Weather Service Report

FXUS65 KPSR 242135

Area Forecast Discussion
National Weather Service Phoenix AZ
235 PM MST SUN JUL 24 2016

.UPDATE...To Aviation Discussion...

An increase in moisture and cloud cover over the region is expected
to take some edge off the heat, and provide somewhat better chances
of showers and thunderstorms through the middle part of the upcoming
workweek. The best opportunity for rainfall will occur over the
higher terrain north and east of Phoenix, with lesser chances heading
west into southeast California. A better chance for showers and
thunderstorms is then forecast for the end of the week and into the
upcoming weekend, as deeper moisture begins to move into the region
from the south and southeast.



Tonight and Monday...

One thing noticeable from the regional weather balloon data was that
monsoon moisture levels in southern AZ have now crept back up to
those seen last week, July 17,18, and 19th, when we had isolated
severe storms and flooding in south central AZ, including parts of
Phoenix. Monsoon moisture has surge dramatically into southern AZ
from Mexico this weekend due to a strong push of humid convective
outflow boundaries. This morning was no different. Last night a
large convective complex south of the border generated northward
moving outflows which hit the lower Colorado River Valley at Yuma
at 4 am with southerly gusts of 43 mph. South and southeasterly
wind gusts to 31 mph at Imperial CA and 35 mph gust at Blythe were
also noted for a short time late this morning. Moisture and
breeziness from southwest AZ has already recurved east and worked
its way through south central AZ including Phoenix this afternoon.

The large convective MCS outbreak in northwest Mexico last night was
located on the difluent west side or leading edge of upper level
inverted trof (IT) that came out of south Texas. Streamline analysis
derived from satellite imagery, including satellite high density
winds and satellite wind computations of divergence fields, highlight
the upper level storm forcing. And, this inverted trof seen in
the model guidance yesterday, is forecast by the models to slow and
weaken before moving into southeast and portions of south central AZ
later tonight through Monday afternoon.

So far at 2 pm mst there has been a late start to afternoon
convection over the southeast Quarter of the state. A slight 
chance of showers and thunderstorms are still forecast for our
portions of south central AZ this evening.  Disconcerting though
are how the models are handling the Mexican IT as it moves into
southeast AZ later tonight and Monday. Very little atmospheric 
response is forecast by the models with this feature which is sort
of counter-intuitive for us. Therefore confidence is low on possible
shwr/tstm effects Monday from this IT feature, therefore we will
continue with the low grade monsoon forecast.

Tuesday and Wednesday...

With the lack of any identifiable upper level disturbances we will
continue the low grade monsoon forecast this period. Afternoon
thermodynamics will be the driving mechanism behind convective storm
development, as usual mainly in the mountains north through
southeast of Phoenix. Again, enough moisture will be present in the
deserts for secondary late day convection from outflow boundaries or
outflow boundary mergers on the deserts.

Thursday through Sunday...

This period gets particularly interesting.  Under increasing 300-250
mb northerly flow, a former Pacific shortwave moving through the
northern tier of the western region, is forced south into AZ
Thursday. This feature slowly migrates into southern AZ through
the weekend, and with the model forecast of excessive amounts of
boundary layer moisture, an upward trend in shower and thunderstorm
potential is expected over southern AZ Thursday through Sunday. Not
sure which day will be more active, however with excessive 
boundary layer moisture predicted (which typically happens only a
handful of days during the summer thunderstorm season), hydrology
should be the issue, or more succinctly, flood potential,.


South-Central Arizona Including KPHX, KIWA, and KSDL: 
Early afternoon conditions will be low on the aviation impacts
directly for the terminals. SCT to BKN mid level cloud cover will
continue over the area, with storm clouds building west to the east
and northeast of the Phoenix area. These storms may be capable of
producing outflow winds that may introduce a east to northeast wind
shift in for the terminals but later into this they are
still rather removed from the Lower Desert elevations. Debris
cloudiness will expand into the area overnight and cannot rule out a
stray shower developing within the clouds, but coverage should not be
enough to warrant even VCSH at this time.

Outflows/Sudden Wind Shifts: 40% 
Blowing Dust: 15%
Storms Directly Impacting the Terminal: 15%

Southeast California/Southwest Arizona including KIPL and KBLH:
Gulf flow up the lower elevations of the CO River will persist
southerly oriented winds for the day, along with lingering slantwise
and longer range field vsbys impacts, for the late afternoon and
evening. Debris clouds associated with a decaying Mexican storm
complex will continue to spread in from the east-southeast generating
BKN to OVC mid level coverage at times. Isolated gustiness may result
within the southerly oriented wind profile and strong southerly winds
overnight are likely. 

Aviation Discussion not updated for amended TAFs.


Wednesday through Sunday...
Monsoon moisture working its way back into the region will bring
chances for afternoon and evening thunderstorms each day. The best
chances for wetting rains will be over the higher terrain east of
Phoenix through the mid week window with lesser chances over the
lower deserts of south- central and southwest Arizona and only slight
chances over southeast California. Some increase in convective
activity is expected on Friday as deeper moisture begins to move into
the region. Temperatures to remain slightly above normal through the
entire forecast period. Minimum humidities should fall in the 15-30
percent range through Thursday and rise into the 20-35 percent range
for Friday and Saturday.


Spotter activation is not expected.




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