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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 221841
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1141 AM MST Sun Sep 22 2019

.UPDATE...Updated Aviation Discussion.

&&

.SYNOPSIS... 
Dry conditions are expected today before tropical moisture 
increases late tonight ahead of a strong and deep upper level 
disturbance that will move into the region for Monday. Scattered 
showers and thunderstorms will develop late tonight across south 
central Arizona before spreading across most of southern and
central Arizona and eastern portions of Imperial and Riverside 
Counties in California for Monday. Isolated severe thunderstorms 
capable of producing damaging winds and hail are expected along 
with locally heavy rainfall capable of producing flash flooding. 
Although rain chances will begin to decrease on Tuesday, elevated 
rain chances will linger through Thursday as the upper level 
disturbance remains near Arizona. This disturbance should finally 
move east of the region by Thursday night, with drier conditions 
expected by Friday into the weekend. 

&&

.DISCUSSION...
Clear skies and dry conditions are in place across the lower 
deserts of Arizona and southeast California as the region is
temporarily under a weak ridge between a shortwave to our east 
and a rapidly digging shortwave moving into the Pacific Northwest.
Tropical Depression Lorena is currently moving into southern 
Sonora with only a small burst of convection near its center as it
continues to rapidly weaken. Lorena is forecast to continue 
moving northward over western Sonora as it dissipates today over 
higher terrain. Although Lorena will weaken, significant moisture 
is expected to move into southern Arizona this afternoon before 
spreading into south central and southwest Arizona later tonight. 
This will be coincident with a Gulf surge induced by both 
Lorena's remnants and the aforementioned shortwave that will 
rapidly dig south into the Great Basin overnight and into 
southeast California and Arizona for Monday. Therefore, conditions
will rapidly change from today's mostly sunny conditions with 
near normal temperatures in the mid to upper 90s across the lower 
deserts and dew points generally in the 30s to much more humid 
conditions tonight into tomorrow as dew points increase into the 
upper 50s and 60s. 

Although a few hi-res models do show some potential for isolated
showers and thunderstorms to slip into northern Pinal County and
southern Gila County early this evening, most prevent convection
from developing until after Midnight Monday morning. The primary
reason for this delay is that weak midlevel stability aloft will
take at least a few hours to erode as moisture rapidly moves into
the region this evening. However, S-SW to N-NE oriented 
isentropic ascent will increase overnight into Monday morning as 
a leading shortwave ahead of the digging upper level trough moves 
through the region, allowing at least isolated to scattered 
showers and a few thunderstorms to develop. Locally heavy rainfall
will be possible with this convection late in the overnight hours
into tomorrow morning, with hi-res models suggesting the best 
chances will be over northeast Pinal and southern Gila Counties 
where isentropic ascent will be best aligned with terrain. A Flash
Flood Watch was considered, but we decided to hold off for now in
coordination with WPC and surrounding offices given the strong 
model agreement in delaying the onset of convection until late in 
the overnight hours and significant uncertainties in storm 
coverage and rainfall amounts. WPC has this area in a marginal 
risk of excessive rainfall through Monday at 5 am MST before 
expanding the marginal risk to much of Arizona with a slight risk 
over portions of central and southern Arizona for Monday. 

Confidence continues to increase that scattered showers and
thunderstorms will develop over most of south central and
southwest Arizona Monday afternoon and/or evening as the upper 
level trough digs further south into the Colorado River Valley. 
Very strong dynamic forcing is expected due to significant 
diffluence aloft combined with a strong jet streak aloft and SW- 
to-NE- oriented isentropic ascent at lower and mid-levels. 
Abundant moisture will be in place as PWAT values increase into 
the 1.4-1.6 inch range close to record values for late September. 
The biggest uncertainty centers around what storm modes will 
initially develop and how much (if any) cloud cover we will have 
during the day on Monday as that could significantly curtail or 
enhance instability. Shear is expected to be in the 40-50 knot 
range with gradually veering winds, with MUCAPE and surface- 
based CAPE values likely above 1000 J/kg assuming skies are clear
enough. Thus, supercellular structures will most likely develop 
along and ahead of a moisture gradient acting as a pseudo-dryline
somewhere near the Colorado River Valley. Damaging winds and hail
up to golf ball size are expected with these storms, which has 
prompted SPC to continue a slight risk for severe storms on 
Monday, something they have only issued for two events on their 
Day 2 outlook this decade (9/27-28/2014 and 10/5-7/2010). SPC 
expanded this slight risk west into Yuma and La Paz Counties with 
a marginal risk over far eastern Riverside and Imperial Counties 
given the expected position of the moisture gradient. 

