Weather

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Area forecast discussion...update 
National Weather Service Las Vegas Nevada 
800 PM PST Monday Jan 5 2009 


Synopsis...high pressure will move over the area through Wednesday 
resulting in occasional high clouds and a warming trend through middle 
week. A low pressure system is expected to bring a slight chance of 
precipitation and cooler temperatures to the area on Friday. 
&& 


Update...as shortwave trough exits the area...overall clearing 
taking place behind the system faster than previously thought. 
However upstream plume a middle/high level moisture to rapidly drop 
southeast in the northwest flow over the County Warning Area tonight and should 
bring clouds once again to the eastern portion overnight. In 
addition variable high level moisture will be over much of the 
remainder of the area. With this said sky cover will be very tricky 
overnight...but will update grids/zones to reflect faster timing of 
exiting system and less clouds over all areas except the eastern 
sections. Will need to monitor temperatures closely next several hours as 
overnight lows may drop more than anticipated due to the less cloud 
cover. Updated zones out shortly. 
&& 


Aviation...for McCarran...light and variable winds through Tuesday 
with variable high level moisture...mainly at or above 25k feet. 


For the rest of southern Nevada...northwest Arizona...and southeast 
California...generally light and variable winds through Tuesday with 
variable mainly high level moisture...mainly at or above 25k feet. 
&& 


Previous discussion...341 PM PST short term...middle and high clouds 
are over most of the area and did bring some light precipitation 
over the Sierra and Owens Valley. Expect clearing over night. Ridge 
will slowly move in over the next few days and will aid in keeping 
skies mostly cloud free. The challenge will be how much warming 
takes place. Expect valley locations to continue to have low level 
inversions that will inhibit mixing and not getting full benefit of 
what little solar energy is available. Did lower temperatures a 
degree or two in most low elevation locations Tuesday and Wednesday. 


Long term...while no major changes were made to the long range 
forecast...there continues to be major differences with the 
operational models as to what takes place with the system prognosticated to 
move through Friday. The last 2 runs of the operational GFS have 
been in a league of their own taking a piece of energy associated 
with the positively tilted upper trough and closing it off into a 
low over California. This is not supported by the 12z Gem/European model (ecmwf) or 
the 12z GFS ensemble mean and thus was discounted by our office as 
well as the surrounding National Weather Service offices. Thus have continues to follow 
the idea of a positively tilted upper trough crossing the area 
Friday with limited moisture but just enough forcing that it could 
squeeze out some light showers across Lincoln County in the 
morning/early afternoon as it moves through as well as the higher 
terrain of northern Mohave County in the evening. Temperatures will fall 
again behind this feature as cold air advection kicks in...dropping readings back 
below normal. Although there is only a 60 kt+ jet behind this 
system...models do show a strong surface pressure gradient setting 
up behind it for Friday afternoon into Saturday evening as a surface 
high moves into southern Idaho. This will create breezy to locally 
windy north winds for Friday afternoon into Saturday evening. A lake 
Wind Advisory looks likely for the Lake Mead nra at some point then. 


A large upper ridge builds in from the eastern Pacific Sunday into 
Tuesday keeping the area in a dry northwest flow aloft. Some high 
clouds may cross the area at times. Otherwise temperatures will warm slowly 
back to above normal in the higher elevations however an inversion 
will limit the amount of warming valleys experience as it keeps 
low-level cold air trapped there. 
&& 


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


$$ 


Jensen/Jacques/stachelski 


Http://weather.Gov/lasvegas