Flash Flood in California, December 26, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
935 AM MST Thu Dec 26 2019

.SYNOPSIS...
A significant weather system will move into the region today
providing heavy snow across mountain areas in southern California
and moderate rainfall in the southeast California and southwest
Arizona lower deserts. Light to moderate rain will spread across
southern and central Arizona this evening into tonight with
lingering showers and possibly a few thunderstorms Friday. Light to
moderate accumulating snows are expected across the higher terrain
north and east of Phoenix tonight through early Saturday morning.
Drier conditions are anticipated for the rest of this weekend, but
temperatures will remain below normal with some lower desert
locations seeing overnight lows near freezing Sunday morning.

&&

.DISCUSSION...
A classic, text book mature cyclone was churning off the southern
California coast this morning as robust ascent fields spread into
the western CWA. A deep PV anomaly just west of Los Angeles was
still surging southward as the system reaches its maximum intensity
later today. 12Z KVBG sounding data sampled midtropospheric heights
at daily record low levels (records started in 1958 for this site)
along with tropopause heights crashing to near 500mb. Given the
center of the cyclone was west of the observing site, these measures
and anomalies are almost certainly more intense than sampled giving
credence to the magnitude of the upcoming weather event.

With the baroclinic leaf and strong isentropic upglide now shifted
into the SE California deserts, a pronounced swath of light-moderate
rain has quickly materialized over the far western parts of the
forecast area. Rainfall rates locally as high as 0.50 inch/hour will
likely translate inland though due to shallow cloud tops, radar
sampling may be overshooting much of the actual precipitation and
estimates uncertain. Regardless, training bands of moderate rainfall
will result in a flood threat for the more susceptible locations of
SE CA the remainder of the day. Extremely heavy snowfall remains a
substantial concern over Joshua Tree NP as Lost Horse observations
(elev 4100 ft) show 33 degrees and lack of precipitation measurement
in the past 2 hours suggesting the change over to snow has occurred
already. Observations from San Diego mountains earlier today
measured 16 inches of snow at 4200 ft, so similar amounts (or more)
seem quite likely. Still appears snow levels may fall as low as
3500ft once deformation banding becomes more organized this evening,
and Winter Storm Warning continues.

Elsewhere, a small finger of dense fog through Pinal County was only
slowly evaporating, though have let the Dense Fog Advisory expire as
conditions should rapidly improve. With the somewhat slower eastward
progression of precipitation echoes through the day, have trimmed
higher POPs westward a bit through the daylight hours, and relegated
most rainfall chances in the Phoenix metro until very late afternoon
at the earliest.

&&

.PREVIOUS DISCUSSION...
/538 AM MST Thu Dec 26 2019/
The strong dynamics associated with the approaching low are very
evident as shown by the moderate to heavy rain band in California
with rainfall rates upwards of 0.50" per hour. As the low shifts
southeastward later today, rain and snow is forecast to push through
the southern California interior mountains into southeast
California. This precip band should bring widespread rainfall of
0.50-1.00" across the southeast California deserts later this
morning into the afternoon before shifting into southwest Arizona
later in the afternoon. Highest QPF amounts should fall across the
eastern half of Riverside County which will be perfectly aligned
just north of the low center. This rainfall should impact more
vulnerable areas providing localized flash flooding and some road
closures.

Higher elevations across southern California will see heavy snowfall
today which includes much of Joshua Tree National Park (JTNP).
Models show JTNP will fall within the moderate to heavy precip band
today beginning sometime later this morning, likely extending
through the evening hours. The combination of snow levels of 3500-
4000 feet and forecast QPF amounts of around 1.5" across JTNP should
result in total snowfall accumulations of 12-24 inches above 4000
feet and as much as 12 inches as low as 3500 feet. The strong
forcing and heavy nature of the precip should allow snow down to
around 3000 feet, but surface temperatures at those elevations
should stay several degrees above freezing likely preventing
significant accumulating snowfall.

