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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 231808
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1108 AM MST Fri Aug 23 2019

.UPDATE...
Updated Aviation section.

&&

.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 will be possible across the Phoenix metro, 
especially later this evening into Saturday morning. Drier air will 
move in later in the weekend, lasting through the first half of next 
week, and basically ending storm chances. Temperatures will climb 
Sunday through the middle of next week with Tuesday and Wednesday 
possibly topping 110 degrees across the lower deserts.

&&

.DISCUSSION...
Quite a convoluted flow pattern and environmental setup this morning 
across the SW Conus as deep troughing over the Great Basin has 
displaced the H5 anti-cyclone into southern New Mexico while 
strengthening northerly jet level winds were objectively analyzed. 
Interrogation of WV imagery suggests increasing anti-cyclonic 
subsidence and drying in the middle troposphere already descending 
into northern Arizona. Boundary layer analysis depicts a 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 across the forecast area. However, sounding data 
also shows 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. 

HREF and other high resolution members suggest a convective 
evolution this afternoon not terribly dissimilar from yesterday 
with storms most prevalent from Cochise/Pima counties into 
southern Pinal and southwest Maricopa counties. Further north, the
environment will be somewhat more hostile for renewed convection 
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 threats. 

Aside from trending cloud cover and dewpoint forecasts towards 
observational data in the short term, have made some targeted minor 
increases in POPs early Saturday morning. There's growing evidence 
among both high resolution CAMs and global models that the 
combination of another robust Gulf surge along with infusion of 
moisture around the H7 layer will result in modest theta-e 
advection through parts of south-central Arizona. There are also 
indications that a deeper ascent structure, some sort of gravity 
wave, and more favorable jet divergence will spread into the area 
potentially yielding a "sunrise surprise" for some isolated 
communities around or south of the Phoenix metro. Confidence is 
not tremendous, but enough modeling output depicts this outcome 
and a few of these isolated sunrise events occur every monsoon 
season. 

&&

.PREVIOUS DISCUSSION...
/410 AM MST Fri Aug 23 2019/
Storm chances late this afternoon and evening do look fairly similar 
to yesterday's activity across southern Maricopa and Pinal Counties 
and this is mostly supported by high-res CAMs. Moisture levels will 
be better today, but the weak ascent from the passing trough will be 
missing. We will also be substantially cooler today (roughly 5 
degrees), yielding slightly less favorable lapse rates and likely 
negating much of the positives from the increase in moisture levels. 
Like yesterday, most storms will need to first form over the high 
terrain with subsequent activity into the lower deserts forming 
along any significant outflows or outflow interactions. Seems likely 
the Phoenix area will see at least some showers and isolated 
thunderstorms, but they will most likely be south and east of 
downtown Phoenix and occur after sunset.

Saturday is now looking like a mostly dry day, even though we will
start out with quite a bit of low level moisture in place. Models
show the upper level ridge reforming to our northwest on Saturday
providing a northerly dry flow aloft. Increasing heights aloft and
subsidence should be enough to hinder storm development for all
areas except for extreme southern Arizona (Pima County). Despite 
the increasing heights and warming aloft, the increased boundary 
layer moisture and weak mixing conditions will keep highs Saturday
slightly below normal. 

The drying conditions will continue on Sunday and into early next
week as the upper level high builds southeastward over Arizona. 
This will dry out the boundary layer, prevent any storm activity 
and bring a decent warming trend through early next week. The
hottest day still looks to be next Tuesday and model guidance has
been trending a bit warmer over the past several runs. After highs
of 106-109 on Monday, Tuesday will likely top 110 degrees in many
lower desert locations. If the warmer model trends continue, next
Tuesday and Wednesday may require an Excessive Heat Watch soon 
for some areas. Though it is still around a week away, there seems
to be some hope monsoonal moisture will be on the increase again.
Both the European and GFS ensemble means show a substantial
increase in monsoon moisture into southern Arizona and even
southeast California beginning sometime late next week. We should 
have a better idea of this potential by early next week.

&&

.AVIATION...Updated at 1807 UTC.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: 
Anticipate isolated convective showers along/near the I-8 corridor
between Gila Bend and Casa Grande this afternoon...though
confidence in that scenario is starting to trend down.
Nevertheless, would anticipate that outflows, if any, would damp
out before reaching the TAF sites. Thus, no distinct wind shifts
depicted in the TAFs due to outflows. Can't rule out the
possibility of an moderate outflow moving in after 07Z originating
from storms near/south of Tucson. Otherwise, storm coverage this
afternoon and evening will be better over southeast AZ and White
Mountains - but little to no coverage over Yavapai County and
western Mogollon Rim. 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 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...
Saturday through Thursday: 
Minimum humidities are expected to remain above 20% through the 
weekend and overnight recovery will be at least Good. Storm chances 
will be quite modest and limited to south-central AZ. Temperatures 
will be at, or slightly below, normal. Humidities trend down 
significantly Monday and Tuesday and temperatures trend up - though 
not as high as we've seen in recent days. Humidities are anticipated 
to trend upward Wednesday and Thursday. Storm chances return as 
well. Apart from thunderstorms, winds will follow familiar warm 
season diurnal patterns. 

&&

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

&&

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

$$

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