Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 251206 AAA

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
505 AM MST Sun Aug 25 2019

.UPDATE...Updated aviation discussion.


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 1205 UTC.

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

Another gulf surge has raised dewpoints this morning to around 70 in
the Phoenix area, and the high levels of low level moisture may
result in another episode of low clouds mainly during the morning
hours. Do not expect CIGS, but FEW-SCT decks from FL040 to FL060 are
likely again, with clouds thinning by later morning/early afternoon
and becoming genly FEW at best. Overnight tonight do not expect
significant cloud issues, just a few mid/high decks. Winds will
again favor the west most of the period, with some gustiness again
during the late afternoon and much of the overnight period. Gusts
into the teens or low 20s are likely as the weather pattern appears
similar to what has just occurred over the past 12-24 hours.

Southeast California/Southwest Arizona including KIPL and KBLH:

Another gulf surge has occurred, and the result is continued high
levels of low level moisture. Thus, we may see another episode of
mainly FEW_SCT decks from FL030 to FL060 this morning. The low
clouds should mainly affect KIPL and to a lesser extent KBLH. Expect
low clouds to dissipate later this morning and early afternoon.
Later this afternoon and overnight we could see some increase in
mid/high clouds moving in ahead of a dying tropical system off the
Baja coast. Winds should continue to favor the southeast at KIPL and
the south at KBLH. Look for some gustiness again this afternoon at
KBLH with peak gusts approaching 20kt. Otherwise sustained speeds
genly 15kt or lower at the TAF sites next 24 hours.

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.




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