Flash Flooding in California, January 20, 2021

Area Forecast Discussion...Updated
National Weather Service Phoenix AZ
500 AM MST Wed Jan 20 2021

.UPDATE...Updated aviation discussion.

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.SYNOPSIS...
A low pressure system will bring somewhat cooler weather and
periodic showers to the region through Thursday. Even a few
isolated thunderstorms and moderate to locally heavy rainfall are
possible today across southeast California and southwest Arizona.
The best chances for showers will eventually shift into central
Arizona by early Thursday. After a brief break in the activity
Friday, a second weather system will dive into the western United
States over the weekend resulting in additional chances for rain,
high elevation snowfall, and even cooler temperatures.

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.DISCUSSION...
The upper level low is currently situated about 250 miles west of
the Baja coast and has shifted around 175 miles directly south of
where it was located yesterday afternoon. Deep south
southeasterly flow across southern California and Arizona has
brought some moisture northward, but based on water vapor imagery
the connection with deeper sub-tropical moisture remains well to
our south and east. PWATs have risen to around 0.5" in the Phoenix
area to as high as 0.8" in Yuma and further moisture advection
through Wednesday night should boost PWATs closer to 0.75-0.9" in
many places. The strongest area of forced ascent has shifted
southward over the past 12 hours as the system slid to the south,
but there remains some modest vorticity forced ascent across
southeast California stretching southeastward through the Yuma
area. Additional jet forced ascent is also seen across south-
central and southeast Arizona which is allowing for some light
high based shower activity early this morning.

The best area for rainfall this morning and likely through much of
today will remain across southeast California and southwest
Arizona closer to the low center and where forcing and moisture
levels will be greater. Scattered showers have been fairly
persistent moving through this area since early last evening, but
much of it has been light with rainfall amounts mostly less than a
tenth of an inch. We do expect this shower activity to continue
through much of today, though most of it should be fairly light
through the rest of the morning. By this afternoon, the low will
have already begun shifting to the east and northeast again
putting southeast California and especially southwest Arizona
under a more a favorable jet forced ascent region. This along with
some model forecast increase in instability should allow for some
heavier showers and isolated thunderstorms over the
aforementioned area. The Flash Flood Watch for much of Yuma,
Imperial and portions of Riverside Counties still looks warranted
given the potential for these heavier showers and thunderstorms
and due to some potential for training echoes.

Starting this evening but especially during the overnight hours
Wednesday night, models are still showing a plume of moisture and
widespread shower activity developing and moving northward out of
Mexico. Through Thursday morning, the bulk of the model guidance
shows most of this shower activity affecting areas just to the
south and east of Phoenix, but it still seems likely at least
some scattered shower activity will affect the Phoenix metro.
HREF guidance along with SPC agree there will likely be a threat
for isolated thunderstorms and thunderstorms embedded within the
main area of rain, but for now it seems areas to the south and
east of Phoenix will be more favored for this stronger convection.
QPF amounts have been trending lower overall for all areas except
for where the Flash Flood Watch is in effect. Rainfall amounts
upwards of an inch or so seem possible within the watch area,
while the rest of the Arizona deserts will mainly see less than
0.25". Higher amounts upwards of 0.5" with locally higher amounts
are still possible over the high terrain east of Phoenix and
across southern Pinal County. Aside from some lingering shower
activity from Phoenix and areas to the east, the bulk of the rain
should be ending fairly quickly by late Thursday afternoon or
early evening.

Models are finally backing off considerably for precip chances on
Friday as we come under a brief period of upper level ridging and
subsidence in between weather systems. There have been some minor
changes with the second weather system as it seems to have
shifted a bit farther to the east with the main low center
possibly tracking over land across California late Friday into
early Saturday. This subtle eastward shift is likely to result in
less moisture being available and this is definitely reflected in
latest EPS and GEFS QPF amounts. Nevertheless, this next system
will be quite deep, fairly cold, and should bring plenty of
forcing for at least scattered shower activity across the lower
deserts and fairly persistent shower activity across the Arizona
high terrain. The best timeframe for rain and high elevation snows
this weekend currently looks to be late Saturday through the
first half of Sunday, which coincides with the main PV anomaly
tracking from west to east across our region. Expected QPF amounts
are still fairly uncertain, but the latest numbers indicate very
little across the wester deserts, to around 0.25" across the
south-central Arizona deserts, and higher amounts over the high
terrain.

