Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 06/29/21
Area Forecast Discussion
National Weather Service Phoenix AZ
452 PM MST Mon Jun 28 2021
.UPDATE...Updated Aviation Discussion.
A major uptrend in humidity and thunderstorm activity will take
place over the next few days. Strong winds will be the main impact
from any storms today and Tuesday. Additional moisture on
Wednesday will allow storms to produce more rainfall. However,
many uncertainties remain exact locations of storms and rainfall
amounts. There is higher confidence in high temperatures trending
down, even dipping below normal by Wednesday and Thursday. Storm
chances are anticipated to slowly decline from west to east during
the latter part of the week and over the weekend. High
temperatures are anticipated to slowly increase as well.
Latest water vapor imagery depicts an anomalous trough across
southwestern NM. The moisture plume from this trough extends
southward towards Hurricane Enrique, off the west coast of Mexico.
Meanwhile, the anticyclone producing historic record breaking
temperatures remains situated across the Pacific Northwest.
Between these two systems, a stronger than normal north-
northeasterly flow is evident across Arizona. Latest KPSR sounding
from 12z measured 30 kt at 500 mb. Mean layer flow off the higher
terrain along with somewhat above normal PWATs will promote
isolated convection reaching and possibly even developing across
portions of the lower deserts this afternoon.
Latest HREF indicates storms will initiate across eastern Gila
County this afternoon. Latest U of A WRFs then suggest a strong
westward-propagating outflow boundary will develop out of this
convection. Latest HRRR indicates a 50% chance of 35+ mph wind
gusts across the Valley, with the highest likelihood across Pinal
County. Consequently, a Blowing Dust Advisory has been issued for
the dust-prone areas of southeastern Maricopa and Pinal Counties
for this evening. The boundary may also be strong enough to
overcome any convective inhibition. Isolated storms are possible
in the Valley tonight, though a general lack of CAPE will limit
the overall coverage of storms. The environment also still remains
more conducive for wind/dust/lightning, rather than heavy rain.
Storm chances will increase Tuesday as total PWATs increase above
an inch. Latest SREF indicates the aforementioned vort max across
southern Mexico will migrate westward into a more favorable
position for synoptic ascent across southeastern Arizona. However,
CAMs are now displaying more variability. Some are developing
activity early and with an expanse as far west as southeastern
California. Others suggest storms will remain concentrated across
south-central Arizona with another threat of an outflow boundary
capable of producing blowing dust. Both scenarios could even
materialize, resulting in a possible outflow boundary collision
across the lower deserts.
Moisture availability continues to increase on Wednesday. That will
enable any storms to be better rain producers. It remains to be seen
what influence the aforementioned inverted trough will have at that
point. Also, it remains to be seen how unstable the boundary layer
will be which will be heavily influenced by the preceding day`s
convection. Despite the increase in moisture, the GEFS plumes don`t
depict much change in the CAPE from Tuesday to Wednesday. However,
the SREF actually shows a decline (possibly due to some boundary
layer stabilization). A lot of uncertainties remain. The big picture
takeaway is that we will be seeing a major uptrend in thunderstorm
activity and associated impacts over the region during the first
half of the workweek. High temperatures trend down as well (below
Ensemble data depict a decline in moisture starting as early as
Thursday over southeast California (remaining steady or even peaking
a little more over south-central AZ). The NAEFS shows a decline over
the weekend. The EPS does as well but more slowly. Meanwhile, high
pressure becomes centered roughly over Arizona during the latter
part of the week. But at this time it doesn`t appear to be overly
strong. The PoPs follow the moisture trends. Temperatures slowly
trend upward during the latter half of the week and into the
.AVIATION...Updated at 2350Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Wind speeds and directions will continue to be the primary aviation
weather concern through this evening. For now, wind directions will
continue to favor a south-southeasterly direction with speeds mostly
between 8-12 kts and occasional gusts of 16-20 kts. Directions may
veer more southerly at times creating temporary crosswinds for any
east-west runway orientations. Meanwhile, attention remains focused on
the thunderstorm activity moving westward from the White Mountains
into Gila County. These storms are expected to send an easterly outflow
through the terminal area late this evening between 2-6Z. There is less
confidence assessing the potential wind strength of the switch for any
particular airport, but gusts between 15-30 kts are anticipated, as is
the potential for blowing dust (30% chance) at KIWA. There is a low
chance for storms tonight, 20% or less, favoring the far eastern
portion of the Phoenix area. The most current anticipated timing and
strength of the wind shift is in the latest TAF package but will be
amended to reflect any updates as the storms and winds evolve.
For tomorrow, thunderstorm activity is likely to increase across the
Mogollon Rim and westward through Yavapai County, thus increasing the
chances for northerly outflows and VCTS in the terminal area late
Southeast California/Southwest Arizona including KIPL and KBLH:
There is some uncertainty regarding aviation weather towards the latter
half of the forecast period. For this evening, southeasterly winds at
8-12 kts and occasional gusts are anticipated at KIPL, while winds
maintain a more southerly heading at KBLH at 7-13 kts with some
overnight gustiness. For tomorrow, there is potential that thunderstorm
outflow winds move through the terminal area from the north-northeast
late in the evening. Convection cannot be ruled out either but
confidence in either of these two outcomes is too low to mention in
the TAFs for now.
Thursday through Monday:
Humidities will peak midweek with Min RH values of 20-30% on the
lower elevations (30-40% higher terrain). Max RH values will be
highest on Thursday morning with readings of 40-60% most places
(locally higher). Storms on those days, if any, would have the
potential to produce localized flooding - especially on burn
scars. High temperatures will also be at their coolest (several
degrees below normal over south-central AZ). Atmospheric moisture
and storm chances slowly decline from west to east late in the
week and over the weekend. However, isolated showers and
thunderstorms will remain a threat across much of the area.
