Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 281545
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
845 AM MST THU JUL 28 2016

.SYNOPSIS...
Strong high pressure will once again keep thunderstorm activity
somewhat isolated today as well as maintaining very hot
temperatures. However, this high pressure system will begin to
weaken starting Friday, with better moisture profiles streaming
north throughout the region. In addition to somewhat cooler
temperatures, a marked increase in thunderstorm activity will impact
the area through the weekend and into the beginning of next week.

&&

.DISCUSSION...
At first glance, today would maybe be a decent day for convection in
our forecast area based on height/wind patterns, but limited MLCAPE
and moderate-to-high levels of CIN will likely result in a fairly
down day. PWAT values from 28.12Z sounding and GPS sounders are in
the 1.5" range, much of that in the lowest levels. Using our forecast
high of 112 (which may be a little generous, will be watching trends
next few hours) yields MLCAPE around 500 J/kg with some CIN
remaining. This is in-line with the latest NAM/HRRR/SREF output.
Thinking is what instability there is will thin as the afternoon
turns to evening, which is unfortunate as the mid-level steering flow
is a solid 20 kt from the northeast. Thunderstorms will impact areas
near and north/east of Globe, but struggle to move into lower
elevations. Outflow from northeast of Phoenix may be the story this
evening, which generally yields little to no dust issues.

Will be looking more closely at the Sat/Sun timeframe; initial review
of available information leads us to concur with the previous shift
that a significant increase in thunderstorm activity is in our
future.

&&

.Previous Discussion...

A sprawling midtropospheric anti-cyclone remains situated over srn
Nevada with anomalously warm temperatures aloft (reaching the
highest 98th percentile) expanding from the central circulation. On
the periphery of this high, several discernible shortwaves were
spilling from the Rockies towards the Southwest, most notably an
open wave over SE AZ this morning. Regional 00Z soundings show stark
differences in moisture profiles, with KTWC data fairly solid 10
g/kg sfc-H7 mixing ratios while KPSR 9 g/kg moisture abruptly stops
near 800mb falling to 6 g/kg yielding significant Cinh. Not
surprisingly, this has resulted in persistent convection over the
Tucson area given the generalized ascent (and aided by a compact MCV
within the easterly flow aloft), while points to the north remain
dry.

The larger SE AZ inverted trough and embedded MCV`s will propagate
into the Gulf of California today, and a convectively contaminated
and overturned environment may not support additional convection in
the SE part of the state this afternoon. However, recovery along the
Mogollon Rim and White Mountains should allow for scattered to
numerous updrafts with good agreement among high resolution ensemble
output this activity will attempt to progress towards lower
elevation to the west. Unfortunately, forecast soundings continue to
depict a worrisome moisture discontinuity above the H7 layer such
that Cinh at lower elevation above 100 J/kg may preclude activity
from surviving off higher terrain. A more organized outflow boundary
supported by deep layer easterly flow does look very probable, but
without additional colliding boundaries from the south, this may be
insufficient to force deep convection (and only yield some lofted
dust in the Phoenix vicinity).

Once again today (and potentially continuing into Friday), excessive
heat will be the largest impact. Observational highs actually
slightly outperformed guidance yesterday, and see almost no reason
why a near persistence forecast would not be valid today yielding
readings at or above 115F throughout population centers of SE CA.
Depending on the influence and potential boundary layer moistening
from outflow tonight, warnings may need to be extended into Friday
for a portion of the area with best performing guidance still
suggesting hot conditions prevailing before more substantial
moisture surges north over the weekend.

Friday still appears to be a convective transition day as heights
and temperatures aloft relax allowing for more widespread activity
and less inhibition area wide. Forecast soundings around the Phoenix
metro still show boundary layer mixing ratios only 8-10 g/kg,
however the depth of these moisture profile increase above 700 mb
while the likelihood of multiple deep colliding outflow boundaries
increases substantially. Have yet to get too aggressive with POPs
Friday evening as ensemble output depicts rather modest chances
given the initial marginal moisture profiles, though certainly
chances are better at lower elevation than seen the past couple
weeks.

