Dust Storm in Arizona, August 11, 2020

Area Forecast Discussion
National Weather Service Phoenix AZ
430 PM MST Tue Aug 11 2020

.UPDATE...
Updated Aviation

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.SYNOPSIS...
The hot and dry weather pattern will persist into early next week
with temperatures running well above normal. Daily highs across the
lower deserts will mainly top out at around 110 degrees through
Wednesday before likely warming to between 112 and 115 degrees
starting Thursday or Friday. Aside from mostly cloudy skies late
Wednesday into Thursday, skies should remain mostly clear most days.
Any rain chances through early next week should be limited to high
terrain areas and mostly across eastern Arizona.

&&

.DISCUSSION...
Although dewpoints are now down a bit from the levels we saw earlier
this morning, they still remain elevated, with 50's across south-
central AZ and 60's being observed across SW AZ/SE CA at this hour.
Although current radar imagery is showing nothing across our CWA at
this time, latest high-res CAMS models are continuing to show the
development of few-sct TS across Pima County later this afternoon,
with a few cells perhaps briefly making their way into SW Maricopa
County. Although significant rainfall is still not expected, there
is an outside chance (10% or so) that outflows from these storms
could kick up some localized blowing dust across NW Pinal/extreme SW
Maricopa Counties late thus afternoon.

At the same time, the models have backed off somewhat on the idea of
an additional moisture surge late tonight/early Wednesday. Thus
there is more confidence that highs in the Phoenix metro will exceed
110 degrees on Wednesday. This warmer solution combined with the
expectation to the Phx urban core will likely see lows near 90
degrees tonight has boosted confidence enough that the decision was
made to upgrade the current Excessive Heat Watch to a Warning.
Wednesday night looks to be fairly cloudy and this should result in
very warm overnight temperatures and lows likely around 90 degrees
again for much of the Phoenix metro to the low to mid 80s across
rural desert areas.

Although Wed afternoon looks like it will be a bit more active
convective-wise due to a modest increase in mid-level moisture, it
appears that the main impact in our CWA will be a better chance for
outflows into the lower deserts of south-central AZ from distant
storms, with the only realistic rain chances remaining across the
higher terrain areas of far southern Maricopa and along the Mogollon
Rim into the White Mountains. There is also some indication of mid
level based virga showers across south-central Arizona early
Thursday morning, but we do not expect much more than some
sprinkles.

Thursday will likely start off fairly cloudy in most areas, but dry
advection and daytime mixing by Thursday afternoon should leave
mostly sunny skies across the lower deserts. The lingering moisture
on Thursday should however lead to another day of high terrain
isolated to scattered storms. However the mainly story for Thursday,
and for several days thereafter will be the excessive heat. Given
the fact that there is now high confidence that forecast highs for
Thursday will rise into the 111-114 degree range, decided to upgrade
the Excessive Heat Watch to a warning from Thursday onward into next
Monday. Even hotter conditions are expected starting Friday as the
subtropical high center is shown shifting directly over Arizona with
500mb heights rising up into the 595-598dm range. From Friday
through early next week, guidance continues to mainly show daily
high temperatures reaching between 113-115 degrees which is very
close to daily records for both Phoenix, Yuma and El Centro. Storm
chances for Friday into the weekend over the high terrain should be
more limited due to the high center being positioned overhead. A
likely shift of the high center to the northwest Sunday into early
next week should lead to a more favorable northeasterly mid level
flow, but moisture still looks to be quite limited. The stronger mid
level flow for early next week and a shift in the high center could
bring increasing storm chances at some point during the early to
middle part of next week.

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.AVIATION...Updated at 2330Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Trends in winds will be the primary weather challenge through
Wednesday evening with just some increase in mid/high clouds through
the period. Actual storms should remain well south of terminal
locations this evening, though there is about a 25% chance of a more
distinct southerly outflow boundary/wind shift progressing into the
city by mid/late evening. At this time, the greatest odds point to a
more subtle wind shift from W to SSW with speeds remaining below
15-20kt. Confidence in timing of wind directions to the overnight
easterly, and back to the daytime westerly late Wednesday morning is
only moderate; and the switch may only be brief for a few hours
after sunrise.

Southeast California/Southwest Arizona including KIPL and KBLH:
Very little weather concern will exist through Wednesday night with
just some increase in high clouds. Trends in wind directions will be
similar to the past several days favoring some measure of a
southerly component.

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.FIRE WEATHER...
Friday through Tuesday:
Above normal temperatures and overall dry conditions will persist
through the early part of next week. Winds will generally follow
typical diurnal/terrain influences with speeds remaining light aside
from some typical afternoon breezes. Thunderstorm activity will be
very limited with isolated storms possible across high terrain areas
of eastern Arizona. Minimum RHs through the period will mostly drop
into the teens each day with overnight recoveries generally between
30-50%.

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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 from 10 AM Wednesday to 8 PM MST Monday
     for AZZ537-540-542>544-546-548-550-551-556-560-562.

     Excessive Heat Warning from 10 AM Thursday to 8 PM MST Monday
     for AZZ530>536-538-539-541-545-547-549-552>555-559-561.

     Heat Advisory from 10 AM Thursday to 8 PM MST Monday for AZZ557-
     558-563.

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

     Excessive Heat Warning from 10 AM Friday to 8 PM PDT Monday for
     CAZ560>564-568.

&&

DISCUSSION...Percha/Kuhlman
AVIATION...MO
FIRE WEATHER...Hirsch/Kuhlman