Weather

748 
fxus65 kpsr 060351 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
850 PM MST Monday Jan 5 2009 


Synopsis... 
leftover moisture is expected to result in some patchy fog in the 
higher terrain east of the greater Phoenix area tonight and Tuesday 
morning. A weak low pressure system from the northwest will bring 
cloudiness across the area tonight mainly this evening with a slight 
chance of showers over the higher terrain east of the greater 
Phoenix area. Otherwise...higher pressure aloft from the Pacific 
will bring dry weather tonight through at least Thursday with warmer 
weather in southeast California and far southwest Arizona Tuesday... 
and across the entire area Wednesday and Thursday. Another passing 
low pressure system is expected to bring temperatures back down 
somewhat Friday and Saturday...but it is expected to remain dry. 


&& 


Discussion... 
clouds were on the increase in Arizona as another cold shortwave trough 
drops rapidly south from Utah. These southward moving systems are 
always a concern due to a quick loss of planetary vorticity...which 
means the relative vorticity (cyclonic circulation) must increase 
or intensify the system. In this case however...500 to 300 mb 
height rise and fall Couplets stretching from the Pacific northwest 
..to Utah/northwest Colorado suggest the system dynamics will move 
just to the east of our forecast area (southeast California to southwest and 
south central az)...in other words a brush-by. 


However what concerns US is over-running cloudiness moving into 
northwest Arizona at 00z. Although there is little upper dynamics 
expected over the forecast area...over running clouds (seen vividly 
on the dra balloon sounding) suggest a few showers over the 
mountainous terrain east of Phoenix...or over our zone 24 southern 
Gila County. Updates in zone 24 for a slight chance of showers 
tonight with the snow level near 5000 feet have been sent. 
Otherwise... the remainder of forecasts look OK. 


Previous forecast discussion below for dry weather Tuesday and 
beyond looks good. No further updates necessary. 




Previous discussion... 
a weak low pressure system from the northwest is expected to bring 
overrunning cloudiness across our area tonight mainly this evening 
with a slight chance of showers over the higher terrain east of the 
greater Phoenix area. With the leftover moisture...there is also a 
possibility of patchy fog there tonight and Tuesday morning which 
has been mentioned in the latest forecast. 


Otherwise...higher pressure aloft from the Pacific will bring dry 
weather tonight through Thursday with warmer weather in southeast 
California and far southwest Arizona Tuesday...and across the entire 
area Wednesday and Thursday. 


Previous discussion... 
Friday into Saturday...medium range forecasts also agree that the ridge 
will flatten as another Pacific shortwave moves southeast and across 
the state. It appears to be a rather dry system for US...and mex MOS 
probability of precipitation for our stations remain in the single digits. We will continue 
to have climatology probability of precipitation in our forecast for this time period just in case 
the system ends up a bit wetter than expected...but for now will 
keep the upper end of those probability of precipitation just below 10 percent. We will see 
a handful of degrees of cooling Friday into Saturday...per the 
associated thickness and height falls with the system. Higher 
pressure will return Sunday for mostly sunny skies and slightly 
warmer maximum temperatures. 


&& 


Aviation... 
south-central Arizona including kphx and kiwa. Middle level broken 
cloudiness based near 12 thousand mean sea level will develop over the area through 
08z Tuesday...with clearing afterward through at least 02z Wednesday. Light 
surface wind. 


For kipl and kblh...light winds...generally less than 8 kts...and 
mainly clouds above 12 thousand feet are expected. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Wednesday through Sunday...drier conditions and a gradual warming 
trend are on tap as high pressure builds over the region. Daytime 
humidity levels will trend down...dropping into the 20s by next 
weekend. Winds will be light...with weak drainage flows at night 
through the period. By Friday afternoon...winds increase and become 
gusty...into the 10 to 20 miles per hour range...mainly along portions of the 
lower Colorado River valley and higher terrain. Less wind is 
expected Saturday...but drier air will overspread the area and force 
daytime humidity levels into the low 20s and upper teens at many 
locations. 


&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...vasquez/Ellis/cb 
aviation...vasquez 
fire weather...paddock