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591
FXUS65 KPSR 181659
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
959 AM MST Wed Sep 18 2024

.UPDATE...
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Updated Aviation section.
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&&

.SYNOPSIS...
A weather system will dive south along the West Coast and pass just
north of the region late this week, which will help keep
temperatures below normal into early this weekend. As this weather
system passes just north of the region, rain chances increase for
portions of Northern Arizona, but the southern half of the state
should remain dry. A ridge of high pressure building to the west
will allow temperatures to increase to the triple digits over the
weekend into early next for the lower deserts.

&&

.DISCUSSION...
The upper level trough that brought portions of eastern Arizona rain
chances yesterday morning is now moving into the Northern Plains as
it continues to eject off to the northeast. Early morning water
vapor imagery depicts longwave troughing remaining over much of the
western US, with a reinforcing shortwave diving south along the West
Coast. This morning is shaping up to be quite cool, with dry air in
place (surface dewpoints in the 20s and 30s for many locations), and
only spotty cloud cover over the eastern CWA. Temperatures are
already around 5 to 10 degrees cooler than this time yesterday.
Early morning lows in the 60s should be common across the lower
deserts, with some rural and higher elevation areas potentially
touching the upper 50s. Lower desert highs today will also be
seasonably mild, around 5 to 10 degrees below normal, mainly in the
lower 90s.

The aforementioned shortwave trough diving south along the West
Coast will eventually start to move onshore late Thursday over
Central California. This is consistent with prior forecasts, but
ensembles in recent runs are trending towards a stronger trough
diving slightly further south. This will make for a stronger
synoptic forcing mechanism to squeeze out any available moisture as
the trough passes overhead later this week, and as such, NBM PoPs
have increased slightly. PoPs along the high terrain on the northern
periphery of La Paz and Maricopa Counties are now up to as much as
15%. This is still quite low, reflecting the poor moisture returns
expected with this system. Southerly flow Thursday through Thursday
night ahead of the trough will bringing ensemble mean PWATs as high
as 0.9-1.0", peaking over the Lower Colorado River Valley. This will
not be enough moisture for precipitation over the lower elevations
of Southern AZ. Synoptic-scale ascent ahead of the trough should be
enough, however, to produce some scattered showers over the
mountains of Northern/Central AZ on Friday.

Aside from chances of precipitation over the Northern AZ high
terrain late this week, the passage of the upper level trough will
help to keep temperatures below normal into this weekend and
bring widespread breezy conditions Thursday and Friday. Gusts
between 20-25 mph are expected during the afternoon/early evening,
with locally higher values upwards of 30 mph over Western
Imperial County. NAEFS mean 700 mb and 850 mb heights/temperatures
will be mainly below the 10th percentile of climatology over the
region through early Saturday. Forecast highs remain below the
Century mark across the lower deserts through Saturday, though
there is up to a 25% chance that some of the typically hotter
locations may touch 100 deg F before Saturday.

The upper level trough will eject northeast of the Desert Southwest
Saturday, allowing H5 heights to rebound to 584-587 dam by Sunday. A
high amplitude ridge then sets up just off the West Coast and
gradually shifts east early next week. With the increase in heights
aloft, temperatures will trend up over the weekend back to near
normal range on Sunday and likely slightly above normal early next
week. WPC cluster analysis still reveals a fair amount of
uncertainty as to the overall pattern next week, with notable
differences in the evolution of ridging over CONUS. No prominent
negative height anomalies appear in any of the clusters, with flat
ridging over the Sonora and strong positive height anomalies over
the Northern CONUS. However, the potential exists for cutoff lows to
form mainly to our east/southeast and over the Eastern Pacific
during the middle of next week, as indicated by at least the GFS
deterministic, so stay tuned for updates on the outlook for the
middle of next week.

&&

.AVIATION...
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Updated at 1658Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT and
Southeast California/Southwest Arizona including KIPL and KBLH:

An upper trough centered off the northern CA coast will maintain
southwesterly flow aloft. Only some minor cloudiness at times
over south-central AZ with FEW-SCT cumulus - mainly over higher
terrain areas east of Greater Phoenix) and FEW-SCT cirrus.
Surface winds will be light (sustained below 10 kts - except
perhaps KBLH mid-late afternoon) and favor familiar diurnal
tendencies with the shifts in direction.
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&&

.FIRE WEATHER...
A drier and cooler airmass encompassing the region will make for
below normal temperatures through Saturday. Winds will follow
generally light and diurnal patterns today, with typical upslope
gusts to 15-20 mph during the afternoon. Slightly stronger
southerly afternoon gusts to 20-25 mph can be expected tomorrow.
Afternoon minRHs in the teens will be common in lower elevations
and closer to a 20-30% range across higher terrain today, whereas
MinRHs will be in the upper teens up to around 25% areawide
Thursday and Friday. Overnight recovery will range widely from
fair to good at 30-70%. As temperatures warm closer to normal over
this weekend and into early next week, humidity levels will
decrease further, with single digit readings more likely in lower
desert communities by Sunday.

&&

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

$$

DISCUSSION...Whittock
AVIATION...AJ
FIRE WEATHER...Whittock