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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 142032

Area Forecast Discussion
National Weather Service Phoenix AZ
130 PM MST Sat Sep 14 2019

An upper level disturbance in eastern Arizona will support good 
chances for thunderstorms and strong winds through much of south-
central and southwest Arizona into the evening. 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 periodic shower chances in the eastern half 
of Arizona. Temperatures into next week are expected to be near 
seasonal normals.


A well defined easterly wave was evident in WV imagery early this 
afternoon, as well as accurately objective analyzed in current 
conditions and model initializations. This feature was helping 
transport better deep moisture profiles into the forecast area, but 
more importantly offering a better ascent background to the regional 
environment. The dynamical energy will likely become a necessary 
ingredient this afternoon as overall thermodynamic profiles are 
rather ordinary with somewhat limited 10 g/kg boundary layer mixing 
ratios and marginal lapse rates yielding MLCape under 1000 J/kg. 
Other than the excellent incoming dynamics, other positive features 
include minimal inhibition with deeply mixed moisture profiles 
through the entire boundary layer and robust DCape above 1000 J/kg 
sufficient for organized outflow boundaries. And given the weak 
inhibition structure, feel outflows won't need to be particularly 
deep or require multiple intersecting boundaries to initiate new 
convective elements. 

There's been little change in modeling with all hi-res CAMs 
developing storms over the Mogollon Rim and White Mountains early 
this afternoon followed by the development of strong easterly 
outflow winds. Forecast soundings still show a conducive profile for 
the production of strong downburst winds, as well as larger 
organized storm outflows developing out ahead of the convection. 
Deep easterly flow should allow for a quick propagating outflow 
boundary with renewed convective development enhancing the forward 
progression. The biggest uncertainty is storm coverage once the 
outflow boundary washes through lower elevations, and in particular 
the Phoenix metro. The preponderance of evidence shows convection 
deteriorating quickly off higher terrain and outflows quickly 
outracing the pool of instability. Nevertheless, sounding profiles 
would suggest a few additional isolated storms popping immediately 
behind the outflow boundary around the entire Phoenix metro before 
exhausting instability in western Arizona. The main wildcard is the 
potential for multiple, intersecting boundaries (i.e. any outflow 
moving north from Pima County behind the outflow racing southwest 
from the Rim) capable of spurring widespread storms in a confluence 
of boundary interactions. 

Regardless, the signal of well organized and long lived outflow over 
the typical dust source regions necessitated a blowing dust advisory 
during the late afternoon and evening. Pinal County would most 
likely experience the worst conditions, though the persistence of 
easterly outflow in CAM output suggests dust spreading well into 
western Arizona regardless of storm persistence. Should more actual 
storm development occur at lower elevations along the outflow 
boundary, DCape values and shear profiles would support a few swaths 
of damaging winds. Heavy rainfall and localized flooding may also be 
a threat with increasing moisture profiles, however storm and 
outflow movement should be fast enough to keep this threat confined 
to the more typical/habitual flood locations and recent burn scars. 

On Sunday, the easterly wave will slow and become quasi-stationary 
across east-central Arizona during the morning before finally 
lifting northeast into New Mexico Sunday evening. Showers and 
scattered thunderstorms will continue through southern Gila County 
into Sunday afternoon with heavy rainfall and localized flooding the 
primary threat. Though models have retreated somewhat with respect 
to overall QPF, there is still signals indicating some areas of Gila 
County (and points east) could experience over an inch of rainfall 
and a potential flood threat. Lower desert communities (including 
much of Phoenix) will likely remain rain-free Sunday with the 
atmosphere overturned from outflows this evening. A few storms 
originating over the Rim could make a push south on the northwest 
periphery of the aforementioned easterly wave, however all current 
modeling shows any of this activity dissipating before reaching 
lower elevations.  

The unsettled weather will continue into Monday as a strong Pacific 
trough digs southeastward into the Great Basin. There should be 
sufficient lingering low level moisture from Phoenix and to the 
north and east for continued shower and thunderstorm chances. The 
bulk of the upper level dynamics will remain well to our north on 
Monday, but much of Arizona should fall under weak upper level 
ascent. Best rain chances will fall over favored southwesterly 
upslope areas just north and northeast of Phoenix Monday 
afternoon/evening. Drier flow will begin to overtake the region from 
the southwest Monday night into Tuesday with mostly lingering rain 
chances over higher terrain areas. The latter half of next week 
should remain dry with temperatures at or just above normal readings.


.AVIATION...Updated at 1842Z. 

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

Current obs show east to southeast winds across much of the metro at 
this hour. Winds will continue to veer more toward the south over 
the next several hours but confidence is low in winds taking on a 
westerly component today. Some HiRes guidance tries to briefly 
switch winds to the west before a strong E/NE outflow pushes through 
later this afternoon but not confidence is not high enough to 
indicate this brief switch in the TAFs.

An active afternoon/evening is expected across south-central Arizona 
as thunderstorms will likely push through the Phoenix metro bringing 
with it strong, gusty winds and potentially some blowing dust. A 
strong outflow is expected to approach from the E/NE reaching the 
terminals between 23Z-00Z. A period of gusts upwards of 34 kts will 
be possible but there's potential for gusts to exceed that. Strong 
gusts may result in reduced visibilities from blowing dust 
especially at KPHX and KIWA. Thunderstorms may directly impact KIWA, 
KSDL, and KDVT with this activity later today so have added a TEMPO 
TS group to these TAFs. Confidence was not high enough to add this 
to the KPHX TAF as storm coverage is more uncertain so decided to 
just keep VCTS at this time. CIGs look to get down as low as 7-8 kft 
as thunderstorms move through. Thunderstorms are expected to exit to 
the west of the Phoenix area by 03Z though a few lingering showers 
and elevated wind speeds upwards of 15 kts will remain possible into 
tonight. Winds gradually taper off later tonight with light easterly 
winds by early Sunday morning.

Southeast California/Southwest Arizona including KIPL and KBLH: 

Light and at times variable winds will persist across southeast 
California through much of the day with speeds remaining below 10 
kts. HiRes guidance shows an outflow originating from south-central 
Arizona moving toward the west and reaching the terminals later this 
evening. Winds should transition to an E/SE component with some 
gusts possible but speeds should generally stay below 10 kts. Winds 
become light and variable overnight with a few mid and high level 
clouds possible. 

Aviation Discussion not updated for amended TAFs.


Tuesday through Saturday: 
A series of low pressure systems will move through Great Basin next 
week gradually pushing moisture and storm chances from the 
districts. Residual lingering moisture may support some upslope 
showers during the middle of the week, however much drier air will 
encompass the region by the end of the week. These storm systems 
will allow temperatures to hover around the seasonal average while 
the drier airmass will pull afternoon humidity levels into a 10-25% 
range. A few periods of breezy conditions will be possible with the 
passage of these weather disturbances. 


Spotters should be prepared to submit reports of severe weather in 
south-central and southwest Arizona this afternoon and evening. 


AZ...Blowing Dust Advisory until 8 PM MST this evening for AZZ531-