge to the east and southeast. Moisture 
though has been slowly increasing. The morning 800 mb analysis showed 
10-15c 800 mb dewpoints wrapping north and NE around periphery of the southeastern 
U.S. Ridge. Surface dewpoints have increased into the upper 50s to 
low 60s over most of the region. 


&& 


Short term...tonight and Sunday... 
models continue to show little eastward progression of the upper air 
features through Sunday afternoon resulting in only a slow eastward 
push of the plains front. This will keep the middle Mississippi Valley 
in the warm sector...continuing the stretch of dry and mild early 
Fall weather. For tonight expect mainly clear skies east. The 
western counties will probably see more high clouds associated with 
weak impulses in the southwest flow over the plains moving through 
eastern NE and western Iowa. Higher dewpoints tonight will keep temperatures 
considerably warmer than last night with mainly 50s east and upper 
50s to around 60 west. For Sunday we should see more clouds than 
the past few days. But...with deeper mixing due to stronger low 
level wind...temperatures should still climb into the upper 70s to low 
80s. While these temperatures are considerably above normal they are still 
below the record highs for the 12th...which are in the middle and upper 
80s. ... 




Long term...Sunday night through Saturday... 
deep upper trough sweeps east early in the week...sending a cold 
front through the area bringing widespread rain and an end to the 
spell of much above normal weather. This trough kicks out in two 
pieces...the initial passing north along the Canadian border Monday 
night and early Tuesday...followed by a secondary upper low over the 4 
corners region that weakens into an open wave overhead early Wednesday. 
12z models appear to be in better agreement with the first wave than 
previous runs...but still have significant differences in timing of 
this lead shortwave passing to the north Tuesday and Wednesday. Kept forecast 
close to compromise between faster GFS and European model (ecmwf)...as NAM notably 
slower and may have suffered from initialization problems. 


Sun night into Monday...have slowed the cold front some...resulting in 
lower probability of precipitation Monday afternoon and warmer temperatures. Will keep some 
mention of thunderstorms in the west for late Monday afternoon...but 
warm cap around 700 mb should limit coverage. Best rain chances will be 
along and behind the front Monday night. Potential for deep 
saturated soundings due to high precipitable water airmass from remnants of tropical 
system...along with lack of strong dynamic forcing will again limit 
thunderstorms chances and kept only isolated wording. Could again 
see a widespread soaking rain with amounts of an inch or more... 
which can be further refined in quantitative precipitation forecast as period nears. 


Tuesday should have a break in the rain in the morning through most 
of the afternoon across at least the north as the front pushes S-southeast 
with weak overrunning confined mainly to the south. Next push of 
lift arrives by evening as 4 corners low induces a wave along the 
stalled front to the south. Models have the high dewpoint 800 mb airmass 
lingering over the far south...or just south of the County warning forecast area...so this 
system will also have potential for significant rainfall. This may 
depend on amount of phasing that may occur with a northern stream 
trough in Canada and related track of the surface low. For 
now...have kept probability of precipitation in chance category Tuesday night...then low chances 
across southeast Wednesday...which may need to be later increased. Surface high 
builds in Wednesday night trough Thursday for period of near to a bit below 
normal temperatures. Models in poor agreement beyond...with European model (ecmwf) 
depicting strong cold front and upper trough Friday night into 
Sat...while GFS has Flat Ridge building aloft into Sat...then weaker 
cold front Sat night. Will maintain continuity for now close to HPC 
solution with slight chances for showers with weak system passing to 
the north Thursday night...then dry Friday and Sat period with near normal 
middle Oct temperatures. ..dls.. 


&& 


Aviation... 
VFR conditions should prevail again through 18z Sunday. Increasing 
low level moisture may result in a brief period MVFR visibilities due to 
fog around sunrise with kmli and kbrl the most likely airports to be 
effected. Will also have to monitor conditions for inclusion of low 
level wind shear later tonight...especially at kmli/kbrl. Surface 
winds are expected to back to the southeast and diminish to 5kts or less as 
winds around 1500ft become southerly and increase to around 30kts. 
Have not included low level wind shear in the 18z tafs due to enough uncertainty 
about the extent of cloud cover and how much the surface wind will 
diminish tonight. ... 


&& 


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


&& 


$$ 


Dls/dlf