FXUS65 KPSR 290541

Area Forecast Discussion
National Weather Service Phoenix AZ
1041 PM MST Thu May 28 2020

.UPDATE...Updated aviation discussion.


Strong high pressure building over the region will bring
temperatures around 10 degrees above normal through this weekend,
with the heat persisting longer across south-central Arizona.
Slightly cooler, but still above average temperatures are likely
early next week as an area of low pressure slowly moves towards
the west coast. Increasing moisture may also bring a slight chance
of thunderstorms to southern Gila County.


Broad anticyclonic flow aloft is firmly in place across the
western CONUS, with a ridge centered across southern California
into the western Great Basin. This feature is forecast to
gradually expand eastward into Arizona over the next 24 hours,
with a subsequent gradual warming trend expected in the lower
troposphere with 700-mb temps warming to near 15C. A bit of mid-
level moisture has also pushed into the region this afternoon as a
weak shortwave brushes by to the north and east, enough to trigger
few-sct showers across the Rim County and White Mountains at this

Ensemble guidance continues to depict a very narrow temperature
spread for the next few days, with the warmest temperatures expected
on Friday, which will likely be the hottest day of this heat event,
as temperatures top out above 110 degrees across most of the Arizona
deserts. A further increase in moisture is expected across eastern
AZ on Friday as mid-level winds become southeasterly, with PW values
approaching 1.0 inch across south-central Arizona. This increase in
mid-level moisture and instability (a few hundred joules of CAPE)
appears it will be now enough to allow the development of isolated
TS activity across the higher terrain of South-Central AZ on Fri
afternoon/evening. Although the potential for wetting rainfall is
very low, there is still the concern that these thunderstorm could
produce gusty outflow winds and lightning-caused fires.

Into the weekend, the above-normal temperatures will persist
across south-central Arizona, remaining above excessive heat
thresholds as highs top out around 110 degrees for Phoenix. More
substantial cooling will occur across southeast California and
southwest Arizona as an upper-level trough approaches the
California coast. The strengthening southerly low-level and mid-
level flow will maintain seasonably high moisture values across
southern and eastern parts of the state, with continued convective
buildups possible on both Saturday and Sunday afternoons. The
Excessive Heat Warning remains unchanged with this forecast
package, and will expire on Sunday evening for south-central

The large-scale pattern evolution remains fairly uncertain into
early next week, particularly with regards to the inward
progression of the upper trough and increasingly compact
associated upper low, and the strength and persistence of the
upper-level ridge across the Southwest. Monday is still likely to
remain very hot, with perhaps a couple degrees cooling over
Sunday. However, a gradual increase in moisture will occur amidst
strengthening southerly low-level flow. GEFS ensemble mean favors
a more eastward trough progression, with modest height falls and
cooling spreading eastward by Monday. This solution would favor an
increase in convective coverage in the Monday-Thursday period,
especially across the higher terrain north and east of Phoenix.
However, couldn't rule out an isolated storm over the lower deserts
within this regime, and greater potential for outflow. The latest
(12Z) ECMWF/EPS output is now beginning to trend towards the GEFS
output, giving somewhat more confidence that a trend towards
cooler/more unsettled weather will occur, with both the GEFS/EPS
now showing more-and-more of their respective ensemble members even
showing measurable rainfall at Sky Harbor on/after June 3rd.
However, given the still-large spread between the ensemble members,
confidence continues to fairly low especially for a typically benign
early June climatologically.


.AVIATION...Updated at 0540Z.

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

Diurnal wind tendencies are favored through Friday afternoon with
speeds around 8 kts or less. There is a slight chance for showers
and an isolated storm or two Friday afternoon east of the metro.
This activity is not expected to spread near any terminals, but
outflow winds from the east to southeast may push into and
possibly through the metro during the late afternoon and evening
hours. This is represented in the TAFs with an east or southeast
wind shift around 1-2Z. Skies should remain mostly clear over the
metro area, but cumulus and towering cumulus will be visible to
the east Friday afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

Clear skies through the Friday afternoon. Winds will favor the
southeast through Friday afternoon at KIPL with speeds increasing
near 10 kts in the afternoon. A strong westerly shift at KIPL is
expected in the early evening with gusts up to 20-25 kts. Wind
will favor the south at KBLH through the TAF period with a period
of variability overnight and breezy winds Friday afternoon with
gusts up to 25 kts.


Sunday through Thursday:

Much above normal temperatures on Sunday to gradually cool closer to
the seasonal average by the middle of next week. There is a slight
chance of an isolated thunderstorm in eastern Gila County on Sunday
and Monday afternoons, albeit with little to no accompanying
rainfall. Somewhat more expansive shower and thunderstorm activity
may be possible towards the middle of next week in eastern
districts, however confidence in this outcome is low. Minimum
afternoon humidity levels falling to around 10% this weekend likely
to rebound closer to 15% by the middle of next week. Overnight
recovery will mostly fall in a 20-35% range though could gradually
improve. Winds may be a bit gusty Sunday and Monday, though well
below critical thresholds. Nevertheless, fire danger will remain
heightened with the prolonged period of very hot, dry weather.



Record highs this week:

Date Phoenix Yuma
---- ------- ----
May 27 111 in 1983 114 in 1951
May 28 113 in 1984 116 in 1983
May 29 112 in 1910 115 in 1984
May 30 114 in 1910 120 in 1910
May 31 109 in 2012 113 in 2012


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ534-537>556-

Excessive Heat Warning until 8 PM MST Friday for AZZ530>533-535-

CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ561>570.



FIRE WEATHER...Percha/Rogers/MO
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