Excessive Heat in Arizona, May 27 to 31, 2020

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
435 AM MST Wed May 27 2020

.UPDATE...Updated aviation discussion.

&&

.SYNOPSIS...
With strong high pressure building into the region, temperatures
will warm substantially the remainder of the week. Readings during
much of the latter half of the week will fall just short of record
levels and around 10 degrees above normal. 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 early next week.

&&

.DISCUSSION...
The warming trend in the lower troposphere observed in more recent
soundings at KTWC will continue through today, with excessive heat
becoming the primary concern for the next several days. The
expansive upper low over the southern Plains continues to shift
eastward, while a well-defined closed low well off the coast of
Baja California slowly drifts northeastward this morning. In
between these two features, a highly amplified upper ridge will
become centered near the lower Colorado Valley, paving the way for
a multi-day excessive heat event. Ensemble forecast trends remain
very consistent with a narrow temperature spread over the next
four days, which is not surprising given the lack of cloud cover
and predictable synoptic-scale pattern. Most desert locations
should remain a degree or two shy of 110 degrees today, before
slight additional warming occurs into Thursday and Friday. In
fact, most model guidance is now highlighting Friday as the
hottest day of the event for south-central Arizona, with Phoenix
peaking at 112 degrees, as a 594 dm upper-level anticyclone builds
across eastern Arizona. Friday may also have the best shot at
breaking a record high temperature for Phoenix. The
aforementioned closed low over the Pacific will begin to shift
towards the California coast on Friday and bring very modest
cooling to southeast California.

The primary change to the forecast this period was to extend the
Excessive Heat Warning into Sunday for south-central Arizona.
There was some concern about the possibility of upper-level
cloudiness and increasing moisture levels, but the synoptic-scale
pattern continues to favor seasonably high mid-level heights
from eastern Arizona into the Southern Plains as the anticyclone
expands longitudinally. NBM ensemble guidance also maintains a
narrow spread for MaxTs on Sunday, with Phoenix topping out at 110
degrees - the same forecast as Saturday. Also of interest is the
increasing levels of moisture this weekend from the south and
east. Precipitable water values of 0.75-1 inch are forecast across
the eastern half of Arizona, which according to the SPC sounding
climatology is close to the 90th percentile for this time of year.
It'll still be very difficult to get any thunderstorm activity
across the lower deserts, but eastern parts of southern Gila
County may have an outside chance for an afternoon thunderstorm
each day.

A more notable change to weather conditions across the Southwest
is expected into next week, though there is still some uncertainty
how exactly it will play out. Model guidance maintains a fairly
persistent signal for mean troughing to remain fixed across the
Pacific coast, with the eastern extent of the trough influence the
biggest question. This introduces more spread in the temperature
and precipitation forecast for next week, with some NBM ensemble
members depicting temperatures closer to or even slightly below
100 degrees by the middle of next week. A more progressive/inland
troughing pattern would also likely result in greater
precipitation chances for the higher terrain north and east of
Phoenix. For now, maintained about a 20 percent PoP chance for
southern Gila County Tuesday and Wednesday of next week.

&&

.AVIATION...Updated at 1135Z.

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

Strong high pressure aloft will result in clear skies across the
greater Phoenix area today and tonight. Winds will follow typical
diurnal patterns, with east wind becoming west after 19z. There will
be a period of gustiness after about 22z where peak gusts will reach
to near 20z occasionally thru about 03z, when winds will then
gradually taper off becoming light west by around midnight. At KPHX
there will be a few hours, beginning around 03z, when the surface
winds start to decouple and west winds at 2k persist at near 30kt.
This will lead to a bit of LLWS; shear will be a bit too low to
mention in the TAFs at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:

Strong high pressure aloft will result in generally clear skies
across the western deserts for the next 24 hours. Winds will tend to
favor the southeast much of the day at KIPL with speeds generally
10kt or less. Winds will favor the south from mid morning into the
evening at KBLH with some gusts to near 20kt at times during
afternoon and early evening hours.

&&

.FIRE WEATHER...
Friday through Tuesday:
Much above normal temperatures late this week will only cool
slightly early next week with afternoon highs 5-10 degrees warmer
than average. Although the majority of the area will remain dry,
there is a slight chance of an isolated thunderstorm in eastern
Gila County Saturday and Sunday afternoon albeit with little to no
accompanying rainfall. Minimum afternoon humidity levels will fall
to around 10% this week, then potentially closer to 15% next week.
Overnight recovery will mostly fall in a 20-35% range. Winds will
be somewhat more gusty Sunday and Monday, though well below
critical thresholds. Nevertheless, fire danger will remain
heightened with the prolonged period of very hot, dry weather.
Thunderstorm chances could increase some into Tuesday across
southern Gila County as a storm system approaches California.

&&

.CLIMATE...

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

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Sunday for AZZ534-537>556-559>561.

     Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Friday for AZZ530>533-535-536.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Friday for CAZ561>570.

&&

DISCUSSION...Rogers
AVIATION...AJ
FIRE WEATHER...Rogers/MO
CLIMATE...MO