Weather

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Area forecast discussion 
National Weather Service Las Vegas Nevada 
321 PM PDT Friday Aug 29 2008 




Synopsis...a significant change in the weather is in store for the 
region over the Labor Day weekend...as an unseasonably strong 
weather system brings thunderstorms to the area Saturday and 
Sunday...and substantially cooler air and gusty winds to the region 
Sunday through Tuesday. 
&& 
Short term...satellite imagery still indicating high level clouds 
across the southern half of the forecast area...with additional 
cirrus over Southern California and SW Arizona expected to move in 
aloft in south/southwest upper level flow...so expect 
Clark...Mohave...and eastern San Bernardino to keep cloud coverage 
the remainder of the day. Convection starting to fire up along the 
Mogollon Rim...so have kept slight chances for thunderstorms in the forecast 
for this evening with slow west to southwest storm motion expected 
out of any storms to our east. The remainder of the forecast area 
will be relatively clear as high pressure continues to transition 
east over the area. By tomorrow morning...moisture laden east to 
southeast flow will be in place with the high center sufficiently 
east of the area and the backside of an easterly wave to the south 
positioned to compliment each other. By middle afternoon...deep 
moisture will be in place over much of the area...with 850 dewpoints 
near 15c and precipitable water values over 1.5 inches across 
Lincoln...Mohave...Clark...and San Bernardino counties. This 
combined with high instability...relatively slow storm motion...and 
vertical wind profiles hinting at training cells...raises the 
probability of flash flooding....and will be issuing a Flash Flood 
Watch for Saturday across these areas today. Hail will be 
another threat tomorrow...with wbz levels below 14kft and very 
moist vertical profiles within hail growth zones of 0 to -20c. 
Sunday brings another set of challenges...as an unseasonably deep 
low pressure system adds strong dynamics to the mix...making Sunday 
similar to Saturday...but in a high vertical shear environment with 
steeper lapse rates. This will likely result in organized convection 
across the eastern third of the County Warning Area...transitioning the main threat 
from heavy rain and flooding to hail and strong winds. Temperatures 
will decrease significantly Sunday and Labor Day...with high 
temperatures 15 to 20 degrees cooler than the past several days. 
Lastly...breezy to windy conditions will develop at most locations 
Sunday and persist into Monday as the strong low pressure passes 
across the northern Great Basin. Winds may be strong enough for lake 
Wind Advisory criteria on Sunday...even with the less than favorable 
wind direction for higher winds in those zones. -Kennedy- 


Long term...Tuesday through Friday...the weekend system should no 
longer be exerting any influence on our area. Fairly good agreement 
between models and ensemble mean. The troughiness will give way to 
northwest flow which will moderate a bit temperature 
recovery...which was the major challenge for the extended. Some of 
the ensemble low members had some significant cool Downs for the 
area. Temperature forecast in this package did lower some highs and 
lows and were a blend of the old forecast and the latest model 
guidance. Models show that by Friday a hang back trough will still 
be over the area and a bit sharper thus keeping US in a north flow. 
-Jacques- 
&& 


Aviation...for McCarran...light and variable winds through early 
this evening will become southwesterly 6 knots or less by 04z 
this evening. Broken to overcast high clouds will likely persist 
through tonight and Saturday. Middle level moisture will increase from 
the southeast Saturday while the air mass becomes unstable leading 
to scattered afternoon and evening thunderstorms...especially over 
the mountains around the valley. Southerly winds will increase 
during the day Saturday with sustained winds 10-12 knots after 18z. 


For the rest of southern Nevada...northwest Arizona...and southeast 
California...generally light winds through tonight with increasing 
southerly winds after 18z Saturday. Mostly high cloud cover tonight 
but middle level moisture will increase from the southeast Saturday 
with the air mass becoming unstable. Scattered thunderstorms are 
expected across the southern portion of the area with isolated 
afternoon and evening thunderstorms up by Bishop. -Harrison-&& 


Fire weather...an unusually strong front will pass through Nevada 
Sunday and Monday. In advance of the front wind gusts to 35 miles per hour or 
greater from the south to west will develop in south central Nevada 
Sunday afternoon and in western San Bernardino County. With these 
winds there will be relative humidities below 15 percent. Nevada 
zone 460...462 and primarily the west part of Nevada zone 461 will 
be affected. In California zones 226 and primarily the west half of 
zone 228. A Fire Weather Watch has been issued for the above 
mentioned zones. Monday will see some strong winds winds in Mohave 
County Arizona but the humidity will be too high for critical 
conditions. -Jacques- 
&& 


Vef watches/warnings/advisories... 
Nevada...Fire Weather Watch from Sunday morning through Sunday evening 
for nvz460>462. 
Flash Flood Watch from Saturday afternoon through Saturday 
evening for nvz015-016-020>022. 
Arizona...Flash Flood Watch from Saturday afternoon through Saturday 
evening for azz001>003-036. 
California...Fire Weather Watch from Sunday morning through Sunday evening 
for caz226-228. 
Flash Flood Watch from Saturday afternoon through Saturday 
evening for caz524>527. 
&& 


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