886
FXUS65 KPSR 192103
AFDPSR

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

.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...Updated at 1803Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Persistence wind forecast is expected again for the next 24
hours. Southerly winds, with SW-SE variability, are expected to
develop by midday and persist through 00-01Z before a more
predominant west to southwest component develops. Diurnal
easterlies then return tonight. Daytime wind speeds will be around
7-10 kts with afternoon gusts occasionally up to 15-20 kts. 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:
A period of variability this morning will be followed by SE winds
at KIPL and S at KBLH. Between 00-02Z, winds will shift W at KIPL,
with gusts up to 20-25 kt during the evening and into the
overnight hours. Speeds will mostly be in the 6-10 kt range at
KBLH. Skies will remain mostly clear. FEW cumulus are expected at
KBLH this afternoon, becoming FEW to SCT by late Friday morning.

&&

.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...Benedict
FIRE WEATHER...Young