If by chance some areas are unfortunate enough to get very little
precip from the first two weather systems, there is likely to be a
third weather system early next week. Specifics with this system
are even more uncertain, but it does look to be very similar to
the weekend trough. This persistent deep troughing over the
Western U.S. will definitely pull colder air southward through our
region with temperature guidance dropping lower desert highs into
the lower 60s by this weekend and likely into the mid to upper 50s
for early next week.

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.AVIATION...Updated at 1200Z.

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

An upper low southwest of Arizona will continue to spread
considerable moisture into the greater Phoenix area today into
tonight, leading to lowered ceilings and a persistent threat of
showers. Many hi-res models support isolated mainly light showers
this morning through around noon, with CIGS in the 6-8k foot range.
As such, VCSH will be mentioned in the TAFs thru roughly 19z. Expect
precip to take a break into the evening hours, and as the upper low
starts to move northeast and into Arizona, showers to become more
widespread after about 04z this evening; look for CIGS to lower into
the 4-5k range as showers increase. There may be some visibility
restriction in moderate showers but confidence is a bit low so kept
vis above 6sm for now. Decided to put PROB30 groups into the TAFs
after 04z and upgrade to predominant -SHRA after 09z.

Winds will be generally northeast to southeast during the 24 hour
TAF period, with sustained speeds occasionally up to 15kt but
overall expect winds 7-12kt through the period. Of course, with
showers in the area there is always the potential for some brief
variability in wind speed and direction.

Southeast California/Southwest Arizona including KIPL and KBLH:

An upper low sitting southwest of Yuma will allow areas of rain to
rotate around the low and into the southwest deserts with the most
significant precip occurring from the Yuma area west into the
Imperial Valley. Look for widespread showers into the late morning
at KIPL with somewhat less coverage at KBLH. CIGS down to around 4k
feet during this time with potential for some visibility restriction
down to around 5sm (although confidence in this is lower so not
mentioned in the TAFs). After 18z guidance suggests coverage of the
showers will diminish so will opt to go with VCSH in the TAFS for
the rest of the TAF period. There will be a continued threat -
especially at KIPL - through most of tonight and CIGS could
occasionally drop into the 3-4k range but feel in general the CIGS
will be 5-8k feet with BKN-OVC decks in place much of the time.
Although showers will lead to some variability in wind speed and
direction, for the most part winds should tend to fall in the north
through east quadrant with speeds 12kt or less.

Given the proximity to the upper low there is an outside chance that
a weak thunderstorm could develop and be embedded within the larger
area of showers; a stray lightning strike would be possible. Do not
feel that storms will be overly significant but could briefly result
in gusty variable wind and brief heavy rain. Due to low confidence
we have not mentioned thunder in the TAFs but chances of a storm are
certainly not zero.

&&

.FIRE WEATHER...
Friday through Tuesday:
A rather unsettled weather period will persist through early next
week with a weather system affecting the region over the weekend
and likely another early next week. Showers will become possible
mainly over high terrain areas late Friday with periods of
scattered showers over the deserts and more widespread rain and
snow over the high terrain on Saturday and Sunday. Breezy
conditions are likely to affect much of the region at times,
especially on Friday and again on Monday. The second weather is
likely to push through the region early next week likely bringing
more rounds of modest precip. The cooler conditions and increased
moisture during the period will lead to continued favorable
humidity readings as minimum daily are likely to stay above 40%
much of the period.

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

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.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Flash Flood Watch through late tonight for AZZ532-535-536.

CA...Flash Flood Watch through this evening for CAZ562>568.

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DISCUSSION...Kuhlman
AVIATION...CB
FIRE WEATHER...Kuhlman