Area forecast discussion 
National Weather Service Phoenix Arizona 
950 am MST Thursday may 23 2013 


Synopsis... 
a low pressure center over the Pacific northwest is expected to keep 
the region under dry southwesterly flow aloft...with breezy to windy 
conditions at times the rest of this week and into early next week. 
Anticipate some modest cooling today with temperatures remaining 
near seasonal normals through the weekend. Additional cooling is 
likely next week with highs across the lower deserts in the upper 
80s to middle 90s. 


&& 


Discussion... 
the upper low centered over the Pacific northwest continues to have 
vorticity lobes rotating around it. The next one is rounding the bottom 
of the trough that the low resides in with the axis of it near the 
California coast this morning. The bulk of it will pass to the north 
of Arizona. Water vapor imagery indicates a small perturbation 
moving into Arizona ahead of the next vorticity lobe which may help 
enhance breeziness temporary today in portions of southern Arizona 
but overall will have little noticeable impact. 12z plot data shows 
winds at 850 mb were lighter than yesterday but at 700 mb they were 
stronger. Net effect should be a little less wind than 
yesterday...more noticeably over southeast California. Anticipate 
high temperatures today to be a little cooler than yesterday...more 
noticeably over southeast California. Made some adjustments to dew 
point/relative humidity grids for today but otherwise no changes at this time. 


&& 


Previous discussion issued 258 am... 
the main weather feature remains far removed from the Desert 
Southwest...but will have some minor impacts over the coming days. 
Breezy to windy daytime conditions remain to be the biggest impact 
with some locations yesterday gusting to near 40 miles per hour. Pressure 
gradients today will still be on the higher side...but not quite 
like yesterday and Friday will be lesser than today. Strongest winds 
today are expected to be across southern Gila County with afternoon 
gusts approaching 35 miles per hour. The dry airmass in place will persist into 
the weekend...but an increase in middle and upper level moisture by 
Saturday night should lead to increasing high cirrus. Temperatures 
over the next several days will change very little...but will be 4-7 
degrees off yesterday/S highs due to the slight cooling influence of 
the deep closed low over Pacific northwest. The blocking upper level 
pattern with the Pacific northwest low and the high amplitude ridge 
over the plains into southern Canada will start to break down by the 
weekend. However...reinforcing shortwave impulses dropping down from 
the northern Pacific will keep a large scale troughing pattern 
across the western United States for the foreseeable future. This 
will keep the Desert Southwest under a modestly warm southwesterly 
flow through the weekend...with temperatures remaining around or 
slightly below seasonal normals. 


Uncertainty increases after this weekend with previous model runs 
differing in the progression of a stronger northern Pacific system. 
Latest model guidance is seems to be in better agreement...but 
confidence is far from high. A strong upper jet maximum is forecast to 
approach the West Coast next Monday...but weakening as it does. Even 
with the depicted weakening...both the GFS and European show a 
developing 500mb low diving southeastward into the Desert Southwest 
Monday night into Tuesday. Both models generally agree with the 
intensity of the system with slight timing differences. Based on the 
overall good agreement now...feel that a cooler first half of next 
week should occur. Have updated temperatures for Tuesday and 
Wednesday next week...lowering highs anywhere from 3 to 10 degrees. 
A decent amount of middle and high level moisture should accompany the 
system...but for now only expecting a couple days of partly to 
mostly cloudy skies with chances for any precipitation less than 10 
percent. 


&& 


Aviation... 
south-central Arizona...including kphx...kiwa...ksdl...southeast 
California/southwest Arizona...including kipl and kblh... 


Dry southwest flow aloft will keep skies clear to mostly clear 
through the taf period. Gusty winds are expected to redevelop at the 
terminals this afternoon...but with speeds remaining lighter than 
what we saw yesterday. Winds will then diminish later this 
evening...and revert back to the usual diurnal trends in direction 
at the phx area terminals. 


Aviation discussion not updated for amended tafs. 


&& 


Fire weather... 
Saturday through Wednesday... 
low pressure is expected to remain along the West Coast from 
Saturday Onward into Monday...then move inland on Tuesday and 
Wednesday...keeping Arizona and southeast California under a dry 
southwest to west flow aloft through the period. Breezy to windy 
conditions will develop during the afternoon and evening hours each 
day...with peak wind gusts in excess of 25 miles per hour possible. Single 
digit minimum humidities will continue through Monday...with 
slightly higher values on Tuesday and Wednesday. Overnight recovery 
will be poor over south-central Arizona and poor to fair over 
southwest Arizona and southeast California. High temperatures will 
remain near or a bit below seasonal normals from Saturday Onward 
into Monday...with somewhat cooler temperatures possible on Tuesday 
and Wednesday as the low pressure moves inland into the Desert 
Southwest. 




&& 


Psr watches/warnings/advisories... 


Arizona...none. 


California...none. 


&& 


$$ 


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Discussion...aj 
previous discussion...Kuhlman 
aviation...percha 
fire weather...percha