Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
305 am MST Friday Sep 4 2015
sufficient moisture will remain over much of Arizona today providing
good shower and thunderstorm chances...particularly across the
eastern half of the state. Somewhat drier air will move into the
region over the weekend...reducing rainfall chances very slightly.
While some thunderstorm chances will persist during the first half
of next week...a substantially warmer and drier airmass will spread
into the area for the latter half of the week.
early morning WV imagery shows a seasonably deep cold core low over
the Pacific northwest...with the longitudinal trough axis extending all the
way south to 20n. This scenario is creating an unusually strong
meridional jet throughout the southwestern Continental U.S. With embedded shortwaves
streaming north from the outer fringes of Tropical Storm Kevin (west
of the southern tip of the Baja California peninsula). Meanwhile...infrared satellite
loops showed numerous Gravity waves pulsing outwards from convection
earlier Thursday...with resultant temporary subsidence in their wake
over much of Arizona. Interestingly...00z/03z regional sounding data
sampled a distinct layer of drier air (7-8 g/kg) in the 800 mb-700 mb levels
hovering above more impressive shallow surface moisture.
However...objective analysis and forecast soundings indicate mixing
and advective processes modifying this boundary layer this morning
closer to 12 g/kg.
Very interesting and tricky forecast for the next 24-48 hours as
numerous subtle features and processes will make or break the
forecast. During the morning hours...an overwhelming amount of high
resolution models initiate elevated convection through much of central
Arizona in association with a speed maximum and jet divergence field.
There is some evidence in model forecasts of Theta-E advection and
weak isentropic ascent...albeit focused more north towards
Flagstaff. Within the mesoanalysis fields...there is hints of a
vorticity center and -10c 500 mb cold pocket propagating north with the
jet segment...and indeed recent satellite images show a
corresponding cirrus streak and altocumulus castellanus field blossoming through
southwestern/central Arizona. Thus...feel model output does lend credence to
showers this morning lifting from the Phoenix vicinity into northern
Amount and scope of afternoon thunderstorm development may hinge on
any atmospheric contamination (or lack thereof) and potential
limited insolation from morning showers and cloud cover. However
given the stronger than normal winds aloft...any debris clouds and
showers will likely be advected into northern Arizona quickly
allowing for rapid airmass recovery and boundary layer warming. With
the boundary layer conservatively holding 11 g/kg moisture
levels...cooler temperatures and steep lapse rates aloft...and
decent surface warming...MLCAPES around 1000 j/kg will be attainable.
Deep layer bulk shear 25-30kt and adequate forward storm motion may
be able to translate higher momentum air to the surface in more robust
convection...however dcape values below 1000 j/kg may preclude a
more substantial widespread severe threat.
Convective potential for the weekend and early next week may be
somewhat more muted than recent days as deep layer winds veer
somewhat more westerly as troughing translates into the Great Basin
and northern rockies. Still...boundary layer moisture will be difficult
to completely scour out and some modest infusion of midlevel
moisture peeling off the remnants of Kevin may be possible.
However...there remains some disconnect with the more quality
tropical moisture to the south...and the best WV transport may be
deflected around the forecast area. In addition...with middle latitude
troughing lifting to the north and heights aloft rising ahead of the
remnant tropical system...models depict substantial midlevel warming
and increased convective inhibition. Kept some modest near
climatology probability of precipitation through Tuesday...however measurable rainfall at
Lower Desert elevations may be difficult to come by.
Some minor uncertainty still exists for the latter half of next week
as subtle differences in the subtropical ridge position will dictate
if and how quickly moisture is completely scoured south of the
forecast area. Mesoscale processes and outflow from sonoran mesoscale convective system/S
may allow for Gulf surges and boundary layer moisture
advection...but definitively forecasting and timing such events
would prove fruitless at this time. Regardless...500 mb height rises at or above
590dm will result in temperatures peaking back above normal under
gradually more compressional subsident flow.
south-central Arizona including kphx...kiwa...and ksdl...
could see a lingering shower or two for the rest of the evening into
the overnight hours while scattered to broken clouds bases remain around 10k
feet. Another shot at some showers and thunderstorms will be possible
Friday afternoon and evening. Wind directions will eventually shift
toward downsloping easterlies by around midnight and generally stay
southerly during much of the daytime hours Friday.
Southeast California/southwest Arizona including kipl and kblh...
aside from high clouds...expect minimal weather-related aviation
impacts through Friday evening. Diurnal winds with occasional late-
afternoon gustiness anticipated.
Aviation discussion not updated for amended tafs.
Saturday through Wednesday...
elevated moisture will remain in place across Arizona this weekend
through the middle of next week...whereas drier than normal air will
reside west of the Colorado River. Humidities will fall into the 20-30
percent range each afternoon across Arizona and into the 10-20
percent range to the west. Overnight recoveries at all sites will be
good/excellent. Winds will follow typical upslope/downvalley patterns
outside of thunderstorm induced outflows.
Spotter information statement...
spotters are encouraged to report conditions according to Standard
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