Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 141843
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1143 AM MST Sat Sep 14 2019

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS... 
An upper level disturbance moving into eastern Arizona today will
increase moisture levels significantly and bring good chances for
thunderstorms, some possibly strong to severe, across south-
central Arizona. The threat for showers and 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 continued shower and thunderstorm chances across much of
Arizona on Monday with lingering chances into early Tuesday. 
Temperatures into next week are expected to be near seasonal 
normals.

&&

.DISCUSSION...
Today still looks to be shaping up to be an active thunderstorm 
day across Arizona as deeper monsoon moisture continues to advect
northward into southeast Arizona. The main impetus of this
moisture return is the modest low level southeasterly flow along 
the northern fringe of an inverted trough (IT) currently situated
across northern Sonora Mexico. This trough helped to support
storms across northern Mexico last evening into early this 
morning which also produced northward propagating outflows 
enhancing the southeasterly moist flow into Arizona. 

Current analysis shows PWATs have increased to near 1.25" south 
of Phoenix and surface dew points have generally increased into 
the middle 50s across southeast Arizona, including Pinal and Gila 
Counties. Moisture levels will continue to increase across eastern
Arizona today, possibly as far west as western Maricopa County. 
Some models have backed off slightly on moisture and instability 
levels, but the general consensus shows plenty of instability for 
storms to initialize across the eastern Arizona high terrain early
this afternoon. There will also be some weak upper level dynamics
enhancing storm development today. Southeast and south-central 
Arizona will fall under a favorable upper level ascent area later 
today with the inverted trough/closed low positioned over 
southeast Arizona and a weak shortwave seen at 250/300mb centered 
just south of Yuma.

Hi-res CAMs nearly all agree storms will first develop over the
Mogollon Rim and the White Mountains early this afternoon followed
by additional storms to the south across Pima County. Forecast
soundings show a conducive profile for the production of strong 
to severe downburst winds under storms as well as larger storm 
outflows developing out ahead of the convection. The IT will 
enhance mid level northeasterly winds across east-central Arizona 
and this should eventually allow for a westward propagating broken
line of thunderstorms into the lower deserts late this afternoon.
Additional storm outflows moving north-northwestward out of Pima 
County will also be possible, potentially intersecting with the 
anticipated outflows coming out of Gila County. There is still 
some uncertainty in the overall thunderstorm coverage over the 
south- central Arizona deserts, including Phoenix, and this will 
likely hinge upon the available moisture. For the most part, 
models show decent storm coverage into the lower deserts, but some
still show mostly scattered storms. Essentially all the CAMs 
indicate a strong outflow will push through Maricopa County 
between 23-03Z producing gusty winds and likely patching blowing 
dust. There is also the possibility of some parts of the outflow 
nearing severe limits in addition to any strong to possibly severe
thunderstorms. SPC currently has a marginal risk of severe storms
from Gila County westward into eastern La Paz and eastern Yuma 
Counties. We should have a better idea of the severe potential 
after seeing the 12Z soundings and model runs. Heavy rainfall and 
localized flooding will also be a threat, but storm movement 
should be fast enough to keep the threat on the lower side. As the
outflow and/or storms progress westward into southwest Arizona 
later this evening, they should eventually weaken as they run into
a drier and more stable environment.

For Sunday, the IT is forecast to remain nearly stationary across
east-central Arizona into the afternoon hours before finally 
lifting northeast into New Mexico by Sunday evening. Showers and 
scattered thunderstorms will continue to be a threat mainly 
across the high terrain east of Phoenix through Sunday evening. 
The main threat for Sunday looks to be heavy rainfall and 
localized flooding with model QPF showing around one inch across 
Gila County. The lower deserts, including Phoenix, should mostly 
be rain free as dry and warmer subsident flow aloft is seen 
southwest of the IT. Cooler temperatures will also be realized for
most of the area beginning Sunday with highs in the upper 90s in 
Phoenix to as warm as 105 across the southeast California deserts.

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.

&&

.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... 
Spotter activation may be needed later today across southern
Arizona.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Blowing Dust Advisory until 8 PM MST this evening for AZZ531-
     533>536-538>540-550-551-553-554-559.

CA...None.
&&

$$

DISCUSSION...Kuhlman
AVIATION...Smith
FIRE WEATHER...Hirsch