FOUS30 KWBC 080052

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Apr 07 2020

Day 1
Valid 01Z Wed Apr 08 2020 - 12Z Wed Apr 08 2020


...Southern California / Southern Nevada...
At 2230 UTC, we removed the Marginal Risk area over NV, and
trimmed a portion of the Marginal over southeast CA, based on the
latest observational trends and most recent high-res CAM guidance.
The cold upper low will support another round of organized
rainfall, especially as a shortwave trough swings northward into
southern California this afternoon and evening. While stout
onshore flow at 700mb, and increasingly stout at 850mb may lead to
some terrain-enhanced precipitation, the fact that this is a
stacked closed low separated from the sub-tropical jet will likely
lead to greater influence from mesoscale deep layer ascent and
convective enhancement rather than being tied to the terrain. The
models that best fit this description are the 12Z WRF-ARW and
WRF-ARW2. More generally, the bulk of deep layer ascent and QPF
from the more recent hi-res and global model runs is placed a
little west of the inherited Slight Risk area. For this reason we
chose to expand Slight Risk west and southwestward, catching more
of the population centers in Southern California where the
mentioned shortwave arrives during peak heating this afternoon and
evening. Expect locally heavier downpours to radiate out to the
east and northeast along outflows, and following the shortwave...
as well as to the northwest, up the coast, following the occluding
portion of the wave.

While there is perhaps increasing concern for hourly rain rates of
a half inch or more in or near coastal Southern California, we did
maintain the Slight Risk area over the deserts, where
convectively-enhanced precipitation producing a quarter inch or
more per hour could train during the overnight hours, leading to
what would be a relatively rare event for the deserts.

...Previous California Discussion...

The low level flow ahead of the closed mid level low behind the
surface cold front continues to feed 0.75 inch precipitable water
air (per the most recent blended TPW precipitable water product)
into Southern CA that supports low topped showers early this
morning. As the closed low drop south, the low level flow starts
to back, and maintains a 15/25 knot inflow. Steepening lapse rates
should allow pockets of 250 J/KG of MUCAPE to form in the moisture
plume, as it becomes focused across portions of Southeast CA later
this morning into this afternoon.

A general increase in showers and low topped storms (some possibly
with small hail) is expected across much of southern CA, as
synoptic scale ascent ahead of the next jet streak approaches
Southern CA, mainly after 07/21z. As the 850/300 mb mean flow
becomes better aligned with propagation vectors (particularly
after 08/00z), the threat for training increases across Riverside
across central San Bernardino into southern Inyo counties. Along
this axis, local 1.50/2.00 inch rainfall amounts are possible,
especially where training occurs.

The threat is expected to continue past 08/06z as the low level
flow remains persistent. Instability in the plume could ultimately
become consumed by the convection and overturning, but until that
occurs, hourly rainfall rates could top out near 0.50 inches
(though the 07/00z HREF probabilities do not necessarily support

Finally, the Marginal Risk was extended from Los Angeles county up
through San Luis Obispo county. While convection here may be more
scattered in nature (with less moisture but slightly better
instability due to the proximity to the mid level low and
steepening lapse rates), local 0.50 inch rainfall amounts here
could cause issues in places that received 3.00/6.00 inches Sunday
night into Monday.

...Ohio Valley...
Mid level lapse rates were especially impressive at greater than 8
deg. C per km across much of the Midwest toward the mid Atlantic.
Low level moisture is seasonably tame, but sufficient, with
dewpoints in the 50s to lower 60s, to support fairly large MUCAPE
numbers later today across the Ohio Valley / Lower Lakes. A very
sharply defined shortwave and associated forcing will plow through
this air mass, and all indications are of a progressive round of
supercellular or quasi-linear convection affecting a broad area
this evening and overnight. If there is any focus where greater
cell mergers could occur, it may be near the surface warm front in
western PA. Given the stout lapse rates and MUCAPE, there is some
concern that instantaneous rain rates may be impressive, and any
cell mergers could lead to flash flooding. Still, the hi-res QPF
signal is pretty variable over the region, and very indicative of
a progressive event that is unlikely to lead to higher-end event
totals. We will maintain Marginal Risk and expand the risk
northward to the warm front position...while watching trends for
any need to upgrade this afternoon.