Eventually the rain band is shown progressing eastward into
south-central Arizona beginning this evening, but the low will be
in a weakening phase and rainfall rates and QPF amounts will lower
than areas farther west. The bulk of the rain in this area should
fall during the evening and early overnight hours. Higher elevation
snows are also expected across central and eastern Arizona with snow
levels falling to around 5000 feet tonight. Scattered showers and
possibly a few thunderstorms (small hail) are expected during the
daytime hours Friday as the low center traverses across Arizona.
GEFS plumes and the European ensemble mean QPF show an average of
0.3-0.5" across south-central and east-central Arizona with
localized higher amounts. Snow accumulations of 4-6 inches are
likely above 5000 feet across far northeast Maricopa Co. and Gila
Co. with an inch or two of accumulation down to around 4500 feet. A
Winter Weather Advisory has been posted for elevations above 5000
feet for tonight through early Saturday.

The low is shown exiting the region Friday night into early Saturday
with some partial clearing skies likely across the lower deserts.
Due to the widespread rainfall, likely clearing areas Friday night,
and relatively light winds fog formation is likely across portions
of the lower deserts Saturday morning. There should also be
potential for fog Saturday night into Sunday morning, but drier
conditions may limit coverage to only the most fog prone areas.

Temperatures will remain quite cool into at least the weekend due
the unsettled weather and cold air mass over the region. Daily highs
mostly in the 50s over the deserts are expected into at least early
next week as another Pacific trough looks likely in or around the
region next Monday into Tuesday. Overnight lows will drop into the
30s for the bulk of the lower deserts this weekend, possibly through
early next week. The coldest rural deserts will likely see some near
freezing temperatures, but widespread freezing conditions are not
expected. Rain and snow chances with the early next week weather
system are still quite uncertain due to the fairly high model spread
and the general model consensus that it may mostly track to our west
and then to our south.

&&

.AVIATION...Updated at 1230Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Fog is already starting to develop in parts of Pinal county this
morning and Casa Grande airport is currently reporting reduced
visibilities due to fog. Patchy fog development this morning
should generally remain over Pinal county and the far west-
southwest portions of the valley. Due to this, fog is currently
not included in any of the TAF sites at this time, but will be
amended as conditions dictate. Otherwise, VFR conditions, with
ceilings aoa 5 kft, are expected through this afternoon. Winds
will gradually veer E-SE to S-SE with speeds around 5-10 kts.

IFR-MVFR conditions will be possible in the early overnight hours
(2Z-7Z) as another storm system moves in from the west. Lower
ceilings, below 5 kft, are expected to redevelop mainly after 00Z
and linger through the end of the TAF period. In addition, there
is the potential for an isolated thunderstorm or two overnight
and tomorrow but confidence on coverage and timing still remains
too low to mention in TAF. Reduced visibilities and erratic winds
will be possible with any shower/tstorm that develops.

Southeast California/Southwest Arizona including KIPL and KBLH:
VFR conditions with light easterly to southerly winds are expected
before conditions rapidly deteriorate after sunrise. Ceilings
should lower below 4 kft with brief periods of IFR-MVFR ceilings
associated with shower activity tied to an approaching upper level
disturbance from the west. The best chance for shower activity
will be between 16-21Z at KIPL and 22-03Z at KBLH, but showers
will be possible as early as three hours before that at both sites
with the potential to continue into the evening hours,
particularly at KBLH. Winds will veer from southerly to westerly
this afternoon with lighter 5-10 knot winds at KIPL relative to
KBLH where 20-25 knot gusts will be possible during the afternoon
and early evening hours. Very isolated thunderstorms are possible
late in the afternoon and evening hours as well, with the best
chances over southwest Arizona in Yuma County.

&&

.FIRE WEATHER...
Saturday through Wednesday:
Drier conditions and low end precipitation chances expected this
weekend. Another weather system will impact the region early next
week and bring another increase in rain chances. Minimum RH
values will generally stay in the 30-40% range, with slightly
higher values Monday and Tuesday. Winds should mostly be on the
light side through the bulk of the period.

&&

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

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Winter Weather Advisory from 11 PM this evening to 11 AM MST
     Saturday for AZZ557-558-563.

     Flash Flood Watch through Friday morning for AZZ530>533-535-536.

     Flood Watch until 4 PM MST this afternoon for AZZ556.

CA...Flash Flood Watch through Friday morning for CAZ560>570.

     Winter Storm Warning until 7 AM PST Friday for CAZ560.

&&

DISCUSSION...MO/Kuhlman
AVIATION...Hernandez
FIRE WEATHER...Hernandez