Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Phoenix Arizona
550 am MST Friday Aug 22 2014
updated aviation and fire weather sections.
low pressure will continue to move through the area today...however
only a few isolated showers are expected across the region. Drier
conditions are expected across the area through the weekend along
with slightly cooler than normal temperatures. A return to a more
typical monsoon pattern is expected next week...with afternoon and
evening showers and thunderstorms returning to the region.
upper level low pressure was situated over north-central Arizona
this morning as of 0830z...with the majority of the storm activity
well ahead of the low in New Mexico. While there were a few showers
noted on radar beneath the upper level deformation zone /located
approximately along Interstate 17/...the storms from Thursday
evening had largely dissipated across the deserts. In the wake of
this storm...temperatures were generally in the upper 60s to lower
70s across the deserts...approximately 10 degrees below normal for
Strong isentropic descent is expected across the region today as the
trough moves out of the area. Bl moisture will remain somewhat
elevated as 1000-700mb mean mixing ratios will still linger in the
8-12 g/kg range...however the lack of upper level support should
preclude much in the way of widespread showers and thunderstorms.
Given the closer proximity to the departing low...I would expect to
see a few pop-up showers north and east of Phoenix with little of
consequence across the deserts. There could also be some activity
across portions of Southeast Arizona that glances Pinal County...but
that would be about it. Hi-res WRF models along with the hrrr paint
a similar solution for today. More substantial bl drying is expected
this weekend as all operational models suggest mixing ratios will
drop to around 3-4 g/kg across southeast California and to 6-8 g/kg
across Arizona. With even less upper level support this
weekend...probability of precipitation will remain substantially below climatology this weekend.
Next week will see a return to a more typical monsoon setup as high
pressure slides east and low level winds turn around out of the
southeast. While there are some differences in the models regarding
a trough moving through Utah by the middle of next week...both the
GFS and European...as well as the 12z naefs indicate a noteworthy
increase in moisture across the area Monday/Tuesday next week. This
could be the result of the passage of a tropical system well south
of the Gulf of California...or it could simply be outflow driven
from convection over Sonora. Regardless...mixing ratios expected to
return to the 10-12 g/kg range early next week which should set the
stage for afternoon showers and thunderstorms each day through at
least Friday. Tough to pick out one active day over another...but
probability of precipitation have been nudged toward climatology /and slightly higher east of
Phoenix/ from Tuesday through Friday.
As for temperatures...the boundary layer will struggle to recover
today and temperatures should remain below normal. Some recovery is
expected on Saturday with a return to normal temperatures expected
Sunday through the end of next week.
south-central and southwest Arizona and southeast California
including kphx...kiwa...ksdl...kipl...and kblh...
a weakening upper low was centered just west of phx at 12z and will
continue shifting east during the day with further weakening.
Anticipate few-scattered bases at 8-10 kft mean sea level...becoming scattered-broken by
midday over higher terrain near and east of Phoenix metropolitan. A stray
shower cannot be ruled out over south-central Arizona including
Phoenix area...more likely further east...but probability too low to
reflect in tafs. Expect clear skies this evening as the system
shifts into New Mexico and weakens even further. Surface winds over
Phoenix area will favor east and southeast directions this morning
before transitioning to westerly around midday. Surface winds over
southwest Arizona and southeast California will hold on to southerly component
longer before trending toward southwest and west this
evening...local gusts to 20 kts this evening Imperial Valley.
Aviation discussion not updated for amended tafs.
Sunday through Thursday...
humidities will trend upward Monday and Tuesday before starting to
trend downward Thursday. Overnight recovery will remain good through
the week. Storm chances will remain slight and favor the higher
terrain of south-central Arizona however they will expand slowly
westward to near the lower Colorado River valley by Wednesday.
Haines index values peak at 5 over portions of southeast California
and southwest Arizona Sunday and Monday before trending downward.
Expect lower values over south-central Arizona due to better
atmospheric moisture content.
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