Flash Flood Warning 35, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
410 PM MST Sat Aug 3 2024

.UPDATE...
Updated Aviation

&&

.SYNOPSIS...
Extreme heat will be the main story this weekend and into Monday as
high temperatures soar upwards of 110 to 116 degrees and overnight
temperatures only cool into the middle 80s to lower 90s across the
lower deserts. Daily thunderstorm chances will continue over the
next few days, but storm coverage will be primarily focused over
Arizona high terrain. An upper level disturbance arriving early next
week will bring better potential for more robust storms and greater
coverage of storms, as well as a cooler temperatures.

&&

.DISCUSSION...
An unseasonably strong ridge of high pressure has become established
over the 4-Corners region with 500 mb hghts now approaching 598 dam.
Hghts aloft have also increase over south-central AZ and are now up
to 595-596 dam according to the latest RAP 500mb analysis. This
expansive area of high pressure is expected to persist into early
next week, resulting in excessively hot temperatures across the
Desert Southwest through Monday.

Today, we will see highs approaching 115 degrees in some locations
such as Phoenix, Yuma, Blythe, and El Centro. There is currently a
75% chance that Sky Harbor will exceed the record high of 114 this
afternoon (see climate section for more information on records). Low
temperatures tonight will be very uncomfortable, with many locations
not falling below 90 degrees. As far as rain chances go today,
instability will be much more limited today (SBCAPE largely below
1000 J/kg) and therefore storms are expected to remain isolated
across our region. The highest rain chances will be around 30-40%
across S Gila County and 20% or less in the lower elevations of
south-central AZ. The only caveat will be a complex of storms
expected across Pima County which could send an outflow boundary
into Pinal and Maricopa Counties this evening, which may result in
patchy blowing dust along I-8. It is not out of the question that a
storm or two could form along said boundary as it progresses towards
the Phoenix Metro area tonight.

On Sunday, the subtropical high over the 4-Corners region is not
expected to move much at all which will allow temperatures to yet
again climb well above normal. Lower desert highs will be similar to
today in the 108-115 range which will result in areas of major to
extreme HeatRisk. Therefore, it is important to continue to practice
heat safety measures to keep yourself safe during this stretch of
very hot temperatures. Latest hi-res guidance including the HREF
does indicate slightly better instability tomorrow afternoon across
south-central AZ up to 1000 J/kg. Forecast soundings also show very
high DCAPEs approaching 2000 J/kg. Thus, convection may be more
robust with slightly higher coverage across the central deserts
Sunday afternoon and evening with the main threats being strong,
gusty outflow winds.

&&

.PREVIOUS DISCUSSION...
Signs still point toward a more active monsoonal pattern, with
greater storm intensity and coverage during the early to middle
portions of next week as an inverted trough (IT) moves in from the
east and undercuts the subtropical ridge. Global ensembles have the
IT reaching the Sonoran Desert as early as Monday afternoon or
evening next week. Global ensemble means have the core vorticity
passing mostly through northern Mexico, but regardless of whether a
track staying just to the south limits the full potential of
thunderstorm activity, there is strong agreement that the wave will
help push moisture levels back up, and thus instability, as well as
bring increased mid-level easterly winds. Global ensembles have PWAT
values climbing back into the 1.6-1.7" range and mixing ratios to
around 11-12 g/kg by Tuesday. GEFS probability for >1000 J/kg of
CAPE respectively goes up to around 40-50% for most of southern AZ
with this moisture increase. NBM PoPs remain highest on Tuesday,
with widespread 30-40% across southern AZ.

This shortwave during the first half of the week should help
moderate and lower high temperatures slightly, but based on the
current NBM forecast not enough to fall below normal. Then by the
end of the week, as the influence of the wave wanes and moisture
levels drop slightly again, temperatures may go right back up with
the persistent strong high still in place over the region.

&&

.AVIATION...Updated at 2310Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Distant outflows causing potentially more than one abrupt wind shift
this evening will be the primary weather issue through Sunday
afternoon. While storms with SCT mid/high cloud decks will be common
around the periphery of the Phoenix airspace, odds of any direct TS
impact is well under 20% and not considered in this TAF package.
Instead, weakening northerly outflow would be most likely to impact
KDVT/KSDL early evening with a separate long traveling southerly
outflow potentially sweeping into KPHX/KIWA mid/late evening.
Confidence in exact directions and timing still remains low with the
possibility of outflows dying before reaching any given terminal. An
eventual shift to easterly should be established after midnight with
a return to a westerly component near/shortly after noon Sunday.
Probabilities for more direct outflow and TSRA impacts appear much
higher Sunday evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
No major weather issues will exist through Sunday afternoon under a
few occasional midlevel cloud decks. Wind directions will vary
between a daytime SE component and a general veering towards an
overnight SW component. TS well east of KBLH may result in a period
of evening easterly winds, however confidence is too low to include
in this TAF package.

&&

.FIRE WEATHER...
Scattered thunderstorms will be possible this weekend across the AZ
high terrain. Although storm chances will remain low (20% or less)
in most lower desert locations, there will still be the risk for
strong outflow boundaries which could result in a sudden wind shift
and gusty winds at active burn areas. Much better chances for
widespread wetting rains will arrive early next week, with
possibility for isolated flash flooding across the new burn scar
locations. Aside from thunderstorm outflows, winds will should
remain light and diurnal with afternoon gusts into the teens.
Minimum RH values will mostly reside in the 10-20% range through
this weekend with overnight recovery to 30-50% most areas.
Temperatures this weekend will be several degrees above normal,
with lower desert highs reaching the 110-116F range. Temperatures
will cool down closer to normal by Tuesday with the arrival of
better moisture and cloud cover.

&&

.CLIMATE...

Phoenix Daily Record Temperatures

         High      Warm Low
      ----------  ----------
8/3   114 (2020)   92 (2023)
8/4   116 (1975)   91 (2018)
8/5   116 (2023)   95 (2009)

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ530>555-559-
     560.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ560>570.

&&

DISCUSSION...Salerno
PREVIOUS DISCUSSION...Benedict
AVIATION...18
FIRE WEATHER...Salerno
CLIMATE...Benedict

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