.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.
AZ...Excessive Heat Warning until 8 PM MST this evening for AZZ530-
Blowing Dust Advisory until 9 PM MST this evening for AZZ550-551-
CA...Excessive Heat Warning until 8 PM PDT this evening for
Area Forecast Discussion
National Weather Service San Diego CA
843 PM PDT Mon Jun 28 2021
Weakening high pressure and an influx of monsoonal moisture into
Southern California through Wednesday, will bring some clouds and
cooling, along with a chance for mainly afternoon and evening
showers and thunderstorms, decreasing on Thursday. The best chance
for showers will be over the mountains and deserts through
Wednesday, while the marine layer and associated clouds hold over
coastal areas with continued cool Summer weather. The moisture
decreases on Thursday, along with the chance for showers, leading to
a fair and dry weekend with seasonal temperatures.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
* Scattered thunderstorms Tue-Thu afternoon mountains/deserts
* Main potential impacts: Lightning strikes, downburst winds
* Less impact: Localized heavier rainfall/low flash flood risk
Marine clouds held firm along the coast and offshore today and
continue this evening, while higher clouds spread southwestward from
the high deserts. A moderate coastal eddy continues to support
marine stratus and south winds along the coast, which should spread
inland tonight with moderate onshore sfc pressure gradients. Our 00Z
sounding remained little changed from 24 hours prior. The marine
inversion was based at 1900 FT MSL, and was about 16C, peaking near
31C at 4000 FT MSL. An elevated moist layer was at 16K FT where
several high-based cumulus congestus developed over the mountains
this afternoon, and spread anvils downwind over the valleys.
The weakening of the ridge aloft is still expected to bring cooling,
so the Excessive Heat Warning and Advisory will expire at 9 PM this
evening. No other forecast changes.
From previous discussion...
Mid-high level moisture will be increasing from the northeast
through Tuesday, underneath the anomalous ridge over the Pacific
Northwest, which is resulting in record-shattering and very high
impact heat there. The moisture on Tuesday will be highest aloft
over San Bernardino County. Hi-res models of the HRRR and local
WRF are continuing to show the only cells developing on Tuesday
afternoon being over the San Bernardino County mountains and the
Mojave desert. Most cells only dropping about 0.10-0.25", though
the local WRF does show heftier cells over the Mojave Desert to
about 0.50"/hr, but given the dry low levels this may be overdone.
HRRR shows less activity and lower rainfall rates. The higher
potential impact looks to be potential downburst winds versus
There could be a few showers and thunderstorms moving southwest
from Nevada Tuesday night, but model consensus keeps the bulk of
this convection over SE California and just outside of our
service area, but will have to monitor this as some of this
could skim the Coachella Valley Tue night-Wed morning.
The coverage of tstorms looks to increase across the mountains on
Wednesday afternoon as better lower level moisture moves in from
the southeast, but not looking too deep/significant as it had
looked a few days ago. The prevailing flow will be westerly, so
the coast and valleys likely will stay dry, except perhaps
portions of the Inland Empire. The tstorms on Wednesday afternoon
will favor the desert mountain slopes with the heaviest cells
occurring over the Riverside (Santa Rosa Mountains) and San Diego
County mountains where cells could drop about 0.50"/hr. Very
localized flash flooding is possible, particularly along the
desert mountain slopes and over the deserts. Again, the
microburst/damaging wind potential looks to be a greater risk,
though that would be localized as well.
With the moisture aloft and fairly dry air continuing in the
lower levels, thunderstorms will be capable of producing strong
and potentially damaging downburst winds, which could lead to
blowing dust over the deserts.
The moisture will be advecting east on Thursday. There could still
be isolated thunderstorms in the afternoon over the mountains and
deserts, but less activity compared to Wednesday, and it is
possible it could be dry if not much moisture remains.
Friday and through the holiday weekend will see a slow warming
trend along with expected dry conditions. Monsoonal moisture will
be just off to our east over SE California and Arizona. The model
consensus keeps moisture at bay here until perhaps later next week
when a moisture return is possible. Until then, the upcoming
weekend and into the following week looks tranquil with above
normal temperatures likely.
290330Z...Coast/Valleys...BKN-OVC stratus will continue to spread
overnight, up to 25 miles inland by 12Z Tue. Bases will be 800-1100
ft MSL with tops to 1300 ft MSL. Local vis 2-4 miles will occur over
higher coastal terrain and in the coastal valleys. Most areas will
clear 15Z-17Z Tue, except 18Z-21Z Tue within 5 miles of the coast,
with BKN/OVC conditions continuing through the afternoon near the
beaches. Conditions Tue night will be similar to tonight.
Mountains/Deserts...SCT-BKN clouds above 20000 ft MSL will prevail
through Tue evening. There is a chance of thunderstorms Tue
afternoon/evening, with greatest likelihood in the mountains. CB
bases will be 10000 ft MSL with tops to 40000 ft MSL, with strong
up/downdrafts and local surface gusts to 50 knots possible in/near
No hazardous marine conditions are expected through Saturday.
An influx of monsoonal moisture will bring a recovery in humidity
Tuesday into Wednesday with a chance for mainly afternoon and early
evening thunderstorms, and mainly in the mountains and deserts.
There is greater potential for dry lightning strikes on Tuesday
afternoon as the moisture begins to increase, then again on Thursday
afternoon as the moisture decreases. Wetting rains are most likely
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.