Flash Flood Warning 45, NWS Phoenix, AZ

Area Forecast Discussion
National Weather Service Phoenix AZ
238 PM MST Fri Dec 22 2023

.SYNOPSIS...
A strong low pressure system will continue moving through the
Desert Southwest, bringing widespread precipitation and the threat
of localized flooding and strong thunderstorm winds. Lingering
scattered showers are likely to redevelop across south-central
Arizona Saturday. Temperatures will remain near or slightly below
their normal values through the first half of next week.

&&

.DISCUSSION...
Latest water vapor imagery reveals a well defined closed low near
San Diego. Ahead of this low, anomalous moisture transport from
the eastern Pacific is directed towards Arizona which has
translated into widespread precipitation across the Desert
Southwest. Latest gage observations indicate between a third of an
inch to a half of an inch of rain has fallen within this IVT
plume, mainly from Phoenix south and eastward. Further west,
rainfall amounts drop off dramatically however earlier breaks in
the clouds have contributed to modest increases in instability and
steeper lapse rates across southwestern Arizona closer to the
center of the low.

HREF has generally captured the evolution of this system quite
well and indicates the potential for organized convection this
afternoon, particularly across portions of Yuma/La Paz and
possibly Maricopa Counties. Although latest mesoanalysis indicates
CAPE generally remains below 300 J/kg, there is abundant
directional and speed shear which may help to generate isolated
strong storms or short linear segments with gusty winds and small
hail. HREF UHIs also suggest conditions are favorable for an
isolated mesovortex or tornado embedded within a QLCS,
particularly in Yuma County. Areas that see repeated cells with
heavy rain will also be susceptible to localized flash flooding.

Although there is a relative lull in the activity across central
Arizona, consensus from the CAMS and global models remains
consistent depicting widespread redevelopment of showers later
this afternoon and this evening associated with strong jet-forced
ascent ahead of the low. A break in the rain is then expected late
tonight, followed by wrap-around precipitation as the center of
the weakening low moves eastward into central Arizona Saturday
morning. Low-level moisture will linger throughout the day,
resulting in mostly cloudy skies and the continued threat of
showers even into the afternoon.

Latest NBM indicates a 70 percent chance of at least an inch of
rain by Saturday evening across the Phoenix area, with higher
probabilities generally located north and east of the Valley.
Given the ample deep-layer moisture, isolated locations could even
pick up as much as 1.5 to 2 inches. This threat extends as far
west as the Yuma area, where convection and short-duration
rainfall is more favorable.

In the wake of the low, a drying trend is expected with a
northwesterly flow persisting through midweek. Latest NBM also
indicates a warming trend, though temperatures will generally
remain within a few degrees of normal with no chance of rain.

&&

.AVIATION...Updated at 1700Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Low cigs and visibilities in an MVFR range along with periodic rain
and shifting winds will all be weather issues through Saturday
morning yielding limited forecast confidence. Steady rain with
widespread MVFR vsbys have been common this morning, and although
there may be breaks in the rain through the mid afternoon and
evening, models suggest large fluctuation between VFR and MVFR
as showers progress through the area. While confidence is low, there
are chances for IFR cigs with any heavier showers, but
probabilities are generally less than 20% (most likely at
KSDL/KDVT).

A more distinct break in restricted flight conditions should occur
later this afternoon and evening, however thunderstorms likely across
SW Arizona should progress towards western Maricopa County.
Confidence is moderate that actual TSRA will dissipate before
reaching the KPHX airspace, however a renewed batch of SHRA with a
westerly wind shift could move into the area mid evening. Another
period of unrestricted flight conditions should occur overnight,
however another round of lower cigs, and possibly MVFR conditions
may arrive again around sunrise Saturday.

Southeast California/Southwest Arizona including KIPL and KBLH:
Periodically lower cigs and visibilities in an MVFR range along with
scattered showers and thunderstorms and erratic wind directions will
all be weather impacts through Saturday morning. Confidence is
moderate that VFR conditions this morning will deteriorate this
afternoon with building SHRA/TSRA. There is at least a 40% chance of
occasional MVFR conditions through early evening with the best
chance of TSRA and highly erratic winds at KBLH. Cigs should
gradually improve this evening as showers exit east, however KBLH
may remain under a deformation band and persistent rain and cigs
just above the MVFR level through midnight.

&&

.FIRE WEATHER...
An unsettled and moist weather pattern will exist over the region
through early next week as temperatures abruptly retreat to slightly
below normal levels. Deep moisture will surge into the region Friday
with widespread wetting rains throughout the day totaling at least
0.50" for almost all locations. Additional showers are likely
Saturday across eastern districts, and upwards of 1-2" of storm
total rainfall is possible over higher terrain areas. Nearly
saturated conditions will be common over the weekend with humidity
levels generally not falling below 60%. Drier weather returns next
week, however humidity levels should remain above a 25-35% range.
Winds should largely remain below 15 mph through the period,
however, showers and potentially an isolated thunderstorm could
create brief periods of gusty, erratic winds.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flood Watch through late tonight for AZZ530-531-533>563.

     Flood Watch through late tonight for AZZ532.

CA...Flood Watch through late tonight for CAZ569.

     Flood Watch through this evening for CAZ560>568-570.

&&

DISCUSSION...Hirsch
AVIATION...18
FIRE WEATHER...Whittock/18

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