496
FXUS65 KPSR 171205
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
505 AM MST Wed Jul 17 2024

.UPDATE...Updated 12z Aviation Discussion.

&&

.SYNOPSIS...
Daily thunderstorm chances will continue through the
next several days, with most of the activity expected over the
higher terrain areas. The chances for thunderstorms over the lower
deserts will increase heading into this weekend and through early
next week as moisture increases. Temperatures are expected to remain
above normal through the next several days, with excessive heat
conditions likely across the lower deserts starting Friday and
lasting through the weekend.

&&

.DISCUSSION...
The latest objective analysis continues to show the subtropical high
centered across central New Mexico. This high through the end of the
week will slowly drift westward through the northern half of Arizona
and then be situated across southern Nevada by the weekend. As it
migrates westward it will also strengthen from 593dm it is currently
to 596-597dm by the end of the week. As the high moves into Arizona,
there will be increased subsidence aloft leading to drying in the
low to mid level of the atmospheric column. Thus, low-level mixing
ratios through the end of the week will generally remain under 9
g/kg across the lower elevations. However, enough moisture will
still be present to result in daily chances for thunderstorm
activity over the AZ high terrain. With the drying in the
atmospheric column from the increased subsidence, there will be
marginal instability as well as decent convective inhibition
across the lower deserts through the end of the week as well.
Thus, thunderstorm activity across the lower deserts will continue
to be hard to come by and the overall coverage will be isolated
at best. Even though storm activity will remain sparse across the
lower deserts, outflow boundaries emanating from the higher
terrain activity could still impact the region with gusty winds.
Latest HREF shows around a 10-30% chance of gusts exceeding 35 mph
across the south-central AZ lower deserts later today and on
Thursday.

A more active monsoon pattern looks to set up heading into the
weekend through the first half of next week. As the subtropical high
positions itself over Nevada and a trough dives through the Plains,
a diffluent flow aloft pattern looks to be setting up across the
region, increasing the upper-level forcing. In addition, moisture
will be on the increase as well with the global ensembles showing
PWATs rising to 1.5"+. This will lead to the low-level mixing
ratios to rise above 10 g/kg. Therefore, the combination of the
increased upper-level forcing and moisture will result in a more
conducive environment for more widespread thunderstorm activity to
materialize, including the lower deserts. At this time, it
appears that the time period between Sunday through early next
week will have the best shot of seeing at least a day of
widespread strong thunderstorm activity. This far out, it is
difficult to pinpoint exactly what day is likely to see the most
widespread convection as it be dependent on the overall mesoscale
details. 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 and early next week
time frame to have an impactful convective event across the
Phoenix area. This is something that bears watching as we get
closer.

In addition to the storm perspectives during the next several days,
the other main weather headline heading towards the end of the week
and into the upcoming weekend will once again be the potential for
excessive heat conditions. As the aforementioned subtropical ridge
moves westward and strengthens, surface temperatures will on a
gradual warming trend through the end of the week. Highs today and
Thursday will generally range between 108-113 degrees across the
lower deserts with widespread moderate HeatRisk expected. However,
by Friday and Saturday, as the 500 mb height fields from the
subtropical high strengthen to around 596-597dm, surface
temperatures are expected to increase even further with highs
ranging between 110-116 degrees across the south-central AZ lower
deserts to between 112-117 degrees across the western deserts.
Temperatures of these magnitudes will result in a widespread
coverage of major HeatRisk and thus an Excessive Heat Watch has
been hoisted for the lower deserts. The Excessive Heat Watch for
south-central and southwest AZ is in effect for Friday and
Saturday but from the Lower Colorado River Valley westward through
southeast CA, it is in effect from Friday through Sunday.
Temperatures on Sunday 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 1200Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
The primary aviation weather concern through the TAF period will
once again be the potential for gusty outflow winds reaching the
terminals this evening, with mostly FEW-SCT skies and cloud bases
remaining aoa 11 kft. After a period of lighter winds (calming to
aob 6 kt sustained) and somewhat variable directions, winds will
begin to favor westerly by mid-late morning, with gusts to 15-20
kt developing this afternoon. Best timing and direction of
outflows this evening will be as early as 02Z for arrival at KSDL
and KDVT out of the N-NE, though this is subject to change.
Confidence is currently too low to mention timing/direction of an
outflow in the TAF for KPHX and KIWA. Otherwise, typical diurnal
tendencies will prevail and E or VRB winds will establish
overnight.

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,
especially by the upcoming weekend. 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...Lojero
AVIATION...Whittock/Young
FIRE WEATHER...Lojero/Berislavich