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Area forecast discussion 
National Weather Service Phoenix Arizona
155 PM MST Thursday Jul 31 2014

Synopsis...
a low grade monsoon evening is anticipated tonight with just a
slight chance for thunderstorms forecast over the lower central
deserts. Expect an uptrend in thunderstorm chances expanding from
east to west Friday through Sunday. Accordingly...temperatures will
not be as hot. By Sunday high temperatures in the central deserts
will likely lower into the 90s. Next week...a downtrend in
thunderstorm chances and steadily warming high temperatures is
anticipated as drier air filters in from the west.

&&

Discussion...
tonight...
upper high remains centered to our south near the Mexican border
this evening...allowing for a somewhat drier southwesterly to westerly flow aloft
to affect southern/central Arizona. At 2 PM infrared imagery showed
considerable mainly high clouds across the Lower Desert of southeastern California
and Arizona...a result of a decayed convective complex that formed
over northwestern Mexico last night. The clouds along with slightly
increased humidity levels allowed temperatures to fall off slightly
compared to yesterday and they dropped below excessive heat warning
thresholds. As a result the heat warning was cancelled earlier this
afternoon for southeast California. Surface dewpoints over the lower
deserts were running in the low to middle 50s early this afternoon and
precipitable water values were around 1.4 inches. With weak wind fields aloft...
unfavorable westerly steering flow and limited low level moisture...
chances for storms tonight over the lower Arizona desert are meager.
Expect a low grade monsoon evening with probability of precipitation running around 10-15
percent under partly cloudy skies.

Friday through Sunday...
model remain very consistent in calling for a marked increase in
moisture during the period...with the flow becoming more classically
monsoonal. Steering flow changes from weak west to stronger out of
the southeast and south...and moisture spreads westward across the
lower deserts with precipitable water values climbing to nearly 2 inches in some
locations by Saturday afternoon. Of course...there are model
differences regarding the disturbances in the upper flow but most of
the guidances suggest that an inverted trough will rotate around the
high and into western Arizona by Saturday...and flow aloft will become
stronger and more difluent by that time. Cape/instability also
upramps and correspondingly we have a significantly increasing pop
trend with the highest numbers for Saturday afternoon and evening.
By that time expect 30-35 percent over most of the lower
deserts...with values around 60 percent in the higher terrain east
of Phoenix. Increasing clouds/moisture will result in a sharp
downturn in high temperatures during the period with well below
normal highs forecast for the weekend. By Sunday...given the
expected convection overnight on Sat night...we should see highs in
the 90s across much of the lower deserts and we lowered the high for
Phoenix to just 97 degrees.

Monday through Thursday...
the first half of the next workweek continues to look like a
downtrend in storm activity as the inverted trough departs and drier
air moves into the region...at first from westerly flow then
northerly. Accordingly...probability of precipitation retreat eastward and temperatures slowly
climb.



&&

Aviation...
south-central and southwest Arizona and southeast
California...including kphx...kiwa...ksdl...kipl...and kblh...

Considerable middle and high cloudiness is expected to continue over
the entire region through the taf period as increasing southerly
flow continues to import moisture into the Desert Southwest.
Although thunderstorms are not expected to affect the terminals at
this time...there is a chance that outflows from distant storms to
the north could affect the winds at the phx area terminals this
evening...with outflows from storms over Southeast Arizona enhancing the typical
late-evening easterly winds later tonight. Winds at both the southeast California
taf sites will remain mostly out of a southerly to southeasterly
direction through the taf period.

Aviation discussion not updated for amended tafs.

&&

Fire weather...
Saturday through Wednesday...
moisture will return to the area by the weekend with showers and
thunderstorms expected. Day to day timing and placement of storms is
hard to pin down but it looks like a decent chance of a wetting rain
even across some desert locations. Humidity values will steadily
increase with minimum values between 20 and 30 percent expected
across the deserts Saturday into the early part of next week.
Correspondingly...high temperatures will fall off...dropping from
around seasonal normals Friday to well below normal values on
Sunday. Slight warming will return by next Tuesday as humidity
values lower somewhat. Strong winds are not expected during the five
day period...and they should typically favor south to southwest
directions during the afternoon and evening hours.

&&

Psr watches/warnings/advisories...

Arizona...none.

California...none.

&&

$$

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Discussion...cb/aj
aviation...percha
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