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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 220320

Area Forecast Discussion
National Weather Service Phoenix AZ 
820 PM MST Wed Aug 21 2019

Excessive heat with record high temperatures continues today 
across south central Arizona into southeast California with only 
very isolated thunderstorms over far eastern Gila County. Moisture
will return by Thursday morning to begin a gradual cooling trend 
and bring isolated thunderstorms back into the forecast for south 
central Arizona Thursday through Saturday, with the best chances 
over Pinal and Gila Counties. A gradual warming trend will begin 
on Sunday and continue into the first half of next week with 
minimal rain chances. 


Exactly as we have been anticipating, today was a very hot day 
across the lower deserts; Phoenix shattered it's old record high of 
110 and set a new mark of 114. Much of the lower desert reported 
highs between 113 and 115 under sunny skies. 500mb heights have 
actually started to taper off and were running around 593dm over the 
lower deserts per the latest plot data, but despite the slightly 
cooling aloft, the boundary layer dried again and allowed high 
temperatures to really climb. At 8 pm surface dewpoints over the 
lower deserts were mostly mid 20s to mid 30s and were running 10-15 
degrees drier than 24 hours ago. Moisture has been pooling up to the 
southeast of our area, and a few cells this afternoon approached the 
San Carlos area and were not too far to the southeast of Casa 
Grande. Moisture does appear to be close by and we should see the 
return of monsoon conditions shortly even if only in low to moderate 
grade mode. IR imagery at 8 pm showed generally clear skies area 
wide and convection over far SE AZ continued to diminish. No issues 
expected for the rest of tonight as the gusty afternoon west winds 
have been tapering off and dry conditions will persist into Thursday 
morning. Grids have been tweaked ever so slightly and forecasts look 
to be on track.

Another very hot day based on August standards continues to 
unfold with an Excessive Heat Warning still in effect through this
evening for much of Arizona and southeast California. Phoenix has
already reached 114 degrees smashing the old daily record of 110 
degrees. Temperatures will approach 115 degrees across all lower 
elevation locations, including Yuma and El Centro. Otherwise, much
of Arizona continues to be dominated by clear skies with dry flow
aloft under a repressive high pressure regime. Some moisture has 
seeped into the state from the east leaving the White Mountains 
and southeast Arizona with the only significant chances for 
afternoon storms. A few cumulus clouds are trying to build over 
the higher terrain in Gila County, and while a shower or two 
is possible, the impact of any storms will be very minimal. 

Beginning tomorrow, a passing trough across the northern US will
dampen atmospheric heights suppressing the H5 high before it 
eventually recenters over our area. This position will favor a 
southerly mid-level flow allowing moisture to seep further into 
central Arizona. Additional low level moisture will also increase 
beginning late tonight/early Thursday morning as a gulf surge 
brings dew points into the mid 50s by the afternoon. Both of 
these conditions also signal the start of a cooling trend with high
temperatures falling close to seasonal normal values by Friday and
lasting through the weekend.

However, the moisture increase is also needed for thunderstorm 
chances. A handful of model soundings suggest a moderate amount of
CAPE and mixing ratios in the 9-10 g/kg range as early as 
Thursday afternoon. Very high DCAPE and inverted V profiles show 
the potential for strong downbursts with any convection, and the 
possibility for longer living/traveling outflows. Even though 
storm coverage will be best to our south, outflow interactions 
from distant convection may provide enough lift for isolated 
thunderstorms in the Phoenix area and northern Pinal County. 
Strong winds will be the primary threat if any storms do form 
given the aforementioned atmospheric profile and DCAPE. Still, 
most areas are more likely to miss out on any precipitation but 
the potential is at least there. The better storm chances will 
increase late Thursday evening/night over southern Gila County. 
Elevated moisture levels will keep low-end precip chances for the 
lower desert through the weekend with storm activity most likely 
over the higher terrain in eastern Arizona. 

Despite the presence of an elongated ridge over the Desert 
Southwest through the first half of next week, rich Monsoonal 
moisture will continue to remain positioned southward across 
northwest Mexico. Although the gradual rise in heights aloft will 
increase high temperatures back into the upper 100s and suppress
convection through subsidence associated with the ridge, this 
moisture should keep temperatures just below excessive heat 
thresholds. Models continue to indicate that a more significant 
moisture increase could occur during the second half of next week,
with some potential for relatively strong dynamics associated with
an inverted trough that splits off a relatively strong trough and
frontal system moving through the eastern two-thirds of the U.S.
on Wednesday into Thursday. Although confidence this far out is
still low, both the GEFS and ECM ensembles show better rain 
chances for the second half of next week that may try to help keep
us from having the driest monsoon season on record for Phoenix.


.AVIATION...Updated at 0220 UTC.

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

No aviation concerns tonight through at least Thursday morning. 
Skies tonight generally clear with potential for FEW-SCT mid decks 
to develop Thursday morning with most bases aoa 14k feet. A few high 
based cumulus possible in the afternoon/evening Thursday as well. 
Winds tonight will follow typical diurnal tendencies; westerly gusts 
to 18kt should taper off shortly and eventually swing back to the 
east after 08z or so (likely after 09z at KPHX). Winds back to 
southwest after 20z Thursday. 

Convective threat well to the southeast of Phoenix increasing on 
Thursday and cannot rule out some east/southeast outflow wind making 
it into the Phoenix area during the evening hours...probably no 
earlier than 03z if it happens at all. Right now confidence is 
rather low so no mention to be made in the TAFs.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation concerns. Look for genly clear skies next 24 hours with 
winds mostly below 15kt. Winds to favor the southeast much of the 
period at KIPL, with some tendency to go southwest this evening. 
Wind favor south to southwest at KBLH with some gustiness likely in 
the afternoon tomorrow; peak gusts could exceed 22kt at times.

Aviation Discussion not updated for amended TAFs.


Saturday through Wednesday: 
Slight chances for storms will favor the higher terrain north and
east of Phoenix, including the Tonto NF, through the first half of
next week. Humidity values will generally improve with minimum RH
values in the 15-25% range in the desert with slightly higher
values the forests. Overnight RH recovery will be in the 40-50% 
range for most locations. The winds will mostly follow typical 
diurnal trends with some afternoon breezes. 



Record High Temperatures

Date        Phoenix         Yuma
----        -------         ---- 

Aug 21    110 in 2007    115 in 1969 
Aug 22    113 in 2011    115 in 1969 
Aug 23    114 in 2011    115 in 2011


Spotters should follow standard reporting procedures.