Scientific Forecaster Discussion
fxus65 kpsr 231132
Area forecast discussion
National Weather Service Phoenix Arizona
432 am MST Thu Feb 23 2017
Update...updated aviation and fire weather discussions...
behind a cold front, temperatures will hover slightly below average
through the remainder of this week and into early next week. Two
fast moving storm systems will move into the region Sunday and again
on Monday. The weather system on Monday will have the better
potential of spreading rain showers across much of the region. Drier
and eventually warmer weather will return to the area by the middle
of next week.
per observational data, fropa was occurring through the Phoenix
metropolitan early this morning with the strongest sfc pressure
rises diving into SW Arizona forcing a continued eastward March of the
front. By the mid morning hours, this front will have cleared the
entire forecast area leaving the region controlled by a much cooler
airmass than the past couple days. Modest 6-12dm 500 mb regional height
falls and objectively analyzed deep cold air advection (some 8c-12c decreases in
the h9-800 mb layer) will translate into afternoon highs some 8f-15f
cooler than Wednesday and 5f-7f below the seasonal normal.
Dry zonal flow will predominate the region Friday and Saturday as
the SW conus awaits the next series of weather disturbances swinging
south from the Pacific northwest. 500 mb heights will remain essentially steady
hovering either side of 564dm through this period. While some
moderation/warming of the boundary layer will occur (especially
saturday), temperatures categorized as slightly below average will
An initial lead shortwave will be swept southeast towards The Four
Corners/southern rockies coincident with the entrance region of a
downstream zonal jet core Sunday. As a result, the wave will
progressively lose amplification and larger scale forcing structures
as it becomes sheared and dampened eastward over northern Arizona. A
narrow plume of midtropospheric moisture may briefly become
entrained along a dying frontal boundary with the only remaining
lift being orographic processes east of Phoenix Sunday afternoon.
Outside of the highest elevation areas, virga and sprinkles would be
the most likely outcome Sunday.
A secondary shortwave will take a similar path into the southwest on
Monday and Tuesday as compared to the aforementioned Sunday lead
wave, however will also absorb a piece of moist subtropical Pacific
energy along its journey through SW California. The primary uncertainty with
respect to this wave is when the best moisture advection and forced
ascent moves into the County Warning Area with models still settling on timing the
passage of the trough axis. In general, model timing has been
slowing with each successive iteration; the European model (ecmwf) and Gem
operational members faster than the operational GFS and towards the
faster edge of the entire naefs suite. However, with excellent
evidence supporting a moderately organized subtropical water vapor
plume along and preceding the trough axis, ascent and precipitation
may begin in line with the faster model output. On the other hand,
recent model trends have certainly pointed towards a slower arrival
so confidence is still rather low and have split pops around the
Monday afternoon/evening time frame hedging towards the evening and
overnight periods for most expansive rainfall. Lingering spotty
showers could persist into the day on Tuesday primarily over higher
terrain of eastern Arizona, though any amounts would be very light.
Post frontal pressure rises and deeper subsidence will envelop the
region by Tuesday evening as mean troughing shifts into the eastern
conus prior to the hemispheric pattern stalling and becoming
retrogressive in nature. The resultant flow pattern for the middle
and end of next week may very well bring a cutoff weakness in the
midlevel height field over the southwest. Ensemble output varies
substantially as to the potential depth of such a system though very
little evidence supports any remaining moisture or precipitation.
Primarily, the magnitude and haste of warming behind the exiting
weather systems next week will be the challenge with the potential
presence of this cutoff.
south-central Arizona including kphx, kiwa,and ksdl:
Skies have cleared behind the cold front and westerly winds will
dominate through the majority of today. May see some gusts out of
the west this afternoon, but only up to 20 mph. Have high
confidence in easterly downslope winds returning late this
Southeast California/southwest Arizona including kipl and kblh:
No aviation impacts through the period under clear skies. Winds
will remain out of the west and generally at or below 10 kts.
Aviation discussion not updated for amended tafs.
Saturday through Wednesday...
dry conditions, but increasing clouds are expected for most areas
through early Sunday as a mostly dry weather system passes by to
the north. A second weather system is likely for Monday into
Tuesday with better chances of wetting rains, especially across
the high terrain of Arizona. Below normal temperatures are
expected through the entire period. Breezy conditions early Sunday
and again on Monday are expected in most areas with breezy
conditions persisting down the lower Colorado River each afternoon
through Wednesday. Humidities will start out dry on Saturday, but
improve through early next week with minimum readings as high 35
to 40% on Monday. Gradual drying thereafter will lead to afternoon
readings closer to 20% on Wednesday.
Spotter information statement...
spotter reports will not be needed through the weekend.
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