Dust Devil in Arizona, May 4, 2018

Area Forecast Discussion
National Weather Service Phoenix AZ
220 PM MST Fri May 4 2018

.UPDATE...Updated Aviation and Fire Weather Discussions.

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.SYNOPSIS...
High pressure will intensify over the Southwest states through the
weekend resulting in extremely hot weather. Record high temperatures
are very likely Sunday with lower elevation readings topping out in
a 105 to 110 degree range. This will lead to a an excessive heat
risk for many areas. Typical late spring heat will persist
throughout next week with most communities still reaching 100
degrees. there are no signs of rainfall anytime in the near
future.

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.DISCUSSION...
High pressure over the southwest is responsible for clear skies
across Arizona and southeast California this afternoon.
Temperatures are in the low 90s across the lower deserts. The
ridge will build more through the weekend, resulting in excessive
heat by Sunday. Many locations will be looking at record high
temperatures for the day on Sunday. An Excessive Heat Warning has
been issued for many of the low-lying areas of our CWA for
Sunday. High pressure will remain over the region for the
remainder of the week with typical highs in the low 100s, about
10-14 degrees above normal. No chances for precipitation for the
next 7 days.

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.PREVIOUS DISCUSSION...
An elongated, positively tilted trough was shearing through New
Mexico early this morning as upstream ridging builds through the
western Conus in response to a series of waves rotating through the
Gulf of Alaska and east Pacific. This configuration will continue to
place the forecast area under deep subsidence and weak NW flow as
heights aloft steadily increase. With full insolation, there is good
confidence via low model spread that high temperatures will bounce
another 8F-12F above readings Thursday, and fall back into a
slightly above normal range.

Pronounced H5 ridging will become aligned directly over Arizona
Saturday with heights easily pushing above 585dm. Given the
seasonally deeper mixing depths and H7 temperatures approaching 10C,
this will mark the beginning of a string of 100+F days for almost
all lower elevation communities. Although the ridge axis begins to
shift east Sunday, the overall magnitude of heights actually
increases south of northern stream jet in persistent anti-cyclonic
flow. H5 heights will almost certainly peak near or just above
588dm, which falls around 2 normalized standard deviations above
normal, or near the 99th percentile climatologically. Remarkably
despite the rarity of these measures, model guidance has been
consistent with minimal spread yielding record breaking forecast
highs Sunday. Current Excessive Heat Watches will likely need to be
upgraded to Warnings with the only notable uncertainty associated
with the potential for thicker cirrus to disrupt better insolation
during peak heating.

A pair of shortwaves will lift through the Great Basin Monday
partially dampening ridging through the SW Conus. The latest suite
of 00Z global models indicate only minimal reduction in H5 heights
and 1000-850mb thicknesses, so the amount of "cooling" expected with
these wave passages may end up being even less than previously
thought. In fact by Tuesday, there are some indications that H5
heights may peak closer to 590dm within a shortwave ridge ahead of
the next wave breaking into the Pacific NW coast. Similar to the
aforementioned uncertainty Sunday, thicker cirrus may be more likely
Monday while weaker winds through the lower troposphere may reduce
mixing depths somewhat. Regardless, have tried to hedge temperatures
closer to the upper end of the guidance envelop based on recent
model trends.

During the latter half of next week, stronger energy moving from the
Pacific NW through the northern and central Rockies will almost
certainty flatten ridging over the forecast area and dampen heights
and thicknesses. The 00Z ECMWF attempts to deepen a shortwave trough
over the four corners during the end of the week, however this is a
distinct outlier compared to the vast majority of NAEFS members.
Have discounted this solution feeling weak quasi-zonal flow and a
flat ridge will be the more likely outcome resulting in temperatures
still hovering around the 100F threshold. As with earlier in the
week, model spread is quite narrow yielding above average forecast
confidence.

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.AVIATION... South-Central Arizona including KPHX, KIWA, and KSDL:

Expect light to briefly moderate and breezy winds roughly following
normal diurnal directions together with periods of variable headings
through about 05Z. For late tonight through Saturday evening expect
light to moderate and occasionally breezy easterly winds. Gusts near
15-20kt are expected by mid morning, although without the common
afternoon switch around to the west on Saturday. Otherwise skies
will remain clear throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH:

With no aviation concerns expect light to light and variable winds
favoring typical diurnal shifts. Skies should also remain clear to
mostly clear with only a few periodic high cirrus passing overhead.

Aviation Discussion not updated for amended TAFs.

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.FIRE WEATHER...
Monday through Friday...

The main fire weather concern is a very dry heatwave pattern that
will prevail throughout the period with well above normal highs
reaching the low to mid 100s. Min RH values will also remain in the
single digits throughout the period. Expect typical breezy to
occasionally very breezy conditions during most of the afternoons
with gusty southwesterly and upslope winds. Overnight recoveries
will be mostly poor, though occasionally fair in SE CA, the lower
Colorado River Valley and the higher elevations in the east.

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.CLIMATE...

Record high temperatures for Sunday:

Date          Phoenix          Yuma
----          -------          ----
May 6        105 in 1947    107 in 1963

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning from 10 AM to 8 PM MST Sunday for AZZ530-
     532-533-536-539-540-543-544-546-548-550-551-553-559.

CA...Excessive Heat Warning from 10 AM to 8 PM PDT Sunday for
     CAZ565>567-569-570.

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DISCUSSION...KW/MO
AVIATION..Sawtelle
FIRE WEATHER...Sawtelle
CLIMATE...MO