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497
FXUS65 KPSR 210019
AFDPSR

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

.UPDATE...
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Updated Aviation Discussion.
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&&

.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...
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Updated at 0019Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Southerly winds will continue until a more prevalent westerly wind
takes hold by 1-2Z. Wind speeds will be up to 10-15 kts with
occasional gusts to 20 kts for the next few hours. East to
southeast winds then develop tonight close to midnight. Saturday,
an earlier west shift, around 17-18Z, is expected. There is a
slight chance (15-20%) for some spotty showers in the metro area
tonight around 5-8Z, but most activity that develops will favor
areas just to the north and east of the area. Cloud coverage will
be FEW to SCT, with lowest cloud bases (6-7K ft AGL) tonight
around midnight. Otherwise, cloud bases will be around 8-10K ft.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather concerns should exist through Saturday afternoon
under periods of mid/high cloud decks. Confidence is good that W
winds will prevail through much of the period at KIPL with some
modest gusts possible through this evening. Directions will be
more variable at KBLH with a W component favored through the rest
of this evening. Ongoing showers are seen in the general region,
but expected to remain outside the 10 sm radius from the
terminals, with only a 10-15% chance at KBLH through 8-9Z.
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&&

.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/18
FIRE WEATHER...Young