Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Phoenix Arizona
1050 am MST Friday Jul 29 2016
Update...updated aviation discussion...
very hot weather will continue today before a much more humid
airmass moves into the Desert Southwest this weekend, causing a drop
in temperatures. There will be a marked increase in thunderstorm
activity over the deserts beginning tonight and continuing into next
week. Saturday evening bears close watching for strong to severe
thunderstorms, mainly across southern Arizona.
conditions are evolving toward a very active, high impact weather
situation across central Arizona this evening. Regional objective
analysis shows the 500 mb anti-cyclone gradually weakening and shifting
east towards The Four Corners. Per WV imagery, there appears to be a
subtle wave on the eastern periphery of this midtropospheric high
pressure over west central nm, and visible imagery shows rapid
manifestation of midlevel clouds in response to this feature (and
possibly a Gravity wave as well). Given these features, its not
surprising that modest cooling and moistening of the middle
atmosphere is forecast for the remainder of the day.
12z ktwc and kpsr sounding data showing improving sfc-500 mb 12-13 g/kg
mixing rations, though kpsr still shows a troublesome drop off to 6
g/kg in the 700-600mb top of the mixing layer. Ktwc moisture in this
critical layer was much better at 8 g/kg, and forecast soundings for
early this evening depict a notable moisture increase aloft (whether
synoptically or by deep outflow processes). Ncar and sseo ensemble
output along with almost every local high resolution model
resoundingly suggests intense outflow winds/boundaries developing
from high terrain convection propagating towards lower elevations.
Deep outflow boundaries (and numerous collisions) should be
sufficient to spur additional convection helping re-enforce strong
surface winds leading to expansive dense blowing dust. Given the
relatively high confidence, have issued a blowing dust advisory as
a watch type product leading into this presumptive large event. Have
boosted pops slightly as well feeling outflow boundaries
combined with terrain features scattered through valley locations
will be deep enough to locally allow for MUCAPE nearing 1500 j/kg to
be released. Large T/TD spreads along with steep midlevel lapse
rates would support isolated svr wx should this evolution come to
/448 am MST Friday Jul 29 2016/
all of the models indicate an increase in moisture today (compared to
thursday) and thus a significant increase in cape. Hi- res models
reflect this with more convection over the higher terrain of Arizona.
However, with the main 500 mb anticyclone becoming centered over
Arizona, there will still be cin to contend with. This is also
reflected in the model output with less storm coverage over the lower
elevations - but more than we had Thursday. We will have storms that
will be efficient wind producers and thus large swaths of the desert
will experience outflow winds. Even though the steering flow is
primarily easterly (and thus not a classic dust-storm orientation),
inserted mention of blowing dust this evening over the south-central
and southwest Arizona deserts plus some of the southeast California
deserts. With an outflow "surge" overnight, Max temps will be
slightly cooler than they would otherwise - most noticeably over
southeast California and southwest Arizona. Thus did not extend the excessive heat
warning. But conditions across the forecast area will still be hot.
Saturday and Sunday...
deep moisture really arrives over the weekend as an inverted trough
reaches the West Coast of Mexico. Accordingly, pops go up
significantly. Thinking is that Friday activity will not stabilize
things a whole lot. Thus Saturday is shaping up to be a big day with
potential for widespread storm activity. If that pans out, then
Sunday will not be as active over south-central Arizona but more so over
southwest Arizona and southeast California. Current forecasts reflect that
scenario. With higher humidities, more clouds, and more rain cooled
outflow air at times, expect Max temperatures to be significantly
lower. Held on to the sub-100 high for Phoenix on Sunday.
Monday through Thursday...
1000-700 mb mean mixing ratios remain in the 10-12 g/kg through at
least Tuesday, keeping above-climo rain chances in the forecast.
GFS trends toward a slow decline of moisture beginning Wednesday and
especially Thursday (gem starts drying on monday). This is reflected
in a gradual eastward retreat pops from naefs and esrl output. European model (ecmwf)
however holds on to moisture longer as it stalls out an inverted
trough over Arizona. Thus held on to pops over south- central Arizona
through Thursday. Temperatures during the workweek will trend slowly
south-central Arizona including kphx, kiwa, and ksdl:
Westerly winds to prevail through the late afternoon hours...with
stronger winds (sustained speeds as high as 12 kts) during the mid-
late afternoon hours today.
Latest high-res computer models are now in much better agreement on
the idea that thunderstorms, that will be developing over the higher
terrain east of Phoenix, will at least send strong outflow winds
this evening into the greater Phoenix area during the 01z-02z hours,
likely affecting all of the phx area taf sites, with even a decent
chance for the storms themselves to affect one or more of the taf
sites. Wind speeds of at least 25 kts, with gusts as high as 35 kts
look likely, with even stronger speeds possible, along with blowing
dust reducing visibilities as well. Heavy downpours, and even small
hail are possible if thunderstorms do, actually reach the
terminal(s). Winds to become lighter easterly later this evening,
once the storms move through.
Southeast California/southwest Arizona including kipl and kblh:
Should be another quiet taf period at kipl, with little chance of
any isolated showers or storms, and winds remaining from a
Winds will continue to favor southerly directions and wind speeds at
kblh through this evening, with stronger winds during the late
afternoon/evening hours. There is fairly high confidence that
outflows from thunderstorms over south-central Arizona will affect kblh
for a time later this evening, with winds becoming gusty from an
easterly direction, along with some blowing dust possible.
Aviation discussion not updated for amended tafs.
Sunday through Thursday...
a series of disturbances from the south and east will maintain
demonstrable monsoon moisture flow over the region through most of
the period. As a result, significant chances of scattered wetting
rains from thunderstorms from the lower deserts to southern Gila
County can be expected. Below normal maximum temperatures are
expected through Monday, and then near normal beginning Tuesday.
Minimum humidities will range from 20 to 32 percent in the lower
deserts to 32 to 45 percent from the tonto foothills to the high
country. Seasonably breezy south to southwest winds in the late
afternoons are also likely. Overnight recoveries will be good to
Spotter information statement...
spotter activation is possible this evening and Saturday.
Arizona...blowing dust advisory from 2 PM this afternoon to 8 PM MST this
evening for azz027-028.
Blowing dust advisory from 5 PM this afternoon to 8 PM MST this
evening for azz022-023-026.
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