550 
fxus65 kvef 071651 
afdvef 


Area forecast discussion 
National Weather Service Las Vegas Nevada 
950 am PDT Thursday Aug 7 2008 


Synopsis...moist and unstable air will remain over southern 
Nevada...northwest Arizona and nearby southeast California today 
resulting in isolated to scattered thunderstorms. Locally 
heavy rain will be possible with some storms. A drier southwest flow 
is forecast to develop for the weekend and early next week with 
thunderstorms confined to northwest Arizona. && 


Update...roughly the eastern half of the forecast area pretty moist 
and fairly unstable today. Convection which developed very early 
this morning in far southeast San Bernardino has been maintaining if 
not expanding. Updated zones and grids earlier to increase probability of precipitation a 
little. Do not anticipate additional updates this morning. 
&& 


Previous update...elevated convection was developing near the 
southern border of San Bernardino County and Mohave County. This 
development was on the northern periphery of the upper level 
disturbance centered around Yuma. Recent radar returns showed cells 
were fairly disorganized and short lived but occasional lightning 
strikes were detected. This area of upper level support should 
slowly push north though 18z. An update was sent to include isolated 
thunderstorms this morning across southern San Bernardino and Mohave 
counties. 


Previous discussion.../415 am PDT Thursday Aug 7 2008/...short term...the 
deep moisture boundary has changed little from Wednesday with the 
western edge extending from eastern San Bernardino County up through 
central Clark and Lincoln counties. A more southerly middle level 
steering flow is forecast today and a weak disturbance evident 
around the 400 mb level is forecast by the 06z NAM and GFS to move 
slowly from near Yuma up through southeast California into southern 
Nevada the next 12-18 hours. The GFS appears to be a little stronger 
with this feature. This may enhance convection over eastern San 
Bernardino and Clark counties as long as surface heating is 
sufficient to initiate...which appears to be the case. The GFS and 
European model (ecmwf) continue to indicate southwest flow gradually drying out the 
middle levels Friday and Saturday...though the European model (ecmwf) resurges low level 
moisture up the Colorado River valley Friday. Do not see a good 
reason for this and it seems overdone. The NAM shows less southwest 
flow Saturday than the GFS and European model (ecmwf). Prefer the consistency of the 
GFS and European model (ecmwf) and will continue to show the diminishing thunderstorm 
coverage from west to east Friday and Saturday. The mention of 
thunderstorms remains confined to Mohave and far eastern Lincoln 
counties Saturday. 


Long term...an upper level trough axis looks to swing through the 
the northern half of the western Continental U.S. On Sunday in association with 
a closed upper low that heads across British Columbia. This trough 
axis will help to turn the flow aloft to a westerly direction which 
will significantly dry out much of the area. The operational GFS 
plunges precipitable waters  below a half an inch as far east as a Pioche to Las 
Vegas to Amboy line and the European model (ecmwf) also shows significant drying 
taking place as well. As a result tweaked probability of precipitation down even further. 


Just how fast moisture will return to the region will depend upon 
how quick the upper flow across the western Continental U.S. Transitions back 
to a ridge. Monday appears to be a transition day...with general 
model consensus not showing the ridge building back in until 
Tuesday. Even then we will see a gradual moistening up and this 
looks to be confined to mainly Mohave County. As we approach 
Wednesday the western ridge looks to amplify greatly in response to 
deep troughing in the east. This should help to gradually turn the 
upper flow across our area back to more of a south to southeast 
direction allowing monsoonal moisture to work back northward across 
the region. However...just how soon this exactly occurs and far 
north it works is a tough call at this point. Until enough moisture 
returns temperatures will warm up and highs should reach about 2 to 4 
degrees above normal given the prognosticated 500mb heights are between 588 
and 591 dm. 
&& 


Aviation...for McCarran...the potential impact from thunderstorms on the 
Airport will again be the main issue today. While winds overall 
should be southerly...any outflow boundaries that do develop will 
create the potential for erratic shifts in wind direction and/or 
increases in speed. For now...plan to leave any thunderstorms out of the taf 
and just carry a cumulonimbus in with the forecasted sky cover for this 
afternoon and evening. The approach corridors will again see showers 
and thunderstorms...mainly this afternoon/evening...with the Mormon Mesa and 
Peach Springs corridors the most likely impacted. Few-scattered clouds 
at/above 12k feet today into tonight. Thunderstorms that do develop should 
wind down between 3z and 6z Friday...and dry conditions are expected 
later tonight with light diurnal winds. 


For the rest of southern Nevada...northwest Arizona...and southeast 
California...another day with thunderstorms being the primary impact across 
the area. Should see the best chances for thunderstorms and best coverage 
across Lincoln...Clark...Mohave and eastern San Bernardino counties. 
The main issues with storms will be locally heavy downpours dropping 
visibility to MVFR levels and lowering ceilings to around 060k feet as well as 
erratic wind shifts and/or increases in speeds due to thunderstorm outflow 
boundaries. Typical afternoon/evening gusty southerly winds around 
20 kts expected around kdra and in the Owens Valley. Otherwise 
generally few-scattered clouds at or above 12k feet today into tonight. Thunderstorms 
should wind down between 3z and 6z Friday...and generally dry 
conditions are expected tonight with typical diurnal winds. 
&& 


Climate...late yesterday evening an outflow boundary moved 
southward through the Las Vegas Valley and cooler air behind this 
dropped the temperature at McCarran International Airport to 78 
degrees. This ended the run of consecutive days with a minimum 
temperature of 80 degrees or better at McCarran at 15 days...making 
it the 4th longest such stretch on record for Las Vegas since 1937. 
The last time the low temperature at McCarran dropped below 80 
degrees was back on July 21st when it reached 78 degrees. In 
addition...a shower dropped 0.01 inch at McCarran yesterday 
evening...giving the Airport measurable precipitation for the first 
time since July 26th. 
&& 




Vef watches/warnings/advisories... 
Nevada...none. 
Arizona...none. 
California...none. 
&& 


$$ 


Harrison/Salmen/Adair/stachelski 


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