Area forecast discussion 
National Weather Service Phoenix Arizona 
435 am MST Wednesday Apr 23 2014 

..updated aviation and fire weather... 

the weather disturbance that brought gusty winds and areas of 
blowing dust to the region on Tuesday is expected to bring cooler 
temperatures today as it moves off to the east. Warmer temperatures 
will make a return Thursday and Friday. Another...much stronger low 
pressure system will then affect the region from Friday Onward into 
the upcoming weekend. Breezy to windy conditions can be expected 
along with much cooler temperatures on Saturday. There is also a 
chance of showers late Friday night into Saturday night...mainly for 
the higher terrain of south-central Arizona. High pressure building 
back northward over the western US is then expected to bring warmer 
temperatures for the early and middle parts of next week. 


Wednesday and Thursday... 
the dry cool front that brought our County Warning Area gusty winds...patchy blowing 
dust...and high fire danger on Tuesday has finally pushed into 
southeastern Arizona this morning. The main impacts from this front today 
will be somewhat cooler temperatures and lower dewpoints...along 
with some gusty afternoon breezes across southwest and south-central 
Arizona. Current forecast soundings for the phx area are indicating that 
850mb temperatures will drop from yesterday's 22c to around 17c by this 
afternoon...which should result in about 8-10 degrees of cooling at 
the surface...with highs remaining mostly in the middle to upper 80s 
today...which is now right around normal for this time of year. 
Nighttime lows tonight should also be noticeably cooler...with most 
outlying Lower Desert locations seeing lows in the 50s under mostly 
clear skies. Weak ridging ahead of the next upper trough is then 
expected to bring somewhat warmer temperatures on Thursday...with 
Lower Desert highs pushing back into the low to middle 90s. 

Friday through Sunday... 
the next Pacific system that will be affecting our area from Friday 
Onward into the weekend can already be seen on latest infrared satellite 
imagery just beginning to move southward from the Gulf of Alaska 
into the eastern Pacific. The latest GFS...Euro...and Gem have now 
come into very good agreement that this next system will be the 
strongest system that we have seen for a while. They all now move a 
rather robust...and cold upper low inland over central and Southern 
California by late Friday evening and early Saturday...then across northern 
Arizona during the day on Saturday...and into New Mexico by Sunday 

The models are all now showing very sharp height falls and 
temperature drops as this system moves through...with 500mb heights 
over the phx area falling from around 572dm on Friday afternoon to 
around 550-552dm by Saturday afternoon...with 850mb temperatures 
falling from around 19c to 7c during the same period. Although there 
will likely not be a whole lot of moisture associated with this 
system...with precipitable waters  only briefly rising into the 0.75-0.85 inch 
range...the impressive dynamics and cooling that this system will 
possess will likely produce at least some shower activity during the 
day on Saturday across south-central Arizona along and just ahead of the 
main front. However...the biggest impact from this system will 
likely be the winds...with widespread advisory-level winds possible 
on Saturday as efficient mixing will likely allow 35-40kt 700mb 
winds to be brought all the way down to the surface...with blowing dust 
likely to be a problem as well. After another day of 90+ degree 
temperatures on now look likely that highs will remain 
well down in the 70s on Saturday. Sunday looks like it will be a 
much quieter day as the system moves off rapidly to the east...with 
highs rebounding into the low 80s across the lower deserts under 
mostly clear skies...with just some afternoon westerly breezes. 

Monday through Wednesday... 
although there are differences between the models...with the Euro 
being the slowest/coolest and the Gem being the 
fastest/warmest...they all are attempting to rebuild ridging back 
northward over the region for the early and middle part of next 
week...with temperatures warming back up well into the 90s under 
mostly clear skies. However...widespread triple-digit temperatures 
do not appear likely at this time...since long-wave troughing over 
the central Continental U.S. Should keep our upper-level flow a northwesterly 


south-central Arizona including kphx...kiwa...and ksdl... 
no significant aviation concerns through Thursday morning...with 
timing of wind shifts the primary forecast challenge. With fairly 
good confidence...a period of variable surface winds will prevail this 
morning despite a persistent west 5-10kt winds 2k-4k above the 
ground. Eventually late this morning...westerly winds will mix to the 
ground. There may be a period of a few higher gusts up to 
20kt...though these conditions should be more temporally limited. 

Southeast California/southwest Arizona including kipl and kblh... 
surface winds will eventually veer to a more northerly direction this 
morning...and confidence is fairly good that a few higher gusts will 
materialize by late morning (particularly at kblh). No other 
aviation concerns as the pressure gradient weakens by late 
afternoon...and surface winds fall back to around 5kt and become more 
westerly in direction with nocturnal decoupling. 

Aviation discussion not updated for amended tafs. 


Fire weather... 
Friday through Tuesday... 
a very strong upper level low will approach the area on Friday and 
move through the districts on Saturday. Ahead of this weather 
system...winds will increase and become frequently gusty Friday 
afternoon and evening. The strongest winds Friday will be 
experienced across southeast California and the lower Colorado River 
valley...and its possible critical thresholds may be met. 
Temperatures on Saturday will plummet to well below normal 
levels...and humidity levels will increase (along with a possibility 
of light rain). This may otherwise limit a much higher fire 
danger...because winds Saturday across the entire region will be 
unusually strong for much of the day (likely gusting at or above 40 mph). 
Cooler weather will linger Sunday with a warming trend early next 
week. As temperatures recover next week...afternoon humidities will 
once again drop into the single digits with only fair overnight 


Psr watches/warnings/advisories... 





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