732
FXUS65 KPSR 190545
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1045 PM MST Sat May 18 2024

.UPDATE...Updated 06z Aviation Discussion.

&&

.SYNOPSIS...
A weather disturbance will slide primarily north of the region early
next week allowing temperatures to retreat closer to the seasonal
normal. This system will also encourage occasionally breezier
conditions during the afternoon and evening hours with Monday having
the strongest winds and greatest impacts. Very tranquil weather with
near normal temperatures will prevail during the second half of the
week.

&&

.DISCUSSION...
Local shortwave ridging was being eroded and dislodged eastward
early this afternoon as a well defined vorticity center and
associated subtropical jet core punches into the SW Conus. This
negative PV center can be linked to a portion of the base of a
central Pacific Rex block which is currently in the process of
decaying. While this block has resulted in a split flow regime over
the western US, the resumption of more progressive flow early next
week will allow the southern circulation center of the block to
phase with northern stream Gulf of Alaska energy into a full
latitudinal western trough. Although this energy will eject into the
plains during the middle of the week, some measure of broad, low
amplitude troughing should be maintained across the western Conus
the remainder of the week.

Over the next 24-48 hours, anti-cyclonic and subsident
midtropospheric flow will be replaced by a weakly cyclonic pattern
with H5 heights gradually falling ahead of the aforementioned
incoming shortwave. With excellent model agreement and narrow
guidance spread, this evolution will shave a few degrees from a
persistence forecast Sunday, then more so Monday where readings will
have retreated to near the seasonal normal. With exceptionally dry
air precluding any rainfall chances, strong gusty winds will be the
greatest concern and impact, as well timed height falls, a tightened
pressure gradient along a frontal boundary, and midlevel jet energy
become coincident over the forecast area Monday afternoon. Although
gusts 20-30 mph will become common as early as Sunday afternoon,
more pronounced gusts 25-40 mph appear likely Monday as mechanical
mixing taps 25-35kt winds in the H8-H7 layer. Unfortunately, recent
changes in mandated NBM initialization have degraded deterministic
wind forecasts with a distinct low bias at lower elevations, however
NBM probabilities reveal greater chances of a far stronger outcome
with better than a 75% chance of advisory criteria being met across
much of SE California. While not as intense as SE California,
stronger gusts into south-central Arizona will result in a
heightened fire danger when combined with low humidity levels and
receptive fuels.

From midweek through the weekend, a near persistence forecast
appears most applicable as the region oscillates between flat
ridging and low amplitude shortwaves passing north through the Great
Basin. During this time frame, H5 heights should waver little in a
narrow range of 576-580dm with minimal uncertainty among NAEFS
membership. Thus, forecast confidence is very good depicting steady
daily temperatures not terribly far from climatology with dry air
continuing to dominate the region. Deep mechanical mixing will also
continue to tap momentum above the H7 layer resulting in frequent
afternoon/early evening gustiness, however there is little evidence
from NBM probabilities of anything much more than 20-30 mph gusts,
sans the typically windiest locations of far SW Imperial County.

&&

.AVIATION...Updated at 0550Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No major aviation weather concerns will exist through the TAF
period under clear skies. Wind directions will exhibit their
typical diurnal tendencies, with afternoon and early evening
breeziness out of the SW-W. The SE switch will take hold between
08-10Z at KPHX, with sustained speeds overnight remaining mostly
aob 8 kt across all the terminals. Confidence is low in a weak
surge of SW winds reaching the terminals over the next few hours,
which would act to temporarily increase speeds and slightly delay
the SE switch. Speeds are expected to be slightly lower (i.e.,
gusts peaking around 20 kt) tomorrow afternoon than what was
observed today.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major aviation weather concerns, aside from gusty winds, are
anticipated over the next 24 hours under clear skies. Wind
directions will remain WSW at KIPL through the period, whereas
directions will favor SW to S at KBLH through much of the period.
Gusts to around 20 kt will persist through the early morning at
KIPL before calming to aob 12 kt sustained, and winds will soon
establish out of the south at KBLH and remain aob 8 kt through
tomorrow morning, albeit with some periods of very light speeds
and variability. Gusts are expected to redevelop tomorrow
afternoon at both terminals, again peaking around 25-30 kt at KIPL
and slightly lower at KBLH.

&&

.FIRE WEATHER...
A disturbance moving mostly north of the districts early next week
will force temperatures to cool closer to the seasonal normal while
also producing a period of strong, gusty winds. Throughout the week,
minimum afternoon humidity levels will fall into the single digits
at lower elevations and the teens across higher terrain areas. This
will follow poor to fair overnight recovery of 20-40%. Widespread
wind gusts as high as 30-35 mph may be common Monday afternoon
yielding an extended period of near critical thresholds when
combined with low RH and dry fine fuels. Land managers should be
cautious for rapid, uncontrolled spread of any ongoing fires or new
starts Monday. Otherwise, afternoon gusts of 20-25 mph should be
more representative the remainder of the week resulting in a
slightly elevated fire danger.

&&

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

$$

DISCUSSION...18
AVIATION...Whittock
FIRE WEATHER...18