761 
fxus63 koax 030311 
afdoax 


Area forecast discussion 
National Weather Service Omaha/valley NE 
911 PM CST Tuesday Dec 2 2008 


Update... 
will push timing of precipitation back a bit tonight southeastern zones but no 
drastic changes made. 


&& 


Discussion... 
main reason for evening afd update was to mention that 00z NAM 
solution not totally bought. Model has been inconsistent in quantitative precipitation forecast 
generation across the oax County Warning Area and 00z run just spread a little into 
far northern zones late tonight and that was it. Meanwhile satellite showed 
vigorous upper trough moving across Rocky Mountains generating occasional lightning 
strikes over southeastern Wyoming/northern Colorado with trough and strong 850-700 
frontogenesis generating showers in southwestern nebr which were not 
picked up at all by 00z NAM. 00z RUC quantitative precipitation forecast was more bullish/further S 
than NAM with 0.10 around kofk by 12z Wednesday. Also last evenings 4km 
NAM composite reflectivity output did hint at southwestern nebr activity 
in addition to South Dakota snow area and it spread/developed returns southeast through 
Oma/lnk area by 03/12z. Thus no major changes at this time to forecast. 


Chermok 
&& 


Previous discussion... /issued 308 PM CST Tuesday Dec 2 2008/ 


Short term...tonight through Friday. 
Snow chances/amounts tonight...then temperatures through 
Friday...are the primary short term forecast concerns. 


Potent middle level short wave was dropping into the central rockies 
this afternoon...led by 120m height falls as per morning radiosonde observation 
data. Significant drying was occurring in the wake of this wave 
over northern California as shown on water vapor loops...and plume 
of middle level moisture was beginning to surge northeast into 
Colorado ahead of the system. Otherwise not much available 
moisture was noted on morning upper air charts...causing a bit of 
concern for precipitation generation across the County Warning Area tonight. 
However...apparent strength of system as noted on water vapor 
imagery suggests sufficient moisture will work into the system 
prior to its arrival in our area. 


At the surface...pressures have been lowering this afternoon in the 
High Plains with system approaching. A cold front was settling south 
to the Nebraska/South Dakota border as of middle afternoon...with 
southwest flow ahead of it helping to warm temperatures into the 50s across 
much of the County Warning Area. However 30s air was surging south through northern 
South Dakota...and should arrive in our northern County Warning Area by midnight. 


Models are fairly similar in timing of southward moving cold front 
into and through the County Warning Area by 12z. And atmospheric temperature and 
moisture profiles are pretty close as well. However models diverge 
on quantitative precipitation forecast placement with the GFS most robust in painting a swath of a 
tenth to a quarter inch of liquid across the County Warning Area...while the NAM 
keeps most of its precipitation along our northern border through much of 
the night. The NAM then paints another stripe from far southeast 
Nebraska to the northeast Wednesday morning. 


Both models support a tight baroclinic zone developing this evening 
across southeast South Dakota into northeast Nebraska. So expect 
precipitation to get going there by late evening and continue 
through early morning. Focus then shifts southeast where 
frontogenetical forcing in the 750-600mb layer is maximized. NAM and 
GFS are similar with this depiction...but the NAM does not produce 
quantitative precipitation forecast between 06z and 12z despite similar relative humidity forecasts. 
Given their similar relative humidity and frontogenesis forecasts...have gone more 
closely with the GFS quantitative precipitation forecast showing a band of precipitation southeast 
of the Interstate 80 corridor late tonight and early Wednesday. A 
quick look at the incoming 18z NAM lends a bit more support to this 
solution as it paints a similar stripe of precipitation from southeast 
Nebraska trough central Iowa. 


Using a combination of Cobb and Garcia methods run on the GFS model 
output yields a broad area of half to one inch snow totals across 
the County Warning Area...with higher amounts near 2 inches along the South Dakota 
border and just southeast of the Omaha metropolitan area and into 
southwest Iowa. Below advisory criteria...but will hit the 
hazardous weather outlook with amounts and timing...as well as a 
caution for the first significant snow of the year for most areas. 


The snow should end by noon across most of the County Warning Area...but flurries 
will still be possible through the afternoon under cold advection 
and slightly cyclonic flow aloft. Temperatures should not rebound 
much from the middle and upper 20s...so have highs a few degrees 
either side of 30. 


We should stay in a cold period through the end of the week as 
northwest flow aloft brings a couple more reinforcing shots of cold 
air to the plains. Could see some light snow in our northeast on 
Thursday as one of those systems dives through the northern plains. 
Otherwise am expecting lows in the teens and highs in the 20s or 30s 
through Friday...especially where snow cover will persist. 


Long term...Friday night through Tuesday. 


Medium range models overall depicting northwest flow prevailing over 
the central Continental U.S. Through the period with a series of shortwave 
troughs and attendant cold fronts moving across the area. First wave 
of concern dives through the upper Midwest Friday night/Saturday 
time frame with cold front passage through the County Warning Area Saturday morning. 
None moisture prognosticated to tap into thus dry frontal passage anticipated. The ecm 
is noticeably more aggressive sweeping this first cold airmass southward 
through the upper Midwest/Great Lakes region...as well as the 
reinforcing shot coming early next week. At that 
time...DPVA/isentropic upglide and area of 700mb-600mb slantwise 
instability along with subtle moisture return Sunday night through 
Monday suggests small chance for light snow reasonable with this 
second wave and associated frontal passage. Otherwise...appears brunt of cold 
airmasses will remain east of the County Warning Area with only weak thermal 
advection anticipated with the fropas. 


Aviation... 
for taf sites kofk/koma/klnk through 03/18z. 


VFR conditions expected as a warm front then a cold front push through 
the area through 06z. Behind the front...MVFR ceilings develop along 
with precipitation. Initially... -dz/-ra may develop however...as the cold 
air moves in the precipitation should change over to snow. Have precipitation 
starting at kofk between 05-07z and koma/klnk between 08z-10z. The -sn 
should diminish to flurries later in the morning. The precipitation 
will be accompanied by gusty northwest winds from 15 to 30kt. 


&& 


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


$$ 


Short term...dergan 
long term...Dee 
aviation...zapotocny 

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