NWS Phoenix, AZ wrote its last area forecast discussion before the first event at Jun 30, 2:37 PM MST, 1 hour 8 minutes earlier.
Area forecast discussion, Jun 30, 2:37 PM MST, 1 hour 8 minutes before
Area Forecast Discussion
National Weather Service Phoenix AZ
237 PM MST Wed Jun 30 2021
.SYNOPSIS...
Scattered showers and thunderstorms will be common over the
higher terrain and more isolated on the lower elevations the rest
of the week. Strong windS, localized blowing dust, and localized
heavy rain/flooding will be the main impacts. A slow decline in
storm activity begins by early next week. Temperatures will be
near, to slightly below, normal the rest of this week before
increasing a bit early next week.
&&
.DISCUSSION...
Strong convection developed early today, before 18Z, over the
northern AZ high terrain, with the strongest activity in the
Prescott area. An MCV also developed in southern San Bernadino
county CA and moved east through La Paz county this morning with
some embedded strong storms and moderate rain. The MCV is still
pushing east into portions of Maricopa county early this
afternoon, with an outflow pushing out ahead of it, but the
convection with it has mostly dissipated. Going forward through
the rest of the afternoon, the atmospheric flow pattern, dynamics,
and outflows may play a significant role in how convection
evolves.
Water vapor satellite shows a prominent upper level circulation
over the Great Basin and weak anticyclonic circulation over
northern Baja California. An enhanced gradient between the two
features may have aided the MCV this morning. A general divergent
flow pattern is seen over the central and eastern AZ with very
week steering flow. 850-300mb mean flow from RAP mesoanalysis is
around 5-10 kts out of the west to southwest. With the week
steering flow, storm motion will be slow and likely driven mostly
by outflows. With slow storm motions and non-existent shear,
ordinary storm modes are expected. With SBCAPE values upwards of
1000-1500 J/kg and PWATS around 1.2-1.5" analyzed over central AZ,
intense saturated updrafts are expected. As has already been seen
along the Rim and in Yavapai county. Strong storms may be brief,
suffering from waterloading, but still could produce significant
impacts. DCAPE is high, around 1000-1300 J/kg, over south-central,
so strong outflows will be a threat through the afternoon. 12Z HREF
probability of >35 mph is around 50-60% with max wind gusts around
45-55 mph, which seems a little low, but could be attributed more
to the moist environment. With the elevated moisture, slow storm
motions, and high instability, localized flooding impacts may be
the biggest concern this afternoon, especially with any storms
that become anchored to mountain peaks and ridgelines and/or
develop over burn scars. Rain rates may be a little tricky at
lower elevations, with HREF showing upwards of 0.5-1.0"/hr. A
gage near Crown King in Yavapai county measured ~1.0" in 10
minutes. So there is evidence of the potential for higher
rain rates.
A special 19Z sounding was launched out of Phoenix and shows a
virtually uncapped surface environment, with ~1000 J/kg SBCAPE,
~1.4" PWATS, and 10.6 g/kg mixing ratio. Forcing is the only thing
missing at this point. Storms so far have been limited to higher
terrain, but given the sounding, a strong outflow may be enough to
fire off storms over the lower deserts. Activity, in general is
not expected to last long past sunset this evening, dissipating
with the loss of heating.
.PREVIOUS DISCUSSION...
For Thursday, some sort of inverted trough may finally move deeper
into AZ - possibly aided by the sheared out mid/upper level
remnants of Enrique (weakens to a Depression today over southern
Gulf of California) and the westward advance of high pressure
centered to our east. The track, or even presence, of any such
perturbations remains quite uncertain. But, there may also be a
little bit more moisture to work with (associated with Enrique's
influence). Ensemble data suggests a little bit more moistening
yet on Friday. As the moisture availability increases, the
potential for storms to produce heavy rain also increases. That
being said, the best precipitable water values look to be over
southwest AZ as opposed to south-central AZ. The threat would tend
to be pretty localized as well. Look for temperatures to be near
average.
A slow downtrend in moisture (PWAT) begins as early as Saturday or
as late as Monday. Meanwhile, the upper high centered over Texas
is advertised to become centered over Arizona over the weekend.
With weak steering flow and decreased instability, that would
tend to hinder storm development on the lower elevations. But, if
enough moisture lingers and steering flow isn't too weak, there
could be a period of relatively better chances for the the
south-central AZ deserts. But, the high isn't anticipated to be
super strong so we're not anticipating an excessive heat event
through Tuesday.
&&
.AVIATION...Updated at 2040Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Thunderstorm activity across southern Yavapai County will likely
help to initiate new convection across Gila County this afternoon.
These storms are then expected to produce an outflow boundary
that will propagate into the lower deserts, including the Phoenix
area. There is a 50% chance of the outflow boundary reaching KPHX
with easterly winds up to 35 kt. Conditions will also remain
favorable for additional isolated storms to develop along the
boundary and near subsequent boundary interactions in the Valley.
There is also a slight (10%) chance of a dust storm this evening.
Otherwise, gusty winds and heavy rain will be the primary hazards.
Southeast California/Southwest Arizona including KIPL and KBLH:
Overall little aviation impacts expected through the period for
KIPL and KBLH. Thunderstorm activity is generally expected to
remain well east across Arizona. Otherwise, southerly winds will
predominate at both KIPL and KBLH.
&&
.FIRE WEATHER...
Saturday through Wednesday:
High pressure is expected to become centered over Arizona over the
weekend. A slow decline in atmospheric moisture is also
anticipated to begin by early next week. Thus, the best potential
for storms with locally heavy rain looks to be this weekend.
Otherwise, a familiar Monsoon season pattern will be in place with
storms primarily focused over the higher terrain producing
outflows that spread to the lower deserts and in turn generate
isolated new storms. Temperatures will start off this period at,
or slightly below, normal followed by a a little bit of warming.
Min RH values will generally be in the 20-30% range through Sunday
before decreasing to 15-25% on Tuesday. Max RH values will follow
a similar trend but with decreases starting sooner. For instance,
Max RH values will be 40-60% most places (less eastern Riverside
County) through Monday morning before decreasing to 30-50% most
places by Tuesday morning. Apart from thunderstorms, wind patterns
will follow familiar diurnal patterns.
&&
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&
DISCUSSION...Benedict
PREVIOUS DISCUSSION...AJ
AVIATION...Hirsch
FIRE WEATHER...AJ/Benedict