Excessive Heat in California, September 4 to 7, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
451 AM MST Fri Sep 4 2020

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
Drier air has settled over the region with thunderstorm chances
essentially neglible through at least early next week. Above
normal temperatures close to record territory can be expected
this weekend. A strong storm system will develop east of the area,
and bring much cooler temperatures, breezy conditions, and a
slight chance of rain to southern Gila County by the middle of
next week.

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.DISCUSSION...
Very hot temperatures are expected today through Monday as the
plan-view 00Z 500-mb observation map shows a 597 dm high centered
over the Great Basin. This feature is expected to continue
strengthening over the next 24 hours, and its influence will be
felt far and wide across much of the Western U.S. Today and
Saturday are expected to be the hottest days of this heat episode,
topping out at 113 degrees in Phoenix and up to 117 in Yuma on
Saturday. These temperatures will challenge the record highs in
both locations. Thursday afternoon soundings also showed dry
tropospheric conditions with PW values around 1 inch, which is
generally not supportive for any convection across the PSR CWA.
High pressure will begin to break down Sunday and Monday as very
fast westerly flow becomes established across the Pacific
Northwest into the Northern Rockies, but the heat will linger for
a few more days while the air mass remains dry.

A more substantial pattern shift will finally bring us a break
from the oppressive summer heat by the middle of next week, as an
anomalous upper trough for early September develops across the
central CONUS. In fact, this system may bring snow to parts of the
central High Plains and Rockies. Model guidance continues to
trend farther west with this feature as it digs across the Rockies
towards the Four Corners. Still quite a bit of longitudinal
uncertainty as to where exactly this will set up, which leads to a
large range of possible temperatures mid-week. However,
temperatures in the low 90s are a distinct possibility on
Wednesday and Thursday. Low temperatures in the 70s are very
likely, and may even see some 60s in rural lower desert locations.
It will certainly be the most pleasant weather the region has had
in quite some time.

Given the strength of the trough, some isolated showers and
thunderstorms mainly from Phoenix and areas north/eastward cannot
be ruled out Tuesday. Moisture will still be fairly limited so any
rainfall totals would be on the light side. Perhaps the most
notable impact will be the strengthening of winds, especially
across southeast California, the lower Colorado River Valley, and
western Arizona. 850-mb wind speeds of 30-40 kt out of the north
appear likely to set up along the western periphery of the
aforementioned trough and in the wake of a low-level cold front,
which could bring strong winds of 20-30 mph and stronger gusts to
parts of the CWA. The official forecast deviated substantially
above NBM guidance to account for this anomalously strong low-
level wind setup. Will have to monitor for the possibility of a
Wind Advisory and/or a Fire Weather Watch if trends continue to
shift favorably.

A warming trend should commence once again during the latter
portions of next week, but high pressure should be significantly
weaker than the current setup. Still, may see a return of
temperatures back above the century mark to close out the week
with continued dry conditions favored.

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.AVIATION...Updated at 1151Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Winds will remain light and diurnal through this evening with
typical afternoon breezes up to 15 kts. Winds will shift easterly
early tonight, around midnight, as 20-25 kt east-southeasterly low
level jet develops. This may lead to an increase in surface winds
up to 10-12 kts through Saturday morning, but should subside by
noon. Skies will remain mostly clear, aside from a few cumulus 15
kft cumulus late this afternoon and evening.

Southeast California/Southwest Arizona including KIPL and KBLH:
Winds will generally remain less than 10 kt through the TAF
period. Southeasterly winds will prevail at KIPL, while winds with
a southerly component will be favored at KBLH. Both terminals will
also experience periods of variability. Skies will remain mostly
clear, aside from a few cumulus this evening.

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.FIRE WEATHER...
Sunday through Thursday:
High pressure in the region will create very hot temperatures and
dry conditions Sunday and Labor Day. Daytime RH values will drop
into the 10-20% range for most places while overnight recoveries
will struggle to surpass 25-35% for elevations below 3000 ft.
Higher elevations will see slightly higher RH values. Winds will
generally remain light through Monday with directions favoring
diurnal/terrain patterns. Strong winds are now a possibility on
Tuesday across southeast California, the lower Colorado River
Valley, and southwest Arizona, with sustained wind speeds
exceeding 20 mph. Combined with low RH, this may lead to an
elevated to perhaps critical fire weather threat on Tuesday
afternoon.

There are hints that moisture could return early next
week, perhaps as early as Tuesday/Wednesday, that would at least
improve the RH values across the board by 5-10%. Any rainfall
would generally be confined to southern Gila County on Tuesday,
with amounts on the light side. Temperatures will also be much
cooler, back to near, or even below seasonal normals by
Wednesday/Thursday.

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.CLIMATE...
Record high temperatures:

Date       Phoenix        Yuma        El Centro
----       -------        ----        ---------
Sep 4    112 in 2019   116 in 1945   115 in 1947
Sep 5    113 in 1945   115 in 1945   114 in 1945
Sep 6    111 in 2013   113 in 1986   114 in 1934
Sep 7    111 in 1979   114 in 1979   112 in 2019

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.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>556-559>562.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Monday for CAZ560>570.

&&

DISCUSSION...Rogers
AVIATION...Benedict
FIRE WEATHER...Rogers
CLIMATE...MO