218
FXUS65 KPSR 172139
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
239 PM MST Wed Jul 17 2024

.SYNOPSIS...
Daily thunderstorm chances continue through the end of the week,
with most of the activity expected over the higher terrain areas.
Thunderstorm chances for the lower deserts start to increase heading
into this weekend and the beginning of next week as moisture
increases. Above normal temperatures will continue, with excessive
heat conditions across the lower deserts starting Friday and
continuing through the weekend.

&&

.DISCUSSION...
Objective analysis this afternoon reveals the subtropical high has
started its slow migration westward, and now stretches from
northeastern Arizona through central New Mexico. This high pressure
system will continue to slowly move westward, moving over Arizona by
the end of the workweek and then over Nevada by the end of the
weekend. As the high moves over Arizona, there will be increased
subsidence aloft causing slight drying in the low-to-mid levels
of the atmosphere, with low-level mixing ratios generally below 9
g/kg across the lower deserts. Despite this slight drying, there
will still be ample moisture to cause daily thunderstorms across
the AZ higher terrain. Similar to the past couple of days, there
will be marginal instability along with decent convective
inhibition across the lower deserts. This will cause storms to
struggle to survive into lower elevations as they move off the
mountains. However, DCAPE values will be still be in the 1500-2000
J/kg range, which will support outflow boundaries coming from the
storms in the higher terrain. HREF guidance continues to show
around a 10-30% chance of gusts exceeding 35 mph across the
south- central AZ lower deserts this evening and on Thursday.

As we approach this weekend and the first half of next week, a more
active monsoon pattern looks to set up. The overall synoptic pattern
becomes more favorable as the subtropical high positions itself over
Nevada and a trough dives into the Plains, resulting in diffluent
flow aloft. Additionally, PWATS will slowly rise and approach the
1.5" mark by the start of the weekend and the 1.6-1.7" range by the
beginning of next week. Due to this increase in moisture we will see
low-level mixing ratios rise above 10 g/kg. The combination of
these factors will lead to increased thunderstorm activity across
the region, including the lower deserts. Sometime Sunday-Tuesday
currently looks to be the best time frame for thunderstorm
activity across the lower deserts. The overall steering flow will
be from the north/northeast and thus the overall flow pattern will
be ideal for storms that develop over the Rim to move into the
lower deserts. As mentioned in previous discussions, historically,
the northeasterly flow events have led to severe weather
materializing across the Phoenix metro area. Thus, the potential
will be there for one day between the Sunday-Tuesday time frame to
have an impactful convective event across the Phoenix area. This
is something that still bears watching as we get closer.

In addition to the thunderstorm chances for the coming week, heat
will also be a big headline through at least the weekend. As
mentioned previously the high pressure system will be moving
overhead over the coming days. As this high moves overhead, it is
also expected to strengthen from 593dm to 596-597dm by the end of
the workweek and beginning of the weekend. Temperatures will
gradually climb through the week, peaking on Saturday. Thursday will
have moderate HeatRisk across the lower deserts with afternoon
high temperatures between 108-113 degrees. Afternoon high
temperatures will climb to 110-116 across the south-central AZ
lower deserts and between 112-117 degrees across the western
deserts on Friday and Saturday. This increase in temperatures will
also result in widespread Major HeatRisk across the lower
deserts. An Excessive Heat Watch remains in place for south-
central and southwestern AZ for Friday and Saturday, and across
southeastern CA and the Lower Colorado River Valley for Friday-
Sunday. On Sunday, temperatures across the AZ lower deserts are
expected to cool down by about 3-5 degrees due to increased low-
level moisture and thus the overall HeatRisk drops into the
moderate category whereas across the Lower Colorado River Valley
through southeast CA, temperatures are not expected to cool off as
much and thus areas of major HeatRisk will still persist.
Therefore, given the very hot conditions that will be
materializing through the weekend, it will be crucial for anyone
partaking in outdoor activities to take all the necessary heat
precautions to prevent any heat-related illnesses.

&&

.AVIATION...Updated at 1845Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
W to SW winds will prevail through this evening, with afternoon
speeds up to 8-12 kts and gusts up to 15-20 kts. Thunderstorm
activity this afternoon and evening is expected to remain north
and east of Phoenix, but there is still good potential for
outflow(s) winds from the north or northeast this evening, mainly
between 2-5Z. Odds of 30 kt outflow winds are around 10-20%.
Diurnal E winds should develop late tonight-Thursday morning.
There is uncertainty in when W winds will redevelop again Thursday
morning at KPHX, as it may occur any time between 14-18Z. FEW to
SCT mid and high clouds will pass over the area, with greatest
coverage overnight and Thursday morning.

Southeast California/Southwest Arizona including KIPL and KBLH:
Typical diurnal winds are expected to continue, with SE at KIPL
and S-SW at KBLH. Limited convective activity across the region
will only provide a very low (<10%) chance of an outflow reaching
either terminal through the forecast period. Skies will remain
FEW-SCT through the period.

&&

.FIRE WEATHER...
Sufficient moisture will remain in place to result in daily
thunderstorm activity through the next several days, with activity
mainly confined to the AZ high terrain through the end of the week.
Thunderstorm chances start expanding into the lower elevations
towards the end of the week, and especially by the beginning of next
week. Gusty, erratic winds will be the biggest hazard with these
thunderstorms, exacerbating issues on any ongoing wildfires and new
starts. Aside from the thunderstorm winds, the typical afternoon
upslope gustiness upwards of 20-25 mph will be common. MinRHs during
the next several days will range between 15-25% with fair to good
overnight recoveries between 30-60%.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Watch from Friday morning through Sunday evening
     for AZZ530.

     Excessive Heat Watch from Friday morning through Saturday
     evening for AZZ531>546-548>555-559.

CA...Excessive Heat Watch from Friday morning through Sunday evening
     for CAZ560>570.

&&

$$

DISCUSSION...Berislavich
AVIATION...Benedict/Whittock
FIRE WEATHER...Lojero/Berislavich