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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

                            
000
FXUS65 KPSR 231059
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
359 AM MST Mon Sep 23 2019

.UPDATE...aviation discussion.

&&

.SYNOPSIS... 
Multiple periods of scattered to widespread showers and 
thunderstorms are expected through Tuesday morning across south
central and southwest Arizona with isolated storms over eastern 
Riverside and Imperial Counties. Locally heavy rainfall capable of
producing flash flooding along with isolated severe thunderstorms
capable of producing large hail and damaging winds are expected. 
Although rain chances will begin to decrease on Tuesday, elevated
rain chances will linger through Thursday as the upper level 
disturbance remains near the region with the best rain chances 
Wednesday night into Thursday. This disturbance will move east of
the region Friday, with drier conditions expected this weekend.

&&

.DISCUSSION...
Tropical moisture associated with the remnants of what was 
Pacific TC Lorena has moved into south central Arizona with 
supplemental moisture from a Gulf surge moving into southwest 
Arizona. Dew points are already in the upper 60s over Phoenix and 
south central Arizona with mid 70s dew points pushing into Yuma 
County. Much drier air is currently present just west of the 
Colorado River over southeast California where dew points are in 
the 40s, resulting in a very sharp moisture gradient. A very 
strong mid- to-upper level trough with 571 dm H500 heights digging
into the Great Basin will dig farther south into the Colorado 
River Valley by this evening as it fills to some extent. This will
maintain very favorable dynamics near and east of the cold core 
low, with significant diffluence aloft, upper-level divergence in 
the right entrance region of a jet streak, and isentropic ascent 
in the presence of near record level 1.4-1.8 inch PWATs through 
Tuesday morning. These conditions will support multiple periods of
scattered to widespread showers and thunderstorms through Tuesday
morning across south central and southwest Arizona with isolated 
storms over eastern Riverside and Imperial Counties. Locally heavy
rainfall capable of producing flash flooding is expected, so we 
have issued a Flash Flood Watch for most of La Paz, Maricopa, 
Gila, and Pinal Counties from 5 AM MST this morning through 11 AM 
MST Tuesday morning. Isolated severe storms capable of producing 
large hail, damaging winds, and possibly even an isolated tornado 
or two are expected by late morning through this evening.

Three main periods or focus areas of convection are expected 
today with varying threats, so we cover them individually here: 

1) Scattered to widespread convection is expected to continue for
the rest of the overnight hours through mid-morning primarily 
from the a line along and east of a Maricopa-Phoenix-Anthem line 
with more isolated convection over southwest Arizona. A weak
shortwave rotating out ahead of the stronger shortwave over the
Great Basin may be enhancing dynamic lift locally right now. Most
of this convective activity should be relatively shallow given we
still have lingering midlevel stability, but this should rapidly 
erode from west to east as height falls and cooler air moves in 
aloft to steepen lapse rates and allow for deeper convection and a
greater coverage of thunderstorms. Locally heavy rainfall will be
the primary threat with these storms, particularly over eastern 
Maricopa and Gila Counties where S-SW- to-N-NE oriented isentropic
ascent is best aligned with terrain. Rain rates should initially 
remain below 1 inch per hour, but as convection intensifies over 
the next few hours with the assistance of terrain and increasing 
instability, 1-2 inch per hour rain rates will be possible. For 
this reason, we have decided to start the Flash Flood Watch at 5 
AM MST this morning as that is when we really expect the heavier 
rainfall and the cumulative effects of ongoing shower activity to 
accentuate the flash flood threat, particularly for Gila and 
northeast Maricopa Counties where WPC has issued a moderate risk 
of excessive rainfall in collaboration with local Arizona offices.

2) By late morning, the ingredients for strong to severe storms 
will be present as the trough provides atmospheric forcing via 
upper level diffluence and a strong jet streak aloft. Gradually 
veering winds and bulk shear values in the 40-50 knot range in the
presence of MLCAPE and CAPE values in the 1500-2500 J/kg range if
not even higher over Yuma County will support discrete supercells
depending on how convection evolves. The moisture gradient 
currently present near or just west of the Colorado River Valley 
should continue to sharpen further to act like a pseudo-dryline 
and provide an initial focus for convection, so there is highest 
confidence in severe storms over Yuma, La Paz, and Maricopa 
Counties. SPC has this area and the Phoenix metro in a slight risk
of severe storms, so a Severe Thunderstorm Watch may be issued 
later this morning or afternoon once confidence is high enough. 
Further east towards the Phoenix metro, there are significant 
uncertainties in how much the ongoing convection and cloud cover 
may temporarily stabilize the atmosphere locally, so we may see a 
brief lull in the action through late afternoon. However, 
convective temperatures are only in the upper 80s so it will not 
take too many breaks in the clouds for the Phoenix metro to 
realize strong instability and favorable wind profiles to get 
strong to severe storms. Large hail up to golf ball size will be 
the biggest threat with these storms, followed by damaging winds. 
Very steep low- level lapse rates with relatively low LCLs and 
cloud bases down to about 5000 feet will support a very isolated 
tornado threat, but any that do occur should be weak. Westerly 
winds expected by this afternoon should keep storm- relative 
helicity values low to keep this threat minimal, but winds backing
to southeasterly by early evening could briefly increase this 
threat depending on how convection evolves.

