189
FXUS65 KPSR 220010
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
510 PM MST Fri Jun 21 2024

.UPDATE...
Updated Aviation section.

&&

.SYNOPSIS...
Elevated moisture content will continue to provide daily
thunderstorm chances during the next several days, with the best
chances for activity across the higher terrain areas of eastern AZ.
Excessive heat conditions will continue today across south-
central AZ before subsiding this weekend. As high pressure
strengthens over the region through the middle of next week,
temperatures are expected to heat up once again.

&&

.DISCUSSION...
The overall synoptic pattern continues to feature a weak trough over
the Great Basin and a retrograding ridge of high pressure centered
over the Tennessee Valley. This overall combination is continuing to
enhance southerly flow, which is advecting moisture from Mexico,
with the majority of it coming from the remnants of Tropical Cyclone
Alberto. Latest SPC mesoanalysis shows PWATs ranging between 1.2-
1.4", mainly across central and southeast AZ along with MLCAPE
between 500-1000 J/KG across eastern AZ, which will be sufficient
to generate thunderstorms this afternoon across the higher terrain
areas to the east of Phoenix. With DCAPE approaching 2000 J/KG,
the environment once again will be favorable for the generation of
strong downburst winds. Given that the steering flow will remain
from the south to south-southwest, any thunderstorm activity that
develops over the higher terrain areas is not expected to impact
the lower deserts. However, gusty outflow winds emanating from the
thunderstorms could reach the Phoenix metro area with the HREF
indicating a 30-50% chance of 35+ mph winds. These outflow winds
could also generate areas of blowing dust as well.

Heading into the weekend, southeasterly flow will continue to bring
surges of moisture into central and southern AZ and as far west as
southeast CA, with ensembles showing PWATs incrementing to between
1.6-1.9", which for this time of the year is close to 250% of
normal. As a result, more thunderstorm activity is expected,
especially across the higher terrain areas of northern and eastern
AZ. Even though the large-scale forcing for ascent will be
lacking, given that the subtropical ridge will be building over
the region, there are a couple of disturbances that will traverse
northern Mexico that could increase the convective coverage, even
across the lower deserts. For Saturday, there is subtle inverted
trough in the 700 mb layer on the northern periphery of the large
moisture plume from the remnants of Alberto that could help
enhance the convective activity with several Hi-Res models
indicating some morning showers across southern and even into
southwest AZ followed by afternoon convection developing across
the Mogollon Rim. With the steering flow expected to shift more
out of the east, the activity across the Mogollon Rim could make
its way down into the lower deserts, including the Phoenix area,
as some of the hi-res guidance is showing for late Saturday
afternoon/early evening. Latest NBM PoPs range between 40-60%
across the higher terrain areas and between 20-40% across the
south-central AZ lower deserts. On Sunday, another inverted
trough associated with a tropical disturbance located over the
western Gulf of Mexico is projected to move into northern Sonora,
which could help increase the convective coverage once again.
However, the convective coverage on Sunday will be dependent on
how Saturday`s activity pans out given that the atmosphere could
be overworked and usually it is not that common to get widespread
activity on back to back days as the atmosphere needs a day of
recovery. With the thunderstorm activity this weekend, strong
gusty winds and locally heavy rainfall due to the higher moisture
content will be the main hazards.

Heading into next week, the subtropical ridge is forecast to
strengthen with 500 mb height fields expected to increment to
between 594-597dm through the middle of the week. The
strengthening ridge will increase the subsidence aloft and thus
moisture content will gradually decrease. Thus, a decrease in
storm activity is likely heading into the middle of next week with
activity confined mainly to the higher terrain areas of eastern
AZ. Ensembles indicate that the ridge will likely weaken heading
into the latter half of next week as a large-scale trough moves
across the Pacific Northwest, however, it appears that abundant
monsoonal moisture will still remain in place to continue to
result in daily chances for afternoon and early evening convection,
especially across the higher terrain areas of eastern AZ.

Excessive Heat will continue to remain an issue today, particularly
across south-central AZ, where high temperatures today are expected
to top out between 110-114 degrees. This will result in areas of
Major HeatRisk materializing and thus an Excessive Heat Warning
remains in effect through this evening. With more cloud cover and
higher moisture content this weekend, temperatures are expected to
slightly cool off with a moderate HeatRisk in place. Heading into
the early to middle portion of next week, as the aforementioned
ridge strengthens overhead, temperatures are expected to heat up
once again with most of the lower desert areas likely to exceed
110 degrees.

&&

.AVIATION...Updated at 0009Z.

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

A surface wind convergence zone is in place over the Greater
Phoenix area with southeasterly winds meeting southwesterly winds.
For most of the TAF sites, this has resulted in southerly or
southwesterly with KIWA being an exception (southeasterly). As
mixing weakens this evening, anticipate an overall trend to an
easterly component due the orientation of the pressure gradient.
However, a major consideration is thunderstorm activity. So far,
activity over the higher terrain to the east of the metro area has
been quite isolated. But there is more significant activity over
eastern Pinal County. Outflow winds from there are expected to
reach the metro area this evening. This will reinforce
southeasterly winds at KIWA and cause a more distinct shift at the
other sites including KPHX. At this time, best estimate of arrival
at the TAF sites is in the 01Z-03Z time frame from southeast to
northwest. In addition, there may be visibilities reduced below
10 miles (mainly at KIWA). This scenario assumes that there will
be not be new storm development over the Valley floor which is not
a certainty. At this time, probability of TS or VCTS is too low
(20-30% to reflect in the TAFs). As for sky cover, anticipate
ceilings remaining AOA FL150. Remnants from storm activity over
Mexico could conceivably produce some pockets of virga late
tonight and Saturday morning but expect that to remain near and
south of I-8.

During the day Saturday, anticipate a change to the wind regime
with significantly lighter winds until perhaps late
afternoon/early evening when storms over Gila and far northeast
Maricopa Counties may spread outflow winds across the Valley
floor.


Southeast California/Southwest Arizona including KIPL and KBLH:

Skies at the TAF sites will be mostly clear except for some minor
mid and high clouds Saturday. Winds at KIPL will have a lot of
variability this evening waffling between westerly and
southeasterly. Expect predominantly southeasterly to prevail late
tonight before more distinct S/SEly winds develop by late morning.
At KBLH, expect a familiar change from southerly early in the TAF
period to southwesterly before nocturnal drainage winds (NWly)
develop late tonight. Expect an uptrend in southerly breeziness
(gusts to 25kts) starting by late morning.

&&

.FIRE WEATHER...
Moisture will continue to increase across the region into the
upcoming weekend bringing daily chances for afternoon
thunderstorms, with the best chances expected across the higher
terrain areas of south-central AZ. MinRH values this afternoon
will range between 30-40% across the far eastern districts to
between 10-15% across the western districts. MinRH values will
then increase for Saturday, ranging from 40% across the eastern
districts to 20-25% across the western districts, with similar
levels through Sunday as well. Winds will follow their typical
daily tendencies, with gusty outflows from any thunderstorm
activity anticipated over the next several days. High pressure is
expected to strengthen over the region by early to middle of next
week leading to even hotter temperatures along with slightly
drier conditions.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ534-
     537>555-559.

CA...None.
&&

$$

DISCUSSION...Lojero
AVIATION...AJ
FIRE WEATHER...Young/Lojero