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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 241814
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1114 AM MST Sat Aug 24 2019

.UPDATE...
Updated Aviation section.

&&

.SYNOPSIS...
Although temperatures will be cooler the next couple days, 
humidity levels will be significantly elevated. Slight chances 
for showers and thunderstorms today will only exist well south of
Phoenix. 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...
Morning WV imagery shows a series of fast moving northern stream 
waves punching into the Great Basin while a flat midtropospheric 
ridge splays across the foreast area. Further south, an easterly 
wave (likely convectively enhanced) was situated over western Sonora 
placing much of south-central Arizona within a region of deformation 
and weak ascent. After another robust Gulf surge last night and mid 
60 to mid 70s surface dewpoints this morning, 12Z sounding data 
sampled very healthy sfc-H8 mixing ratios of 14-16 g/kg. However, 
sounding data also depicts quite a H8-H7 inhibition layer; and 
lacking a more substantial gravity waves or other impetus for 
stronger lift, only shallow cloud decks around the H8 level have 
been observed this morning. Had there been some better ascent 
mechanisms and/or erosion of this capping layer, instability on the 
order of 1500-2000 J/kg could have easily been released (something 
CAMs seemed to pick up on yesterday). Given this scenario, have 
increased cloud cover through the morning hours while trending many 
other parameters towards recent observational data. 

&&

.PREVIOUS DISCUSSION...
/529 AM MST Sat Aug 24 2019/
The rest of the today will still pose a small threat for isolated
afternoon showers and thunderstorms south of Phoenix, but the 
drying aloft will also be mixing out some of the lower level
moisture, gradually decreasing instability by this evening. Sunday
looks even more quiet with virtually no chance of storms other
than south of Tucson. Temperatures this weekend will be somewhat
more comfortable and right around seasonal normals, but increased
surface dew points will make higher humidity levels more
noticeable. 

The high pressure ridge centered to our northwest on Sunday will
gradually shift southeastward over our region early this week 
with heights staying near steady-state, but low level 
conditions will be warming as the atmosphere dries out more each 
day. The warm air aloft and fairly dry conditions will keep storm 
chances near zero on Monday and Tuesday. Models continue to show 
high temperatures quickly trending upward early next week with 
highs 3-5 degrees above normal on Monday and 6-8 degrees above 
normal by Tuesday. The possibility for a marginal excessive heat 
episode is becoming a bit more likely for Tuesday and Wednesday 
for the warmest deserts with median guidance showing highs topping
out 109-112 on Tuesday, but will hold off on any headlines for 
now.

Eventually, models mostly show the high center shifting just to 
our northeast, likely setting up a more favorable southeasterly 
moist flow. This should bring back storm chances as early as 
Wednesday across the eastern Arizona high terrain and possibly 
over the Arizona lower deserts at some point late next week. 
Heights aloft later next week still look to be toward the upper 
end of climo normals, but the likely increase in low level 
moisture should lower temperatures to just a bit above normal.

&&

.AVIATION...Updated at 1815 UTC. 

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

Humid air in the lower levels has been streaming in from the Gulf
of California. It has been moist enough to generate cumulus at 
unusually low altitudes for this time of year with bases at
FL050-070. The cumulus has been scattered for the most part and 
is expected to remain that way. But, in localized areas (especially
near mountains) expect broken conditions into the early afternoon. 
Anticipate coverage to decrease after 20Z. Otherwise, anticipate a
slow increase in the height of the cloud bases. Winds at the
surface and lower levels will favor westerly directions the rest
of the day and overnight. Expect speeds to be strongest late this
afternoon and evening with gusts in the 15-25 kt range. There is 
an outside possibility of another round of cumulus/stratocumulus 
Sunday morning but confidence too low to depict ceilings in the 
TAFs. As for storms, expect activity to remain outside of metro 
Phoenix with the main area over far southeast AZ.


Southeast California/Southwest Arizona including KIPL and KBLH:

Humid air in the lower levels has been streaming in from the Gulf
of California. It has been moist enough to generate cumulus at 
unusually low altitudes for this time of year with bases at
FL035-055. The cumulus has been broken over many areas near and 
west of the Lower Colorado River Valley. Anticipate coverage to 
decrease after 20Z. Otherwise, anticipate a slow increase in the 
height of the cloud bases. Winds at the surface and lower levels 
will favor southerly directions the rest of the day and overnight.
Expect speeds to be strongest late this afternoon and evening 
with gusts in the 15-25 kt range.


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...AJ
FIRE WEATHER...MO