151 
fxus63 kdvn 190914 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
310 am CST Wednesday Nov 19 2008 


Synopsis... 
south winds 5 to 10 miles per hour ahead of strong and seasonably cold Canadian 
high pressure system keeping temperatures a few degrees below 
freezing at 2 am despite mostly clear skies. Leading edge of cold 
front extends from Minnesota/ND border southeast to near Rapid City South Dakota 
moving south-southeast at 35-40+ miles per hour. Low clouds riding south just behind front 
within a few hours after passage. Strong surface pressure rise/fall 
couplet with system supports little change in speed of system. AWIPS 
distance-time features suggesting front to arrive in far northwest sections 
of forecast area about 18z and passing through entire area by 22z. Northwest 
winds of 15-25+ miles per hour to occur with falling temperatures in northwest sections 
by middle afternoon and increasing clouds. Analysis supports 
considerable low clouds to envelop region for at least 3-6 hours or 
well into late evening hours with bl subsidence from strong cold air advection to 
allow for partial clearing after midnight. Upper air analysis 
indicates cold air to envelop region next several days on persistent 
northwest upper level jet axis of 90-120+ kts. 


&& 


Short term...today and tonight... 
strong inversion above 1500' above ground level per all solutions and also indicated 
on latest Nowcast tools suggests below average mixing ahead of front 
despite SW winds of 10 to 20 miles per hour and at least partial sunshine. Will 
apply non-hydrostatic processes and boost temperatures a couple 
degrees higher but strong inversion aloft should keep highs in middle to 
upper 40s NE 3/4 with lower 50s SW 1/4. Skies to become mostly 
cloudy from north to south during afternoon hours. Considered 
applying falling temperatures north of Highway 30 after middle day but 
delayed onset of clouds suggests significant falling temperatures to 
occur after 3 PM...pass for day shift to reassess. Tonight...clouds 
and strong bl mixing to limit strong cold air advection with lows 20 to 25 degrees 
as northwest winds continue at 10 to 20+ miles per hour with some gusts occasionally 
around 25 miles per hour. ..Nichols.. 


Long term...Thursday through Tuesday... 
predominant northwest flow pattern continues throughout...continuing below 
normal temperatures. Shortwave energy currently topping the western 
ridge axis over southern British Columbia to drop southeast across the area on Thursday. 
Latest models continue to trend stronger with this shortwave...which 
results in a deeper upper trough to the east. This allows colder air 
to spill south into especially the eastern half of the County warning forecast area. The 00z 
models have cooled temperatures aloft over the eastern portion of the 
County warning forecast area by a couple degree c and also strengthened the magnitude of the 
surface high that drops south into the Central Plains. With good 
agreement...have followed this idea and lowered temperatures several 
degrees closer to cooler guidance values with highs Thursday in the lower 
to middle 30s...then Friday possibly not reaching 32 over much of the area. 
Lowered Thursday night mins a couple degrees as well...staying mainly in 
10 to teens range. These may need to be further lowered into single 
digits in favored cool spots if it becomes more apparent skies will 
completely clear out. With the steep lapse rates developing Thursday and 
relative humidity layer depicted below 800 mb in weakly cyclonic flow...will keep 
scattered flurries and extended mention into the evening hours. 


Friday night into Sat...European model (ecmwf) has continued to trend closer to GFS... 
and now the NAM...with shortwave in northwest flow tracking through the 
County Warning Area. Still some differences in timing and support for flurries or 
sprinkles. Overall...the system looks to be trending a bit faster 
and brought in mention of flurries to late Friday night in the northwest. This 
shortwave will be overhead or just southeast of the area on Sat with no 
reflection at the surface other than a weak warm front developing to 
the north. With little support or moisture other than early 
morning...removed mention of flurries on Sat. Developing warm air advection as 
surface ridge pushes east and troughing develops out west...should 
allow temperatures to recover back into the upper 30s to lower 40s. 


Medium range models coming into slightly better agreement with next 
shortwave carving out a surface and upper low over the Great Lakes 
early next week. This system sends a cold front through the area sun 
to cool temperatures back to 30s by Monday. Will maintain slight chance 
probability of precipitation for mixed precipitation sun and Monday with no changes beyond Sat. 
.Sheets.. 


&& 


Aviation... 
VFR conditions with mostly sunny skies to give way to MVFR conditions 
from broken-overcast ceilings of 2-3,000' above ground level between 20-24z this afternoon 
behind strong cold front. Northwest winds of 15 to 25+ miles per hour in PM and strong 
cold air advection support light-moderate turbulence within 1,000' of ground from 
mixing well into the evening hours. MVFR ceilings to continue until 
at least sunrise with gradual clearing during day with some mainly PM 
light flurries possible. Visibilities to remain at or above 7 miles through 
period. ..Nichols.. 


&& 


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


&& 


$$ 


Nichols/sheets