Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
205 PM MST Tuesday Jul 7 2015
thunderstorm chances will be significantly lower than usual for the
middle of the monsoon season...and primarily relegated to the
mountains of northern and eastern Arizona the next several days.
Despite the lack of storm activity...temperatures will remain near
to slightly below normal. Beyond this upcoming weekend...high
pressure will reposition and strengthen again leading to warmer
temperatures and possibly an increase in moisture.
unusually strong southwesterly flow aloft continues across the southwestern Continental U.S.
Ahead of an anomalously deep gyre off the central California coast.
12z ktwc and kpsr sounding data still showed lingering pesky surface-700 mb
moisture (mixing ratios 10-11 g/kg)...with substantially drier air
aloft. However...surface dewpoints through southwestern Arizona have tanked
significantly today...with widespread readings in the Lower/Middle 40s.
This drier airmass (mixing ratios below 8 g/kg) has advected
northeast beginning to mix and scour higher boundary layer moisture
out of central Arizona.
As such...deep convection has been primarily tied to terrain
features north and east of Phoenix...with strong steering flow
pushing debris clouds/anvils well to the east. Given the long
awaited introduction of this drier airmass unsupportive for
organized storms...have cut probability of precipitation through this evening towards just
the periphery of the forecast area...with minimal chances of
convection impacting Lower Desert locations.
Wednesday through Friday...
as the upper low center approaches and moves over central/Southern
California...upper level winds may back to a more southerly
direction (especially thursday) temporarily increasing moisture over
Southeast Arizona...with a very sharp drop off into central Arizona.
A vorticity lobe and jet segment will swing through southeast California
and western Arizona Thursday...brushing eastern Arizona and the
midlevel moisture plume. Associated dynamical forcing looks to be
less than robust over the area with the jet axis remaining to the
west. However...nudged up the probability of precipitation over zone 24 as even subtle
influences can make a difference this time of year. The low quickly
shifts off and weakens late in the week...but will be replaced by
renewed troughing and anomalously strong southwest flow.
With 500 mb heights falling into a 576-582dm range later in the week and
1000-850mb thicknesses only touching 143dm...high temperatures will
fall into a below normal category (barely eclipsing the 100f
threshold at lower elevations). Concurrently...with dewpoint/wetbulb
temperatures substantially lower than is typical for the middle of
the monsoon season...overnight lows will also fall below normal
leading to very comfortable weather for middle July. In fact...the
entire pattern and atmospheric thermal profiles are more
reminiscent of middle may versus July.
Saturday through Monday...
models continue to depict a westward expansion of the high centered
to our east with moisture improving over the lower deserts...as southwesterly
flow lifts north. The GFS continues to be more generous with the
moist advection than the European model (ecmwf)....likely due in part to more of a
southerly flow component and deeper coastal troughing depicted by
the GFS. Accordingly...it also shows stronger winds aloft over our
forecast area. The operational GFS shifts the trough northeastward
Sunday with a jet streak moving into the Great Basin...placing the
entrance region briefly over our forecast area. Taking a
conservative approach due to inherent uncertainties...only slight
chance probability of precipitation over the south-central deserts are advertised Sunday
(which is an westward expansion compared to previous forecasts).
There is an even greater difference between GFS and European model (ecmwf) moisture
profiles on Monday...as the European model (ecmwf) holds onto the West Coast
troughing nudging it further east and yielding additional drying.
The GFS meanwhile expands the high further west for a bit of
additional warming aloft. Both of those scenarios...especially
European model (ecmwf)...could have a negative impact on storm potential and so
pulled back probability of precipitation from the lower elevations of south-central Arizona
south-central Arizona including kphx...kiwa...and ksdl...
thunderstorm activity will generally remain confined to higher
terrain well north and east of Phoenix late this afternoon. The
likelihood of thunderstorm impacts at the terminals is extremely
low...with possibly only subtle wind shifts from long traveling
outflow boundaries. Otherwise...winds will be light and follow
usual diurnal patterns.
Southeast California and southwest Arizona including kipl and
no aviation impacts under mostly clear skies through Wednesday
afternoon. Surface winds could occasionally include higher gusts late
afternoon/early evening...though will retain lower speeds by
Aviation discussion not updated for amended tafs.
Friday through Tuesday...
the flow pattern will transition from southwesterly and unfavorable
for extensive moisture and storms during the first part of the
weekend...to more traditional monsoon season southerly early next
week. Thunderstorm chances will mostly be limited to higher terrain
locations north and east of Phoenix...though could begin moving
westward next week. Afternoon humidity levels in a 10-20 percent
range during the weekend will increase to a 15-25 percent range next
week. Overnight recovery will be good. Diurnally driven winds speeds
and directions will predominate much of this time...though stronger
than usual afternoon upslope gustiness will still be evident across
much of the district.
Spotter information statement...
spotters are encouraged to report any significant weather conditions
according to Standard operating procedures.
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