249
FXUS65 KPSR 131802
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1102 AM MST Sat Jul 13 2024

.UPDATE...Updated 18Z Aviation Discussion.

&&

.SYNOPSIS...
A strong ridge of high pressure will continue yield above normal
temperatures across the region through the weekend. This ridge
will finally migrate over the 4-Corners region this weekend,
resulting in a noticeable increase in moisture levels and a slow
cooling trend. Isolated showers and storms will be possible
primarily across the northern Arizona high terrain today with the
best chances for storms reaching the lower deserts on Sunday.
Drier conditions are expected to gradually spread over the area
during the first half of next week.

&&

.DISCUSSION...
Radar earlier this morning showed showers and isolated
thunderstorms across parts of the Valley thanks to a northeasterly
outflow that helped spark the nocturnal convection. This activity
led to gusty winds with isolated gusts in excess of 40 mph, while
rainfall amounts have been less than 0.10" for most areas that ended
up under these showers and isolated thunderstorms. We will
continue to see activity weaken and diminish this morning with
quieter conditions anticipated through the rest of the day.
Meanwhile, above normal temperatures continue as the upper level
ridge gradually shifts eastward today with the center of the high
positioning itself over the Four Corners. Global ensembles show
the ridge strengthening some today with 500 mb heights rising to
around 595-597 dm across the forecast area.

However, as low level moisture increases this weekend temperatures
will gradually trend downward, dropping HeatRisk into the Moderate
category for most places. On the other hand, the increase in
moisture, with surface dew points around 60-70 degrees, will help
push heat indices upwards of 115-118 degrees for areas along and
west of the Lower Colorado River Valley today and tomorrow. Thus,
despite HeatRisk being mostly in the Moderate category, the
Excessive Heat Warning for southeast California and along the
Lower Colorado River Valley has been extended through Sunday
given these dangerous heat indices. Across the Arizona lower
deserts, afternoon temperatures today are forecast to top out
around 108-114 degrees before falling below 110 degrees for most
places Sunday afternoon. The Excessive Heat Warning for the
Arizona lower deserts is set to expire this evening. For those who
have outdoor plans this weekend, the proper heat safety
precautions, such as staying hydrated and limiting time outdoors,
should be taken to avoid any negative health impacts from the
heat.

The latest HREF keeps the best instability out across the western
half of the CWA this afternoon, though only isolated convection
along terrain features such as the Kofas may occur. Instability
across eastern and southeastern Arizona will be minimal today,
keeping things quiet across south-central Arizona this afternoon and
evening. HREF paintball plots keep the bulk of activity confined to
far southern Arizona along the international border and across parts
of northern Arizona today. However, this will change Sunday as gulf
surges and moisture fluxes from Sonora thunderstorm complexes push
better moisture into Arizona. Ensemble guidance show PWATs increasing
from around 1.1" today across south-central Arizona to around
1.4-1.5" Sunday. The increase in boundary layer moisture will
boost our instability across the region with GFS forecast
soundings continuing to show CAPE values around 1500 J/kg, though
with some modest CIN present. Hi-res guidance suggests convection
will fire up along the Rim and across southern Arizona Sunday
afternoon before potentially sending strong outflows toward the
Valley with the potential for outflow collisions to promote new
thunderstorm development across the lower deserts. Forecast
soundings also indicate strong DCAPE with values upwards of
1500-2000 J/kg, which will support strong to potentially severe
winds. Thunderstorms Sunday will be capable of producing strong
winds capable of generating dense blowing dust along with locally
heavy rainfall.

Starting Monday, drier air will begin working its way into southeast
California and into western portions of Arizona, while moisture
across the eastern half of Arizona will remain adequate to support
another round of showers and thunderstorms Monday afternoon. Drier
air will continue to works its way in across the region going into
the middle part of the week next week as global ensembles show PWATs
falling to around 1". This will limit any convection to the Arizona
high terrain, while the drier air will once again boost temperatures
near or in excess of 110 degrees across the lower deserts. Moisture
then looks to increase again into the region late next week and
yield an increase in thunderstorm chances once again.

&&

.AVIATION...Updated at 1802Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
W to SW winds should develop at all terminals by 18-19Z with
speeds up to around 8-12 kts and occasional gusts up to 15-20 kts
through this afternoon. An enhancement of the SW winds are
expected to push speeds up to 12-16 kts with an occasional gust up
to 25 kts for a few hours this evening. There is also a low
chance (<10%) for a thunderstorm outflow boundary from south late
this evening. Winds subside later tonight, but confidence is very
low in diurnal E winds developing Sunday morning. No thunderstorms
are expected in the Phoenix area during this TAF period. Skies
will remain mostly clear through this afternoon, followed by FEW
to SCT mid and high clouds this evening/tonight.

Southeast California/Southwest Arizona including KIPL and KBLH:
VCSH, with an isolated lightning strike or two, at KIPL should
diminish over the next 1-2 hours. KIPL will see SE winds prevail
through the TAF period with speeds up to 10-15 kts and gusts up to
20-25 kts at times this afternoon. KBLH will see prevailing S
winds and see similar wind speeds and gusts. At both terminals
there is a slight chance (20-30%) for a thunderstorm outflow to
pass through. At KIPL, the favored direction will be from the W or
SW and at KBLH it will be from the E to NE. The potential for
VCSH/VCTS at the terminals is low (<10%) this afternoon and
evening. FEW to SCT mid and high clouds will pass over the area at
times.

&&

.FIRE WEATHER...
Very hot temperatures will be seen again today with thunderstorm
chances remaining primarily confined to the northern Arizona high
terrain this afternoon. Temperatures will slowly retreat over the
weekend while humidity levels increase. Storms will initially
produce only isolated wetting rain chances in eastern districts
with areal coverage likely becoming far more widespread by Sunday.
Moisture levels will be on the rise through this weekend,
pushing MinRH values up from 15-25% today to upwards of 20-30% on
Sunday. 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 15-25 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.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ530.

     Excessive Heat Warning until 8 PM MST this evening for
     AZZ531>555-559.

CA...Excessive Heat Warning until 8 PM PDT Sunday for CAZ560>570.

&&

$$

DISCUSSION...Smith
AVIATION...Benedict
FIRE WEATHER...Smith/Kuhlman