Excessive Heat in California, August 6 to 7, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
958 AM MST Mon Aug 6 2018

...Updated Aviation Discussion...

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.SYNOPSIS...
Well above normal temperatures are expected through Tuesday as
thunderstorms remain relegated to the mountains in northern and
eastern Arizona. More significant moisture will return around the
middle of next week as high pressure builds back to the northwest
resulting in a return of thunderstorm chances, some of which may be
strong to severe. The return of moisture will allow high
temperatures to decrease back closer to normal for the second half
of the week.

&&

.DISCUSSION...
Early this morning, a strong east to west oriented upper level high
pressure ridge was situated across the southwest CONUS, with the
axis suppressed into far southern Arizona, keeping southwest flow
aloft in place over much of the lower south-central deserts. Latest
Phoenix sounding showed PWAT down to just 1.17 inches. Latest 500mb
plot showed heights running 595-596dm over southern Arizona, and
surface dewpoints over the central deserts have fallen into the
upper 40s to mid 50s. Little change in the weather pattern is
expected today; upper ridge remains strong and in a similar position
keeping drier air in all but the very far southern zones, and 850mb
temperature over the Phoenix area is actually forecast to rise at
least one half degree C today compared to Sunday. High in Phoenix
was 113 on Sunday and we're looking at a high of 114 today. Most of
the best performing guidances keep the bulk of the deserts at or
above 113 today and we have raised our highs 1-2 degrees most
places. This puts most of the CWA under excessive heat and the
warning has been expanded to all but a few zones.

On Tuesday, upper ridge really does not change much and the
atmosphere remains hot and relatively dry although flow aloft starts
to become weakly out of the east allowing some moisture to creep
westward; we may see isolated evening storms Tuesday over higher
terrain areas east of Phoenix. The heat is still going to be a big
issue Tuesday, and 850mb temps only lower a degree or so from
Monday, with values similar to those seen Sunday evening. Also,
NAEFS percentile data for 850mb temps on Tuesday actually show
higher readings over the lower deserts than today, with most of the
lower central/western deserts above the 97th percentile. Heat risk
data also shows that much of the Phoenix area as well as portions of
far southeast California will see high/very high risk of excessive
heat on Tuesday. Thus the Excessive Heat Warning has been extended
through 8 pm on Tuesday for a portion of the lower deserts,
including the greater Phoenix area.

For Wednesday into the weekend...operational guidance from GFS, NAM
and ECMWF all call for the upper high to gradually build north of
Arizona allowing increasing easterly flow to move into the area on
the southern periphery of the high. As this occurs, a hurricane is
forecast to slowly move westward...passing by well to our south...
allowing deeper tropical moisture to spread north and into southern
Arizona. For Wednesday into Friday, a slow moving inverted trof to
the north of the hurricane slides by far southern Arizona and we may
see some dynamics from the passing of this system. Upper streamlines
look to become rather chaotic with time, with deformation zones and
areas of difluence aloft moving through from time to time to
potentially aid convection. Certainly the increasing easterly flow
will be able to spread more moisture westward and across the deserts
and if the GFS is correct, lots of deeper moisture from the passing
hurricane will be available - especially over our southern zones
nearer the Mexican border. For now we will simply go with broad
brushed trends; POPs will increase quite a bit Wednesday into Friday
with chance values spreading into the south-central deserts. High
temps will fall below normal for a few days, bottoming out around
103. Depending on how much moisture we get, or how active convection
gets, highs could certainly fall below that in Phoenix or other
lower desert areas. It should be noted that latest GFS is REALLY
bullish on this hurricane - it is MUCH stronger than what is
depicted by the European and probably overdone. Based POPs on a
blend between GFS and ECMWF and also used NAEFS POPs for trend
guidance but not so much actual values (because NAEFS is often
overdone). We should be seeing more active monsoon weather as the
week wears on, with temperatures falling well below the excessive
heat values that we are currently dealing with.

&&

.AVIATION...
South-Central Arizona including KPHX, KIWA, and KSDL:
There are no aviation weather concerns through tonight as winds
will stay fairly light, outside of a few afternoon gusts, and
follow typical diurnal tendencies. There is no chance for storms
in the terminal area and any storm activity should stay well to
the east/southeast portions of Arizona. Otherwise, skies will be
mostly clear outside of a few passing clouds.

Southeast California/Southwest Arizona including KIPL and KBLH:
There are no aviation weather concerns as skies will remain clear
and winds will follow typical diurnal tendencies.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Significant moisture will return to the area from the east and
bring an increase in thunderstorm activity through the period.
Storms are expected to be most prevalent over the higher terrain
north and east of Phoenix but even lower elevations will have a
shot at precipitation each afternoon. Minimum humidities will end
up between 20 to 30 percent over the deserts each day following
fair to good overnight recoveries. Apart from thunderstorms, winds
will be light except for typical afternoon and evening upslope
breeziness. Temperatures each day will remain at or just below
seasonal normals.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should not be needed through Tuesday.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Tuesday for AZZ537-540>544-
     546-548>551-556.

     Excessive Heat Warning until 8 PM MST this evening for
     AZZ530>536-538-539-545-547-552>555-559>562.

     Heat Advisory until 8 PM MST this evening for AZZ557-558.

CA...Excessive Heat Warning until 8 PM PDT Tuesday for CAZ561-566-567-
     570.

     Excessive Heat Warning until 8 PM PDT this evening for CAZ560-
     562-563-568-569.

&&

DISCUSSION...CB
AVIATION...Deemer
FIRE WEATHER...Wilson/Kuhlman/CB