FXUS65 KPSR 192327

Area Forecast Discussion
National Weather Service Phoenix AZ
425 PM MST Mon Aug 19 2019

Updated Aviation


Temperatures will remain several degrees above normal as moisture
and thunderstorm activity will be very limited through Wednesday.
Another Excessive Heat episode with daily record high temperatures
is expected tomorrow and Wednesday across southern Arizona and
southeast California. Moisture will return by Thursday beginning a
gradual cooling trend and bringing isolated thunderstorms back into
the forecast for south central Arizona during the second half of
this week, with the best chances on Friday.


Our region still lies between weak troughing to our west and strong
ridging to our east, resulting in another day of dry southwesterly
flow a loft. The aforementioned ridge has shifted towards western
Texas today and has caused our 500 mb heights to increase a bit too
(591 dm across our far western forecast area to 594 dm across the
eastern forecast area). This increase in heights combined with dry
flow aloft will bring the forecast area mostly clear skies today
with temperatures a few degrees warmer than what was observed
yesterday. Lack of substantial moisture is evident in the current
SPC mesoscale analysis which only depicts 0.5-0.8 inches of PWAT
over the region. Across the state, the greatest moisture exists
across far southern Arizona, close to the border. With a stable
airmass aloft and limited moisture, storm chances will remain pretty
much non-existent today, even across southeast Arizona. The HREF
depicts that the best chances are around and south of the AZ/Mexico
border. Thus, another quiet day is in store for us with high
temperatures approaching 110 degrees this afternoon.

Temperatures will increase above excessive heat thresholds for most
lower desert locations tomorrow and Wednesday as the mid- to-upper-
level anticyclone broadens and become more zonally oriented.
Forecast H500 heights will increase to around 594 dm on Tuesday
before dropping a bit on Wednesday as the high pressure center moves
west, whereas forecast H850 temperatures will increase to 30-32 deg
C on Tuesday and 31-33 deg C on Wednesday, about a degree warmer
than last week's excessive heat episode. Therefore, our Excessive
Heat Warning across most of the lower deserts of Arizona and
southeast California continues, with daily records likely being set
on Wednesday and possibly on Tuesday. A Gulf moisture surge
associated with an increase in convection on both sides of the Gulf
of California on Wednesday should occur over the lower deserts
Wednesday night into Thursday to bring an end to this excessive heat
episode for most locations. Although there is a chance that this
warning will need to be extended into Thursday for some portions of
southeast California, higher confidence in the moisture surge across
the lower deserts should only allow for isolated locations to exceed
excessive heat thresholds, with temperatures on Thursday being
similar to today.

An easterly wave currently located east of the Yucatan Peninsula
that will trigger the aforementioned convection on Wednesday will
move northwest on Thursday and Friday to help cause a northward
expansion of strong convective complexes over the Sierra Madre. In
addition, nearly all models show the development of a tropical
cyclone just off the coast of Mexico by the middle of the week that
should track northwest about 100-200 miles west of Cabo San Lucas by
Friday. This should help advect significant upper level moisture
into southern Arizona and southeast California for Friday and
Saturday with regular Gulf surges maintaining dewpoints in the mid
50s to mid 60s. At the very least, decreased heights aloft and
increasing cloud cover will allow temperatures to decrease back to
near normal levels in the lower to mid 100s over the lower deserts
with isolated to scattered thunderstorms each afternoon and early
evening over the higher terrain of Gila County.

Unfortunately, models are slightly less bullish on convection for
the lower deserts for the second half of this week than they had
been before. The biggest culprit appears to be the lack of a well
defined high pressure center which was previously forecast to move
west or northwest into the Great Basin, but is now ill-defined. This
prevents favorable north-northeast steering flow from developing on
Thursday or Friday and prevents wave energy and mid-to-upper level
moisture associated with a frontal system moving into the Central
Plains to move into the Desert Southwest to help trigger higher
terrain thunderstorms. Although significant outflows from the north
and/or east do not appear to occur on Thursday or Friday, scattered
to widespread convection should develop over southern Arizona that
may push outflows and additional moisture into the south central
Arizona, so we are still maintaining 10-20 POPs on Friday. We cannot
rule out isolated convection on Thursday either, but midlevel
stability will most likely suppress convection. The best chance for
north- northeast steering flow now appears to occur on Saturday as
the ridge builds in from the northwest, but we may be on the
subsident side of the inverted trough associated with the tropical
cyclone tracking northwest parallel, but just offshore Baja
California. Thus, we have 10 POPs for Saturday before keeping PoPs
limited to the higher terrain of Gila County for Sunday as the ridge
moves over northern Arizona. Although we should have a break in
convection for early next week, the monsoonal moisture will stick
around to help set up a potentially busier convective period for the
second half of next week with deeper moisture that the broader model
ensemble continues to latch onto.


.AVIATION...Updated at 2325 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Unusually dry air and a weak pressure gradient will result in little
to no aviation concerns through Tuesday evening. Much like the past
several days, prolonged periods of variable wind directions will be
common during the typical diurnal variation.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation issues through Tuesday evening under clear skies. Wind
directions and speeds will be very similar to the past several days
favoring some aspect of the southerly direction most of the time at
KBLH and swinging between SE and WNW at KIPL.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
Moisture will return by Thursday to begin a gradual cooling trend
and bring back isolated thunderstorms into the forecast for the
higher terrain of Gila County and perhaps the lower deserts
including the Phoenix metro whose best chances will be on Friday.
Thunderstorm chances will begin to decrease on Sunday and Monday,
but elevated moisture levels should keep temperatures within a few
degrees of normal. Minimum relative humidity values will be below
10 percent on Wednesday before an upward trend commences on Thursday
through Monday to keep most areas in the 15 to 25 percent range.
Winds will follow typical diurnal patterns with afternoon and early
evening gustiness.



Record High Temperatures

Date Phoenix Yuma
---- ------- ----

Aug 19 113 in 1986 116 in 1915
Aug 20 112 in 1986 114 in 1982
Aug 21 110 in 2007 115 in 1969
Aug 22 113 in 2011 115 in 1969
Aug 23 114 in 2011 115 in 2011


Spotters should follow standard reporting procedures.


AZ...Excessive Heat Warning from 11 AM Tuesday to 8 PM MST Wednesday
for AZZ530>546-548>556-559>562.

CA...Excessive Heat Warning from 11 AM Tuesday to 8 PM PDT Wednesday
for CAZ561>570.

Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for



FIRE WEATHER...Hernandez/Hopper
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