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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 251031

Area Forecast Discussion
National Weather Service Phoenix AZ
331 AM MST Sun Aug 25 2019

After near normal temperatures and elevated humidity levels 
today, a significant warming trend will begin on Monday. Drier 
air will also move into the region through at least the first 
half of the week while strong high pressure remains in place. This
will allow high temperatures Tuesday and Wednesday to top out at 
or just above 110 degrees in many lower desert communities. 
Moisture levels may increase enough during the latter half of the 
week to provide minimal chances for storms and a slight cooldown, 
but temperatures will continue to run several degrees above 


Current upper level analysis shows an elongated upper level high
pressure ridge over the Southwestern U.S. with the upper level
remnants of T.D. Ivo moving northward through central Baja. The 
00Z area soundings continued to show ample low level moisture and
plenty of elevated instability, but a very strong mid level 
inversion remains. The northerly dry flow in the mid levels will 
continue today reinforcing the capping inversion and prevent any 
storms across the region. This scenario will continue into early 
Monday with another low level moisture surge tonight as well as a 
period of high cloudiness from Ivo. However, during the day 
Monday, mixing should be more efficient than over the weekend 
eroding away some of the excess lower level moisture. Since the 
upper level heights will be near steady-state Monday into Tuesday,
the lowering boundary layer moisture will allow temperatures to 
quickly warm up. Highs Monday should top out a few degrees above 
normal, but fall short of 110 degrees.

Little has changed with the forecast thinking for another likely
heat episode beginning Tuesday and lasting through at least
Wednesday. The elongated upper level ridge will become better
organized beginning Tuesday with a high center forming somewhere
across the Desert Southwest. Forecast 500mb heights of 592-594dm
and 850mb temps climbing to around 30C should yield daily highs of
110-112 degrees across the lower deserts for at least Tuesday and
likely Wednesday. This will be enough for Excessive Heat
headlines, but the issue starts with what happens after Wednesday.
For several days, models were showing the high center shifting to
our northeast opening up much of the area to an easterly moist 
flow, but the past couple model runs now show the high center 
either staying directly over Arizona or even shifting over 
southern California. This subtle shift means moisture levels are 
probably going to remain on the low side later in the week and 
consequently keep temperatures higher than previously thought. 
Because of this we have increased forecast temperatures for 
Thursday through Saturday with highs generally 107-110 degrees. 
Any higher and we will have to worry about a prolonged period of 
marginal excessive heat conditions. Storm chances are still in the
forecast beginning Wednesday, but PoPs have been trimmed from 
previous forecast packages and are mainly confined to higher 
terrain areas across eastern Arizona. This may very well change 
again if models shift the high center back to the east, but for 
now the bulk of the ensemble guidance shows a drier and slightly 
warmer latter half of the week.


.AVIATION...Updated at 2330 UTC. 

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

Debris cloudiness from dissipated thunderstorms to the east are
drifting over south-central Arizona towards the west. This will
result in BKN ceilings AOA FL250 into the early morning, before 
gradually thinning out. Another moisture increase from the south 
should result in cumulus development near sunrise around FL050,
but coverage should mainly remain SCT. Winds will maintain a west
component at most area terminals overnight, except for KIWA, which
may experience a brief southeasterly shift. No thunderstorms are
expected anywhere near Phoenix on Sunday. 

Southeast California/Southwest Arizona including KIPL and KBLH:

Another moisture surge is occurring tonight and will bring a
potential increase in low-level cloudiness near FL050 by sunrise 
Sunday. Could potentially reach BKN coverage in some spots, but 
confidence in affecting either KIPL or KBLH is still low. 
Otherwise, low-level cloudiness will dissipate by late morning,
before more extensive upper-level cloudiness spreads northward
late in the day. A few thunderstorms may exist across the coastal
range of southern California, and over Joshua Tree NP, with the
only potential impacts at KIPL and KBLH being a possible outflow
boundary, but confidence is low. Winds should maintain general
south-southeast directions with speeds between 10-15 kt. 

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday: 
Dry conditions will persist into the middle of the week with 
increasing thunderstorms chances later in the week focused mainly 
over higher terrain locations. Before the dry airmass gives way to 
more humid weather, temperatures will once again warm near excessive 
levels as afternoon highs peak some 5F-10F above average. Midweek 
afternoon humidity levels will fall into the teens, but increase 
closer to a 20-30% range late in the week. Occasionally gusty 
upslope terrain winds are likely through the week, though nothing 
particularly unusual for the season. 


Spotters should follow standard reporting procedures.