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Area forecast discussion 
National Weather Service Phoenix Arizona
918 am MST Thursday Jul 24 2014

Synopsis...
a strong high pressure system is forecast to continue over the
region through the end of the week...resulting in very hot
temperatures through Friday. Sufficient moisture will be in place for
at least a slight chance of thunderstorms on most days through the
middle of next week...more so over the higher terrain of south-
central Arizona.

Discussion...
the intrusion of dry air already making its presence known this
am...both in balloon data and satellite images. Precipitable waters at ktwc have
dropped below 1.20 inches this am and by the afternoon surface
dewpoints across Southeast Arizona and into portions of south-central Arizona
could mix down into the 40s. Early and middle morning skies across
southeast and east central Arizona remain mostly clear...also indicative
of the drying. Most of northern down through southwest Arizona skies
still holding scattered to broken middle level clouds...with plenty of
moisture and subtle instability maintained those decks. Will continue
to see clouds and that elevated moisture surface transition towards
the Colorado River valley through the day with the easterly mean
atmospheric flow...which could amount to some isolated development
later in the day...covered so far in the forecast.

Outside of the moisture decline and its potential on storm
development for today...very warm temperatures remain the primary
concern across the Lower Desert elevations today. We had another warm
morning start in Phoenix with temperatures only cooling to 93f this
am at Phoenix Sky Harbor. With the potential for ample sunshine and
the expectation of moisture to thin through the day from the
east...have no plans to adjust the excessive heat warning headline
and will leave that in effect until 8pm this evening.

&&

Previous discussion /issued at 205 am MST/PDT/...
after a more active day on Wednesday and overnight...in spite of the
excessive heat...a bit of a downtrend trend in convective activity
seems to be in the cards for today. Somewhat drier air is expected to
spread into the region behind the weak shortwave that triggered the
thunderstorm activity over the region on Wednesday as the upper high
center that has brought US the very hot weather the last few days
repositions itself off to the east over nm. The reduction of the
amount of cloud cover and drying of the lower levels of the
atmosphere...along with another very warm start to the day (90f+ lows
at many Lower Desert locations) is expected to allow enough warming
to approach or reach excessive heat criteria across the lower deserts
for one more day...in spite of some reduction of the 500mb heights
over the region...from 595-596dm yesterday to 592-592dm today. Given
the relocation of the main moisture plume to the west...it appears
the best chance for any isolated thunderstorm activity today will be
across western Pima County...perhaps extending into southwest
Maricopa County and into eastern Yuma County.

Friday through Saturday...
slightly cooler afternoon temperatures are forecast for Friday as
the upper-level high weakens a bit...and a little more monsoon
moisture spreads into the region. A better threat for afternoon and
evening convection is expected on Saturday as the upper high center
shifts a little further east...allowing a deeper southeasterly flow
to develop over the region...importing more moisture into the Desert
Southwest. Another weak shortwave embedded in this flow appears that
it will move into the region...further enhancing the convective
activity as MUCAPES are pushed up into the 500-100 j/kg range.
Thus...have decided to raise probability of precipitation over the region and lower maximum
temperatures on that day.

Sunday through Thursday...
it appears that a low-middle grade monsoon will likely prevail through
the extended period as weak southerly flow aloft continues to pull
moisture into the region...but with the upper high center attempting
to reposition itself westward back over the region...stabilizing the
airmass somewhat and perhaps putting a bit of a lid on the
convective activity.

&&

Climate...

Record high temperature for the middle of the week.

City July 24 July 25
---- ------- -------
Phoenix 114 in 2006 115 in 1943
Yuma 117 in 1943 120 in 1943

&&

Aviation...
south-central Arizona including kphx...kiwa...and ksdl...broken to overcast
middle and high level debris cloudiness will gradually thin through the
day. Cloud bases will remain at or above 15 kft. East to southeast surface
winds will continue through this evening...with middle-day and
afternoon gusts up to 25 kts possible.

Southeast California and southwest Arizona including kipl and kblh...
some high debris cloudiness may work its way into southeast California
through the day...though skies are expected to remain mostly scattered.
Winds at kipl will remain light and variable today...while at kblh
southeast winds with gusts to near 25 kts are expected middle-day to
early evening. There is a slight chance of thunderstorms east of the
Colorado River this afternoon and evening.

Aviation discussion not updated for amended tafs.

&&

Fire weather...
Saturday through Wednesday...
slightly cooler afternoon high temperatures are expected through the
period...as upper level high pressure weakens and slides further to
the south. Even so...readings will still be slightly warmer than
normal. An increase in moisture is possible as the high slips to the
south...though minimum afternoon daily humidity levels will still
dip below 20 percent. Overnight recovery should however be moderate
to good. There will be a daily threat of thunderstorms...but these
will be mainly concentrated in the higher terrain of eastern Arizona.
Outflow boundaries moving into the lower deserts will be capable of
producing strong wind gusts. Further west across southeast California...the chance
for rain and thunderstorm impacts will be even lower. Winds to
generally follow diurnal and terrain driven headings...with mostly
light speeds.

&&

Psr watches/warnings/advisories...

Arizona...excessive heat warning until 8 PM MST this evening
azz020>023-025>028.

California...excessive heat warning until 8 PM PDT this evening caz031>033.

&&

$$

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Discussion...Nolte
climate...percha
aviation....McLane
fire weather...Nolte/McLane/Hirsch

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