Special Weather Statement, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
305 PM MST Sat Aug 3 2019

.SYNOPSIS...
Isolated storms will persist across the higher terrain of eastern
Arizona today. Otherwise, a warming and drying trend will occur
through Monday as the Monsoon High meanders across the Desert
Southwest. Main impact will be excessive heat the next few days
before temperature return to normal Wednesday. Richer moisture
will also return by the middle of next week along with the chance
of thunderstorms, particularly east of Phoenix.

&&

.DISCUSSION...
The large anticyclone across the Southwestern U.S. remains in
place with the high center currently located across west-central
New Mexico. Just to the west of the high center, residual low and
mid level moisture is still plenty sufficient for isolated to
scattered storms today. The lack of morning cloud cover allowed
the atmosphere to quickly heat up and destabilize across the
southeastern 1/3 of Arizona with ML CAPES of up to 1000 J/kg over
the high terrain. Early afternoon temperatures across the lower
deserts have already topped 110 degrees in many spots, but drier
westerly flow will lower boundary layer moisture enough to mostly
prohibit storms across the lower deserts. However, we can't
completely rule out a stray storm popping up along an outflow
collision around the outer fringes of the Phoenix metro.

The biggest forecast concern for today through Monday will be the
Excessive Heat across the lower deserts. After highs today of
110-112 across the south-central Arizona deserts and 112-115
across the southeast California deserts, very little will change
through Monday as the high center remains parked near the
Arizona/New Mexico border. For reference, 500mb heights of around
596dm fall around 97% of climo for the time period. Median
temperature guidance is still running too cool, so kept with
pushing forecast temperatures toward the upper end of guidance
through Monday.

With the Monsoon High drifting into eastern Arizona Sunday and
Monday, low-level mixing ratios will continue to decline. Steering
flow will also be quite light and not favorable for much
convection. Ensembles suggest that PWATs will increase again by
Tuesday/Wednesday as the ridge drifts eastward. Weak steering flow
will generally remain unfavorable for widespread storms across
the lower deserts, however overall moisture will be sufficient for
at least isolated activity each day beginning Wednesday.

&&

.AVIATION...Updated at 1810 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Expect light to moderate winds to favor common diurnal directions
during the period. Storm chances are slight and not expected to
affect the terminals this evening, although they can not be
completely ruled out. There is a slight chance for an outflow to
move into the greater Phoenix area this evening but the
probability is low. Skies will be partly to occasionally mostly
cloudy with FEW-SCT mid level clouds near 10-18kft and SCT-BKN
thin cirrus.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will follow familiar warm season diurnal patterns along with
periods of light and variable conditions. Winds will also favor
the south to southwest at KBLH and light and variable at KIPL
before switching to the west beginning this evening with some
evening gusts near or above 20kt. Skies will partly to
occasionally partly cloudy with FEW-SCT mid clouds near 15kft and
SCT-BKN cirrus through this evening with decreasing clouds
beginning late tonight.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Isolated to scattered showers and thunderstorms will continue over
the higher terrain of south-central Arizona each day. There is
some uncertainty on which day will be more active and allow
thunderstorms to make it into the lower deserts, however Thursday
and Friday could potentially be more favorable. High temperatures
will start out several degrees above seasonal norms on Tuesday
including over hotter western deserts into far SE CA with the
hotter readings around 110 degrees. A cooling trend with closer to
normal max Ts is expected by Wednesday through the second half of
the week. On Tuesday, minimum RH values will improve, but still
drop to around 15 percent over the deserts. Humidity will
gradually increase during the week and by Wednesday minimum RH
values will climb to between 20 and 30 percent over the southwest
and central Arizona deserts. Winds each day will generally be on
the light side, with typical afternoon and early evening gustiness
favoring the south over the western deserts and the southwest
into the south-central deserts.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ531-532-534-
     536-537-540>544-546-548-550-551-556.

     Excessive Heat Warning from 10 AM Sunday to 8 PM MST Monday for
     AZZ538-539.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ566>568-570.

&&

DISCUSSION...Hirsch/Kuhlman
AVIATION...Sawtelle
FIRE WEATHER...Kuhlman

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