Area forecast discussion 
National Weather Service Phoenix Arizona 
452 am MST Friday Oct 31 2014 

Update...updated aviation discussion... 


another day of well above normal temperatures is expected today 
under mostly sunny skies. Significantly cooler readings are then 
expected for the weekend... as a low pressure system and cold front 
push through the Desert Southwest. Highs in the lower deserts will 
drop into the upper 70s or low 80s on Saturday and fall to the lower 
70s on Sunday. In addition to the cooler temperatures...periods of 
breezy conditions can be expected throughout the weekend. Rain 
chances will be slight and confined mostly to the higher terrain 
east of Phoenix. A gradual warming trend will begin Monday into the 
middle of next week. 


strong upper level ridge that led to record high temperatures in 
some of the warmer desert locales yesterday continued to shift to 
the east early this a deep and cold upper trough 
approached the West Coast. A fair amount of cold air cumulus was visible 
on infrared imagery just off the Pacific northwest/northern California coast...and when this cold 
system passes through Arizona this weekend we can expect high temperatures 
to fall 20 degrees or more over the lower deserts. Upper ridge axis 
has already shifted east of Phoenix with the axis moving into New 
Mexico early this morning...and the increasing southwesterly flow aloft will 
lead to some breezy conditions this afternoon over southeastern California and the 
western Arizona deserts. At 3 am variable amounts of high clouds were 
pushing east across the deserts...but we should see some clearing 
this morning...with skies becoming partly to mostly sunny. Low level 
winds backing ahead of the approaching trough will pull some moisture 
into far eastern Arizona this afternoon...and 850mb dewpoints will 
approach 8c across the higher terrain east of Phoenix. As such we 
have a slight chance for thunderstorms mainly east of Globe 
developing into the afternoon and early evening hours. As far as 
temperatures should be the last 90 degree day that we see this 
calendar year. 850mb temperatures will stay elevated...and will only drop 
off by a degree or so over the central deserts. As such we will call 
for low 90s again over the warmer western and central deserts with a 
high of 92 forecast for Phoenix. 

Model guidance...including GFS ensemble members...remain consistent 
in calling for the deep and cold trough to push across Arizona this 
weekend...bringing much cooler temperatures...a slight chance for showers 
across south central Arizona...and breezy/locally windy weather on 
Saturday. The trough will move through as a series of waves...with the 
first wave quickly racing northeastward and across the state 
Saturday. The first cold front Saturday will lead to about 10 
degrees of cooling to go along with breezy or locally windy 
southwest winds. Moisture and dynamics will be somewhat limited so 
precipitation will be confined to areas mainly north and east of 
Phoenix...across the higher terrain...and probability of precipitation will stay mostly 
below 15 percent. The second...and stronger...wave moves across 
the state Sunday into Sunday night...and the GFS actually develops a 
closed upper low that treks across northern Arizona during the day. Stronger 
dynamics are being the 300 mb jet dives further to the 
south putting south central Arizona under the favored front left quadrant of 
the jet. impressive bullseye of middle level q convergence is 
forecast to push across much of central and northern Arizona. With temperatures 
being much cooler relative humidity values will be higher...and it will be easier 
to wring smaller amounts of quantitative precipitation forecast out of the atmosphere. As such we 
raised probability of precipitation into the 15-20 percent ballpark over south central Arizona on 
Sunday...including the Lower Desert areas. High temperatures are expected 
to drop another 10 degrees or so with highs in the greater Phoenix 
area to fall to near 70 degrees. Those numbers would represent highs 
more than 10 degrees below seasonal normals. 

By Monday morning the trough axis will have shifted southeast of the 
greater Phoenix area...putting most of the County Warning Area under a subsident 
northwest flow aloft. Moisture will continue to thin...with skies to 
become mostly sunny by early afternoon over all but the high terrain 
of southern Gila County. Little change in high temperatures is 
cooler air continues to filter over the lower deserts. The forecast 
high for Phoenix Monday will be just 72 degrees. 

Dry north to northeast flow aloft continues across the area in the 
wake of the departed upper trough on Tuesday...for generally sunny 
skies. High temperatures will rise a few degrees as heights/thicknesses 
start to climb...but overall temperatures will stay below seasonal normals 
with Phoenix sitting in the middle to upper 70s. 

Quite a bit of model uncertainly developing for the latter portion 
of the extended period. GFS keeps high pressure aloft in place 
across the Desert Southwest...and develops a closed low well off to 
the southeast...over portions of Texas and northern Mexico. On the other 
hand...the European model (ecmwf) develops that same upper low much further to the 
west...and actually moves the low center into southeastern Arizona by Wednesday 
afternoon. The low starts to advect quite a bit of moisture into 
southeastern Arizona...some of that moisture tropical in nature. Very low 
confidence in the European model (ecmwf) solution as model run to run consistency has 
been very low. For now...will keep a dry forecast in place across Arizona 
for Wednesday and Thursday...but will keep some single digit probability of precipitation in 
place from Phoenix eastward...and add in some cloud cover as a nod to the model uncertainty. 


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

Periodic middle and high clouds to affect the region through the period 
as low pressure approaches. Winds across the Phoenix area are 
expected to remain out of the east for the majority of the period 
with the possibility of going variable or switching out of the west 
for an hour or two late this afternoon. Winds across southeast 
California will become breezy late this afternoon and evening with 
gusty westerly winds at kipl likely persisting through the overnight 
hours tonight. 

Aviation discussion not updated for amended tafs. 


Fire weather... 
Sunday through Thursday... 
slightly cooler than normal temperatures will filter into the 
district the first part of next week...before moderating the latter 
half of the week. Occasionally gusty afternoon northwest and west 
winds will be common Sunday and Monday...with gusts to around 25 miles per hour 
possible along ridge tops and through favored River Valley 
locations. Minimum relative humidity values will initially hover in 
a 20-40 percent range Sunday but will fall into the teens the 
remainder of the week as drier air gradually overspreads the area. 
Otherwise...overnight recovery will be good with only minimal 
chances of wetting rains over higher terrain early next week. 


Psr watches/warnings/advisories... 





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