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000
FXUS65 KPSR 210549
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1049 PM MST Sat Apr 20 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
Unseasonably warm temperatures with mostly sunny skies will persist
through the first half of next week. The warmest days of Sunday and
Monday will yield widespread minor heat-related health risks. A
series of weather systems later next week will result a gradual
cooling trend while also supporting periods of strong, gusty winds.

&&

.DISCUSSION...
As a low amplitude shortwave trough exits into New Mexico early
this afternoon, an expansive flat ridge was building into the SW
Conus providing the next stretch of dry, tranquil weather for the
region. Model output remains very consistent in advertising H5
heights just breaching 580dm with overall widespread synoptic
subsidence prevailing the next 72-84 hours. As a result, guidance
spread is extremely tight yielding forecast temperatures around 10F
above the seasonal normal with tomorrow and Monday being the warmest
days of the week. NBM probabilities still indicate little chance
(less than 10%) of extensive 100F readings with this warming
episode, however a few of the warmest lower elevation locations
should certainly be flirting closely with this threshold (avg first
100F day for major population centers of the CWA range from the end
of April through beginning of May). Nevertheless, widespread minor
HeatRisk with localized areas of moderate risk will be common
through the first half of next week.

Towards the middle of next week, a Pacific trough incorporating an
active subtropical jet will advance towards the SW Conus
encountering the aforementioned high pressure ridge. Trends among
NAEFS membership along with historical precedent suggests a slower
forward progression to the wave structure, and it would not be
surprising if future forecasts indicate an even slower arrival of
this system. Regardless, the first vestiges of midlevel height falls
arrive Tuesday heralding an imminent cooling phase with more
pronounced changes sweeping through the region Wednesday and
Thursday. At this time, the most optimal combination of a prefrontal
pressure gradient and midtropospheric height falls incurring strong,
gusty winds is forecast Wednesday where expansive afternoon gusts 25-
30 mph should be common.

While the initial shortwave trough ejects into the plains during the
latter half of the week, ensemble output generally argues that mean
longwave troughing will settle over the western Conus with several
disturbances emanating over the northeast Pacific digging and
intensifying over the four corners region. Timing and depth of any
given shortwave carries very large uncertainty, however a resurgence
of afternoon gusty winds and reinforcing of the cooling trend late
in the week appear likely. In fact, by next weekend, temperatures in
a slightly below normal range are generally preferred. As is typical
this time of year, even with the trough passages, dry weather is
most likely with limited moisture advection and more muted cold air
aloft. However, the most intense ensemble members do introduce a
robust cold core with the negative PV anomaly such that a few
convective showers could conceivably be squeezed out with daytime
heating.

&&

.AVIATION...Updated at 1715Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will be light (speeds aob 10 kts) and follow diurnal
tendencies. Extended periods of light and variable winds are
expected, mainly at KDVT and KSDL. At the other terminals, expect
a few hours of light and variable winds during the wind shifts.
Other than a FEW high clouds passing over the region later in the
forecast window, skies will be mostly clear.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will be light (aob 10 kts) and follow diurnal tendencies.
Extended periods of light and variable winds are expected for the
overnight and morning hours before a S-SE component wind develops.
Clear skies will give way to a FEW to sometimes SCT high clouds by
this evening but will clear by tomorrow afternoon.

&&

.FIRE WEATHER...
Dry conditions with above normal temperatures will persist through
early next week while winds remain light following diurnal trends
with limited upslope gustiness. Min RHs will continue to fall into
the 5-15% range while overnight Max RHs mostly stay in a 25-45%
range. A cooling trend will arrive with a series of weather
disturbances during the latter half of next week, eventually
bringing temperatures back closer to the seasonal normal. Breezy to
locally windy conditions will also periodically impact districts
with afternoon gusts commonly reaching 20-30 mph. Humidity levels
are likely to stay low next week, however should exhibit some
improvement late in the week with daily MinRHs only falling into a
15-25% range. Given this combination of winds and humidity levels, a
periodic elevated fire danger should be planned for during the
second half of next week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...18
AVIATION...Young
FIRE WEATHER...18/Kuhlman