Excessive Heat in California, May 6 to 7, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
416 AM MST Wed May 6 2020

.UPDATE...Updated Aviation Discussion...

&&

.SYNOPSIS...
High temperatures will be 10-15 degrees above normal today and
Thursday and thus an Excessive Heat Warning is in effect for the
lower elevations. A gradual cooling trend starts Friday but above
normal temperatures will continue into next week. The hot conditions
combined with very dry vegetation will allow for heightened fire
potential for at least the next 7 days. Increasing humidity over
the weekend may allow for isolated dry thunderstorms over the
high terrain Sunday and Monday.

&&

.DISCUSSION...
A narrow high amplitude ridge of high pressure is in place over
western North America. 850mb temperatures are running about +2
standard deviations per both NAEFS and ECENS. Latest ensemble
guidance is about subtly cooler for the high temperatures today
and Thursday but not enough to merit canceling the Excessive Heat
Warning. The good news is that strong high pressure won't be in
place ad infinitum. This is thanks to troughing (currently
centered over the far northeast Pacific) that will eject a series
of short waves inland and weaken the ridge. The first one actually
starts today but the modest cooling effects won't be felt over
our forecast area until Friday. Though the ridge temporarily
rebounds, the heights/thicknesses don't return to previous levels.
It gets flattened next week by a more substantial wave. the net
result will be a gradual cooling trend starting Friday. A downside
to the cooling trend will be enhanced afternoon/evening
breeziness (most noticeably beginning Sunday) that will exacerbate
wildfire potential.

After the peak of the heat and before we get to the more
noticeably cooler (though still above normal) temperatures next
week, there will be an increase in deep layer humidity over the
weekend. There is already a large area over northern Mexico with a
positive anomaly of precipitable water. To help advect that
moisture northwestward, there will be at least one weak cyclonic
disturbance that develops near the Baja Peninsula. Models depict
it arising in the southern branch of a split in the Westerlies.
The feature will be slow to move eastward due to lingering
anticyclonic flow over the Borderlands region. The prolonged
residence time facilitates the moist advection. Both the NAEFS and
ECENS depict PWAT values of +1 to +2 standard deviations slowly
moving northward and peaking over AZ and far southeast CA on
Sunday. However, those ensemble mean values stay below 1 inch
since normal PWAT values for this time of year are a lot lower
than during the Monsoon. It appears the initial disturbance
dissipates and is followed by a weak wave that moves our region.
Thus, there will be an opportunity for precipitation. But this
looks most likely to be in the form of isolated high-based storms
over the higher terrain of Arizona due to meager CAPE. Of note,
the GEFS mean depicts light precip over a considerably larger area
than the ECENS. For our area, PoPs start on Sunday. On Monday,
the southwesterly flow pushes the moisture slowly eastward but
with a lingering slight chance over our easternmost area. We're
not anticipating any significant rain. But rather, a more
significant concern is the potential for lightning caused brush
fires.

&&

.AVIATION...Updated at 1115Z.

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

No aviation impacts through at least Thursday morning. Winds will
continue to follow typical diurnal patterns with speeds less than
10 kts. Skies will remain mostly clear except for some thin high
cirrus.

Southeast California/Southwest Arizona including KIPL and KBLH:

No aviation impacts through at least Thursday morning. Winds will
continue to be on the light side, favoring the southwest at KIPL
and the south at KBLH with speeds less than 10kts. Skies will
remain mostly clear except for some thin high cirrus.

&&

.FIRE WEATHER...
Friday through Tuesday: A gradual cooling trend is expected to
begin Friday as high pressure weakens. However, temperatures will
still be above normal. The factors driving the weakening of the
high pressure will also mean enhanced afternoon and evening
breeziness - most noticeably starting Sunday. Before lower
elevation high temperatures drop below 100, there will be an
increase in atmospheric humidity this weekend. In fact, it will be
enough to generate a slight chance of thunderstorms over Gila
County on Sunday and Monday. The thunderstorms, if any, will
struggle to produce wetting rain. Thus, there will be the
potential for lighting starts that could be spread by thunderstorm
winds. On a positive note, there will be a modest improvement in
minimum humidities (from around 10% to the teens) with more
noticeable improvement in the maximum humidities (from 20-30%
to 30-40% on the lower elevations). There will also be more
cloudiness around. The moisture aloft will be cleared out by
Tuesday but humidities will drop much due to the cooler
temperatures.

&&

.CLIMATE...

Record High Temperatures

Date        Phoenix         Yuma
----        -------         ----
May 6     106 in 2018    107 in 1963
May 7     108 in 1989    108 in 1989

Record Max Low Temperatures

Date        Phoenix         Yuma
----        -------         ----
May 8      76 in 1989    76 in 1987
May 9      81 in 1989    76 in 1987
May 10     77 in 1987    78 in 1987

&&

.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
     Thursday for AZZ530>540-542>544-546-548>551-553-554-559.

CA...Excessive Heat Warning from 10 AM this morning to 8 PM PDT
     Thursday for CAZ562-565>567-569-570.

&&

DISCUSSION...AJ
AVIATION...Kuhlman
FIRE WEATHER...AJ