Winter Storm in California, December 26 to 27, 2019

Area Forecast Discussion
National Weather Service Phoenix AZ
415 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...
There is currently a break in the rain and snow across southeast
California and Arizona, but this will change later today as
another Pacific storm system enters the region. Current water
vapor imagery and upper air analysis show an impressive looking
very wound up low pressure system with a well developed baroclinic
leaf along the southern California coast into central California.
The low center is located just off the California coast between
LA and San Francisco. The 00Z Vanderberg AFB raob sampled a 500mb
height of 5370m which is 10m deeper than the previous record for
the date and possibly in the top 10 since records began for the
location. The low should maintain its strength through the rest of
the morning before it begins its transition to a weakening phase
as the low center tracks across southern California later today
and then through Arizona on Friday.

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 this morning, 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 0559Z.
South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
Aside from patchy fog that should generally remain over Pinal
County and the West Valley, VFR conditions are generally expected
through Thursday morning. Patchy fog is trying to develop closer
to KCGZ where saturated conditions already exist and calm winds
are expected overnight. Fog potential is more uncertain at the
Phoenix terminals where mixing has been stronger with persistent
easterly 5 knot winds and saturation is a bit less likely except
for portions of the West Valley near KGEU and the southeast Valley
near KIWA. In addition, high clouds moving in from the southwest
overnight along with a slight potential for FEW-SCT ceilings near
5 kft make radiation fog development less likely. Therefore, fog
is currently not included in any TAFs at this time due to low
confidence, but will be amended as conditions dictate. Any fog
that develops should lift quickly and set up a day with gradually
veering E-SE to S-SE 5-10 knot winds and SCT-BKN ceilings around 6
kft. Ceilings will lower below 4 kft after 02Z Friday as showers
associated with an upper level disturbance from the west affect
the terminals, with some potential for IFR-MVFR conditions into
the early overnight hours.

Southeast California/Southwest Arizona including KIPL and KBLH:
VFR conditions with light easterly to southerly winds are expected
overnight 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 18-21Z at KIPL and 21-03Z 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
tomorrow 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...
Friday through Tuesday:
Rain chances will diminish from west to east on Friday with drier
conditions and low end precipitation chances expected this weekend.
Another weather system has the potential to impact the area early
next week and bring another increase in rain chances. Minimum RH
values will generally remain in the 40-60% range through Friday,
lowering closer to 30-40% this weekend. 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 from 6 AM MST this morning through Friday
     morning for AZZ530>533-535-536.

     Flood Watch through 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...Kuhlman
AVIATION...Hopper
FIRE WEATHER...Hernandez