Excessive Heat in California, August 30 to 31, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
415 AM MST Fri Aug 30 2019

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Thunderstorm activity will mainly be confined to eastern Arizona the
next few days. Meanwhile, a warming trend is expected through
Saturday as the Monsoon High migrates into northern Arizona. High
pressure will continue to slide towards the Four Corners Sunday,
resulting in an easterly flow and a more favorable setup for
increasing moisture and thunderstorm activity.

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.DISCUSSION...

A hot day is on tap across the Desert Southwest where high heat risk
exists across portions of the lower deserts. An Excessive Heat
Warning will go into effect later this morning and run through
Saturday evening for portions of south-central and southwest Arizona
and southeast California. Current SPC mesoscale upper-air
analysis shows an anticyclone centered just off the coast of
northern Baja California, allowing for dry northwesterly flow
aloft to set in across the state. This will help to keep things
dry across the lower deserts over the next couple of days with
dangerous heat being the main concern for today and tomorrow.
Saturday looks to be hotter than today with NAEFS Percentiles
showing 850mb temperatures above the 90th percentile. High
temperatures will generally range from 109-112 degrees across
south-central and southwest Arizona and 111-115 degrees across
southeast California tomorrow.

Over the next couple of days, thunderstorm activity will continue to
stay confined to the higher terrain locations. HREF members show
activity staying mainly over Gila County and the Mogollon Rim today
and tomorrow with virtually little to no activity expected across
lower desert locations. HiRes guidance shows any outflows generated
by these thunderstorms washing out for the most part before reaching
the desert floor but can't rule out a weak outflow getting into
eastern Maricopa County. More favorable chances for monsoon activity
enter the forecast by the end of the week into early next week as
the anticyclone migrates toward the northeast over the next couple
of days.

GFS and ECMWF guidance continue to show the aforementioned
anticyclone shifting further northeast reaching the Four Corners
region by late Sunday. This will allow for a more favorable flow
pattern to set up with moisture advection increasing across the
region. NBM PoPs reflect this change to a more favorable monsoonal
pattern with chances for thunderstorms increasing slightly beginning
Sunday afternoon and continuing through mid next week.

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.AVIATION...Updated at 1110UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Easterly winds will continue across the terminal area through the
mid morning before becoming westerly between 17-19Z. Wind speeds
will remain mostly at or below 10 kts but a few late afternoon
gusts upwards of 20 kts are possible at all sites. Most thunderstorm
activity should remain north and east of the terminals but a late
evening/night easterly outflow cannot be ruled out. However, the
timing of any outflows is too uncertain for any mention in the
TAFs at this time.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no major aviation weather concerns for today. The winds
will remain light through this morning with variable directions at
times. A general westerly shift will occur during the early
evening with a few breezes before becoming light again Friday
night.

Aviation Discussion not updated for amended TAFs.

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.FIRE WEATHER...
Sunday through Thursday:
Storm chances will increase early in the week and remain in place
each day favoring the higher terrain in Arizona. However, storms
are possible across the lower desert into southeast California as
well. Daytime minimum humidity values will fall into the 15-20%
range with overnight recoveries into the 30-50% range. Outside of
any outflows, the winds will favor diurnal trends with typical
afternoon breezes.

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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
     Saturday for AZZ530>532-534-536>538-540>546-548>551-553>556-
     559-560-562.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Saturday for CAZ563-566-567-569-570.

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DISCUSSION...Smith
AVIATION...Deems
FIRE WEATHER...Deems