Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Phoenix Arizona
905 PM MST sun Aug 30 2015
drier westerly flow will slowly push the moisture to the east by
Tuesday...limiting storm chances primarily to higher terrain areas
to the east of Phoenix. There will also be a gradual cooling trend
this week with central deserts high temperatures dropping to around
seasonal normals by the middle or latter portion of the week.
although anti-cyclonic flow had shifted east earlier today bringing
deeper layer westerly trajectories into the forecast area (usually
unfavorable for organized storms in the Phoenix metro)...residual
boundary layer moisture held tight over central and eastern Arizona
(mixing ratios at or above 10 g/kg). 00z kpsr sounding data indicated a
rather notable capping inhibition at the 700 mb level (cinh near 50
j/kg)...though increasing upper level divergence and ascent supported
deep convection through Pima and Pinal counties. With dcape values
in excess of 1500 j/kg...numerous outflows were common helping erode
the capping inversion and mix moisture to the top of the boundary
Showers and storms will continue through northern Pinal and possibly
parts of southern Gila County through the evening...with much of Maricopa
County worked over by outflows and convection. In general...probability of precipitation
were consolidated to match radar trends and conceptually adjusted
high resolution ensemble blends. Otherwise...only cosmetic changes
to account for precipitation effects were needed for other
/236 PM MST sun Aug 30 2015/
starting on Monday...operational runs of the GFS and European model (ecmwf) as well
as ensemble guidance continue to call for the start of a gradual
drying and cooling trend as troughing settles further along the West
Coast and gives a deep southwesterly flow aloft to southeastern California and
Arizona. Where earlier forecasts have called for this flow to scour
out and sweep the deeper moisture surface to our east at a fairly
quick rate...deterministic and ensemble forecast guidance still point to
a southerly 850mb flow into the region...maintaining some semblance
of elevated dewpoints and ll to ml mixing ratios through midweek.
Confluent flow through the ml and ul won't be overly supportive of
storms...but cannot rule out at least slight chances for the limited
areas of higher terrain across south-central Arizona. Otherwise...best
locales for storms will be primarily confined to higher terrain
areas east of Phoenix. Except for some high clouds at times moving
through the southwest flow aloft skies will be mostly sunny over the
lower deserts through most of the work week. As heights slowly
lower...high temperatures will drop off day by day. Monday will
still see high temperatures well above normal with highs around 107
expected at Phoenix but by Wednesday the central deserts should
lower to around 104 degrees...and much closer to seasonal
normals...with little change then expected into the weekend.
There is a bit of a wrinkle in the forecast out at the end of the
extended period...over the coming weekend...as medium range models
continue to call for some tropical moisture to be drawn northward by
the broad trough over the western Continental U.S....and then spread into the
lower Arizona deserts leading to at least a slight chance of
afternoon and evening showers or thunderstorms by Saturday. Probability of precipitation
have been raised over the weekend accordingly...and will be slightly
greater than climatology values for now.
south-central Arizona including kphx...kiwa...and ksdl...
rain showers and thunderstorms and rain will generally slide south of terminal locations
through this evening and overnight. Timing of wind shifts is low
given multiple outflow boundaries...however should obtain the
traditional easterly flow after midnight. Moderate confidence that
storm activity will remain east of the Phoenix metropolitan Monday
Southeast California/southwest Arizona including kipl and kblh...
no aviation impacts through Monday afternoon with only occasional
high cirrus. Surface winds will periodically become gusty...south at
kblh and west at kipl...up to 25 kts in the afternoon and
evening...with tendency for light and variable winds during the middle
Aviation discussion not updated for amended tafs.
Wednesday through Sunday...
a low pressure system across the Pacific northwest will displace the
monsoon high to the south and east through the week. The trend will
be towards cooler and drier conditions across the Desert Southwest.
However...enough moisture will linger across eastern Arizona for
isolated afternoon thunderstorms. For the weekend...latest guidance
is hinting at a return to deeper moisture and better rain
chances...particularly south and east of Phoenix.
Spotter information statement...
spotters are encouraged to report conditions according to Standard
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