Excessive Heat in California, September 12 to 13, 2021

Area Forecast Discussion
National Weather Service Phoenix AZ
450 AM MST Sun Sep 12 2021

.UPDATE...Updated Aviation Discussion...

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.SYNOPSIS...
Unseasonably warm temperatures and dry conditions will prevail
through at least early next week as strong high pressure remains
over the region. High temperatures today will run 3 to 5 degrees
above normal before fairly widespread excessive heat with highs
between 107 to 112 degrees impacts the lower deserts on Sunday and
Monday. A very gradual cooling trend is then expected through the
rest of next week, but dry conditions will most likely persist
for the foreseeable future.

&&

.DISCUSSION...
Unseasonably oppressive temperatures remain the primary weather
focus today and tomorrow. Current surface observations show temps
cooling at a pace slower than recent nights. Most population
centers are now in the 80s but a surprising number of stations
dispersed across southwestern Arizona are still in the 90s! For
some areas, this could mean starting today off a bit warmer than
recent mornings.

For today and tomorrow, a midlevel high pressure ridge will expand
farther west today and tomorrow, mandating additional drying of
lower level moisture, while modest increases in atmospheric heights
and thickness will dictate warmer temperatures. Current dew
points, which range from the mid-50s to mid-60s across the lower
desert, are nothing more than a prestidigitation, as the last
several upper air soundings reveal substantial drying throughout
the boundary layer. By Monday afternoon, the drying will force
its way down to the surface with common sub-50 dew points. The
same can be said for sub-40 dew points by Wednesday afternoon.
Moreover, comparison between the 11/12z and 12/00z KTWC, which is
closer to the ridge's center, showed H5 height increases of 1-2 dm
and up to 595 dm. A similar increase can be expected for our forecast
area and well into southern California as the aforementioned ridge
expansion occurs. These heights are fairly uncommon this time of
year, as they would place at or above the 99th percentile of
climatology, but we will have to wait to see the next couple of
KPSR and/or Yuma area soundings to see just how far west 595 heights
extend.

This regime will force temps up slightly today and tomorrow. For
south-central AZ, highs will lean towards the 106-109 degree range
while readings in western AZ, along the lower Colorado River, and
into southeastern California will reach 108-112 degrees. Just how
hot will the next two days be? NBM probabilities are most bullish
for KNYL, where there is a 60% chance of reaching 110° either day,
a 20-25% chance to reach 112 deg, but nearly 0% chance for anything
warmer than 114°. For KPHX, NBM probs of are relatively cooler,
with a 50% chance to reach 109 deg, just one degree warmer than
yesterday, and 25-35% to reach 110 , but asking for much more is
an overreach with 0% for anything hotter than 112 deg. Even though
the afternoon highs might end only a degree or two hotter compared
to the last few days, any heating combined with the residual heat
buildup is enough to mandate expansive High Risk on the NWS
Experimental HeatRisk for most of the populated lower desert areas
from Phoenix to El Centro through Mon evening. Therefore, the
Excessive Heat Warning was kept and is in effect beginning at 10
AM today and ending at 8 pm Monday.

There is good news to focus on as temperatures are set to cool by
a degree or two each day beginning Tuesday as passing Pacific
troughs prevent the ridge from completely rebounding. The first
trough, currently located over the northern Pacific, will compel
temps to drop by 2-5 degrees Tuesday and restricting any chance
for widespread excessive heat this week. Pressure gradient winds
are also expected Tuesday afternoon in response to the trough’s
passing, with gusts frequently reaching 20-25 mph. Subsequent
troughs later in the period, combined with shrinking daylight
hours, will build momentum for the cooling trend. Highs are expected
settle right around seasonal normal between 98-102 deg by next
Sunday. The date of next Sunday, 9/19, is also the latest 110 deg
on record for Phoenix which gives an exceptionally confident
feeling and unbridled excitement to say the window for 110°+ in
Phoenix will close in two days after Monday. Unfortunately for
Yuma and El Centro, that kind of confidence is not quite present
yet given historical records, but is at least very close, as
medium range guidance suggests your window might be closed just
the same.

Otherwise, the forecast continues to lean heavily towards no clear
signal for rain this week. Tropical moisture may surge northward
out of Mexico late this week but forecast guidance does not give
much chance for rain with PoPs remaining below 5% everywhere.

&&

.AVIATION...Updated at 1150Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation impacts expected through at least Monday morning. Light
winds with typical diurnal patterns are expected through the
period, while skies remain clear to mostly clear.

&&

.FIRE WEATHER...
Tuesday through Saturday:
Strong high pressure will persist over the region through at least
early next week providing dry conditions and above normal
temperatures. High temperatures through Tuesday are expected to
run several degrees above normal with some lower desert areas
likely reaching 110 degrees on Monday. A gradual cooling trend is
then expected into the latter half of next week. Min RH levels
will mostly be between 10-15% through the period, while max RHs
generally range from 30-40%. Winds will remain fairly light
through the period while following typical diurnal patterns.
Localized afternoon breeziness is also likely through the period
with gusts up to around 20 to 25 mph, and isolated gusts of 30-35
mph over any exposed ridgetops.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM this morning to 8 PM MST
     Monday for AZZ530>544-546-548>551-553>555-559.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Monday for CAZ561-565>567-569-570.

&&

DISCUSSION...12
AVIATION...Kuhlman
FIRE WEATHER...Kuhlman/12