Excessive Heat in California, June 3 to 4, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
437 AM MST Wed Jun 3 2020

.UPDATE...Updated Aviation Discussion.

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.SYNOPSIS...
Expect hot temperatures today and tomorrow as strong high pressure
builds over the region. A low pressure system will move through
the region Friday, causing winds to increase and temperatures to
begin cooling. Winds will decrease by Saturday as temperatures
drop to below normal values and remain as such into early next
week.

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.DISCUSSION...
An upper level cut-off low remains off the coast of southern
California this morning, with some increasing mid-level clouds
over our forecast area. A mid-level shear axis will continue to
lift northward through Arizona the remainder of this morning,
interacting with some upper level diffluence. This is well
depicted at the 320K isentropic level on the GFS. Condensational
pressure deficit values may become low enough across southeast
California and southwest Arizona that the upglide flow (in an area
of steep lapse rates and non-zero CAPE) will trigger some
convective showers. This forcing will lift north of our area by
15-18Z Wednesday morning, leaving behind deep southerly flow on
the east side of the cut-off low.

The overall synoptic pattern will remain fairly stagnant the rest
of Wednesday and into Thursday, with a warm southerly flow over
the forecast area, promoting increasing height/thickness values.
H7 temperatures per the HREF mean will climb from around 12C
this past Tuesday afternoon to 14C Thursday afternoon. This
should yield an increase in surface temperature of 3-5 deg F.
While afternoon record high temperatures seem mostly out of reach,
record warm lows remain possible through Friday morning. NBM-
based probabilities for reaching/exceeding HeatRisk "High" levels
remain near 70 percent for Wednesday and Thursday. The Excessive
Heat Warning expands in coverage today and will remain in place
through Thursday.

A jet streak diving southeastward out of the Gulf of Alaska will
finally dislodge the cut-off low from its current position,
ejecting it into Arizona on Friday. This has been the forecast
scenario for a while now and latest ensembles are showing fairly
good agreement on this scenario. The low will be transitioning
into an open wave as it moves northeastward, which may trigger
some showers and thunderstorms, mainly over northern/eastern
Arizona. A few ensemble members do keep it a closed low a little
longer as it lifts into Arizona, and should that happen, the
potential for showers/storms would be higher for central Arizona
than currently forecast. Winds will increase across the region as
the pattern change occurs, leading to gusty conditions Friday
afternoon. Forecast wind/RH values at this time appear to remain
below critical thresholds. Wind gusts may climb high enough to
warrant an advisory in southwest Imperial County, with some
potential to extend into the Imperial Valley. Another impact will
be cooling temperatures Friday, which will cause the heat warning
to come to an end Thursday.

For the weekend and into early next week, the upstream jet streak
will cause a pattern realignment, with broad troughing through the
Western CONUS. Temperatures will fall into the below normal
category for several days. Heights may begin to show a rebound
toward the middle of next week, with temperatures creeping back up
into a more typical range for mid June.

&&

.AVIATION...Updated at 1135Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:

Westerly flow is present at KPHX with easterly flow at the other
terminals. Light easterly flow will likely set in at KPHX in the
next 1-2 hours, but intermittent light westerly intrusions will
remain possible. By late morning the flow should shift back to the
southwest or west at all TAF sites with speeds of less than 10
kts. SCT-BKN clouds near 15kft and 25kft this morning will
gradually thin out with just a few mid and high level clouds by
the afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:

SCT to BKN clouds near 15 kft with isolated showers and
thunderstorms will continue near KBLH for the next few hours.
Precipitation will likely be spotty with mainly virga, leading to
potentially gusty and erratic winds at times. These showers are
expected to remain to the east of KIPL. By this afternoon just a
few mid and high level clouds are expected. Winds at KIPL will be
light and variable for much of the day, but by the late afternoon
westerly winds should increase to 10-15 kts. At KBLH the dominant
wind direction will be light westerly this morning, shifting to
southerly at 10-15 kts this afternoon.

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.FIRE WEATHER...
Friday through Tuesday:
Expect an increase wind winds across the region Friday as a
weather system moves in. Winds will be strongest during the
afternoon hours, but will mostly stay below critical thresholds.
Likewise, temperatures will cool slightly and humidity levels will
increase slightly. That said, critical thresholds could be hit in
some areas for brief periods. There may be enough moisture for
some showers and thunderstorms, primarily focused into the higher
terrain areas of northern and eastern Arizona. Should the system
come in a little stronger and with a little more moisture, storm
activity would be possible at lower elevations. Any storms can
produce gusty winds in excess of 30 mph and numerous lightning
strikes.

Saturday and beyond, winds will be much weaker and show typical
diurnal patterns. Temperatures will be several degrees below
average. Humidity values will remain low, near 10 percent, during
the afternoon with overnight recovery in the 30-40 percent range.
Even though these elements remain well below critical thresholds,
the fire danger will remain heightened.

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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 until 8 PM MST Thursday for AZZ537-
     540>544-546-548-550-551.

     Excessive Heat Warning from noon today to 8 PM MST Thursday for
     AZZ530>536-538-539-545-547-549-553>556-559>562.

CA...Excessive Heat Warning from noon today to 8 PM PDT Thursday for
     CAZ561>570.

&&

DISCUSSION...Iniguez
AVIATION...Hodges
FIRE WEATHER...Percha/Smith