915 
fxus63 koax 080148 
afdoax 


Area forecast discussion 
National Weather Service Omaha/valley NE 
848 PM CDT sun Sep 7 2008 


Discussion... 
have updated the forecast mainly for precipitation trends this evening and 
tonight. Band of rain showers is progressing northward out of northestern Kansas into southeastern 
Nebraska...and have increased probability of precipitation there. Meanwhile...main precipitation area 
is pushing eastward out of north ctrl Nebraska/southern South Dakota...mainly associated with 
frontogenetic forcing around 750-700mb. That area of precipitation should 
persist through the rest of the evening...as indicated by going 
categorical probability of precipitation after 06z. In the southern County Warning Area...some very weak 
elevated instability is noted overnight...along with negative 
Theta-E lapse rates. Could see some isolated thunder there...but 
not expecting widespread thunderstorms. Have scaled back thunder 
mention to isolated wording with prevailing rain showers. Made no other 
changes to going forecast. 


Updated grids/zone forecast product already sent. 


&& 


Previous discussion... /issued 247 PM CDT sun Sep 7 2008/ 


Short term...main forecast challenges revolve around 3 distinct 
chances for rain over the next seven days as a series of fronts 
move through the region. First chance comes tonight and Monday 
with categorical probability of precipitation forecast...then next chance for precipitation Wednesday 
through Thursday with the next cold front...then next weekend. 


12 subjective upper air analysis indicated a 110kt jet streak 
moving through the Dakotas with an ample supply of low/middle level 
moisture. Scattered showers had developing in the right entrance 
region of this streak and continue at 19z from northeast Nebraska 
all the way back to western South Dakota. Meanwhile...a weak front 
was also moving through the County warning forecast area...located across southeast 
Nebraska and west central Iowa at middle afternoon as well. This 
front will likely push into northeast Kansas and southern Iowa 
before stalling. An additional short wave with associated 100kt jet 
streak noted on water vapor imagery moving out of British Columbia 
will be focus for developing precipitation overnight. There should 
be two areas. The previously mentioned area of showers will likely 
expand in coverage through the night into northeast Nebraska and 
northwest Iowa. Strong isentropic lift over an 850 mb boundary should 
develop precipitation in southern County warning forecast area by daybreak as well with potential 
for thunder as h850 lifted indices overrun the front...as 850 mb 
dewpoints return to +8 to +12...within zone of strongest thetae 
advection. The only area that precipitation may hold off till after 6am 
Monday could be parts of west central Iowa...but could see some very 
light precipitation beginning there late tonight with the better chance 
setting up on Monday. 


Precipitation should end from west to east by Monday evening...although 
it will be a rather cool day across the region with highs only in 
the upper 50s to lower 60s...when combined with northerly winds at 
15 to 25 miles per hour. 


For Monday night...surface high pressure builds into the lower 
Missouri River valley. This should allow for clearing skies and 
potentially plunging temperatures. New guidance suggests upper 30s to 
lower 40s...with remarkable agreement in favored cold valleys. 
Should also see some patchy fog develop...and possibly areas of 
locally dense fog where the wind can go completely calm given 
previous rainfall amounts of one half to one inch. This would 
mostly be along and south of Interstate 80...as weak return flow 
is already beginning to set up across northeast Nebraska by 
daybreak Tuesday. 


Have warming temperatures conditions Tuesday/Wednesday as return flow 
sets up across the area. Did leave in a chance of rain on 
Wednesday as suggested by latest GFS/ECMWF...but NAM still 
suggests that the moisture return may not make it that quick. 


Long term...Wednesday night through Sunday. The next front will be 
moving into the region early in the period which will bring a decent 
chance of rain that should linger through much of Thursday. Went 
with the slower GFS for frontal positioning with front moving 
south of the region into Kansas/Missouri by early Friday. Also 
went cooler than HPC guidance given low insolation and expected 
precipitation. Went dry for Friday...but then followed HPC guidance 
pretty closely for Friday night through Sunday as model solutions 
really diverge by this time with no real continuity with any 
particular model. Thus overall confidence is low...but model 
blends suggest that moisture and energy may begin to move back 
into the area from the west by Friday night and slowly move 
through the region through the weekend. This is a departure from 
previous dry forecast and have inserted small probability of precipitation for showers 
latter half of ot the extended period. 


Aviation... 
for taf sites koma/klnk/kofk. Valid through 08/18z. 


Warm air advection will continue the middle clouds at the taf sites today with isolated 
-ra at kofk. The approach of a shortwave trough will result in a 
strengthening frontal system from Kansas into MO...with precipitation 
expanding to the north of the fnt. The taf sites with see worsening 
conditions as the night wears on into Monday. Rain will be on the 
increase between 04-09z at the taf sites...with ceilings becoming MVFR/IFR 
into Monday. At this time held off on the mention of thunderstorms and rain for late 
tonight and early Monday...however this made need to be added. 


&& 


Oax watches/warnings/advisories... 
NE...none. 
Iowa...none. 
&& 


$$ 


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