Excessive Heat in California, June 3 to 5, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
225 PM MST Sun Jun 3 2018

.SYNOPSIS...
Dry conditions will persist through the week as strong high pressure
remains situated just to the southeast of the region. This will keep
afternoon highs stuck in an above average range. An Excessive Heat
Warning is in effect for portions of southeast California, as well
as portions of the Phoenix area through Monday. Temperatures will
retreat somewhat Tuesday and Wednesday before potentially warming
back to around 110 degrees late in the week.

&&

.DISCUSSION...
WV imagery depicts a compact shortwave de-amplifying and lifting
into SW Colorado helping support a rather uncommon organized severe
weather event across the Continental Divide of New Mexico this
afternoon. In the wake of this feature, modest height rises were
spreading into Arizona along with a warming atmospheric column. As
expected, temperature were trending 3F-6F warmer than Saturday and
pushing excessive criteria, eventually landing in a 105F-110F range
later today. Little change in overall conditions will exist Monday
with the exception of very minor height falls in SE California
corresponding to the next shortwave/elongated trough axis
approaching the northern/central CA coastline. Regardless, Excessive
Heat Warnings will continue for the more health sensitive regions of
the CWA into early Monday evening.

In association with the aforementioned elongated trough, deeper SW
flow will begin to overtake the forecast area, such that even low
level winds will increase out of the south-southwest Monday
afternoon. While not a classic Gulf surge, this evolution will allow
some moisture to briefly get pulled off the Gulf of California into
Arizona and SE California Tuesday and Wednesday. Heights aloft will
oscillate in response to waves lifting into the Great Basin through
the midweek time frame; and the combined influence of the increased
moisture (albeit marginal, at best) and flow aloft will yield
temperature a few degrees cooler than the excessive levels we're
experiencing today and tomorrow.

Eventually this trough will erode and lift away from the coast
allowing 500mb heights to climb back toward 590dm. Model consensus
still shows a decent likelihood of temperatures approaching
Excessive Heat criteria in some areas as early as Friday and
possibly extending into the weekend. While the 00Z ECMWF and 12Z GFS
have depicted expanding 591dm+ H5 heights westward, these models
were more outliers as compared to the majority of 00Z NAEFS ensemble
members. However, the 12Z ECMWF has become more in line with the
ensemble mean showing a more pronounced trough entering the Pacific
NW essentially stifling the westward retrogression of the central
ridge axis. Another possible limiting factor would be the potential
for mid and high level moisture edging into the region helping limit
full daytime insolation. As a result, have kept forecast highs over
the weekend closer to the 50th percentile of model output, albeit
still 5F-10F above the seasonal average.

&&

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

No aviation concerns through the TAF period. Winds will follow
normal diurnal tendencies with speeds aob 10 kts. Skies will
remain clear.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday:
Very hot and dry conditions will continue through at least the
weekend as temperatures remain 5-10 degrees above average. Minimum
relative humidity values will generally fall into the single digits
with just a hint of marginally better moisture seeping into the
eastern districts by the end of the weekend. Overnight recovery will
also generally be poor in most areas except for parts of southeast
California and the lower Colorado River Valley where southerly low-
level flow will help bring in richer surface moisture leading to
fair recovery. After modestly breezy conditions early this week,
winds will relax and only be locally gusty in the typical upslope
locations the remainder of the week.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation will not be needed.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Monday for AZZ540-543-544-
     548-550.

CA...Excessive Heat Warning until 8 PM PDT Monday for CAZ561-562-
     566>568-570.

&&

DISCUSSION...MO/Kuhlman
AVIATION...Hernandez
FIRE WEATHER...Hopper/MO