Weather

270 
fxus63 koax 210901 
afdoax 


Area forecast discussion 
National Weather Service Omaha/valley NE 
301 am CST Friday Nov 21 2008 


Discussion... 
precipitation chances tonight...and temperatures through the 
weekend...are the primary forecast concerns. 


Surface high pressure was settling into the County Warning Area this morning. Clear 
skies...light winds and dry air has allowed temperatures to plunge 
into the teens already...with parts of northeast Nebraska into the 
single digits. After a few more hours of radiational cooling this 
morning...temperatures should start the day quite chilly. Though 
southerly flow will return during the day...increasing high clouds 
will offset potential warming somewhat this afternoon. Have 
generally gone below mav MOS numbers for highs today...with most 
areas in the middle 30s. 


Our potential precipitation maker for tonight was moving through the 
northern rockies into Montana this morning. This shortwave is 
forecast to dive southeast through South Dakota into Iowa 
overnight...then quickly race off to the east Saturday morning. 
Ahead of this system...a quick shot of pretty decent isentropic 
upglide along the 295k surface under frontogenetical forcing in the 
850 to 700mb layer will produce precipitation in or near our County Warning Area. Strongest 
forcing is forecast north and northeast of the Norfolk area after 
06z. Cross sections and forecast soundings suggest moisture 
profile will be most conducive to precipitation just northeast of the County Warning Area 
as low level dry air will be tough to overcome in much of the County Warning Area. 
Temperature profiles also show a wedge of warm air greater than 0c 
just off the surface in warm advection zone...suggesting sleet 
will likely mix in with any snow that occurs. Snowfall amounts 
should be generally light given dry air and short duration 
event...but did paint a stripe of half inch amounts along our far 
northeast periphery...generally northeast of an Onawa to Harlan 
line. 


As shortwave zips east by late Saturday morning...should see 
decreasing clouds as isentropic downglide scours out any remaining 
cloud cover. However that downglide is quickly replaced by warm 
advection as another short wave rolling through the fast upper flow 
begins to lower pressures in the High Plains. Not much time for the 
warm advection to help temperatures Saturday afternoon...but will 
still be about 10 degrees warmer than Friday. 


Light southerly flow combined with relatively clear skies Saturday 
night will make low temperature forecast tricky. Will likely see a 
lull in winds during the evening...allowing temperatures to drop off 
quickly...with steady temperatures overnight as mixing kicks in with 
increasing flow. Have lows in the upper 20s or low 30s for now...but 
could see some spots in western Iowa in the lower or middle 20s. 


Another cold front will roll through the County Warning Area Sunday afternoon behind 
our second shortwave. First...dynamics and moisture are limited in 
our area...so no precipitation expected. Temperatures again will be tough 
to pin down as northern half of the County Warning Area should see frontal passage 
by early afternoon...with southwest winds in the south potentially 
warming that area up nicely. Looks similar to Wednesday when we had 
a good 18 degree spread between north and South County warning area...but Sunday 
system is not quite as strong. Have highs ranging from the middle 40s 
north to the middle 50s south. 


Cold advection will take control Sunday night...but winds should 
stay up keeping temperatures from falling too far past the upper 
20s. Sunshine will help to offset light northwest winds on 
Monday...pushing temperatures into the 40s again. Monday night still looks 
cold with high pressure in the area providing light winds and clear 
skies. Lows should settle into the lower 20s if not a tad bit colder. 


Did not change the forecast past Tuesday. Latest European model (ecmwf) and GFS 
forecast are diverging for the Holiday travel time...but there seems 
to be some convergence between various medium range models and 
ensemble members in showing upper ridging over the center of the 
country. However there are varying degrees in amplitude and thus 
storm tracks. Will let extended forecaster make decisions/changes 
based on new guidance later on this morning. 


&& 


Aviation... 
for taf sites kofk/koma/klnk through 22/12z. 


Surface high pressure over eastern Nebraska will slide to the east today with south 
flow returning ahead of a shortwave trough that is over Montana. Forecast 
soundings moisten up the higher parts of the atmosphere at the 
cirrus level this morning...then work downward during the afternoon 
as the shortwave trough moves into the Dakotas. 
Upward vertical velocity/frontogenesis/isentropic lift associated with the shortwave will 
produce clouds and if there is enough moisture light snow/ip/ 
flurries. At this time the track of the wave has the more favorable 
area for snow to the north of the taf sites. May mention some 
flurries/snowshower/ip when upward vertical velocity is greatest/cloud temperatures will be 
favorable with ice crystals/steep lapse rates/high relative humidity. Ceilings should 
remain VFR. This afternoon...surface winds will strengthen to 10 to 20 
kts with gusts 20kts to 25kts especially at kofk and klnk. In 
addition...between 04z-10z the low level winds (h9) are forecast to 
increase to around 40kts and will likely continue mention of low level wind shear. 


&& 


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


$$ 


Discussion...dergan 
aviation...zapotocny