Weather Service Report

FXUS65 KPSR 251633

Area Forecast Discussion
National Weather Service Phoenix AZ
933 AM MST Sun Jun 25 2017


High pressure will keep temperatures hot today and Monday. A chance 
of afternoon thunderstorms will also return to the mountains of 
eastern Arizona. A series of Pacific weather disturbances are 
forecast to move into the western states north of Arizona Tuesday 
through Saturday. Developing dry westerly winds will weaken the high 
pressure system for cooler temperatures, and push most of the 
moisture east and out of Arizona. 



Morning sounding from Phoenix observed a PW of 1.07 inch, with
most of the relatively richer moisture confined to above 5 km AGL,
while the boundary layer remains fairly dry and well-mixed. A
relatively shallow layer of richer moisture was also noted near 
the surface, as evidenced by mid-50s dewpoints across the lower 
deserts, but this should mix out by this afternoon. Appears that 
isolated thunderstorms developing over the higher terrain north 
and east of Phoenix today is a good bet, and hi-res models 
(including the latest HRRR runs) have latched onto this. However, 
nearly uniform southwesterly flow aloft will result in storms 
moving in a general eastward direction today away from the lower
deserts. Precipitation totals will also be light in most spots, 
given the lack of more substantial lower-level moisture. Still 
will have to monitor for the potential of thunderstorms/rainfall 
over the Pinal Fire burn scar near Globe today. Made some slight 
adjustments to the PoPs over the higher terrain of south-central 
AZ. Excessive heat also remains a concern, and mid-high level 
cloudiness should remain sparse/thin enough to have minimal effect
on temperatures today.


Today and Monday...

Significant boundary layer moisture from New Mexico continued to 
push deeper into southeast AZ at 2 am this morning. Last evenings 
weather Balloon soundings El Paso, Midland, and Chihuahua Mexico all 
detected 850 mb southeast winds in the 15 to 25 knots range. In fact 
low level moisture convergence along this moisture front fueled the 
large Meso Scale Convective storm in far southern New Mexico last 
evening. Perhaps outflow winds from this system in combination with 
the synoptic scale back door low level easterly winds will send 
appreciable humidity all the way to Phoenix this morning.

Models continue to forecast a weak upper level disturbance, at and 
above the 300 mb level, to move into east and southeast AZ by 
evening. This should lead to a larger area of convective threats, 
including portions of our mountainous southern Gila County Zone east 
of Phoenix. However enhanced 850/700 mb westerly flow, typical for 
the month of June and is climatological, should help push most of 
the boundary layer moisture on the south central deserts into the 
mountains east and southeast of Phoenix. Therefore the only 
convective threats in our forecast area, southeast CA to south 
central AZ, will be in southern Gila County. One caveat though. If 
large early evening convection develops closer to Tucson, there is a 
possibility of gusty northward moving outflows and blowing dust on 
the deserts between Phoenix and Casa Grande.

Following the Sunday evening upper level disturbance in southern AZ 
a more anti-cyclonic flow regime at and above 300 mb will likely 
stabilize the atmosphere over most of our forecast area Monday. 
However weak upper level deformation is still forecast by the models 
along the eastern AZ border where convective threats can be 
expected. Otherwise partly cloudy skies, and dry and hot weather 
will cover our forecast area Monday. The exception is a slight chance 
of thunderstorms over the higher terrain of southern Gila County 
bordering the White Mountains.

Very hot weather will continue Monday and with slightly hotter 
temperature forecasts, The Excessive Heat Warning has been updated 
to now cover all the lower deserts of southeast CA, and southwest and 
south central AZ. 

Tuesday through Saturday...

A series of weak Pacific disturbances are still forecast to move 
into the western states mainly north of AZ this period. Resulting 
dry westerly flow AZ will weaken the regionally strong high 
pressure, and push most of the boundary layer moisture out of AZ 
into New Mexico Tuesday, then into west Texas Wednesday through 
Saturday.  As a result, cooler afternoon temperatures are forecast 
along with mostly clear skies.


South-Central Arizona including KPHX, KIWA, and KSDL:  

Persistence forecast with typical diurnal wind shifts and
afternoon breeziness today across the Phoenix area. Any 
thunderstorm activity that develops during the late afternoon is 
expected to remain confined to the higher terrain of Gila County. 
However, there is a slight chance that an outflow boundary 
propagating from the east or southeast could produce brief wind 
gusts mid to late evening.

Southeast California/Southwest Arizona including KIPL and KBLH: 

No aviation weather impacts expected. Winds will retain a 
southerly component at KIPL/KBLH with winds generally less than 10
to 15 kt.

Aviation discussion not updated for amended TAFs.


Tuesday through Saturday:

A high pressure system will remain over the southwest allowing 
temperatures to continue to soar well above normal. Temperatures 
will begin to decrease through the week as the high pressure weakens 
and some moisture moves into the region. The moisture is primarily 
moving into southeastern California/ southwestern Arizona and 
eastern Arizona. The best chances for any thunderstorms will be 
confined to the mountainous regions including the Rim and southern 
Gila County. However, dry lightning will be a concern as the amount 
of precipitation with any storm may be limited. Otherwise, minimum 
relative humidity will be in the 8-12 percent range for desert 
locations, a little higher over the higher terrain, and both with 
meager overnight recoveries. Finally, winds look to be the strongest 
Tuesday evening and into Wednesday with gusts up to 30 mph possible 
though much of the area. 



Record highs this week:

Date           Phoenix            Yuma
----           -------            ----
6/25         120 in 1990        119 in 1994 
6/26         122 in 1990        122 in 1990 


Spotters may be needed in southern Gila County on Sunday 


AZ...Excessive Heat Warning until 8 PM MST /8 PM PDT/ Monday for 

     Heat Advisory until 8 PM MST Monday for AZZ557-558.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ560>570.



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