Area forecast discussion 
National Weather Service Phoenix Arizona 
845 am MST Thursday 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. Precipitable water 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 Southeast Arizona this morning. Regional 00z soundings show Stark 
differences in moisture profiles, with ktwc data fairly solid 10 
g/kg sfc-700 mb 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 mesoscale convective vortex 
within the easterly flow aloft), while points to the north remain 
dry. 


The larger Southeast Arizona 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 southeast 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 700 mb 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 southeast California. 
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 southeast flow will aid in moisture advection 
throughout the atmospheric column. Forecast soundings for Saturday 
afternoon/evening become very supportive for strong convection with 
sfc-700 mb 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 Southeast Arizona storms reaching taf sites. 


Later today, expect storm development over the usual higher terrain 
locations of central Arizona 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 Southeast Arizona 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... 
Arizona...excessive heat warning until 11 PM MST this evening for azz020. 


California...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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