221
FXUS65 KPSR 200536
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1036 PM MST Fri Jul 19 2024

.UPDATE...
Updated Aviation section.

&&

.SYNOPSIS...
Thunderstorm chances will persist across the area through at least
the middle of next week with the highest probabilities over higher
terrain areas of Arizona. Far better odds for thunderstorm survival
into lower elevations will arrive over the weekend and early next
week. Otherwise, widespread excessive heat will impact much of the
region the next few days with temperatures better than 5 degrees
above the daily normal.

&&

.DISCUSSION...
WV satellite imagery and objective analysis early this afternoon
shows the 596dm H5 monsoon high is centered directly over north-
central AZ with overall very weak steering flow (5 kts) around
the region. There has not been too much change environmentally and
synoptically since yesterday, besides the slight shift of the
high center. Today`s thunderstorms began initiation around 11 AM
MST and the storms will continue through this evening, once again
focused mainly in the high terrain of northern and southeastern
AZ. The latest HREF places the highest probability of thunderstorm
clustering from the White Mtns northwest to just west of
Flagstaff and southeast of Tucson. The environment remains
favorable for strong thunderstorm outflow winds, with DCAPE values
up to around 1500 J/kg over the lower deserts. The HREF has a
high neighborhood probability (70%) for downburst winds >35 mph in
the height Mtns and through Gila County this afternoon, with
probabilities dropping off into the South-Central AZ lower
deserts. Like yesterday, there is still no strong signal for
severe thunderstorms as MLCAPE is expected to remain at low to
moderate magnitudes, generally close to 500 J/kg, but a few
isolated severe cells will remain possible. High MLCIN absolute
value magnitudes over the lower deserts will make it difficult for
convective development off of outflows later this evening. Still
a preponderance of some high-based ACCAS showers and isolated
storms could make there way/develop over the lower deserts during
the overnight and early morning hours.

For Saturday, the steering flow, although still weak, becomes a
little more N-NE over the eastern half of of AZ, which is more
favorable for Rim activity to drift toward the South-Central AZ
lower deserts. HREF has a stronger signal for storm intensity and
coverage around the White Mtns and down through Southeast AZ
tomorrow afternoon and evening, with even higher probabilities for
thunderstorm outflow winds >35 mph (70-90%) than today. This
stronger wind signal increases confidence in the potential for
outflow winds to generate blowing dust tomorrow afternoon and
evening, especially in Pinal county. There is also a little bit
better of a signal for convection in the South-Central AZ lower
deserts, but this will likely be dependent on a very deep outflow
or colliding outflows from the north and south as the cap will
still be pretty strong (MLCIN around -50 to -100 J/kg).

Even if neither today nor Saturday sees convection survive and
initiate over the lower deserts of South-Central AZ in the evening
hours, then Saturday at least looks to help set the stage for an
even more active period Sunday-Monday as moisture increases, with
warming dew point temperatures and mixing ratios rising up to
11-12 g/kg. N/NE mid-tropospheric flow will also increase up to
20-25 kts as the high shifts further to the northwest, which will
be more favorable for Rim-to-Valley storm motions and add a little
more shear, which will add a little organization to the storms.
Storm threats heading into Sunday and Monday will include strong-
severe wind gusts, dense blowing dust, and localized flash
flooding.

HEAT: An Excessive Heat Warning is in effect today and tomorrow
for South-Central and Southwest AZ, and through Sunday for
Southeast AZ. With the 595-596dm H5 heights near the 90th
percentile of climatology and warm lower tropospheric
temperatures, afternoon highs today and tomorrow will reach the
111-116F range, resulting in widespread Major HeatRisk. It
wouldn`t be impossible for outflow moisture intrusions or
extensive residual clouds to disrupt the extreme heat, however no
tangible evidence in model output suggests this outcome. Thus,
confidence is excellent that temperatures 4F-8F above the daily
normal will be common the next few days, through still a few
degrees short of record territory.

Forecast confidence deteriorates quite rapidly for the remainder
of next week as uncertainties in both the mesoscale and larger
synoptic patterns grow substantially. Daily convective trends will
almost certainly be modulated by previous days activity and
ability for atmospheric recovery after the influence of convective
complexes. However, there is convincing evidence that 10-12 g/kg
mixing ratios will be maintained through at least the middle of
the week resulting in a continuation of MLCape above 1000 J/kg and
convective potential after peak heating. Adding to the
uncertainty towards the middle of the week is a small handful of
ensemble members indicating the Southwest subtropical high re-
strengthening (most aggressive members near 600dm) and shifting
back into northern Arizona resulting in warming aloft and larger
scale subsidence. However, while the majority of ensembles also
shift the high pressure center back southeast, its magnitude is
far weaker in response to deep troughing approaching/entering the
NW Conus. Given this model spread, mandated NBM forecasts are only
advertising temperatures in a slightly above normal range with
POPs essentially set near climatology, though the range of
possible outcomes is far wider than the official deterministic NBM
forecast.

&&

.AVIATION...Updated at 0536Z.

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

Northwesterly surface winds (remnant outflow from now-dissipated
storm activity that was over far NW portions of Greater Phoenix
area) will slowly weaken with light and variable conditions
becoming common between 08Z-10Z. One caveat is storm activity near
Gila Bend which may send modest southwesterly outflow winds
between about 08Z-10Z. Southwesterly surface winds develop during
the afternoon Saturday. As for sky cover, a batch of weak showers
is drifting off of higher terrain near and upstream of the
Superstition Mountains. Thus could see a stray shower over the
Valley floor (most likely at KIWA) between about 07Z-09Z.
Otherwise, anticipate broken layers AOA FL200 with SCT layers
roughly between FL100-FL200. Storm potential for Saturday evening
looks to be a bit better for Greater Phoenix than this evening
(Friday evening) but too early to pin down any details on outflows
and potential for TSRA.


Southeast California/Southwest Arizona including KIPL and KBLH:

Typical diurnal wind patterns are expected at the TAF sites with
Wly/SWly transitioning to Sly/SEly (first at KBLH then KIPL). As
for sky cover, anticipate increasing cirrus (more noticeably at
KIPL) but overall only minor cloudiness.


&&

.FIRE WEATHER...
Scattered afternoon thunderstorms will be common over higher terrain
areas of eastern districts through at least the middle of next week
with storms becoming more likely in lower elevations and western
districts early next week. The main threats with these storms will
be strong, gusty outflow winds which could exacerbate issues with
any ongoing wildfires or new starts. Winds outside the influence of
thunderstorms will generally exhibit typical afternoon upslope
tendencies with gusts commonly 20-25 mph. Minimum afternoon humidity
levels will mostly fall in a 15-30% range through the weekend, albeit
improving closer to a 20-35% range next week. Similarly, overnight
recovery should also improve from a 20-45% range to a 30-70% range.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Excessive Heat Warning until 8 PM MST Sunday for AZZ530.

     Excessive Heat Warning until 8 PM MST Saturday for AZZ531>546-
     548>555-559.

CA...Excessive Heat Warning until 8 PM PDT Sunday for CAZ560>570.

&&

$$

DISCUSSION...Benedict/18
AVIATION...Berislavich
FIRE WEATHER...18