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Area forecast discussion 
National Weather Service Phoenix Arizona 
442 am MST Thursday Nov 20 2008 


Update...updated aviation and fire weather discussions. 


&& 


Synopsis... 
high pressure aloft will continue to weaken today under mostly sunny 
skies. By Friday and Saturday...some high level clouds will drift 
into the area...but temperatures should remain above the seasonal 
normals. By the middle of next week...a major change in the pattern 
will lead to cooler weather...along with a chance for showers. 


&& 


Discussion... 
strong upper level high pressure ridge...which has produced several 
days of near record warmth over our lower deserts...finally starting 
to weaken. 00z plot data showed 20-30m height falls...with heights 
down to around 582dm. Airmass continues to be very dry this morning 
with precipitable water values below 0.10 inches...and 2 am surface dewpoints over 
the lower deserts ranged from upper teens to middle 30s. Infrared imagery 
early this morning showed clear skies across the area...with areas 
of high clouds spreading into central California ahead of a low pressure 
system approaching the Pacific northwest coast. 


As a series of shortwaves move across the northern tier of 
states...the ridge over Arizona will continue to weaken and flatten...and 
this will allow increasing high clouds to overspread the area. In 
addition...thickness values are forecast to slowly lower about 
10-20m each day...leading to a steady falloff in maximum temperatures today 
through the weekend. Still...highs will remain well above seasonal 
normals. Forecasts indicate a baggy trough will develop off northern Baja California 
Friday...then move slowly eastward with time...but this feature will 
do little more than help spread high cloudiness ahead of it and into 
Arizona. The increasing high clouds will lead to partly cloudy skies from 
time to time...instead of clear skies that we have seen the past few 
days. 


For the extended period...Monday through Wednesday...the GFS and 
European have come into surprising agreement with the forecast 
solution and as such confidence is growing that we will be seeing a 
significant change in the weather pattern by the middle of next 
week. Both forecasts forecast an upper low to develop off the Southern California 
coast by Monday afternoon...and this low is then forecast to slowly 
push east and approach the California/northern Baja California coast by Wednesday. No 
weather is expected with this system Monday through early Tuesday...but 
we should see variable middle/hi clouds spread across the area ahead of 
the low. By Tuesday afternoon...GFS advertises a frontal band to move 
into southeastern California ahead of the main low...with quantitative precipitation forecast spreading into our 
western County Warning Area. Have added slight chance probability of precipitation from the Colorado River west on 
Tuesday...then spread the threat for showers into the southwestern Arizona 
deserts overnight Tuesday night. Although the upper low is forecast to 
remain just offshore through Wednesday afternoon...it looks as if a 
substantial subtropical moisture tap will develop and spread 
considerable moisture into our County Warning Area ahead of the main low on 
Wednesday. As such have raised probability of precipitation quite a bit Wednesday...into the 20-30 
percent range...for the entire County Warning Area. Also lowered maximum temperatures several 
degrees to near or a bit below seasonal normal values. It appears 
that the most significant precipitation and cooling will occur beyond the 7 
day forecast period...when the main low finally moves inland across 
Arizona. 


&& 


Aviation... 
high cirrus clouds will begin to spread over the phx County Warning Area through 12z 
Friday. Winds will remain light...as the County Warning Area is still under the 
influence of high pressure. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Saturday through Thursday... 
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...cumulonimbus 
aviation...paddock 
fire weather...paddock