493 
fxus65 kpsr 211139 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
440 am MST Friday Nov 21 2008 


Updated...updated aviation discussion. 


&& 


Synopsis... 
high pressure aloft will continue to give the southwest deserts warm 
weather through the upcoming weekend...with periods of mainly high 
clouds. The upper level high will begin to break down as next week 
progresses...with low pressure expected to take shape off the West 
Coast. This system will bring much cooler and unsettled weather to 
the entire southwest United States by midweek. In fact...there will 
be a chance of showers most areas by about Wednesday or Thanksgiving 
day...depending on the speed of movement of the low pressure system. 


&& 


Discussion... 
early morning satellite pictures showing an area of high cloudiness 
moving into the phx County Warning Area from the far eastern Pacific. Meanwhile...a weak 
backdoor cool front is pushing westward through the eastern part of the state 
early today. This feature is accompanied by a tightening surface 
pressure gradient...which will make for breezy to locally windy 
conditions this morning over parts of south-central Arizona...especially 
south and east of the kphx. Gradients will relax by afternoon for 
lessening winds. Otherwise...high pressure aloft is prognosticated by 
models to actually rebuild over the region the next few days in 
response to an expected amplification of trough/ridge features over 
our portion of the hemisphere. Because of the amplifying ridge...we 
can expect continued dry and unseasonably warm weather through about Tuesday... 
at which time it will move east of the region. 


By midweek...GFS and European model (ecmwf) paint a different pictures for the regional 
weather...especially with regard to timing of features. The GFS is quite 
a bit faster and brings precipitation into the County Warning Area by as early as Tuesday 
..whereas the slower Euro holds off until Wednesday. Given the buckling 
nature of the flow pattern around the eastern Pacific/western US next week... 
along with the fact the Euro seems to have the best run-to-run 
consistency...will go along with its speed and timing of the 
expected precipitation event. The folks at HPC also concur with this idea. 
Therefore...will ignore probability of precipitation that mex MOS introduces as early as Monday 
night and Tuesday over western County Warning Area...and will instead have probability of precipitation over 
most of County Warning Area by Wednesday into Thanksgiving day. Obviously...the timing of 
these events are still in doubt...so we may need to adjust as the 
event gets closer. Will be issuing an Special Weather Statement to give travelers and 
those with outdoor interests a heads up what with all the upcoming 
Holiday activities coming up. In addition to the threat of precipitation... 
will also bring a considerable amount of cooling to the County Warning Area by 
the Thanksgiving Holiday...with maximum temperatures falling to near seasonal 
norms by then. 


&& 


Aviation... 


Low level wind shear expected this morning.. 


Kphx/kiwa and surrounding airfields... 
light easterly surface winds with periods of high clouds expected 
through 17 UTC. However...winds from about 1 to 3k feet above ground level will be 
much stronger...on the order of 25 to 35 knots...and could cause issues 
with low level wind shear until about 18 UTC when surface winds are 
expected to increase to around 12g22kt. Winds will stay out of the 
east all day...decreasing in strength after 23 UTC. 


Kipl/kblh/knyl and surrounding airfields... 
scattered-broken high clouds and relatively light winds expected with 
continued VFR conditions and no aviation concerns to speak of at 
this time. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Saturday through Wednesday... 
as an upper level low pressure system develops off the California coast...we 
will begin to see changes in the weather pattern. Cloud cover will 
increase...temperatures will begin to cool into the 70s and the 
chances of precipitation will begin to increase starting in 
southeastern California then spreading eastward into the southwestern Arizona 
deserts by Wednesday morning. As a result...humidity levels will 
begin to trend upward. Winds will remain light for the beginning 
portion of the week. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...estle 
aviation...wanek 
fire weather...paddock