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Area Forecast Discussion (AFD)

Issued by NWS Phoenix (PSR)

FXUS65 KPSR 201021

Area Forecast Discussion
National Weather Service Phoenix AZ
320 AM MST Fri Sep 20 2019

Dry and mostly clear conditions will affect much of the region
through Saturday with temperatures within a few degrees of
seasonal normals. As early as late Sunday, likely lasting through 
the first half of next week, rain and thunderstorm chances greatly 
improve, especially across Arizona. This next weather system will 
have the potential of dropping heavy rainfall across portions of the 
area early next week.


Early this morning, plot data showed the presence of a large upper 
level low pressure trof over the western CONUS, with Arizona on the 
southern periphery of the trof and situated under dry southwest flow 
aloft. IR imagery at 2 am showed generally clear skies area-wide and 
surface dewpoints were mostly running from the low 30s to low 40s. 
The upper trof is expected to remain over the area through Saturday, 
keeping dry west to southwest flow aloft in place, for sunny days, 
clear nights and near normals highs in the upper 90s. The trof moves 
off to the east on Sunday as a brief, transitory upper ridge moves 
into the desert southwest, keeping dry conditions but allowing high 
temperatures to climb to near 100 degrees. The dry conditions along 
with slightly above normal temperatures won't last long because big 
changes in the weather pattern will be coming starting Sunday night 
and continuing into the early to middle part of next week.

Although conditions across Arizona are currently dry and benign, a 
look to the south shows the presence of not one but two tropical 
weather systems...Lorena and Mario. The two systems are nearly on 
top of each other; Lorena was located just south of the southern tip 
of Baja, and Mario was just off to the southwest of Lorena. Tropical 
storm track guidance continues to suggest that over the next few 
days, Lorena will move north along the Baja coast before eventually 
dissipating while Mario is progged to gradually move off to the 
west/northwest and out into the east Pacific. While we do not expect 
the circulation system associated with Lorena to move inland, we 
should certainly see a good portion of remnant moisture drawn north 
and into Arizona, starting Sunday night. The deep tropical moisture 
will then interact with another rather deep upper level closed low 
poised to develop and drop south into the desert southwest with the 
low center likely to move into Arizona by later in the day Monday.

At this time it appears fairly certain that deep moisture will 
indeed move into the area - mainly from the lower Colorado River 
valley east into east/central Arizona. The moisture will interact 
with the developing low resulting in locally heavy rain, potentially 
severe storms, and flooding/flash flooding. What is NOT as certain 
is the timing and track of the upper low as well as the instability 
that may or may not develop across the deserts in advance of the 
low. Operational model guidance, such as the latest GFS and ECMWF 
runs, have been very inconsistent with their handling of the upper 
low. The latest GFS is MUCH more progressive with the low, taking it 
much quicker to the east; by 5 pm Tuesday the GFS puts the low 
center in west-central New Mexico, while the ECMWF has the low just 
northeast of Yuma. GEFS ensemble members are all over the map with 
low centers spread across the desert southwest, lingering across the 
area through Wednesday. Should the ECMWF be correct, there would be 
a much greater potential for heavy rain to linger over central 
Arizona through Tuesday night into Wednesday, whereas much drier air 
would already be spreading into the area behind the exiting low 
should the GFS be correct. These sorts of uncertainties also impact 
the storm total rainfall that will occur with this event. For now, 
we still expect locally heavy rain to occur, with many areas across 
the west/central deserts likely to see between 1 and 2 inches of 
rain. Favored areas in the east/northeast quadrant of the low could 
see totals approach 3 inches, as depicted by the latest WPC QPF 

At this time, it appears the best window for heavy rain will be from 
early in the morning on Monday through Tuesday morning, before some 
amount of drying starts to take place from the west. POPs continue 
to be raised, with much of central and south-central Arizona seeing 
values climb to 50-70 percent Monday into Tuesday. Should the heavy 
rain occur, with amounts exceeding 2 inches over the deserts, 
flooding and flash flooding would be possible if not likely. At this 
time, given the uncertainty, it is a bit too early to consider a 
flash flood watch, but one may eventually be needed.

Another potentially significant impact from this upcoming weather 
event is the chance of severe thunderstorms. Given the strength of 
the wind fields with the developing "bowling ball" closed low, bulk 
shear values of 50kt are possible and the favored veering with 
height wind profiles would be likely, supporting rotation within the 
developing storms and increasing their severe potential. Should 
colder air aloft closer to the core of the low move into the 
picture, freezing levels lower and instability increases allowing 
for possible large hail. Instability is a wild card - too much cloud 
cover over the area would reduce surface heating and limit 
instability/CAPE values. Should there be breaks in the clouds, 
better heating potential would support stronger and possibly severe 
storms. In any rate, the potential is there but the timing and exact 
location for strong storm development is still a bit up in the air.

Of course, given all of the expected clouds, showers and 
thunderstorms for early next week, we can expect much cooler days 
with highs in the south central deserts falling into the low to mid 
90s on Monday and Tuesday. Cannot rule out at least one day where 
highs stay in the 80s. We should see warming develop on Wednesday as 
the low moves off to the east and drying sets in from the west 
allowing for more sunshine.

Again, depending upon the evolution, timing and track of the upper 
low, and the exact amount of deep tropical moisture moving into the 
area from hurricane Lorena, some locations across southern Arizona - 
east of the lower Colorado River Valley - could see very heavy rains 
with totals well in excess of 2 inches. Strong and even severe 
thunderstorms are also possible - likely some time during the day on 
Monday. Stay tuned to this developing potentially significant and 
high impact weather event!


.AVIATION...Updated at 0550Z 

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

Dry southwest flow aloft will keep skies mostly clear through
Friday. Wind directions will favor typical diurnal tendencies.
Wind speeds should remain aob 10-12 kts with gusts to up to 15-20
kts Friday afternoon. Could see a couple hours of southerly 
crosswinds again around midday Friday as winds veer towards the 

Southeast California/Southwest Arizona including KIPL and KBLH: 

Skies will remain mostly clear under dry southwest flow. West to
southwest winds should gradually veer northwesterly Friday morning
and then persist out of the north/northwest through the day with 
speeds aob 10 kts and limited afternoon gustiness.

Aviation Discussion not updated for amended TAFs.


Sunday through Thursday: 
A low pressure system will lead to cooler temperatures and an
increase in rainfall potential, particularly Monday and Tuesday,
for most locations east of the Colorado River. In addition,
minimum RH values will improve as well as overnight recoveries.
Otherwise, expect some stronger afternoon breezes, especially on
Monday, with gusts as strong as 20-30 mph.


Spotters should follow standard reporting procedures.