Scientific Forecaster Discussion
fxus65 kpsr 221151 aaa
Area forecast discussion...updated
National Weather Service Phoenix Arizona
450 am MST sun Sep 22 2019
Update...updated aviation discussion.
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.
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 north-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 light snow shower
to-NE- oriented isentropic ascent at lower and mid-levels.
Abundant moisture will be in place as precipitable water 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 Storm Prediction Center 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). Storm Prediction Center
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 1150z
South-central Arizona including kphx, kiwa, ksdl, and kdvt;
southeast California/southwest Arizona including kipl and kblh:
Across the greater Phoenix area through the day we do not expect
impacts from former Hurricane Lorena. Except for some high clouds,
look for generally sunny skies into the afternoon before few high
based cumulus decks form. Winds will be on the light side, favoring
typical diurnal tendencies, with southeast winds becoming
southwest/west after 21z. Tropical moisture will start to be drawn
north into the Phoenix area during the evening, with scattered-broken decks
around 8-10k feet developing by around midnight, and as we move into
the morning hours on Monday, isolated showers will likely start to
develop. After midnight few-scattered decks near 5-6k feet may start to
form. Thus the mention of vcsh in the tafs after 08z. Cannot rule
out thunderstorms forming but confidence is rather low and timing
and exact location of storms too difficult to pinpoint at this time
so we will wait until at least the next package before adding
thunder into the tafs.
No impacts expected out west during the taf period. Winds to be
rather light and somewhat variable, favoring the south to southwest
this afternoon and evening at kblh and favoring southeast during the
afternoon at kipl before gradually swinging towards the southwest.
Tropical Storm Lorena, currently centered over guaymas Mexico,
will continue to weaken with time. In fact, upper portions of the
storm will be shearing toward the northeast. Through this
afternoon, there will be little discernible impact to the taf
sites. An approaching trough in the westerlies will pull remnant
moisture northward Sunday night and Monday leading to an upward
trend in storm chances - most notably during the daytime and
evening hours Monday (mainly east of the lower Colorado River
Aviation discussion not updated for amended tafs.
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.