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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 240501
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1001 PM MST Fri Aug 23 2019

.UPDATE...Updated aviation discussion. 

&&

.SYNOPSIS...
Although temperatures will be cooler the next couple days, humidity 
levels will be significantly elevated. Thunderstorm chances will be 
relegated mostly to the south of Phoenix through Saturday, but a few 
isolated storms could sneak into the Phoenix metro, especially late 
this evening into Saturday morning. Drier air will move into the 
region through the first half of next week, basically ending storm 
chances. Temperatures will start climbing again early next week with 
Tuesday and Wednesday possibly topping 110 degrees across the lower 
deserts.

&&

.DISCUSSION...
A very complicated flow pattern and environmental setup persists 
this afternoon as deep troughing progressing through the Great Basin 
has displaced the H5 anti-cyclone into southwest New Mexico keeping 
light southerly flow through most of Arizona. However, the H3 jet 
core has already passed eastward yielding anomalously strong 
northerly flow while the H7 high pressure center was supplanted into 
northern Sonora providing a drier westerly flow just above the 
boundary layer. This strange configuration of anti-cyclone centers 
along with interrogation of WV imagery suggests increasing anti-
cyclonic subsidence and drying in the middle troposphere enveloping 
much of the northern 2/3rd of Arizona. 

Boundary layer analysis depicts a totally different picture with 12Z 
KPSR and KTWC sounding data sampling 12-13 g/kg mixing ratios in 
conjunction with widespread sfc dewpoints in the low 60s to low 70s. 
However, sounding data also showed this moist layer relatively 
shallow (sfc-800 mb at Phoenix and only sfc-900mb at Yuma) and very 
susceptible to mixing of dry air from aloft. In fact, 20Z sfc 
dewpoints had already fallen into the middle 50s to low 60s 
suggesting boundary layer mixing ratios down towards a 10-12 g/kg 
range. Really, no large forecast change into this evening as HREF 
and other high resolution members continue to suggest a convective 
evolution this afternoon not terribly dissimilar from yesterday. 

Storms will most likely become prevalent from Cochise/Pima counties 
into southern Pinal and southwest Maricopa counties though 20Z 
satellite imagery and objective analysis only shows Cochise County 
favorable for deep convection. Further north and west, the 
environment will be somewhat more hostile for convective development 
with notable inhibition on forecast BUFR soundings despite holding 
10-11 g/kg sfc-800 mb mixing ratios. Not to say a deeper outflow 
couldn't spark a few storms, but the preferred colliding outflows 
are not likely and the preponderance of model evidence along with 
the underlying inferred weak sinking motion in the atmosphere argues 
against much storm activity this afternoon and evening into major 
population centers. Dissipating outflows and localized blowing dust 
still appear as the greatest (albeit limited) threats south of the 
Phoenix metro. 

Further investigation of 12Z model output only reinforces evidence 
that some combination of another robust Gulf surge, infusion of 
residual moisture around the H8-H7 layer through outflows, a 
potential gravity wave, and favorable jet divergence will result in 
modest theta-e advection through parts of south-central Arizona. If 
these influences align correctly Saturday morning as suggested by 
several CAMs, a couple isolated communities around or south of the 
Phoenix metro could get a "sunrise surprise" of elevated storms. 
Confidence is not tremendous, but enough to increase POPs above 
automated guidance as a few of these isolated sunrise events occur 
every monsoon season (although very difficult to forecast). 

Drier air will steadily erode moisture from above as an elongated 
east Pacific H5 ridge builds eastward resulting in predominant NE 
flow above H7, while also promoting substantial warming aloft and 
synoptic scale subsidence. Due to the prevalence of recent Gulf 
surges and lingering thick low level moisture, deep westerly flow in 
the sfc-H7 layer will counter the drier air aloft and keep humidity 
levels somewhat elevated through the weekend. Each successive day 
through the middle of next week should yield a drier daily outcome. 
With such warming above the boundary layer, convective inhibition 
will be quite strong and prohibitive of storm development. As has 
been common this monsoon season and inability to hold moisture in 
place, an extended Sunday-Wednesday time frame will result in 
POPs less than 5% given this scenario. 

As a 594dm H5 anti-cyclone eventually becomes centered over Arizona, 
heat will once again become the main weather theme. The hottest day 
still looks to be Tuesday and model guidance has been trending a bit 
warmer over the past several runs. After highs 2F-5F above normal 
Monday, Tuesday and Wednesday will likely top out some 5F-8F above 
average and likely touching 110F in many lower desert locations. If 
the warmer model trends continue, next Tuesday and Wednesday may 
require an Excessive Heat headlines though current indications show 
a marginal/borderline event with respect to local criteria. Though 
still around a week away, there seems to be some indication of 
robust moisture return, more favorable dynamics, and thunderstorm 
chances returning during the latter half of next week. Both the 
European and GFS ensemble means show a substantial increase in 
monsoon moisture into southern Arizona and even southeast 
California, and a better idea of this potential should be attainable 
by early next week.

&&

.AVIATION...Updated at 0500 UTC. 

South- Central Arizona including KPHX, KIWA, KSDL, and KDVT:

There are a few showers popping up south of Phoenix but they do
not look like they will have an immediate impact on aviation.
Currently, an outflow is moving towards the Valley but is most
likely to impact KIWA. Otherwise, the westerly winds will mostly
prevail overnight with some gusts. The winds at KIWA are a little
more suspect and may be variable at times. 

Late tonight toward sunrise, there are indications that a batch 
of more modest intensity showers will develop over Pinal County 
and into southern/eastern portions of metro Phoenix. If that were 
to happen, most likely timing would be between 11Z-16Z. Confidence
remains too low at this point to insert -SHRA into the TAFs. 

Southeast California/Southwest Arizona including KIPL and KBLH:
South and southeasterly winds will prevail today and tonight as
one Gulf surge damps out and possibly another comes in late 
tonight. Not enough instability for storms though. In fact, 
anticipate clear skies except perhaps for some hazy conditions in 
the morning over the Imperial Valley - depending upon development 
of another surge. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: 
Unusually dry conditions for late August will prevail through the 
first half of next week with only slight chances of high terrain 
thunderstorms later in the week. With this dry airmass, temperatures 
will once again warm near excessive levels as afternoon highs peak 
some 5F-10F above average. Afternoon humidity levels will fall into 
the teens during the early part of the week, but increase closer to 
a 20-30% range late in the week. Occasionally gusty upslope terrain 
winds are likely through the week, though nothing too unusual for 
the season. 

&&

.SPOTTER INFORMATION STATEMENT... 
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...MO/Kuhlman
AVIATION...Deems/Rogers
FIRE WEATHER...MO