The subtropical high shifts more to the east and weakens further
over the weekend providing the best chance of repeated widespread
convection throughout the forecast area thus far this summer. Not
only will convective outflows help moisture the boundary layer, but
deep layer synoptic SE flow will aid in moisture advection
throughout the atmospheric column. Forecast soundings for Saturday
afternoon/evening become very supportive for strong convection with
sfc-H7 mixing ratios solidly near 12 g/kg resulting in MLCapes 1500-
2000 J/kg. A notable capping layer still persists in these
forecasts, though any good trigger (shortwave or deep outflow) would
be sufficient to release this energy. If not during peak heating,
both deterministic models and ensemble output are picking up on
strong theta-e advection Saturday night/Sunday morning possibly tied
to decaying tropical activity and/or a stronger inverted trough.
Regardless, the surge of moisture with this feature may be quite
impressive; and it seems the area experiences one of these
overnight/early morning moisture flux events every summer. Have
started boosting POPs Sunday morning given the reflection in models,
while also cutting Sunday afternoon temperatures in anticipation of
extensive cloud cover in the wake of this system.

A high moisture content environment will likely persist through much
of the region for the bulk of next week keeping the potential for
rainfall valid most days. However, overall lapse rates will likely
suffer by this point and it will take better forcing mechanisms to
allow for thunderstorm survival into lower elevations. At this
point, it is virtually impossible to identify any particular wave
that would support persistent convection more than 3-4 days in
advance. Thus, generalize POPs not tremendously far from
climatology are advertised understanding there may be one very
active day during the middle of next week while adjacent days would
be less convectively active.

&&

.AVIATION...
South-Central Arizona Including KPHX, KIWA, and KSDL:
Isolated thunderstorms will continue over southeast Arizona through
about 14Z before dissipating. Otherwise, storms are expected to
remain outside of Phoenix metro through at least 22Z. Very low
probability of outflows from SE AZ storms reaching TAF sites.

Later today, expect storm development over the usual higher terrain
locations of central AZ trying to move west-southwestward. Storms
will likely struggle to survive over the valley floors. However,
there will be a good chance of outflow winds from those storms
spreading over large portions of Phoenix metro after 00Z. Too early
to be able to insert specific timing, directions, and magnitude of
the wind shifts in the TAFs. At this time, it seems SE AZ will
have less activity this afternoon/eve than they did on Wed.
Otherwise, surface winds will follow familiar diurnal patterns.

Southeast California/Southwest Arizona including KIPL and KBLH:
Expect mostly clear skies to continue the rest of the night and much
of the day. Late this afternoon/early evening, thunderstorms
from southern Yavapai and Mohave Counties will drift over northern
La Paz County after 00Z but dissipate in the process. However,
outflow winds from those storms are quite possible. Outside
possibility those winds could reach KBLH. Confidence too low to
reflect in the TAF. Yuma County will be less likely to see
outflow. Elsewhere, no storm activity expected. Otherwise, surface
winds will follow typical diurnal patterns.  There is a
possibility of a weak surge/seep causing enhanced southerly winds
during the daytime but confidence too low to reflect in KIPL TAF.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Saturday through Wednesday...
Major upswing in atmospheric moisture content is expected to
arrive Saturday before beginning to decline from west to east on
Tuesday. The result will be higher humidities, substantially better
chances for thunderstorms, and more cloudiness. Temperatures will be
cooler over the weekend, before warming back to normal, or slightly
above, Monday through Wednesday.

&&

.SPOTTER INFORMATION STATEMENT...
Spotter activation is not anticipated.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 11 PM MST this evening for AZZ020.

CA...Excessive Heat Warning until 11 PM PDT this evening for
     CAZ031>033.

&&

$$

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DISCUSSION...Iniguez/MO
AVIATION...AJ
FIRE WEATHER...AJ


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