Weather

829 
fxus63 kgid 211930 
afdgid 


Area forecast discussion 
National Weather Service Hastings NE 
230 PM CDT Thursday Aug 21 2008 


Short term....short term...through 12z Sunday. Forecast concern is 
first chance for rain in quite some time Friday evening with the 
passage of a cold front. 


County warning forecast area currently wrestling with the slowly retreating low clouds on the 
back side of weak circulation north of Kansas City. These clouds should 
continue to slowly exit the area through early this evening. 
Numerical guidance suggests a return to some haze/fog tonight. Winds 
will be stronger below 850 mb than recent days...and will turn west-southwest 
above 850 mb after midnight. Believe better mixing will keep any low 
clouds ceilings from forming...with only some renegade stratus 
passing by around dawn Friday and the occasional haze report. 


On Friday...the GFS is quicker with the front than the NAM. Both 
hedge toward capping keeping convection at Bay until about 00z or so 
and have kept that idea going. It appears the front will trigger 
storms to our east first...which will build back to the southwest. 
Later in the evening...low level let sets up further west toward 
the southwest County warning forecast area and will likely spell more rain/storms. Would 
expect a general small mesoscale convective system type rain event for the southern half 
of the County warning forecast area through Friday...with likely precipitation chances there. 
The northern County warning forecast area could escape with not much of the anything as 
the front slides by so only small rain chances there. Stronger 
shear stays behind the front some distance. Instability is pretty 
good through the evening hours...so a few strong storms are 
possible... but should generally be non severe in nature. 
Precipitable water values of 1.5 to 2 inches suggest some locally 
heavy rain may occur...more likely over the southeast County warning forecast area. 


As can be the case...Post frontal weather daytime weather is heavily 
influenced by the prior evening/S convection. The convection will tend 
to leave behind plenty of clouds...or could even start to clear 
things out if the storms were on a large enough scale. Believe these 
types of scenarios will impact our Saturday weather...with only 
limited and small chances for precipitation during the day Saturday. 
The far northern County warning forecast area could end up being fairly clear if some of 
that 50 degree surface dewpoint air can be realized. Do expect 
plenty of clouds through much of the day in the southern 
County warning forecast area...and have kept 20-30 percent chances focused more down 
there. Highs forecast in the lower to middle 80s may be a bit 
optimistic if the clouds hold firm. Also kept the small chance of 
showers/storms Saturday night. No real confidence in some of the 
GFS/UK big rain scenarios for that time period...so persistence 
was used. 


Long term...12z Sunday through Thursday. 


Forecast concern during this time frame will be off and on chances 
for precipitation. Early in this timeframe we will be in northwest 
flow aloft with ridging over The Four Corners region. Extended 
models all showing various shortwaves rotating around the ridge and 
across our County Warning Area. The problem lies in the timing as not much agreement 
from run to run or between models. May also be some convective 
feedback issues to deal with. Surface boundary seems to wash out on 
Sunday as well. Looking at a time/height plot of forcing mechanisms 
from the GFS could have a short wave trough or two slide across on 
Sunday and Monday so will go with a slight chance of thunderstorms 
during that period. 


Tuesday should be mainly dry before next system approaches 
Wednesday. Winds will be a bit higher on Tuesday as Lee trough 
develops over the High Plains. GFS advertising a sharp middle level 
trough to swing through with a surface front pushing across the County Warning Area. 
The GFS is a little further south with this wave than the some of 
the other extended models and usual timing differences come into 
play. However the latest European model (ecmwf) similar in generating some 
precipitation on Wednesday so will continue with a chance of rain. 


High temperatures should hover around the middle to upper 80s during 
the extended which is pretty close to the average for this time of 
year. 


&& 


Previous discussion... /issued 1228 PM CDT Thursday Aug 21 2008/ 


Update...went for another round of updates to account for slowly 
diminishing clouds across the County warning forecast area. We are finally seeing a slow 
migration east of western edge...and some thinning/mixing out from 
the south. Models suggest far northeast County warning forecast area will be last to clear 
and that seems to be coming true based upon satellite trends. Made 
some slight modifications to afternoon temperatures with only 
around 80 likely for that far northeast County warning forecast area area. 


Aviation...18z taf. IFR ceilings should quickly rise to MVFR ceilings 
by 19z...and then VFR ceilings by 20-22z at kgri. Satellite and 
observation trends supporting the likelihood of improved 
conditions shortly. By evening...prevailing VFR conditions are 
expected. A short period of lowered visibilities in haze is possible 
near dawn Friday...but it should not be nearly as lengthy nor 
widespread as previous days. 


&& 


Gid watches/warnings/advisories... 
NE...none. 
Kansas...none. 
&& 


$$