Flash Flood Warning 205, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
549 AM MST Wed Aug 18 2021

.UPDATE...Updated 12Z Aviation Discussion.

&&

.SYNOPSIS...
Shower and thunderstorm chances will persist today across south-
central Arizona with heavy rainfall the primary threat. However, a
major shift in the weather pattern will bring substantially drier
air into the region starting Thursday. This will result little to no
chance of storms through at least early next week. Temperatures
slightly below average will slowly warm back to near normal levels
over the weekend.

&&

.DISCUSSION...
Early morning WV imagery shows a deep trough and seasonably strong
PV anomaly digging into the Great Basin heralding a large shift in
the flow pattern across the Conus. Subtropical ridging previously
centered over southern NV (which providing the convectively
favorable flow pattern) has already eroded and retrograded into the
central Pacific. Meanwhile, a more innocuous upper low was ejecting
northward from the Baja peninsula and will eventually become
absorbed into the aforementioned Great Basin trough. This evolution
will quickly yield deep westerly flow promoting substantial drying,
subsidence, and dearth of storm activity starting Thursday and
persisting into at least early next week.

The most interesting and active portion of the forecast will occur
over the next 24 hours as several subtle forcing mechanisms act to
encourage thunderstorm development. In the northern stream,
objective analysis shows the leading edge of the 700mb trough axis
and cooling aloft moving into the western CWA. Meanwhile, strong jet
level divergence can be inferred from satellite and sounding data in
relation to the Baja circulation while closer investigation suggests
a small vorticity center entering far SW Arizona. Arcing upper level
cloud bands and in situ midlevel cloud development can easily be
seen in IR imagery indicative of the growing ascent structure.

Moisture and total instability should not be an issue with deep 12-
13 g/kg mixing ratios and MUCape in excess of 1000-1500 J/kg.
Forecast soundings indicate saturation and lift primarily centered
in or above the 700-800 mb layer, so actual energy release may only
encompass a small portion of the total available (typical for
elevated nocturnal convection). The greatest uncertainty reflected
in HREF membership is the intensity and coverage of elevated
showers/storms through the morning hours with a range of outcomes
ranging from concentrated storms forming over the same area to more
pedestrian scattered showers. Regardless, any shower/storms that can
develop may be relatively shallow topped, yet incorporating
efficient warm rain processes. Thus the Flash Flood Watch continues
given the recent rainfall and moist soil conditions promoting
effective and enhanced runoff.

The leading edge of drier air aloft can already be seen trailing the
Baja circulation, and will begin fully advecting into the forecast
area this afternoon. Boundary layer moisture will be slower to erode
with 10-11 g/kg sfc-700mb mixing ratios still common this afternoon.
However, marked midtropospheric warming and loss of better lapse
rates will occur behind this wave limiting instability (not to
mention the likelihood of synoptic scale subsidence). Forecast
soundings show development of deep westerly flow, such that forced
orographic ascent of the residual moist boundary layer may be the
primary ascent means. HREF and other CAM output hint at this
scenario with just isolated, transient cells popping during peak
heating. The increasing flow aloft will result in mean storm motion
much faster than typical for monsoon, and reduction of residence
time of any particular storm core. This should begin to limit the
flood threat more towards the most susceptible locations (i.e. burn
areas).

Very little change in ensemble output from Thursday through the
weekend can be seen from previous model iterations. The rapid drying
trend becomes pronounced Thursday with 500-850mb mixing ratios
falling below 8 g/kg. Diurnal mechanical mixing will steadily erode
boundary layer moisture while anti-cyclonic midtropospheric warming
allows 500 mb temperatures to warm to at least -4C. This combination
will essentially preclude any deep convection within the forecast
area (much less the entire state of Arizona), and NBM POPs have
finally fallen appropriately into the lower single digits. Not
surprisingly, temperatures will warm with this drying trend though
the nearby presence of the Great Basin trough will keep heights and
temperatures in check only returning closer to normal by the first
part of next week (100F-105F at lower elevations around the Phoenix
area, and potentially 105F-110F in the Yuma/El Centro areas).

During the first half of next week, uncertainty still exists in
relation to how subtropical ridging over the southeast and southern
plains retrogrades back into the SW Conus. Fewer NAEFS members hold
deeper troughing through the Great Basin, but a majority also do not
portray a particularly convectively favorable outcome. While the
specifics obviously remain murky, the breadth and elongated
orientation of the ridge axis may displace easterly waves well south
and east of the forecast area while only encouraging thunderstorm
development/maintenance over higher terrain locations. Regardless,
there is still nothing in the ensemble spectrum that suggests a
return to heightened thunderstorm activity throughout the forecast
area for the next 7-10 days.

&&

.AVIATION...Updated at 1248Z

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Scattered showers and isolated storms have developed this morning,
mainly east of the terminals. Activity through the rest of the
morning is expected to continue to favor these areas, but a slight
chance (20-40%) for development in the vicinity of terminals will
continue through 14-15Z. Through the rest of the morning skies may
become more FEW to SCT and winds shift more southerly with speeds
increasing up to 10-12 kts. Vicinity shower and storm chances
(40-50%) return in the afternoon with development beginning around
20-21Z. Winds will remain southerly through the afternoon with
gusts up to 20 kts with the exception that brief stronger gusts up
to 25-30 kts and wind shifts will be possible in the vicinity of
storms. Cloud bases should remain aoa 6-7 kft. Storms should
diminish after 2-3Z and skies will become mostly clear.

Southeast California/Southwest Arizona including KIPL and KBLH:
Breezy conditions will be the primary impact through the TAF
period. At KIPL winds will pick up out of the west to southwest by
20-21Z and peak in the evening with speeds up to 20-25 kts and
gusts up to 30 kts. Similar magnitudes are expected at KBLH, but
beginning a little earlier around 18-19Z and with directions
favoring the south. Winds should gradually subside late this
evening, with gusts falling off by midnight. Winds may generate
localized blowing dust in the Imperial Valley and briefly lower
visibility.

Skies will remain mostly clear, but there is a non-zero chance
(5%) of isolated showers and/or storms developing this afternoon
between 22-02Z. Mainly east of the terminals.

&&

.FIRE WEATHER...
Friday through Tuesday:
Unusually dry conditions will dominate all districts through the
weekend with almost no chance of thunderstorms or wetting rains.
Chances only increase marginally over eastern districts during the
first part of next week. Temperatures should rebound into a near
normal range over the weekend. Minimum afternoon humidity levels
will generally range between 20-40% Friday, then likely fall closer
to a 10-30% range over the weekend and early next week. Overnight
recovery will be good to excellent in a 40-80% range. Winds should
be relatively light with an afternoon upslope component and
overnight drainage common. Some enhanced afternoon and evening gusts
will be possible at times in western districts.

&&

.SPOTTER INFORMATION STATEMENT...
Spotters should follow standard reporting procedures.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through this evening for AZZ537-540>558-
     560>563.

CA...None.
&&

DISCUSSION...18
AVIATION...Benedict
FIRE WEATHER...18

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