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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 140524
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1024 PM MST Fri Sep 13 2019

.UPDATE...Updated aviation discussion. 

&&

.SYNOPSIS... 
Conditions will continue to moisten through Saturday with chances 
for strong to severe thunderstorms increasing markedly across 
south-central Arizona. The threat for storms will continue Sunday, 
but will mainly be relegated to eastern Arizona. A low pressure 
system moving into the Great Basin early next week will allow for 
mostly slight chances of showers and thunderstorms on Monday and 
Tuesday before drier and somewhat cooler air overspreads the Desert 
Southwest.

&&

.DISCUSSION...
Western extension of the southern CONUS ridge remains sprawled 
across the Desert Southwest. In the lower levels, conditions are 
exceptionally dry with dewpoints in the 30s, though with the ridge 
axis lifting northward, low-level mixing ratios will increase 
steadily to around 10 g/kg through tomorrow. Latest runs of the HRRR 
indicate that isolated showers will remain relegated to southeastern 
Arizona this afternoon. Biggest weather impact will instead be the 
above normal temperatures, however highs will fall well short of 
daily records. 

Focus will shift to an increased threat of monsoon hazards Saturday 
as the models suggest ingredients will come together to produce 
strong to severe storms across portions of the area. Latest 
streamline analysis depicts an upper low near the northern Baja 
Peninsula, which will drift eastward into northern Sonora, yielding 
widespread jet-forced ascent across eastern Arizona. Convection will 
ensue across southeastern Arizona, which the models suggest 
will induce an organized inverted trough/vort max across eastern 
Arizona. This will consequently increase the easterly component to 
the steering flow off the higher terrain. Local/regional CAMS 
indicate that additional storms in the lower deserts are expected 
to initiate, given the abundant MUCAPE > 1000 J/kg. Although model 
soundings indicate inhibition (due to a mid-level warm layer) will be 
present to some degree, it is expected to erode during the afternoon 
and should be overcome by any downslope-propagating outflow boundary.

Latest HREF > 40 dbz paintball and max surface winds suggest the 
most intense cells Saturday will be capable of producing strong to 
severe winds, microbursts and frequent lightning. Although PWATs are 
not projected to be off the chart by any means, heavy rain and 
subsequent flash flooding cannot be ruled out, particularly across 
the urban areas of the Valley. Forecast PoPS were increased above the 
latest NBM guidance and are some of the highest we've seen so far 
this Monsoon season. SPC has also placed much of the forecast area 
into a Marginal Risk for severe storms, however this could be 
upgraded as the situation evolves.

A more distinct low pressure system (as depicted in both the 
operational GFS and ECMWF) will likely emerge across eastern Arizona 
Sunday, again resulting in favorable conditions for thunderstorms. 
However, the southwesterly flow will generally focus activity across 
eastern Arizona, though there will be a discernible risk of flash 
flooding across portions of the Gila County, where PWATS remain 
anomalously high. Further west in the lower deserts, rain coverage 
will likely not be quite as widespread, given the environment will 
be more subsident behind the departing low pressure system.

&&

.PREVIOUS DISCUSSION...
The track of the Pacific trough for early next week now looks to 
be even farther north with the low center tracking across northern
Nevada into southern Idaho. We will still fall under cyclonic 
flow aloft early next week and there should be enough lingering 
low level moisture for some showers and thunderstorms mainly on 
Monday and mostly over the high terrain north and east of Phoenix.
Slight chance PoPs are still in for the south-central Arizona 
lower deserts, but it seems likely most of the activity will be 
over the more favored upslope foothills areas just north and east 
of Phoenix. Drying aloft will increase into Tuesday, ending rain 
chances across the lower deserts, but there may still be some 
isolated showers and possibly a few thunderstorms over the 
eastern Arizona high terrain. Temperatures will lower back to near
normal readings beginning Sunday with highs sticking around the 
century mark across the deserts through the middle of next week.

&&

.AVIATION...Updated at 2345Z. 

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

Easterly wind shift will occur by early Saturday morning at all
area terminals, and will stay out of that direction throughout the
entire day with no westerly shift anticipated. Speeds should
generally remain under 10 kt through mid-Saturday afternoon. 

A very active late afternoon and evening period is expected for 
convection and strong winds across the Phoenix area. Latest
guidance for timing suggests a strong outflow boundary from the 
east will move westward, with the forecast time period expected to
range from 23Z to 01Z for initially reaching area terminals. Wind
gusts of at least 25-30 kt, and perhaps locally stronger, with 
areas of blowing dust and some reduced visibilities are 
anticipated. Thunderstorms are also likely to develop across the 
Phoenix area, and may impact some terminals especially after 00Z. 
Ceilings may fall as low as 8000 feet during this period. Most 
thunderstorm activity should shift west of the Greater Phoenix 
area by 03-04Z, though lingering showers remain possible, along 
with elevated wind speeds over 15 kt. Winds will weaken into early
Sunday morning with quieter conditions returning. 

Southeast California/Southwest Arizona including KIPL and KBLH: 

Light winds with variable directions will prevail through late
Saturday afternoon. FEW-SCT high clouds will be possible through 
the TAF period. An outflow boundary may approach KBLH from the 
east late in the TAF period, around 06Z, with the possibility of 
convection accompanying it. However, most guidance suggests any 
impacts near the lower Colorado River Valley should occur beyond 
06Z. 

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Monday through Friday: 
A series of low pressure systems will move through the 
Pacific Northwest and Great Basin next week. Conditions will remain 
sufficiently moist ahead of the first system for a chance of 
showers, mainly north and east of Phoenix Monday and Tuesday. 
Otherwise, these storm systems will reinforce the westerly flow, 
which will result in a gradual drying trend and near normal 
temperatures.

&&

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

&&

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

$$

DISCUSSION...Hirsch
PREVIOUS DISCUSSION...Kuhlman
AVIATION...Rogers
FIRE WEATHER...Hirsch