Weather Service Report

FXUS65 KPSR 202117

Area Forecast Discussion
National Weather Service Phoenix AZ
217 PM MST Fri Jul 20 2018

Thunderstorm chances and areal coverage will progressively wane 
through the weekend as strong high pressure gradually builds over 
the region. Into the first half of next week, storms will likely 
become relegated only to the highest terrain areas as hot 
temperatures begin to bake the lower elevations. In fact, the 
hottest weather of the year will cover the area with afternoon highs 
nearing record levels. 


.DISCUSSION...Current mesoscale analysis showing 500 to 1000 J/kg
across southwest Arizona all the way into southeast California 
and 1000 to 2000 J/kg across Maricopa county eastward. Very little
CIN exists across the southeastern and eastern parts of the state
and this is where current radar imagery is showing isolated to 
scattered thunderstorm development. The latest PSR sounding was 
done at 18Z this morning and it showed 1.65 inches of PW, 1084 
J/kg of MLCAPE, slightly lower CIN than from the 12Z sounding, 
DCAPE value of 1497 J/kg, a convective temperature of 106 F, 
pretty steep lapse rates with a 700-500 mb lapse rate of 7.5 C/km,
and weak shear. Although it is evident that moisture in the 
atmosphere has decreased today, according to the SPC Sounding 
Climatology page our PW is still above the max moving average for 
this time of year. This combined with sufficient daytime heating 
will enhance the instability this afternoon. Into the overnight 
hours, there is also hints of a well defined MCV moving into 
Arizona from Mexico, with a northwest trajectory starting in Pima 
County around 09Z and then into Maricopa County (between 12 to 
15Z) and then into La Paz and far eastern Riverside county 
(between 15Z and 21Z). 

So for the rest of the afternoon into tomorrow morning we expect 
additional development in eastern and southeastern Arizona this 
afternoon, with storm motion to the north-northwest. An outflow 
boundary will likely develop from the east or south or both and 
push towards the Phoenix area. With DCAPE values between 1000 and 
1500 J/kg in eastern/southeastern Arizona, stronger downdrafts 
will be likely and thus stronger outflows will be a possible this 
afternoon with the potential for some blowing dust. If outflows 
can make their way into the lower deserts and are strong enough, 
the potential is there for additional redevelopment. Any storm 
that does end up developing today will have the potential to 
produce damaging winds and hail due to large instability and 
steep lapse rates. With the influence of the MCV overnight, expect
activity to remain elevated through the overnight and morning 
hours. However, any storm that develops should remain relatively 
weak due to the lack of daytime heating. 


The New Mexico anti-cyclone will expand in coverage and intensity
beginning Sunday and become the driver for the regional weather 
hazard much of next week - extreme heat. This intensifying high 
pressure will enhance larger scale subsidence while also 
contribute to the drying of the lower troposphere. BUFR soundings 
suggest sfc-800mb mixing ratios falling below 10 g/kg as early as 
Sunday afternoon with total column PWATs progressively edging down
towards 1.25 inches (and even becoming even more dessicated by 
the middle of the week). Even high terrain convection may be 
difficult to come by as H5 temperatures increase to -4C or warmer 
limiting much available instability. 

The H5 ridge will become parked directly over Arizona and SE 
California during the Monday-Thursday time frame with heights 
likely flirting with 600dm. H8 temperatures will easily exceed 30C 
yielding widespread lower elevation temperatures well above the 110F 
threshold. While anomalies of these magnitudes are quite rare, there 
is tremendous ensemble agreement resulting in better than average 
forecast confidence of near record highs in the Mon-Wed time frame. 
Thus, excessive heat seems a near certainly with many lower 
elevation locations reaching 115F, and would not be out of the 
question for a few locations to tease near 120F. There is some 
uncertainty regarding the duration of the most intense heat as 
typically the thermal intensity will quickly draw moisture inland 
resulting in higher humidity levels and more convective coverage. 
Model spread seems to incur this evolution as soon as Thursday, but 
certainly by Friday as the expanse of the high pressure ridge 
shrinks and begins to shift north. 


South-Central Arizona including KPHX, KIWA, and KSDL: Isolated 
strong storms over Yavapai, Rim, White Mtns., and Pima County will 
increase to scattered coverage by 00Z. Anticipate isolated strong 
storms over lower elevations after that in the 01Z-08Z time frame. 
There will be potential for dense blowing dust over Pinal County and 
southeast fringes of metro Phoenix. Overnight, anticipate 
remnants of Sonora storms to bring scattered light showers between
09Z-17Z. Away from thunderstorm activity, anticipate 
predominantly southwest/westerly winds. 

For metro Phoenix specifically: areas most likely to have a 
thunderstorm move over top of them will be near and outside of 
the 101 Freeway loop. However, that does not preclude areas inside
of that. Timing for that activity looks to be 02Z-07Z. Anticipate
PHX to at least be affected by outflow winds - most likely from 
the east after 03Z. SDL and IWA will have better chances of direct
hit from TSRA than PHX - but still less than 50%.

Southeast California/Southwest Arizona including KIPL and KBLH: 
Except for northern La Paz County, anticipate quiet conditions
through 08Z. However, remnants of Sonora storms will bring
scattered light showers between 09Z-19Z - mainly near and east of
Lower Colorado River Valley. Away from thunderstorm activity, 
anticipate predominantly south and southeasterly winds. 

Aviation Discussion not updated for amended TAFs.


Monday through Friday: Anticipate little to no thunderstorm 
potential Monday through Wednesday - except perhaps very slight 
chances over the higher terrain of south-central AZ. Instead, much 
above normal temperatures can be expected with highs around 115 
common on the lower deserts by Tuesday. Minimum humidities 
decrease to near 15% lower elevations and 20-30% higher terrain. 
Overnight recovery decreases to fair to good. Winds will favor 
westerly directions. During the latter half of the week, 
humidities trend upward and temperatures trend downward. Slight 
chances of storms slowly expands over south-central AZ. 



Record high temperatures next week:

Date         Phoenix         Yuma
----         -------         ----
July 23    114 in 2014    117 in 2014
July 24    116 in 2014    117 in 2014
July 25    115 in 1943    120 in 1943


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Warning from 10 AM Monday to 8 PM MST Wednesday 
     for AZZ532-535-536-539>544-546-548-550-551-553-554-559.

     Excessive Heat Warning from 10 AM Tuesday to 8 PM MST Wednesday 
     for AZZ530-531-533-534-537-538-545-547-549-552-555-556-

     Heat Advisory from 10 AM Tuesday to 8 PM MST Wednesday for 

CA...Excessive Heat Warning from 10 AM Monday to 8 PM PDT Wednesday 
     for CAZ567-570.

     Excessive Heat Warning from 10 AM Tuesday to 8 PM PDT Wednesday 
     for CAZ560>566-568-569.




Return to Home page

Western Regional Climate Center,