Flash Flood in Arizona, January 15, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
230 PM MST Tue Jan 15 2019

.SYNOPSIS...
Scattered to widespread showers will continue to expand east into
the lower deserts of Arizona today as an upper level disturbance
moves through the region. Rain chances will gradually taper off this
evening and overnight across the lower deserts. A brief period of
dry weather and decreasing clouds is expected on Wednesday before
another disturbance pushes through the region Thursday night into
Friday bringing another slight chance of showers. Clearing skies
and high pressure will follow behind this system for the weekend
as high temperatures generally remain within a few degrees of
normal.

&&

.DISCUSSION...
Widespread showers continue to stream northeast over La Paz,
Yuma, and Maricopa Counties ahead of a secondary shortwave that is
pushing through central Arizona right now. Deep moisture with
precipitable water values near and exceeding one inch interacting
with ample dynamic lifting due to PVA aloft and low-level
isentropic ascent is continuing to sustain these showers. Hourly
rainfall amounts are generally remaining below a quarter inch with
a few spots observing up to a half inch. Several locations along
the La Paz County border with Maricopa and Yavapai Counties have
received 1.5 to 2.0 inches of rain over the past 24 hours, but the
lower rain rates have only allowed for minor flooding issues so
far handled by advisories. The orientation of the best SSW-to-NNE-
oriented isentropic ascent currently over western and central
Arizona will gradually slide east during the afternoon and early
evening hours.

Additional average rainfall amounts of around a quarter inch are
expected for the Phoenix metro with higher amounts of around a
half inch for northern Maricopa County and lower amounts closer to
a tenth of an inch for northern Pinal and southern Gila Counties.
The convective nature of these showers will support isolated
amounts up to an inch, with the best chances for those in northern
Maricopa County where the isentropic ascent will be best aligned
with terrain. Model soundings continue to show some marginal
instability capable of generating very isolated thunderstorm
activity, so we have retained the mention of that in the grids
primarily for western Maricopa County later this afternoon and
evening. All in all, a healthy soaking for the region today with
only minor advisory-level flood impacts expected.

Activity should start to dwindle down after 8 PM, with skies
gradually clearing from west to east. Areas that got a good
soaking today and that will clear up a good amount tonight,
primarily northern and western Maricopa County westward, will have
a chance to see patchy fog in the early morning hours.

A brief period of drier weather and decreasing cloudiness is
expected tomorrow. High temperatures will be a bit warmer than
observed today, thanks to more sunshine, with highs rising into
the mid to upper 60s. Another disturbance will move in from the
northwest Thursday night into Friday and bring another slight
chance of showers to the region. As that system digs to the east
and southeast of the region expect some breezy conditions to
develop as cooler air filters in from the northeast. This pattern
will bring some cooler nights to the valley over the weekend, with
higher elevations to our east in southern Gila county forecast to
get low temperatures near freezing and sub freezing during the
weekend and early next week.

&&

.AVIATION...

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

Light to moderate rain is expected to push into the metro early this
afternoon. As this happens, CIGs will likely drop to around 3-4 kft
AGL with FEW to SCT as low as 2 kft. Model soundings show some
modest instability developing over Phoenix this afternoon, which may
support few isolated thunderstorms, primarily between 22Z-3Z, but
confidence is low. Winds will persist around 7-12 kts out of the S-
SE before switching westerly with the arrival of the rain, upon
which may also see a few isolated gusts up to 15 kts. Shower
coverage should decrease across the area by 03Z before moving out
later tonight. CIGs drop a little further to around 2-3 kft AGL,
with SCT ceilings down to around 1 kft, and winds shift back
easterly at 5 kts or less. Conditions improve after sunrise
Wednesday, with scattered cumulus bases around 3 kft by midday.

Southeast California/Southwest Arizona including KIPL and KBLH:

Radar shows the heaviest rain is now moving out of SE CA. Some light
rain or drizzle may still persist for the next few hours with CIGs
around 4-5 kft, falling briefly into MVFR and IFR conditions with
any additional rain. Sky conditions should improve by late this
afternoon with some dry air advection from the SW. Leaving only FEW
to SCT clouds around 6-8 kft. Winds will be light and variable
through tonight. After the rainfall from today, conditions Wednesday
morning may support the development of some patchy fog with enough
radiational cooling and the light winds, especially in Imperial
Valley. Light winds persist through the day Wednesday with scattered
high clouds and possibly some cumulus over ridgetops.

Aviation Discussion not updated for amended TAFs.

&&

.FIRE WEATHER...
Thursday through Monday:
There are no major fire weather concerns for this time, although
the area will undergo a slight drying trend through the upcoming
weekend lasting into next week. However, minimum RH values will
remain above critical thresholds. Somewhat breezy conditions are
expected during the weekend, especially for in the higher terrain
and ridgetops.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

DISCUSSION...Hernandez/Hopper
AVIATION...Benedict
FIRE WEATHER...Deems