Greater uncertainty occurs late in the afternoon and evening hours
as storms move into south central Arizona with a relatively strong
surface cold front, assuming storms do not develop on their own 
earlier in the warm sector. Hi-res model runs currently depict a 
wide variety of solutions ranging from initially supercellular 
storms merging into a line of storms to discrete modes being
supported deep into the evening and overnight hours ahead of a 
line of storms. There is still a chance for very little coverage 
of strong to severe storms if instability is reduced due to cloud 
cover and/or the timing of the wave comes through a bit later than
expected after the loss of daytime heating, but this appears to 
be the least likely scenario at this time. How storms evolve 
during the evening and overnight hours and how much hail occurs 
with these storms will go a long way towards determining how much 
rainfall and ultimately what our flash flood threat will be. The 
GEFS and ECM ensembles are currently in stark disagreement with 
the GEFS suggesting midlevel dry air supporting steeper lapse 
rates and more severe storms and the ECM supporting more moist 
conditions at midlevels with less severe storms and heavier 
rainfall. This decrease in the GEFS and more cellular modes 
depicted by hi-res models has decreased our rainfall forecast for 
most areas more in the 0.5-1.0 inch range, but we still have 
widespread 1-2 inch totals for Gila and northeast Pinal Counties. 
Locally heavy rainfall amounts of 3-4 inches are still possible
anywhere, particularly any places where cell training occurs. A 
Flash Flood Watch may be issued later today as more hi-res models
come in and we get a better sense of how storms may evolve. 

Convective activity decreases on Tuesday morning with lingering 
chances for showers and storms through Thursday as the upper level 
disturbance remains near Arizona before finally ejecting to the 
northeast as a strong longwave trough moves into the western U.S.
Rainfall chances with this system are currently expected to stay 
over northern Arizona for Friday into the weekend because we 
should be under drier southwest flow aloft further south. 
However, any changes in the position and orientation of the trough
could provide better dynamic forcing that could allow our period 
of active weather to continue. Regardless, this trough will allow 
below normal temperatures to continue through next weekend as 
daytime high temperatures remain 5-10 degrees below normal.

&&

.AVIATION...Updated at 1840Z 

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

Visible satellite this morning shows mostly clear skies over the 
Phoenix metro with some mid and high level clouds beginning to 
push into the region. Light diurnal winds will continue through 
today with wind speeds generally remaining below 10 kts. 

As we head into tonight, increasing tropical moisture will result
in increasing cloud cover with SCT-BKN decks around 8-10 kft and
25 kft along with a few decks developing as low as 5 kft. Going 
into Monday morning, isolated showers will begin to develop across
south-central Arizona so VCSH has been mentioned in the TAFs. 
Can't rule out a few thunderstorms in the morning but confidence 
is pretty low. The best chances for thunderstorms look to occur 
during the early afternoon/evening timeframe. Went ahead and add 
VCTS for KPHX but confidence in exact timing is low and is subject
to change in future TAF updates. Some of these storms could be 
strong to severe with gusty winds, frequent lightning, and severe 
hail possible. CIGs may get as low as 5 kft with any storms that 
pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:

No major aviation concerns across southeast California at this
time. Winds will continue to be light and at times variable
through much of the period. Cloud coverage is expected to increase
with FEW-SCT decks around 9-10 kft and SCT-BKN clouds around 25
kft developing later today and into tomorrow. Can't rule out a
vicinity shower or thunderstorm developing early tomorrow
afternoon near KBLH but confidence is too low to mention in the
TAF.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Tuesday through Saturday: 
Lingering isolated showers and thunderstorms will be possible
Tuesday through Thursday as a cutoff upper level disturbance
remains near Arizona. The best chances will be on Thursday as a
much larger and stronger disturbance moves into the western U.S.
to help the aforementioned disturbance move across and east of the
region. Aside from gusty thunderstorms, winds should generally
remain below 15-20 mph until Friday and Saturday when stronger
winds are expected with the approaching disturbance. Minimum
relative humidity values will remain in the 20 to 30 percent range
for most locations except a few spots in southeast California that
may drop into the upper teens. 

&&

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

&&

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

$$

DISCUSSION...Hopper
AVIATION...Smith
FIRE WEATHER...Hopper