-- Discussion containing changed information from previous version are highlighted. --
209
FXUS65 KPSR 200014
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
514 PM MST Thu Sep 19 2024

.UPDATE...
-- Changed Discussion --
Updated 00z Aviation Discussion...
-- End Changed Discussion --

&&

.SYNOPSIS...
A low pressure system will pass through southern California and
Arizona tonight through Saturday. In the process, there will be
cooler temperatures and breezy conditions. There will also be a
chance for showers and thunderstorms in some areas including
eastern Riverside County, La Paz and northern Maricopa Counties,
and portions of Gila County. Better chances will be north of
there. Next week, a ridge of high pressure will advance inland and
result in a warmup with desert highs reaching triple digits
again.

&&

.DISCUSSION...
A positively tilted upper trough lies across much of the western
and north-central CONUS. Within that, a low center is located near
the central CA coast. Meanwhile, high pressure is centered to the
southeast over northeast Mexico. With this flow pattern, there is
a distinct temperature gradient aloft with rather warm
temperatures over southeast AZ and rather cool temperatures near
the CA coast. Precipitable water has increased a little compared
to yesterday and there are some areas of few/scattered cumulus.
But, there isn`t enough moisture/instability for convection
concerns today - just some pockets of few-scattered cumulus.

The upper low will begin entering southeast California tonight and
exit Arizona by Saturday evening. In the process, it will bring a
measure of dynamical forcing and some destabilization with cold
air aloft for our forecast area plus a weak cold front. The
system will be positively tilted before becoming neutrally tilted
late Friday night which is not ideal for moisture advection. But
the other aforementioned factors will help to make up for the
modest moisture availability leading to an opportunity for showers
and thunderstorms for northern portions of our forecast area
(generally eastern Riverside County, La Paz, northern Maricopa,
and portions of Gila County) with better chances to the north of
there. Best chances for our forecast area are over Joshua Tree
National Park Friday afternoon. For the Greater Phoenix area,
precip chances will be limited to northern portions (mainly north
and northeast of the 101 Freeway in the north Valley). As an
aside, PoPs have trended up a bit from the previous forecast
package. The passage of the system will also lead to cooler
temperatures though not a dramatic drop. The coolest day for SE CA
and SW AZ will be Friday; for south- central AZ it will be
Saturday. Daytime and evening wind speeds will also pick up
(breeziest on Friday) but fall short of Advisory conditions. An
exception may be the southwest corner of Imperial County. But HREF
probabilities of exceeding 40mph there are currently less than
50%.

Heights and thicknesses slowly rebound next week and in turn
temperatures rebound as well. In fact, triple digits return
beginning Sunday and peak on Tuesday with little change through
Thursday. But the probability of reaching 110 remains less than
10% for each of those days. Though there are indications of
follow-on shortwaves carving out a cutoff over/near the CWA, it
would likely have little impact other than muting the warm up a
little but. Of note, even the 25th percentile high temperatures
for PHX and YUM are in the 100-105 range next week.

&&

.AVIATION...
-- Changed Discussion --
Updated at 0000Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Southerly winds, with SW-SE variability, are expected to persist
during the next couple of hours before a more predominant west to
southwest component develops by around 02z. Easterly winds will
return overnight tonight. A similar wind pattern is forecast for
Friday, with easterly winds in the morning switching out of the
south with SE-SW variability during the late morning through most
of the afternoon hours before eventually switching out of west to
southwest after 00z. Wind speeds will generally remain aob 10
kts, although occasional gusts into the mid to upper teens will be
possible during the afternoon and early evening hours. FEW to SCT
cumulus will be common through the TAF period with varying bases
between 6-10K ft AGL (lowest overnight and highest in the
afternoon)

Southeast California/Southwest Arizona including KIPL and KBLH:
At KIPL, SE winds will switch out of the west by early this
evening and remain out of the west through the rest of the TAF
period. At KBLH, southerly winds are expected through most of the
period, with a period of light and variable winds early Friday
morning. Wind speeds at KIPL will pick up early this evening with
gusts upwards 20-25 kts, before diminishing somewhat late Friday
morning and early afternoon before picking up once again late
Friday afternoon. Wind speeds at KBLH will remain aob 12 kts. FEW-
SCT cumulus will be common throughout the TAF period.
-- End Changed Discussion --

&&

.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 and tomorrow. MinRH`s will hover around 15-25%
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...Lojero
FIRE WEATHER...Young