Weather

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Area forecast discussion 
National Weather Service Las Vegas Nevada 
901 PM PDT Wednesday Aug 27 2008 


Synopsis...a large high pressure area will develop over Nevada... 
resulting in very hot afternoon temperatures across the lower 
deserts Thursday and Friday. An increase in moisture on Saturday 
will lead to a chance of thunderstorms during the Labor Day weekend. 
&& 


Update...water vapor imagery depicts upper ridge building in over 
norcal and shifting eastward as advertised by models. Boundary layer 
continues to dry...surface dewpoints are in the 20s and 30s over most of 
the forecast area. Exception is in the southern extremity...south of 
a line from eed-igm...where dewpoints are still in the 50s. Dra radiosonde observation 
shows precipitable water down to 0.46" with an average of 2c warming from surface-600mb 
over the last 24 hours. We mixed out to about 650mb today...similar 
mixing tomorrow would push the high in leaving to 108...which is the 
excessive heat threshold for this time of year. Updated to issue an 
excessive heat watch for Thursday afternoon and eliminate mention of 
thunderstorms for southern Mohave County Arizona. -Runk- 


Previous discussion...from 255 PM PDT...generally quiet conditions 
currently across the forecast area...but current satellite imagery 
shows thunderstorms beginning to develop along the Mogollon Rim and 
north into the Colorado plateau. With strong northeasterly midlevel 
flow...any thunderstorms east of the area could potentially move 
into Mohave County and combine with an airmass which is quite 
unstable and capable of organized convection....with cape values of 
over 2000 j/kg and 0-3km sunrise helicity of over 200. Multiple other 
indices indicate a good possibility of organized convection across 
the area...but will unlikely see convection initiated on its own. It 
will take a thunderstorm moving in from the east or a strong outflow 
boundary moving into the area. Precipitable water values are still quite high / 1.5 
inches along the southern edge of the County Warning Area...so there is the 
potential for very heavy rain if storms move over the area. Have 
updated the forecast for the remainder of the day and into the 
evening to reflect this possibility. If things do get initiated 
across our area...it will most likely be during the later part of 
the afternoon and more so into the evening...with severe weather 
certainly a possibility over Mohave County to just west of the 
Colorado River. If little development occurs over the next several 
hours and/or the 00z soundings indicate more then expected stability 
aloft as the ridging pushes in...will need to remove thunderstorms from the 
forecast. Tomorrow and into Saturday...high pressure will move over 
and east of the area...which will suppress thunderstorm chances to the 
extreme south and eastern portions of the forecast area Thursday and 
Friday...but result daytime high temperatures well above 
average...with the current forecast showing high temperatures for the Las 
Vegas Valley just one degree below excessive heat warning criteria. 
Have opted to let the middle shift make the final call on this 
one...hoping 00z model guidance will lend a hand. Either way...its 
going to feel quite warm then next couple days. Saturday...the 
center of high pressure will be just east of The Four 
Corners...opening the door for addition moisture to be advected 
across the County Warning Area with south to east flow over most of the area. Have 
increased thunderstorm chance for Saturday areawide. -Kennedy 


Long term...Sunday to Wednesday...Sunday looks to be the day of 
concern and most difficult to forecast for. Monsoon moisture will 
have been moving into the area since Saturday. However models have a 
moderately strong trough moving into the area. This should start to 
remove the moisture from our western zones and hence lower the probability of precipitation 
there Sunday. Did increase probability of precipitation a bit in eastern Lincoln County and 
northern Mohave. Differences arise between models in depth and speed 
of sundays trough. The GFS/ECMWF/GFS ensemble mean were fairly close 
in timing and went with this trio. The Gem was a bit deeper and 
slower and steered more towards the trio again. For now models do 
not bring any significantly strong winds Sunday and for now have no 
fire weather concerns. Monday will see some residual probability of precipitation in Mohave 
County as west flow will be over the area and scour out most of the 
County Warning Area. Tuesday and Wednesday look to be under zonal to maybe weak 
ridging depending on solution used. In any case fairly inactive 
weathers. Have lowered temperatures a bit more towards climatology which 
combined with drier air behind sundays system seems more reasonable. 
-Jacques- 
&& 


Aviation...for McCarran...light and variable winds today and 
tomorrow with the northeast the favored direction each afternoon. 
The winds at night will be southwest and light. No significant cloud 
cover expected with only a slight chance of a thunderstorm in the 
Peach Springs corridor. 


For the rest of southern Nevada...northwest Arizona...and southeast 
California...the only thunderstorm activity today will be confined 
to northern Mohave County and for Tuesday a slight chance for them 
in southern Mohave County. Aside from that clod cover will be 
minimal and winds area wide light and follow Mountain/Valley diurnal 
patterns. 
&& 


Vef watches/warnings/advisories... 
Nevada...excessive heat watch Thursday afternoon for nvz016-017-020>022. 
Arizona...excessive heat watch Thursday afternoon for azz002-003-036. 
California...excessive heat watch Thursday afternoon for caz520-522>527. 
&& 


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