039
FXUS65 KPSR 092145
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
245 PM MST Tue Jul 9 2024

.SYNOPSIS...
Strong high pressure will allow extreme heat to persist across the
region throughout the week as many high temperature records will be
threatened. Moisture will slowly move back into the area over the
next several days providing modest chances for isolated
thunderstorms over eastern Arizona high terrain. Over the weekend,
far better moisture should be imported into lower elevations
supporting improved rainfall chances, as well as pulling
temperatures away from excessive levels and closer to seasonal
normals.

&&

.DISCUSSION...
Expansive ridging continues to dominate western North America this
afternoon with objective analysis depicting the center of a 596dm H5
anti-cyclone situated over the lower Colorado River valley. Through
the remainder of the week, the Conus synoptic pattern will be slow
to advance, however progressive troughing entering the Pacific NW
along with the movement of downstream remnant tropical wave action
will allow the stubborn H5 center to constrict, shift east into the
central Rockies, and eventually weaken. This process will back
prevailing winds aloft from a northerly trajectory into an
east/southeast heading far more supportive for higher quality
moisture intrusion and thunderstorm sustenance.

In the short term, excessive heat remains the greatest threat as
temperatures continue to hover around 10F above normal and very near
record levels (see Climate section below). Models depict very little
variety in thermal profiles with H8-H7 forecasts solidly better than
2 normalized standard deviations above normal, or around +33C and
+20C respectively. With readings of this magnitude only occasionally
experienced, widespread major HeatRisk will continue through Friday
before the introduction of thicker moisture profiles detracts from
the extreme warming.

Although deep moisture profiles are currently compromised, shallow
moisture has been imported from outflow from northern Mexico/New
Mexico as well as via a modified Gulf surge yielding around 8 g/kg
mixing ratios. While insufficient towards supporting lower elevation
convection, current moisture levels have been capable of sparking
isolated storms from the Whites across the Rim. HREF members show
northerly steering flow directing showers and decaying storms across
Gila County with modest chances for minor rainfall accumulations and
weak outflow boundaries. Upper level support appears to wane
somewhat Wednesday with more notable convergence aloft and weaker
updrafts reflected in limited storm coverage via HREF output.

The latter half of the week will be characterized by gradually
increasing moisture levels in both magnitude and depth, as well
increasingly favorable synoptic flow. A more pronounced deep
easterly/southeast flow will become established favoring northward
and westward moving outflow boundaries initiating over higher
terrain. Each outflow should progressively deepen moisture profiles
such that Friday should be a pivotal transition day where values
finally become sufficient for isolated storm surviving in lower
elevations. Early in this evolution, strong outflow winds sweeping
through lower elevations will be the greatest threat.

In combination with an upper level cyclone propagating into central
New Mexico, the position of the high pressure system shifting into
Colorado will provide an excellent jet level wind field with a
tremendous divergence field. Juxtaposed with more favorable moist
flow and mixing ratios jumping near and over 10 g/kg, regional
MLCape in excess of 1000 J/kg should be released over mountains with
well defined outflows initiating additional storms downhill. Greater
uncertainty exists whether atmospheric conditions will support more
widespread lower elevation storms Saturday, however by Sunday,
conditions look even more favorable for aggressive storm activity.
While convective evolution on any given day starting this weekend
may dictate the coverage and ability for thunderstorm formation the
following day, the overall pattern will remain rather favorable for
robust storm development and sustenance across much of the CWA.

&&

.AVIATION...Updated at 1735Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation impacts are expected through the forecast period. Winds
will follow typical diurnal tendencies with a westerly shift
anticipated by 20Z-21Z across the Phoenix Metro. A brief period of
light and variable winds will be possible before this westerly shift
occurs. There could be gusts into the upper teens to 20 kts this
afternoon, but speeds should diminish after sunset. Hi-res models do
show convection developing over the higher terrain east of Phoenix
this afternoon and evening. Therefore, there is a low chance (around
10%) for an outflow boundary emanating from distant storms reaching
KIWA this evening, mainly between 00Z-03Z. Skies should remain
mostly clear with the exception of a few high clouds.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation concerns are expected through the forecast period. Winds
should remain southeasterly at KIPL and southerly at KBLH through
most of the period. Occasional gusts up to 15-20 kt will be possible
at KBLH this afternoon. Skies will remain mostly clear over
southeast California.

&&

.FIRE WEATHER...
Abnormally hot conditions will continue the next several days before
increased moisture spreads into the area late in the week pushing
readings much closer to the climatological normal. Minimum afternoon
humidity levels will fall into a 5-15% range the next few days with
moisture initially increasing over eastern districts by Friday.
Corresponding overnight recovery will remain poor to fair in a 15-
40% range. Humidity levels should improve markedly over the weekend
with minimum levels struggling to reach a 15-20% range as shower and
thunderstorm activity becomes more expansive. Isolated storms will
be possible over the higher terrain of eastern districts the next
few days, however wetting rain chances are, at best, only 10%. Far
better chances for beneficial rainfall arrive over the weekend with
activity moving into lower elevation locations. In general, winds
will be fairly light with occasional afternoon gusts around 20 mph.
However, strong gusty outflow winds and abrupt wind shifts will
become more prevalent through the week as storm activity increases
over higher terrain.

&&

.CLIMATE...

Record Highs

           Phoenix      Yuma      El Centro
          ---------    -------   -----------
July 9    116 (1958)  119 (1958)  117 (1958)
July 10   115 (1958)  118 (1959)  116 (2021)
July 11   118 (1958)  118 (1958)  117 (1975)
July 12   116 (2020)  119 (1939)  118 (1964)

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Friday for AZZ530>555-559.

CA...Excessive Heat Warning until 8 PM PDT Friday for CAZ560>570.

&&

$$

DISCUSSION...18/Kuhlman
AVIATION...Salerno
FIRE WEATHER...18/Kuhlman
CLIMATE...Berislavich/Kuhlman