Excessive Heat in California, August 3 to 5, 2019

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
540 AM MST Sat Aug 3 2019

.UPDATE...Updated aviation and fire weather discussions.

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.SYNOPSIS...
Isolated storms will persist across the higher terrain of Gila
County today. Otherwise, a warming and drying trend can be expected
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.

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.DISCUSSION...

Anticyclonic flow predominates across the Desert Southwest with the
Monsoon High presently centered in its climatological mean position
across western New Mexico. Latest operational models remain in good
agreement, depicting the high meandering over the next several days.
In terms of sensible weather, this will result in a warming and
drying trend through early next week.

High temperatures the next few days will likely not reach daily
records, however height anomalies exceeding the 90th percentile
will translate into areas of excessive heat. Due to the
recent poor performance in the National Blend, the forecast was
instead derived using the 80th percentile of a multi-model ensemble.
This equates to high temperatures in Phoenix above the 110 degree
mark with low temperatures likely not falling below 90 degrees
through at least Monday. Thereafter, temperatures will return to near
normal values by Wednesday.

Residual low-level moisture will be sufficient for isolated storms
this afternoon. CAMs generally suggest convection will remain
confined to the higher terrain east of Phoenix including eastern Gila
County and across southeastern Arizona. However, latest HREF
indicates mean MUCAPEs > 500 J/kg and there is a non-zero
probability (less than 10 percent) that colliding outflow
boundaries could initiate isolated convection across the Valley.

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.

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.AVIATION...Updated at 1240 UTC.

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

Strong high pressure aloft centered east of Arizona, and a weak
southerly steering flow will tend to keep convection focused over
the high terrain east of Phoenix today and tonight. As such, storm
chances are slim and not expected to affect the terminals this
evening. There is an outside chance for an outflow to move into the
greater Phoenix area this evening but confidence is very low and
details such as direction and speed are sketchy at best at this
time. For the most part will just change the prevailing
afternoon/evening west wind to variable at 6kt around 04-05Z with
winds then becoming generally east later after midnight.

Otherwise expect variable mid/high cloud decks this morning with
clearing into the afternoon. Expect some high based cumulus with
bases around 10k feet this afternoon and evening with more BKN mid
and high level debris decks to move back in to the area from the
south/southeast after 04z tonight.

Finally, chances of storms into the lower deserts this evening is
not zero - at least one mesoscale model forecasts storm cells over
the terminals during evening hours, but that models is somewhat of
an outlier so we will go with the preponderance of guidance keeping
storms to the east.

Southeast California/Southwest Arizona including KIPL and KBLH:

Anticipate minor cirrus tonight with mostly clear skies Saturday.
Winds will follow familiar warm season diurnal patterns, favoring
the south to southwest at KBLH and the southeast during the day at
KIPL shifting to the west overnight tonight.

Aviation Discussion not updated for amended TAFs.

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.FIRE WEATHER...
Monday through Friday:
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 Wednesday
and Thursday could potentially be more favorable. High temperatures
will start out several degrees above seasonal norms on Monday
including over hotter western deserts into far SE CA with the hotter
readings in excess of 110 degrees. A cooling trend with closer to
normal max Ts is expected by Wednesday through the second half of
the week. On Monday, minimum RH values will be on the low side
ranging from around 10 percent over the deserts of far southeast
California to the mid teens in south central Arizona. 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.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 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 from 10 AM this morning to 8 PM PDT
     Monday for CAZ566>568-570.

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DISCUSSION...Hirsch
AVIATION...CB
FIRE WEATHER...Sawtelle/Hernandez/CB