Weather

919 
fxus65 kpsr 101649 
afdpsr 


Area forecast discussion 
National Weather Service Phoenix Arizona 
948 am MST Friday Oct 10 2008 


South-central and southwest Arizona...and southeast California 
forecast discussion 


Synopsis... 
an intense upper level low pressure system will move across the 
southwest states today and Saturday ushering in windy and much 
cooler weather. Moisture entering the region from Mexico will also 
give a chance of showers to parts of south-central Arizona tonight 
through Saturday...mainly from the Phoenix area eastward. There will 
be less wind on Sunday as the low pressure system weakens...with 
cool temperatures remaining. Higher pressure aloft next week will 
result in dry and warmer weather. 


&& 


Discussion... 
weather across the region is now rapidly changing as the well 
advertised Pacific northwest trough dips into the southwest. Overall models 
appear to be handling the situation relatively well and forecast 
trends are still looking on target. 


One noticeable change though is that winds will be stronger today 
than previously thought as the 500 mb system is moving in a bit quicker. 
NAM/GFS both develop a 30-40kt SW/NE 850 mb jet this afternoon over 
Pima/Pinal/Gila counties. Those areas could flirt with windy 
advisory criteria this afternoon and evening /sustained 30 miles per hour or 
gust 40 miles per hour/ but feel that it will only be isolated areas and 
mountain tops that actually reach criteria and will therefore hold 
off. We did add areas of blowing sand/dust to SW Maricopa and northwest 
Pinal counties this afternoon...and most areas along and west of the 
lower Colorado River for this afternoon into the overnight hours. Will 
also have to ascertain how strong winds will be across southeast California this 
evening/tonight as frontal passage will enhance already strong winds there. A 
windy advisory may be req/d. 


Will have to look at windy advisory and Fire Weather Watch that is 
currently in effect for Saturday. Faster progression of the system 
may indicate that winds will be a bit weaker across Maricopa/northwest 
Pinal counties than previously indicated and may be slightly below 
Wind Advisory/red flag criteria. Decision on these two products will 
be made with afternoon package. Will also fine-tune probability of precipitation for tonight and 
tomorrow. Still appears that most of the precipitation will be out of our 
County Warning Area by Sat evening. 


Web forecast updated...zones out shortly. 


Previous discussion... 
WV imagery showing the strong trough developing over the western US as 
upper ridge builds over the eastern Pacific near 140w. Models in very good 
agreement now with the expected intensifying and moving of this 
trough/upper low over the next couple of days. As trough drops southward... 
it will begin to tap some of Norbert/S low/middle level moisture in 
developing southeasterly ageostrophic flow. Dynamics will be strong enough to 
lift this moisture and give increasing threat of showers to our far 
eastern County Warning Area later tonight and through much of Saturday. It appears that the 
greater phx area will be on the far western edge of precipitation shield...with 
most of the significant quantitative precipitation forecast amounts staying over zone 24. Could 
easily see one quarter to one half inch of precipitation from about Globe 
eastward with this event. By Saturday afternoon...very dry air will wrap 
northeastward into the low circulation...thereby cutting off precipitation over all 
but extreme eastern County Warning Area by Saturday afternoon. 


To be sure...wind will be the main weather feature to contend with 
next 24 to 48 hours. As height/pressure gradients tighten across the 
region with the low to our north...we should see a marked increase in 
wind speeds by afternoon...especially over higher terrain areas of 
far western County Warning Area. Decided to hoist a Wind Advisory for Joshua Tree np 
from 21z-04z. And...since humidities will be near 15 pcnt have also 
issued a red flag warning same area today. On Saturday as the cold 
front sweeps across the area...we should see strongest wind speeds 
shift to scntrl Arizona zones...with some gusts to near 40 miles per hour possible. 
Wind Advisory has therefore been issued for these areas late Sat 
morning into early Saturday evening. In addition...much drier air 
advecting into the region on Saturday will give localized surface 
humidities less than 15 percent over the entire County Warning Area except for zone 24. 
Have issued Fire Weather Watch to account for this. 


By Sunday...the upper low will begin lifting northeastward into the northern 
rockies...leaving behind general troughiness across the area that 
will persist through Monday. Will be breezy on Sunday...but much 
less wind than Saturday. Main focus area for wind will be down the 
lower Colorado River valley both Monday and Tuesday. Much cooler airmass 
that will settle in over the region over the weekend will gradually 
modify back to near or slightly above normal values by the middle of 
next week under higher pressure aloft and increasing thicknesses. 


&& 


Aviation... 
kphx and kiwa airfields...clear skies and light surface winds expected 
through 16z...with much stronger winds of 20-30 kts through 2 thousand feet 
above ground level. Local areas of low level wind shear of 20 kts is possible this 
morning. Increasing moisture is also underway...dew points ramping 
up across most of southern Arizona...will lead to scattered clouds...occasional broken 
in eastern portions. Slight chance of -shra to the east of kiwa after 
0200z. Sharp cold front expected to sweep through the Phoenix area 
Saturday for stronger west winds...but not confident enough to 
introduce any blwg dust and lower visibilities in kphx or kiwa tafs. 


Southeast California and the Colorado River valley...including kipl...knyl 
and kblh...a sharp cold front...near Las Vegas at 12z will sweep 
into the southeast California area by this afternoon...and strong 
gusty surface winds...possibly some patchy blwg dust...is expected. 
Mostly clear skies are also forecast this period. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
an approaching cold front will lead to windy conditions this 
afternoon and evening...especially in southeast California. The 
combination of winds...low humidities...and fuel conditions has 
prompted the issuance of a red flag warning for Joshua Tree National 
Park from 2 PM to 9 PM. The dry front is expected to move from 
southeast California late tonight and across southwest and 
south-central Arizona Saturday. Thus a Fire Weather Watch is in 
effect for all zones during the day Saturday except az133...southern 
Gila County and far northeast Maricopa County. Az133 is anticipated 
to have high enough humidities to avoid critical thresholds. In fact 
there will be a slight chance of showers and thunderstorms in that 
zone today and tonight...increasing to a chance Saturday. There will 
also be a slight chance of showers in far eastern portions of 
az132...specifically the Phoenix metropolitan area and northwest Pinal 
County...tonight and Saturday morning before the front pushes 
through and brings in much drier air for the afternoon sufficient to 
produce critical thresholds. 


Sunday through Tuesday... 
winds will decrease in all zones on Sunday as the upper low moves 
through The Rockies and high pressure filters into California. Low 
relative humidity is expected to continue Sunday into Tuesday...mainly across the 
deserts. Generally clear skies will prevail during the period. Winds 
are expected to remain below critical fire weather thresholds. 


&& 


Psr watches/warnings/advisories... 


Arizona...Fire Weather Watch from Saturday morning through Saturday 
afternoon for azz131-132. 


Wind Advisory from 10 am to 6 PM MST Saturday for 
azz022>024-027-028. 


California...Fire Weather Watch from Saturday morning through Saturday 
afternoon for caz230>232. 


Red flag warning from 2 PM this afternoon to 9 PM PDT this 
evening for caz230. 


Wind Advisory from 2 PM this afternoon to 9 PM PDT this evening 
for caz030. 


&& 


$$ 


National Weather Service Phoenix is on the internet at 


Weather.Gov/Phoenix 


Discussion...iniguez/estle 
aviation...Sipple 
fire weather...Sipple 

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