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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 242110
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
210 PM MST Sat Aug 24 2019

.SYNOPSIS...
With temperatures hovering near average for the remainder of the 
weekend, humidity levels will remain significantly elevated. 
However, the atmosphere will not be favorable for much, if any 
thunderstorm activity. Drier air will move into the region through 
the first half of next week allowing temperatures to rebound back 
above normal. High temperatures Tuesday and Wednesday could easily 
exceed 110 degrees in many lower desert communities. Moisture will 
begin to return to the area by the end of next week yielding modest 
storm chances. 

&&

.DISCUSSION...
Afternoon WV imagery shows multiple shortwaves in strong NW flow 
punching into the Great Basin while a flat midtropospheric ridge 
becomes more elongated through the forecast area. Further south, an 
easterly wave (likely convectively enhanced) appears to be quasi-
stationary 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. Vertical boundary layer mixing seems to be less 
efficient early this afternoon as these higher dewpoints have budged 
very little, and fair weather cumulus remain abundant throughout 
much of the area. 

Despite the anomalous surface moisture combined with some jet level 
divergence and weak ascent, there remains very little evidence among 
all types of model guidance that much deep convection will initiate 
this afternoon. This is primarily due to a strengthening inhibition 
layer above H8 keeping unusually healthy instability (widespread 
MLCape above 2000 J/kg) in place. The only exceptions may be very 
isolated cells in the typical locations of SW Maricopa County, the 
Kofa range in far western Arizona, and descending south from the 
Bradshaw range. While the heightened instability parameters are 
worrisome, even the typically more aggressive model output only 
yields brief storm activity late this afternoon.

The center of the high pressure ridge currently centered just off 
the northern California coast will intensify and build southeast 
into the SW Conus over the next 48 hours. In addition to warming 
temperatures aloft and strengthening convective inhibition, the 
resulting flow pattern will gradually aid in eroding boundary layer 
moisture precluding the chance for any storms into the middle of 
next week. There is good model consensus that H5 heights around 
594dm will settle directly over the CWA allowing temperatures to 
flirt with excessive levels once again. Model spreads is rather 
narrow depicting high temperatures quickly trending upward next week 
with highs 3F-5F above normal on Monday and closer to 6F-8F above 
normal Tuesday and Wednesday. The possibility for a marginal 
excessive heat episode is becoming a bit more likely Tuesday and 
Wednesday for the warmest desert communities with median guidance 
showing highs topping out 108F-112F on Tuesday.

Uncertainties focused in subtle shifts in the H5 anti-cyclone 
position late in the week cast some doubt on the veracity of more 
pronounced moisture return and storm chances during the latter half 
of the week. Operational 12Z GFS and ECMWF depict the high center 
shifting into southern NV/UT providing a favorable flow pattern for 
Mogollon Rim storms and outflows from SE Arizona to collide over 
central Arizona. However, some NAEFS members keep the anti-cyclone 
centered over Arizona or shift it eastward reinforcing the influence 
of dry air aloft. Thunderstorm chances may start increasing as early 
as Wednesday across the eastern Arizona high terrain, and 
subsequently move into lower elevations depending on the flow 
pattern and moisture availability late in the week. Heights aloft 
later next week still look to be toward the upper end of 
climatological normals, but the potential increase in low level 
moisture should sequester temperatures closer to a slightly above 
normal range. 

&&

.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...
Tuesday through Saturday: 
Dry conditions will persist into the middle of the week with 
increasing thunderstorms chances later in the week focused mainly 
over higher terrain locations. Before the dry airmass gives way to 
more humid weather, temperatures will once again warm near excessive 
levels as afternoon highs peak some 5F-10F above average. Midweek 
afternoon humidity levels will fall into the teens, but increase 
closer to a 20-30% range late in the week. Occasionally gusty 
upslope terrain winds are likely through the week, though nothing 
particularly 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