Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 261210

Area Forecast Discussion
National Weather Service Phoenix AZ
510 AM MST TUE JUL 26 2016

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


Strong high pressure over the desert southwest is expected to keep
monsoon thunderstorm activity to a minimum and many lower desert
high temperatures at or above 110 degrees through Thursday, in spite
of lingering moisture over the region. A return to strong easterly
flow aloft as the high pressure weakens and shifts to the north will
allow deeper moisture to move into the region from Friday onward
into early next week, bringing cooler temperatures and a increase in
thunderstorm activity, with locally heavy rain and gusty winds
likely at some locations.



Today through Thursday...

A relatively quiet period weatherwise appears to be in the cards for
this period as strong ridging (500m heights in the 593-596dm range)
remains locked over the region. Although moisture levels will be
near average for this time of year (PWATS in the 1.50-1.70 inch
range), rather warm air aloft (700mb temps in the 25-27C range) and
light winds through the column is expected to keep the airmass
fairly stable and limit convective activity across the region,
especially across the lower elevations. Although the HRRR and NAM-
4km high-res models are showing isolated-scattered showers/storms
developing each afternoon over the higher terrain of northern and
southeast AZ, they are also showing that almost all of this
convective activity dissipates before moving into the lower deserts,
with the main impacts being gusty winds, and perhaps some blowing
dust as the outflow boundaries most across the lower elevations.
Along with the suppression of convective activity, the very warm air
aloft and mostly clear skies is also expected to allow lower desert
temperatures to reach, or exceed 110 degrees each afternoon, with
temperatures even approaching 115 degrees at some of the warmest
southeast CA locations.

Friday through Monday...

Confidence is building that this will be a rather active period
weatherwise across the region. As long-wave trofing begins to dig
southward into the central US, the strong ridging that is now over
the Desert Southwest is expected to weaken and shift northward. This
pattern change is now showing up in the operational gfs/euro/gem
model suites, and many of their respective ensemble members. This
pattern change is expected to allow more/deeper tropical moisture to
move northward into the SW US, increasing the convective activity
noticeably over our region.

It now appears that the 1st active afternoon/evening will perhaps be
on Friday, with the gfs KPHX forecast sounding now showing 15-20 kt
easterly winds between 700mb and 400mb, with northwesterly winds
above that level, which is a classic sounding for back-building
storms into the lower deserts of South-Central AZ. The soundings
that evening are also showing noticeable mid-level cooling as well,
allowing CAPES to rise into the the 1500-2000 J/Kg range, with LI`s
falling into the -3 to -6 range, which would likely result in
widespread strong-severe thunderstorm activity, if the sounding does
verify. PWATS then rise into the 1.80-2.00 inch range on Sat/Sun,
which is high enough to allow showers/storms to produce locally
heavy rains, with the risk of localized flooding. This active period
could last into early next week as well, although the models do show
a gradual decrease in available moisture as ridging attempts to
rebuild over the region.


.AVIATION... South-Central Arizona Including KPHX, KIWA, and KSDL:
Today should end up similar to the last couple days with mostly high
terrain storm activity during the afternoon and evening with some
degree of outflow activity over the lower deserts. Timing of any
potential wind shifts is difficult and confidence is too low at this
point to include in area TAFs. There is a remote chance thunderstorms
may impact the Phoenix metro this evening. Outside of any
thunderstorms or outflow induced winds, overall winds should
mostly follow typical wind shift patterns but lean toward a more
dominant westerly component.

Southeast California/Southwest Arizona including KIPL and KBLH:
Periodic mid and high clouds through the period and only a remote
chance of any shower or thunderstorm activity this afternoon and
evening. Wind directions out of the south will be fairly dominant at
both KIPL and KBLH.

Aviation Discussion not updated for amended TAFs.


Thursday through Monday:
A warmer and somewhat drier day on Thursday is expected through
Thursday as high pressure builds across northern Arizona. An increase
in convective activity is expected Friday through early next week as
deeper moisture quickly moves back into the region. Temperatures to
remain slightly above normal through Friday followed by slightly
cooling into the weekend. Minimum humidities should fall in the 15-30
percent range on Thursday and rise into the 20-35 percent range
starting Friday.


Spotter activation is not expected.




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