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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 231110

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

.UPDATE...Updated Aviation Discussion. 


Additional increases in humidity can be expected through Saturday
along with lower high temperatures. Shower and thunderstorm
chances will mainly be south and southeast of Phoenix through
early Saturday, but a few isolated showers and storms will be 
possible across the Phoenix metro. Drier air will move in 
beginning late Saturday, lasting through the first half of next 
week, basically ending storm chances. Temperatures will climb 
Sunday through early next week with next Tuesday and Wednesday 
possibly topping 110 degrees across the lower deserts.


As anticipated, much of southern Arizona saw moist southerly low
level flow yesterday with surface dew points climbing into the 
50s across the south-central Arizona deserts. The increase in 
moisture led to just enough instability to combine with a weak 
trough moving through Arizona for a few showers and storms to 
develop mainly south of Phoenix along an outflow from the 

Another Gulf of California moisture surge is currently underway 
and this will add to the already semi-moist boundary layer. This 
should bring low level mixing ratios closer to 9-10 g/kg, not 
quite optimal for storms but enough if provided a trigger. 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 

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 1110 UTC.

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

A somewhat atypical wind pattern is expected later this morning 
as moderate southerly crosswinds develop across the area during 
mid to late morning and into the early afternoon hours as periods
of Gulf surge/seep continue to occur. A very slight chance of 
isolated thunderstorms and showers exists for regions just south 
of the Phoenix metro this afternoon and evening so storm outflows 
can not be completely ruled out at the terminals. However the 
chances and confidence remain too low to include mention in the 
TAFS at this time. Skies will remain mostly clear with periods of
FEW low, mid and high level clouds. 

Southeast California/Southwest Arizona including KIPL and KBLH:

No significant aviation concerns anticipated through the TAF 
period under mostly clear skies. Expect mainly southerly winds at
KBLH and southeasterly winds at KIPL to become breezy this 
morning and this afternoon respectively with a few gusts to 22 
kts. Winds may also become light and variable at times overnight 
and into the morning at KBLH. Skies will remain clear to mostly 
clear with periodic FEW low and high cloud decks. 

Aviation Discussion not updated for amended TAFs.


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. 


Spotters should follow standard reporting procedures.