Day 2
Valid 12Z Wed Apr 08 2020 - 12Z Thu Apr 09 2020


...Southeast California into far western Arizona and southern
...20Z Update...
Minor changes made to the overall footprint of QPF and thus only
minimal adjustments to the ERO risk areas.  The main concern
remains the potential for training shower/thunderstorms along the
eastern flank of the mid-level closed low were better synoptic
scale lift, moisture and instability resides. The best overlap of
the moisture and instability will occur Wednesday across portions
of south-eastern CA as the closed mid-level low moves inland
helping to steepen lapse rates thus increasing instability, while
simultaneously reducing the column moisture.  Hourly rain rates
may approach 0.50 inches/hour briefly, but with the aforementioned
moisture flux decreasing over time, expect rain rates to average
around 0.25 inches/hour. Given some locations will receive
multiple hours of moderate (and at times heavy) rain, felt the
Slight Risk was appropriate, though it has been refined to reflect
the latest QPF.  The Marginal Risk area still encapsulates the
region where localized flash flooding could occur. 

...Previous Discussion...
The strong closed low moving southeast off the Southern California
coast day 1 will begin to push more towards the  east into the
Southwest during day 2.  Timing differences from previous runs
have resolved with the previously more progressive GFS clustering
well now with the previously preferred slightly slower guidance. 
The anomalous pw axis on the eastern and northeastern side of the
strong closed low will continue for much of the upcoming day 2
period with a broad region of values 1.5 to 3 standard deviations
above the mean stretching from southeastern California into
western Arizona and southern Nevada.  Training of cells in a south
to northerly direction possible early day 2 on the east side of
the closed low over Southeast California into western Arizona. The
slight risk area was drawn for these areas and northward into the
upslope of the southern Sierra where southeast low level flow will
enhance totals.  Hourly rainfall totals of .25-.50"+ possible
early day 2 in areas of training.   Elsewhere..the marginal risk
area was maintained over the remainder of Southern California for
additional locally heavy rainfall amounts from slow moving cells
in the comma head deformation band wrapping cyclonically around
this strong upper low.


Day 3
Valid 12Z Thu Apr 09 2020 - 12Z Fri Apr 10 2020


...Southern California...
...20Z Update...
Increased the overall QPF footprint and amounts across portions of
southern CA based on latest model trends.  As the closed low over
southern CA/NV churns through Day 2, the low will start to slowly
shift south with energy rounding the surface feature where
moisture will move through southern CA from northwest to
southeast. Models are split in how this system will evolve on Day
3 with the non-NCEP camp being quick to shift the QPF southward
with enhancements along the terrain while the NCEP models are
showing a slower progression, thus keeping QPF farther north.
After some review of the model continuity and consistency, sided
with the EC/ECENSmean. Given this region has received an abundance
of precipitation recently, felt the Marginal Risk area inherited
was warranted with only some refinements made.  While the
Transverse Mountains will likely observe the highest precipitation
totals due to orographic effects (as evident by the 850mb flow),
it will be in the form of snow in the higher elevations above 4500
ft; below this level is where rain could result in localized flash
flooding concerns. 

...Previous Discussion...
The strong closed low moving into the Southwest day 2 may become
more separated from the northern stream flow passing well to its
northeast across southeast Canada and the northeast U.S. during
the day 3 period.  This may allow for slowing of the eastward push
of the strong closed low with timing differences reappearing
during day 3 with the 1200 UTC Monday and 0000 UTC Tue ECMWF and
UKMET on the slower side with the NAM and GFS more progressive but
still showing the slowing trend.  The EC mean from 0000 UTC has
slowed considerably from its 1200 UTC Monday run and is now on the
slow end with the operational EC.  The more western/slower
solutions of the strong closed low show heavier precip wrapping
cyclonically into Southern California day 3.  While the slightly
more progressive GFS and NAM are not as heavy as the ECMWF and
UKMET, they do both show anomalous PW values wrapping cyclonically
back southward into Southern California.  PW anomalies 2+ standard
deviations above the mean in the GFS and 1.5 to 2 standard
deviations above the mean NAM push southward into Southern
California day 3.  For this reason...can not rule out additional
heavy amounts forming in this anomalous pw axis...with a marginal
risk area maintained for isolated runoff issues, especially over
recent burn scar areas.


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt