678
FXUS65 KPSR 210554
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1054 PM MST Fri Sep 20 2024

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
A low pressure system over southern California will track across
Arizona tonight and Saturday. This will lead to isolated showers
and thunderstorms into Saturday morning. The best chances are over
eastern Riverside and La Paz Counties this afternoon and evening.
Lesser chances will be over northern Maricopa, northern Pinal, and
southern Gila Counties tonight into Saturday morning. This system
will bring below normal temperatures for a short time followed by
a warming trend early next week with desert highs in the 100 to
105 range Monday through Friday.

&&

.DISCUSSION...
An upper low is centered over far southwest CA early this
afternoon. The combination of dynamical forcing and thermodynamic
destabilization has been generating thunderstorms over/near
Joshua Tree National Park. This was handled well by the hi-res
models. Anticipate some development further east later on today
over La Paz County expanding to portions of northern Maricopa,
northern La Paz and Gila County tonight as the system tracks
eastward. However, it appears the activity will be isolated per
HREF and this is not surprising due to limited moisture
availability as well as the timing of the colder air aloft
(nighttime/morning for south-central AZ) for less optimal
destabilization. For the Greater Phoenix Area, the window of
opportunity is roughly 8pm tonight - 8am Saturday. But, coverage -
if any - will be limited. The system will exit AZ by Saturday.

With the passage of the system, temperatures will trend down (more
noticeably today over SE CA and SW AZ; Saturday for south-central
AZ). Breeziness picks up as well but speeds are expected to remain
solidly below Advisory criteria for the large majority of the
forecast area.

For next week, a Pacific ridge slowly advances into the western
CONUS leading to a warmup with highs at most lower desert
locations in the 100-105 range Monday - Friday. Probability of
reaching 110 any given day during the that time frame remains at
or below 10%. Something of note is that follow-on short waves
(subsequent to the system currently over SoCal) in concert with a
bit of fold-over by the advancing ridge, are looking more likely
to lead to a very weak upper low/weakness over/near AZ. For now,
that would likely only result in slight chances of storms over
the White Mountains on Wednesday.

&&

.AVIATION...Updated at 0554Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
VCSH with potential brief -SHRA impacts is anticipated now through
8Z at the latest tonight. SCT to BKN clouds down to 7-8K ft is
expected during this time period, with clearing thereafter. Winds
will also shift S-SE during this time and prevail through the rest
of the morning. There is another low chance (10%) for VCSH/VCTS
Saturday morning, around the 15Z timeframe, but any activity may
stay just north of the metro area. Confidence and probability is
too low to include this in the TAF. An early W wind shift is
expected Saturday, as early as 15-16Z. Wind speeds will remain
light through the TAF period, mostly aob 8 kts.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather concerns should exist through Saturday under
periods of mid/high cloud deck tonight, then followed by mostly
clear skies. SHRA in the region of KBLH, but outside the 10sm
radius, may continue through 9-10Z in the region before fully
dissipating. Current N winds at KBLH are expected to slowly turn
counter-clockwise through the next few hours before becoming VRB.
KIPL will favor a W winds tonight before also becoming VRB. VRB
wind conditions are then expected at both terminals for much of
Saturday.

&&

.FIRE WEATHER...
A drier and cooler airmass encompassing the region will result in
below normal temperatures through Saturday. Stronger southerly to
southwesterly will develop this afternoon, with gusts to 20-25 mph
expected today, focused across higher terrain areas of the western
districts and southcentral AZ. MinRH`s will hover around 15-30%
region-wide over the next couple of days, with some slight drying
first across the western districts starting Saturday and spreading
eastward on Sunday, with MinRH`s more in the 10-20% range by the
beginning of next week. Overnight RH recoveries will be fair to good
across the region over the next couple of mornings, before degrading
to generally fair to poor in spots going into next week. Warming
temperatures to above normal are expected to develop late this
weekend and persist through most of next week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...AJ
AVIATION...Benedict
FIRE WEATHER...Young