790
FXUS65 KPSR 161846
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1146 AM MST Fri Aug 16 2024

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS...
High pressure shifting east of the region this weekend will allow
thunderstorm chances to return to parts of the area. Storm chances
will be highest in the favored higher terrain of Arizona with only
isolated storms progressing in the lower desert communities.
Thunderstorm probabilities look to remain modest through next week.
Temperatures should remain slightly above the seasonal normal with
highs around 110 degrees over the warmest lower desert communities.

&&

.DISCUSSION...
While strong zonal jet energy continues to punch into the Great
Basin early this morning fostering dampened subtropical ridging
across the SW Conus, deep negative height anomalies can already be
seen digging through the northeast Pacific poised to force a
downstream amplification phase. This evolution will result in
strengthening high pressure over the southern high plains this
weekend and deep southeast flow across the forecast area capable of
importing better quality moisture poleward. As moisture deepens,
thunderstorm chances and coverage should slowly increase though more
widespread activity entering lower elevations may be more difficult
to achieve. Nevertheless, gusty outflow winds and dust will return
as more common weather impacts even with limited rainfall.

Larger scale subsidence with dry air dominating the top of the
boundary layer will persist today as high pressure aloft migrates
east across the CWA. Once again, deep convective activity will be
relegated to points well south of the forecast area, although
finally leaking back north over the international border. The flow
pattern tonight will revert back to typical monsoon with forecast
BUFR soundings indicating a surge of deeper moisture with PWATs in
excess of 1.75" and shallow mixing ratios 12-13 g/kg moving through
south-central AZ. Despite this improved moisture profile, enough
drier air looks to reside around the H7 layer yielding CinH around
50 J/kg over lower elevations making it difficult for many storms to
survive/redevelop off the higher terrain initiation points. Rather,
outflow winds sweeping into lower deserts will become more common
through the weekend with the official NBM advertising rather low
POPs.

Although excellent ensemble agreement exists regarding the larger
synoptic pattern next week, forecast confidence is actually somewhat
low based on subtle details of the subtropical ridge position and
magnitude. While there is good agreement that this ridge will
attempt to retrograde into New Mexico as a result of East Pacific
energy lifting north, H5 heights may be restricted as low as 594-
596dm, or possibly up to a 596-599dm range. NBM temperature
forecasts have trended slightly warmer early next week with the 75th
percentile of model spread still showing flirtation with major
HeatRisk and excessive heat criteria. Initially, better forcing
mechanisms and inverted troughs will be relegated to northern
Mexico, however a growing number of ensemble members are indicating
a constriction of the H5 high pressure center and more favorable
conditions for better moisture import, lower inhibition, and forcing
to spread north into south-central AZ. There are some subtle signs
in mandated NBM POPs suggesting this trend, though probably still
lagging a reliable forecast. Based on the preponderance of evidence,
feel there will be a convectively more active day sometime during
the middle of the week.

&&

.AVIATION...Updated at 1846Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Westerly winds will prevail through this afternoon and evening
with speeds mostly at or below 10 kts along with typical afternoon
gusts into the middle teens. No VCSH/VCTS expected this afternoon
and evening, but there is a low chance for an outflow from the
south later this evening. During the overnight hours there is some
uncertainty in wind directions, but an easterly shift is still
forecast to occur, with as late as 12-13Z at KPHX. There is also
indications elevated convection, with potential for lightning, may
develop near or in the metro tomorrow morning, generally after 9Z.
Confidence in this activity is still too low at this time to
include in the TAF. Westerly winds are expected by 18-19Z
tomorrow, with some light variability during the wind shift. SCT
to BKN clouds are expected tonight and tomorrow morning, with
bases aoa 13-14K ft AGL.

Southeast California/Southwest Arizona including KIPL and KBLH:
Wind speeds at KIPL will generally be aob 10 kt favoring the east
to southeast, but with a brief west to southwest anticipated this
evening. At KBLH, winds will gust up around 20-25 kt this
afternoon and early evening, prevailing out of the south to
southwest. FEW-SCT mid and high clouds will pass over the area in
the afternoon and evening timeframe.

&&

.FIRE WEATHER...
Slightly above normal temperatures will prevail through much of next
week as conditions remain somewhat drier than normal by mid monsoon
standards. Higher terrain areas of eastern districts will stand the
best chance for thunderstorms over the weekend, however wetting
rainfall should be somewhat isolated. There may be a period of
better storm chances and accumulating rainfall at some point next
week. With the pattern shifting towards better moisture intrusions,
minimum afternoon humidity values between 15-25% will be common.
Overnight recovery should be fair to good between 30-60% with some
local areas into the excellent recovery range. Wind patterns should
follow a more typical upslope/drainage pattern with afternoon
upslope gusts of 20-25 mph. Chances for gusty thunderstorm outflow
winds will increase over the weekend and occasionally next week.

&&

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

$$

DISCUSSION...18
AVIATION...Benedict/Gibbons
FIRE WEATHER...18