Area forecast discussion 
National Weather Service Phoenix Arizona 
205 PM MST Tuesday Sep 23 2014 

high pressure and a moderately dry airmass is expected to keep skies 
mainly clear over the region through Thursday...with just a slight 
chance for showers and thunderstorms over the higher terrain east of 
Phoenix late Thursday. A Pacific weather system will bring a return 
of storm chances to most of our region from Friday into 
Sunday...along with cooler temperatures. Dry conditions...along with 
near...or slightly below normal temperatures are then on tap for 
early next week. 


tonight through Thursday... 
persistent westerly flow...along with high pressure aloft has 
allowed drier air that has been holding over Southern California eastward 
across the northern 1/2 of Arizona that last few push 
southward across all of our County Warning Area this morning...with precipitable waters  at most 
locations now at or below 1.00 inch. The combination of this drier air and 
ridging aloft is now keeping skies clear across most of the 
region...with just some cumulus now forming across the higher terrain of 
southern Gila County. Other than perhaps an isolated shower or two 
over the highest now appears...especially if the latest 
hrrr hi-res model is correct...that the vast majority of our County Warning Area 
should remain dry today. Further drying in the column as ridging 
aloft continues to strengthen is expected to lead to dry conditions 
on Wednesday...and through most of Thursday as well...with 
temperatures remaining above the 103-108f range across 
the lower deserts. Slight chances for showers/thunderstorms return 
to the higher terrain east of Phoenix on Thursday as the flow aloft 
becomes more southerly as the next upper trough approaches the region 
from the west...and begins to pull a bit of moisture back northward 
into that region...with precipitable waters  climbing back above 1.00 inch. 

Friday through Sunday... 
although it still appears that rainfall chances will return to the 
Desert Southwest as a rather deep upper trough approaches from the 
west...both the GFS and the Euro now take the parent upper low 
center on a more northerly track...from central California into the 
central/northern Great Basin...with the GFS now falling in line with 
the slower Euro. now appears that the best chances for 
measurable rainfall will be on Sat across southeast California and SW Arizona...and from 
late Sat into sun across south-central Arizona. Given the more 
northerly track of the the parent upper low center...the latest GFS 
model suite is bringing less cape and lower precipitable water values into the 
region...with precipitable waters  now just briefly exceeding 1.50 across 
south-central Arizona on Saturday and maximum convective available potential energy remaining mainly at or below 800 
j/kg...with the Euro forecasting even lower values. However...there 
still appears that there will be at least enough moisture/dynamics 
to produce at least scattered showers/thunderstorm activity across 
the region over a somewhat longer period of time. Thus...have 
increased probability of precipitation a bit over SW Arizona early Sat and over south-central Arizona 
on late Sunday to reflect the slower movement of the system...but 
have kept maximum pop values in the "chance" range for now. The 
combination of cooler air aloft and increased cloudiness is expected 
to keep temperatures down as well over the weekend...with highs not 
getting out of the 90s at most Lower Desert locations on Saturday 
and Sunday. 

Monday and Tuesday... 
the Euro and GFS are both now forecasting westerly flow aloft to 
develop across the region as the aforementioned upper trough lifts out 
to our north and east...with the main long-wave upper trough remaining 
over the Pacific northwest...with dry conditions and near...or slightly 
below normal temperatures to prevail through the period. 


Previous discussion... 
today through Thursday... 
water vapor imagery shows an upper low continuing to shift eastward 
onto the Great Plains and ridging becoming more consolidated over 
the interior western states. Upstream is a deepening trough within 
which is a well developed low centered off the Pacific northwest 
coast. Blended tpw imagery shows moisture hanging on over mainly 
Southeast Arizona. Early morning metars indicate a bit of a reversal 
in the dew point trends. Metars also show and easterly oriented 
pressure gradient over portions of New Mexico and Arizona associated 
with remnants of a back door front. Models continue to show a ridge 
building over over the southwest with the middle level anticyclone 
becoming centered near the Arizona/Utah border by Wednesday. What is left 
of meaningful moisture over our forecast area will trend down more 
as northerly flow aloft advects some drier air. However...models 
indicate that there will still be enough moisture by midday/early 
afternoon over zone 24 to get isolated storm activity. So held on to 
slight chances there. Wednesday looks to be the driest day with no 
storm chances forecast for our area. By Thursday...a Pacific trough 
deepens along/near the West Coast which begins a northeastward shift 
of the ridge. This also allows a for an inverted trough over Texas 
to shift westward a little bit. With more easterly flow...some 
moisture expands westward from Southeast Arizona but at the same 
time...some drier air from the north temporarily mixes in as well. 
The net effect is a return of slight chance probability of precipitation to zone 24. As for 
temperatures...anticipate a little more warming today and Wednesday 
with little change Thursday. 

Friday through Monday... 
on Friday...GFS and European model (ecmwf) show the Pacific trough deepening and 
moving a little further inland with the axis along the Pacific 
northwest/northern California coast and extending southward. In the 
process...flow becomes more southerly and deeper moisture begins to 
spread northward over our forecast area. The European model (ecmwf) is still a bit 
deeper and slower than the GFS by Friday. The GFS has trended toward 
European model (ecmwf) in terms of deeper moisture...per 1000-700 mb mean mixing spreading it into California. Conversely GFS also shows 
a dry pocket over Southeast Arizona Friday. Over the weekend...the 
timing of the eastward movement of the trough gets closer between 
the GFS and European model (ecmwf) as the system slows down and becomes a closed low 
centered over the Great Basin. They both continue to show the brunt 
of the system and the jet axis passing north of our forecast 
area...though the GFS is a bit further south than in some previous 
runs. Qg forcing per divergence q field begins in our western areas Friday 
night and tracks eastward during the day Saturday. However...the 
system begins tracking northeastward during the day Saturday for 
more of a brush-by of the divergence q for our eastern areas. Also on 
Saturday...a weakening cool front moves across the forecast area. 
Another potential source of dynamical lift will be upper level 
divergence which looks to affect mainly eastern portions of our 
forecast area...better to our north. Theta-E advection brings brings 
notable cape into play. Deep layer shear looks like it will become 
strong enough to get organized storms. 

Best window of opportunity of opportunity of showers and 
thunderstorms for our forecast area looks to be late Friday through 
Sunday morning as things transition from west to east. There are of 
course caveats as it is far from certain just how the system will 
play out in terms of timing...track...and intensity. This in turn 
affects the moisture advection...cape...dynamical forcing...and 
temperature trends. looks like a transition event 
with storm chances and cooler temperatures in store. 


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

Updates with 23 sept 18z taf package...little change in forecast 
with clear skies to remain. Some cumulus and altocumulus castellanus clouds building 
across southern Gila County today should not impact area taf sites. 
Mostly light and diurnally driven wind patterns will prevail with 
light and variable at times as well. 

Aviation discussion not updated for amended tafs. 


Fire weather... 
Friday through Tuesday... 
shower and thunderstorm chances will slowly increase Friday and 
become more appreciable by Saturday along and ahead of a 
seasonably strong cold front. Winds may become locally gusty Friday 
and Saturday...however this will be accompanied by increasing 
moisture levels mitigating a higher fire danger. Drier and somewhat 
cooler air will overspread the district by early next week with 
minimum humidities falling back into a 10-25 percent range after 
substantially more humid conditions Friday and Saturday. Overnight 
recovery will be good to excellent through the period. 


Psr watches/warnings/advisories... 





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Previous discussion...percha/aj 
fire weather...MO 

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