Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 212055

Area Forecast Discussion
National Weather Service Phoenix AZ
155 PM MST Sat Jul 21 2018

Updated Aviation and Fire Weather sections.


Thunderstorm chances and areal coverage will progressively wane
through the weekend as strong high pressure builds over the
region. Into the first half of next week, storms will likely
become relegated only to the highest terrain areas as hot
temperatures begin to bake the lower elevations. In fact, the
hottest weather of the year will cover the area with afternoon
highs nearing record levels by Tuesday. Cooler and more humid
conditions are expected starting Thursday as high pressure
weakens and easterly flow aloft starts to spread monsoon moisture
back into the area.


A strong mesoscale convective system moved through south-central
and southwest AZ this morning bringing winds gusting 40-50 mph
over a large area. We had isolated reports of 60+ mph winds but
were confined to southern Maricopa County in the Barry M.
Goldwater Air Force Range. In addition to the winds, the storms
kicked up dust with reports of 1/4 to 1/2 mile visibility in north
Phoenix. Areas further west along I-10 towards Blythe saw
visibilities anywhere from 1 to 2 miles. Thankfully, this weather
has all since moved out while only some remnant clouds remain in
western AZ and southeast CA.

Looking towards this afternoon, the NAM bufkit sounding has a
large cap over the Phoenix area with mixed layer convective
inhibition above 200 J/kg. In fact, most models shows a
convective cap being present throughout our entire CWA this
afternoon which is expected to limit convective activity to the
high terrain north and east of Phoenix this evening.

A strong ridge of high pressure currently over the TX/NM border
will gradually begin expanding westward over the Desert Southwest
this weekend and early next week pushing 500 mb heights near
558-600 dm by Tuesday afternoon. Heights of this magnitude have
only been recorded once before during the month of July (per the
SPC sounding climatology) and would certainly be a record for the
date. Consequently, surface temperatures will be extremely hot and
perhaps record breaking. Currently, our Tuesday forecast calls
for 116 and 117 F in Phoenix and Yuma, respectively. This would
tie the record for the date (see climatology section below) and
would be the warmest reading so far this summer in Phoenix.

The excessive heat will last through at least Wednesday before
the high shifts off to our west come Thursday. This will allow
upper level winds to switch out of the north and east while low
level moisture may be replenished over south-central AZ by a gulf
surge. However, there is still a lot of discrepancy as to when
moisture will return as the GEFS keeps PW below 1.25 inches
through early Saturday while the GFS/ECMWF push PWs north of 1.5
inches by Thursday. As such, we have started introducing
thunderstorms for areas of south-central AZ Thursday and expanded
the areal coverage Friday and Saturday into eastern Maricopa
County but confidence is low. Throughout this period, SE
California should remain dry while all lower desert areas cool to
the low/mid 100s by Friday.


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

An exiting weather disturbance moving into the Mohave Desert early
this afternoon will leave behind more stable conditions. Thus only a
very slight chance of thunderstorms the rest of the afternoon and
evening over the forecast area (storm activity remaining primarily
near and north of a line from SOW-EED). Otherwise, just few-sct
cumulus. Anticipate surface winds to remain west and northwest
until around 06Z before becoming light and variable and slowly
trending toward downvalley/drainage patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:

A weather disturbance centered near HII as of 21Z will continue to
shift north and leave behind more stable air with only a very slight
chance of renewed thunderstorm activity. Otherwise, decreasing mid
and high clouds. Surface winds will generally favor north and east
components this afternoon before transitioning to light and variable
(favoring downvalley patterns).

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday: Tuesday and Wednesday are shaping up to be
as hot or hotter than any other days this year so far. Minimum
humidities will be in the low to mid teens on the lower deserts and
up to the mid 20s for the higher terrain areas. Overnight recovery
will be only fair. Expect only very slight thunderstorm chances
which will be confined to the higher terrain of south-central AZ.
During the latter half of the week, high temperatures decrease
significantly as high pressure weakens and atmospheric moisture
content increases. As a result, humidities increase with minimum
readings on the lower elevations returning to the 20s. Storm
chances slowly expand to the lower elevations.



Record high temperatures next week:

Date         Phoenix         Yuma
----         -------         ----
July 23    114 in 2014    117 in 2014
July 24    116 in 2014    117 in 2014
July 25    115 in 1943    120 in 1943


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Warning from 10 AM Monday to 8 PM MST Wednesday
     for AZZ532-534>556-559>562.

     Heat Advisory from 10 AM Tuesday to 8 PM MST Wednesday for

     Excessive Heat Warning from 10 AM Tuesday to 8 PM MST Wednesday
     for AZZ530-531-533.

CA...Excessive Heat Warning from 10 AM Monday to 8 PM PDT Wednesday
     for CAZ561-566>568-570.

     Excessive Heat Warning from 10 AM Tuesday to 8 PM PDT Wednesday
     for CAZ560-562>565-569.



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