755
FXUS65 KPSR 211010
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
310 AM MST Fri Jun 21 2024

.SYNOPSIS...
Monsoonal moisture has made its appearance into the Desert
Southwest, with well above moisture levels continuing over the
next several days. As a result, daily thunderstorm chances will
continue, peaking over the weekend. Excessive heat conditions will
continue today before subsiding this weekend. Temperatures are
expected to warm once again early next week as thunderstorm
chances diminish.

&&

.DISCUSSION...
Satellite imagery early this morning depicts a plume of moisture,
courtesy of former tropical cyclone Alberto, streaming into
southeastern Arizona. While the cyclone center will remain well
south of the region, elevated moisture levels arrived yesterday,
marking the first signs of monsoonal thunderstorm activity. As a
mid-level high continues to retrograde westward across the country
from the Ohio Valley region over the next several days, elevated
moisture levels will continue through at least this weekend as the
general flow pattern will remain southerly to southeasterly.
Thus, the general outlook over the next several days is an uptick
in thunderstorm activity through this weekend, and cooler
temperatures (although apparent temperatures will remain in the
upper 100s to lower 110s) across the region.

An Excessive Heat Warning continues today for the lower desert
areas of southcentral AZ, as areas of Moderate HeatRisk continue
as lows this morning are now in the low to mid 80s (upper 80s to
low 90s in the Phoenix metro) due to the increased moisture levels
and highs today ranging from 110-115 degrees. While mid-level
heights will continue to build due to the aforementioned high
center moving westward, the increasing convective activity is
expected to keep somewhat cloudier conditions, thus moderating the
high temperatures across the region through this weekend.

Looking closer at the moisture levels, PWAT`s generally around
1.25"-1.50" have entered southcentral Arizona (dewpoints in the
upper 50s to low 60s) this morning. Ensembles depict PWAT`s
continuing to rise upwards of 1.75" (generally the consensus
ensemble means) going through this weekend. Thus, today continues
this transition period to better thunderstorm chances, as the main
activity is expected to remain primarily in the higher terrain
areas of southcentral AZ once again this afternoon and through the
evening hours. However, strong winds and areas of blowing dust in
association with any outflows remains a decent threat today. The
latest HREF guidance depicts another corridor of enhanced outflow
wind potential across the higher terrain and edging into the
Phoenix metro for this afternoon/evening, with upwards of 50-70%
(20-40%) chance of 35+ mph wind gusts across the higher terrain
(Phoenix metro) and a sizable 10% contour of 55+ mph wind gust
potential across the higher terrain. Thus, very isolated severe
thunderstorm potential continues today for strong/damaging winds,
as was the case yesterday where there were a few severe wind
reports across eastern Arizona. Given that there are pockets of
1000 J/kg surface based CAPE and DCAPE in excess of 1000-1500 J/kg
available as antecedent ingredients for today, this severe wind
threat should come as no surprise. The general steering flow
continues to be southerly across the region, so storm progression
should preclude most, if not all, activity from the lower deserts,
as depicted by most hi-res guidance at this time. Thus, general
thunderstorm chances are 15% or less across the Phoenix metro (up
to 15% from the east valley to near 0% for the west valley) and
30-40% across the higher terrain areas.

.PREVIOUS DISCUSSION...
Heading into the weekend, the model guidance is showing
additional surges of moisture moving into central and southern AZ
and as far west as the southeast California lower deserts with
PWATs increasing to between 1.6-1.8". However, at the same time
that the moisture is on the increase, the upper-level ridge is
expected to move directly overhead, which will increase the
subsidence aloft. Therefore, the overall convective coverage will
likely be limited to the higher terrain areas of AZ. The one
caveat is that guidance is indicating that an inverted trough
moving westward through northern Mexico will reach southern AZ on
Sunday, which could potentially provide greater convective
coverage. This is reflected in the latest NBM PoPs, which shows
the greatest areal coverage for Sunday. High temperatures over the
weekend, with the increasing moisture and potential cloud cover
will be slightly cooler with overnight lows expected to more
elevated. In fact, overnight and early low temperatures are
forecast to remain near to just above 90 degrees across central
Phoenix. Thus, even though the overall HeatRisk is expected to
drop into the moderate category, the necessary heat precautions
should still be performed, especially that there will not be as
much relief during the overnight hours.

Ensembles are strongly favoring the subtropical ridge to strengthen
even further heading into the first half of next week with 500 mb
height fields climbing to between 594-597dm. The high will likely
be centered between eastern AZ and western NM with subsidence
aloft increasing even further. This will cause some moisture
erosion and thus convective potential will decrease as a result
starting on Tuesday. With the strong ridge positioned directly
overhead, extreme heat will be a concern with temperatures heading
into the middle of next week rising to near 115 degrees with the
areal coverage of major HeatRisk becoming more widespread. By the
latter half of the week, there are indications from the ensembles
that the ridge will gradually weaken as a trough moves through
the Pacific Northwest and thus a slight cooling trend would
ensue.

&&

.AVIATION...Updated at 0544Z.

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

Thunderstorms late this evening are isolated over the eastern
half of AZ and will continue to wind down overnight. Easterly
surface winds have overspread the Greater Phoenix area and east or
southeast winds winds will remain in place through Friday morning.
At most TAF sites, speeds are currently below 15kts (except KSDL).
It is expected there will be an uptrend in speeds (gusts to 25kts) which
could occur as early as 09Z or as late as 16Z and be
southeasterly. Directions are expected to trend to southerly by
late morning then southwesterly by mid-late afternoon. Anticipate
improved surface and slantwise visibilities during the day Friday
(compared to Thursday) though it remains to be seen just how much
improvement there will be.

Isolated to scattered thunderstorms are expected once again over
the eastern half of AZ Friday afternoon. The main impact at the
the TAF sites (if any) is expected to be a shift to southeasterly
winds as early as 00Z or as late as 04Z. However, there is a
20-30% chance of thunderstorms occurring at any given TAF site
which would mean substantially more significant impacts to winds
(and ceilings).

Southeast California/Southwest Arizona including KIPL and KBLH:

Dry southwesterly flow aloft will begin weakening during the day
Friday as an upper trough weakens and high pressure centered to
the east begins to expand westward. Surface winds will generally
follow familiar warm season patterns. A change for KIPL will be
the lack of a distinct shift to westerly in the evening.
Otherwise, clear skies.

&&

.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 for today will
range around 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 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.

     Wind Advisory until 5 AM MST early this morning for AZZ560-562.

CA...None.
&&

$$

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