170
FXUS65 KPSR 102101
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
200 PM MST Wed Jul 10 2024

.SYNOPSIS...
Strong high pressure will allow extreme heat to persist across the
region through Friday as many high temperature records will be
threatened. Moisture will also slowly move back into the area
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...
The pronounced anti-cyclone responsible for the current extended
period of excessive heat has only shifted from SE California
yesterday into NW Arizona this afternoon. As a result, with central
H5 heights continuing to hover near 597dm, high HeatRisk with
temperatures around 10F above normal and near record highs will
persist the next couple days (see Climate section). Guidance spreads
remain quite narrow resulting in seemingly excellent forecast
confidence, however consequences from thunderstorms in the form of
outflows, moisture surges, and debris clouds provide more
uncertainty than usual given the gradual increase in convective
activity the remainder of the week. By the weekend, high pressure
will have finally shifted towards SW Colorado leading to greater
easterly flow with more prevalent outflows and moisture intrusions,
as well as lowered heights/thermal profiles aloft precluding any
additional short term excessive heat.

Early afternoon WV imagery shows a pocket of drier air actually has
rotated around the periphery of the anti-cyclone from central
California into eastern Arizona over the past 24 hours with
objective analysis indicating low level mixing ratios falling below
8 g/kg. With this limited instability and primarily convergent flow
aloft not particularly conducive towards thunderstorm sustenance,
convective activity this afternoon should be less vigorous across
higher terrain locations. However, HREF membership still suggests
modest convective development in central/southern Yavapai County
likely sending gusty outflow winds towards the Phoenix area, albeit
weakening on their travels south.

With models suggesting some form of a Gulf surge overnight in
response to storm complexes over Mexico, moisture should increase
again across southern Arizona Thursday resulting in better
thunderstorm chances with upslope winds driving the shallow moisture
into higher terrain. Forecast BUFR soundings Thursday afternoon show
modest low level instability (MLCape up to 1000 J/kg) feeding
convective activity, although CinH in excess of 100 J/kg should
preclude storms from propagating much past foothills surrounding the
city. On the other hand, DCape values will peak in excess of 2000
J/kg across the region fostering long-lived, and likely multiple
outflow boundaries. While the preponderance of HREF output suggests
convection over eastern Arizona targeting the Tucson area with the
strongest winds and impacts, hi-res models are forecasting at least
a 50% chance of 35 mph winds sweeping into the forecast area. Based
on historical outcomes, feel more widespread 35 mph winds are
likely with some blowing dust in the more preferred area of recent
disturbed land use south of the metro.

The high probability of multiple deep outflows and renewed Gulf
surges Thursday evening acting to deepen better quality moisture
will set the stage for more widespread direct thunderstorm impacts
through the weekend. Activity Friday may mirror that of Thursday
though more uncertainty exists considering the influence of previous
days evolution on the flow pattern and ascent structure.
Nevertheless, the improved moisture profiles Friday may allow for
some high terrain convection to survive closer into lower elevation
population centers of south-central Arizona with higher NBM POPs
displaying more constriction around the Phoenix metro. Gusty outflow
winds should still be the primary threat for the majority of the
area, though with mixing ratios increasing around 9-10 g/kg and
total column PWATs approaching 1.50", heavy rainfall and localized
flooding will start to become a greater threat with time.

The best potential for more widespread and potentially strong
thunderstorm activity arrives Saturday and Sunday when models
continue to show excellent upper level support in the form of
enhanced divergence from the combination of an upper level low over
central New Mexico and the high pressure system in SW Colorado.
Forecast BUFR soundings show a favorable combination of 1000-1500
J/kg MLCape and DCape capable of support both strong, damaging winds
and excessive rainfall. Conceptually, this pattern matches some very
active thunderstorm days, though NBM POPs lag over most lower desert
locations, and have pushed values closer to a 30-40% chance across
much of the Phoenix metro (and this may even be conservative). At
this time, forecasts don`t differentiate much between the weekend
days though historically, if Saturday happens to be very active,
then Sunday would expect to be far more quiet due to convective
overturning of the atmosphere.

This enhanced thunderstorm potential could easily carry over into
early next week as the upper low is forecast to remain over New
Mexico providing continued favorable divergence. By this time,
mesoscale influences may be the greater determining factor in
convective potential as adequate moisture should remain in place
with daily modest instability ready to be tapped. Mandated NBM POPs
appear too low given the overall flow pattern and setup, however
have made no changes given the heightened uncertainty. By midweek,
an upper level trough should reach the East Pacific while mean
longwave troughing becomes re-established over the Great Lakes
causing a re-alignment of the subtropical high. Unfortunately, there
is notable spread among ensemble members on where the central anti-
cyclone core shifts with a split between a shift back into
California and intrusion of drier air cutting off storm activity
(heavily EPS membership), or remaining near the four corners and
supporting continued deep convection (mostly GEFS members). Due to
this uncertainty, POPs towards the end of the forecast period
naturally trend towards climatology.

&&

.AVIATION...Updated at 1754Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
W to SW winds will continue through this afternoon at all terminals,
with occasional gusts up to 15-20 kts. There is moderate confidence
in an outflow from the N to NW late this afternoon and evening, with
a wind shift generally between 01-03Z and wind gusts up to 20-30
kts. There is uncertainty in how far the outflow will travel and
could stall in the metro area. Later this evening/tonight, the
combination of a gulf surge and long traveling outflow from Sonora
storms will keep winds elevated through midnight and lead to some
uncertainties in timing of the diurnal E wind shift. FEW to SCT
clouds will pass over the area at times with bases between 15-20K ft.


Southeast California/Southwest Arizona including KIPL and KBLH:
At KIPL, winds will favor the E to SE, whereas at KBLH, winds will
favor the S. Wind speeds will generally be 8-12. A gulf surge from
the south may cause winds to gust up near 20 kt for a few hours
beginning around 07-08Z. FEW to SCT clouds around 15-20K ft will
pass over the area at times, especially tonight.

&&

.FIRE WEATHER...
Unusually hot weather will persist the next few days with
thunderstorm activity steadily increasing over eastern districts.
Moisture will steadily increase over most districts through the
weekend allowing temperatures to retreat while humidity levels and
rainfall chances increase markedly. Storms will initially produce
only isolated wetting rain chances in eastern districts with areal
coverage likely becoming far more widespread by the weekend. Minimum
humidity values will continue to fall into a 5-15% range through
Friday, however by Sunday, values should struggle to fall below 20%.
Some slight drying may wash over the districts by the middle of next
week when minimum values fall closer to a 10-20% range. Occasional
afternoon upslope gusts near 20 mph will be common the next few
days, and the increase in thunderstorm activity will produce more
frequent strong outflow winds across a larger expanse.

&&

.CLIMATE...

Record Highs

           Phoenix      Yuma      El Centro
          ---------    -------   -----------
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...Benedict/Berislavich
FIRE WEATHER...18/Kuhlman
CLIMATE...Berislavich/Kuhlman