3) There is perhaps stronger model agreement in that another round
of storms is expected late this evening into the overnight hours
as the synoptic front pushes through the region. This should
result more linear storm structures in the the from multicellular
line segments, for which damaging straight line winds will be the
primary threat. Hail should generally be smaller since rotating
updrafts will generally not be expected and instability should
begin to decrease. However, any cells that are able to train or 
occur over areas that have already experience heavy rainfall will 
have the potential to cause flash flooding. Although agreement for
this third period of storms is strong, there is considerable 
uncertainty in how widespread storms will be and what time they 
will actually occur. Fortunately, this uncertainty should decrease
over time as hi-res models come into clearer focus based on how 
the aforementioned two periods of convection evolve. 

Activity should wane in our area by late Tuesday morning although
enough moisture will be present that lingering showers will be 
possible. However, given the close proximity of the upper-level
disturbance that should eventually stall over the northern Gulf of
California and uncertainties in when third more linear period of
storms will occur, we elected to keep the Flash Flood Watch in
place through 11 AM MST Tuesday. Some hi-res guidance shows 
isolated showers and storms over Gila and eastern Maricopa 
Counties Tuesday afternoon as a cutoff low takes shape, but there
is still considerable uncertainty there. Residual moisture and a 
second Pacific trough later in the week justify elevated PoPs in 
the forecast, primarily for Maricopa County eastward, late 
Wednesday and Thursday as the trough finally opens up and ejects
to the east as it is kicked out by a much stronger longwave trough
upstream. As for temperatures, the persistent troughing and 
shortwaves will keep a cooler airmass in place resulting in 
daytime temperatures below seasonal normal values through the 
forecast period. 

&&

.AVIATION...Updated at 0527Z 

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

Light diurnal winds will continue through the early part of the
evening with speeds mainly below 6 kts. As we head into the 
overnight hours, increasing tropical moisture will result in 
increasing low and mid-level cloud cover with BKN-OVC cloud decks
currently near 8-10 kft, dropping closer to 5 kft after 10Z. 
Going into tomorrow morning, shower and storm chances will 
continue to ramp up and hi-res models are now indicating a 
slightly earlier onset over terminals and this has been reflected
in the most recent TAF package. Latest model guidance is 
now suggesting that isolated showers could begin to develop 
near terminals as early as 9Z, with better coverage after 12Z. 
There then seems to be a hint at a break during the early part of
the afternoon, however there very much so could be showers and 
thunderstorms still lingering near or over terminals throughout
the afternoon hours. Thereafter, models continue to indicate a 
line of strong thunderstorms approaching the Phoenix terminals 
from the west after 22Z. These storms will be the strongest storms
of the day and could become severe with gusty winds, frequent 
lightning, and severe hail possible. CIGs may get as low as 4 kft 
with any storms that pass through.

Southeast California/Southwest Arizona including KIPL and KBLH:

Winds will continue to be light and at times variable through 
the morning hours before becoming gusty out of the south-southwest
in the early afternoon. In addition, low-level cloud cover is 
expected to increase through the TAF period with FEW-SCT decks 
around 5-8 kft. There is a little bit more confidence that 
showers and storms could be possible over KBLH in the early to 
mid afternoon. Any storm that develops will have the potential to
produce gusty winds and frequent lightning. Due to this, it is now mentioned in the latest TAF
package.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Wednesday through Sunday: 
No changes to the outlook forecast...Lingering isolated shower
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. Overnight recoveries will remain decent as
long as the moisture remains in the area. 

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through Tuesday morning for AZZ531-533-534-537-
     538-540>558-560>563.

CA...None.
&&

$$

DISCUSSION...Hopper/Deems
AVIATION...Hernandez
FIRE WEATHER...Deems/Hopper