Scientific Forecaster Discussion
fxus65 kpsr 240753
Area forecast discussion
National Weather Service Phoenix Arizona
1253 am MST Wed Jul 24 2019
updated aviation discussion for Phoenix terminals.
expect storm development to continue over higher terrain
locations this afternoon, with activity gradually spreading into
the lower deserts. Locally heavy rain and areas of blowing dust
are possible. A Flash Flood Watch is in effect from 3 PM till
midnight for Maricopa, Pinal, and southern Gila County. High
pressure will slowly become centered over Arizona for a downtrend
in storm chances starting Wednesday. High temperatures will also
increase...especially during the latter half of the week.
convection continued to diminish as of 8 PM this evening across the
area; infrared imagery depicted far less of the cold cloud tops that
dominate eastern Arizona yesterday evening and the main strong mesoscale convective system
remaining is well south of the border in Mexico. Latest Storm Prediction Center
mesoanalysis graphic shows the MLCAPE over the lower deserts is
somewhat diminished, mostly 500 j/kg or less and wind shear is also
less than 24 hours ago. Effective bulk shear is southeast 20kt or
less with southeast winds only 10kt as shown by the latest 00z
Phoenix sounding. Due to diminishing convection, and after
collaboration with Tucson, decided to cancel the Flash Flood Watch
for tonight. So far we haven't received any flash flood reports from
south central Arizona and any storms that re-develop tonight should
not produce sufficient areal coverage of heavy rain to support the
watch. Otherwise expect lots of debris clouds and some lingering
showers across mainly south central Arizona rest of tonight with an
isolated embedded storm possible mainly during the rest of the
high pressure is centered over northwest Utah, with
east/southeasterly flow over the region. Moisture values are
exceptionally high, with phoenix's pwats above the 90th percent
moving average. Earlier today, we were really relying on cloud
cover to thin out to maximize daytime heating and instability,
especially over the lower deserts. Phoenix's convective
temperature today was 105 degrees, and since clouds eventually
did thin out, we've had no trouble warming up to 107 degrees and
we could possibly warm up a little more. With the high being
further north today, it's been evident, even by looking at
aircraft sounding, that mid-level temperatures have cooled off a
bit, which will also aid in storm development today. As a result,
we currently see thunderstorms favoring the high terrain in south-
central Arizona and even in southeast California. We can expect
this coverage to continue to fill in today and tonight, and
possibly even continue into the early morning hours tomorrow
(primarily across southwest Arizona and southeast California early
tomorrow morning). With significant moisture in place, and very
slow storm motion, flooding will be the biggest impact today. Thus
we have issued a Flash Flood Watch for Maricopa, southern Gila,
and Pinal County from 3 PM till midnight tonight. The Flash Flood
Watch may be expanded westward if warranted.
Wednesday through Monday...
there is general model agreement that the storm activity over the
lower deserts trends down beginning Wednesday. This is due in large
part to an increase in stability caused by the the high becoming
centered over Arizona. That also decreases steering flow which
makes it harder for storms to advect from higher terrain. There
also looks to be a west to east decline of moisture with time. The
moisture doesn't drop to nothing. In fact, 1000-700mb mean mixing
ratios don't drop dramatically over most of Arizona. Thus, there
will still be higher terrain storms (mainly outside of our cwa).
With less storms and clouds, and the location of the high, Max
temps will climb. At this time, it looks like we will remain below
excessive heat thresholds. Over the weekend, there are some
indications that the high center may shift westward a little bit
which would make for somewhat more favorable steering flow. It
remains to be seen. For now, have temps trending closer to normal
with "low grade" monsoon pops.
Aviation...updated at 0753 UTC
South-central Arizona including kphx, kiwa, ksdl, and kdvt:
Weak midlevel instability present on aircraft soundings should
continue to increase over the next few hours to support isolated
to scattered showers between 09-13z at the Phoenix terminals.
Convective showers have started to develop in Pinal County and
along the Mogollon Rim in response to a weak inverted trough
northern Sonora into central Arizona. Widespread showers are not
expected because the low-levels remain dry, but relatively high
dcape values may support strong wind gusts up to 35 knots with
these showers or virga. An isolated thunderstorm or two with
stronger gusts is not out of the question, but confidence is too
low to include thunderstorms in the vicinity given at this point given limited instability.
East-southeasterly flow with scattered-broken 8-10 kft ceilings should
prevail in areas not experiencing convective showers.
Once the showers end, look for some clearing later on Wednesday.
We cannot rule out another bout of late afternoon storms for the
greater Phoenix area this evening, but confidence is low so no
mention of thunder has been included at this time. Model guidance
also not very bullish on easterly outflows Wed evening, but we
have added a wind shift around 03-04z to the northeast or east.
Confidence in this shift is actually not very high. Otherwise
expect winds to favor the east tonight through Wednesday morning
with winds lightly turning to the west after about 20z tomorrow.
Southeast California/southwest Arizona including kipl and kblh:
Monsoon moisture continues to move north around the upper high...and
across the western deserts. Chances for showers/storms not zero but
too low to mention weather in the tafs. Will continue to keep
variable mid/high cloud decks with bases mostly 10k and up. Cigs are
likely at times mainly above 15k feet. Look for few-scattered high based
cumulus decks Wed afternoon with bases 8-10k. Winds to favor the
south next 24 hours at kblh with speeds mostly 15kt and below. Winds
favor southeast most of the period at kipl.
Aviation discussion not updated for amended tafs.
Friday through tuesday:
expect thunderstorms to be limited to the higher terrain of south-
central Arizona. Max temperatures will climb to several degrees above
normal Friday and Saturday before beginning to trend down. Any
thunderstorms that manage to form will be very effective at
producing strong winds but with little wetting rain. With the
rising temperatures, minimum humidities decline as well but are
expected to remain well into the teens. Overnight recovery
declines some but remains at least fair (better over higher
Spotter information statement...
spotters should follow Standard reporting procedures.