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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0821Z Apr 07, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Tue Apr 07 2020

Day 1
Valid 12Z Tue Apr 07 2020 - 12Z Wed Apr 08 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHERN CALIFORNIA...

...Southern California/Far southern Nevada...
A strong and persistent closed mid level low drops south just off
the Central and Southern CA coast today. Ahead of the mid level
system, a deep moisture plume with pockets of instability could
support convection capable of producing excessive rainfall across
portions of southern and eastern CA, especially in upslope areas.
Overall, there was a general trend for a slower solution with the
mid level system, which limits the eastern extent of the deeper
moisture and instability. With this in mind, the WPC Excessive
Rainfall was based on a multi model blend, including the 00z WRF
ARW and 00z NAM CONUS Nest.

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
this). Based on this, a Slight Risk was extended across the
abovementioned areas, with a Marginal Risk surrounding it and
extended portions of southern NV (outside of the higher terrain),
as heavy rainfall Monday has resulted in wet soils.

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...
Two rounds of convection are expected across the eastern portion
of the Ohio Valley during Day 1. The initial shot of convection is
expected to track along and just south of a warm front snaking
from low pressure over northeast SD into western PA. The
convection is expected to move along the northern edge of the
500/1000 J/KG of MUCAPE extending along the front and impact IN/OH
into portions of northern WV through the morning hours.

There should be a relative lull in the activity before a short
wave in the mid level flow tracks from the Upper Great Lakes into
the western PA and western NY state after after 07/21z. While this
round of convection is expected to remain fairly progressive (and
outflow dominated), there is a multi high resolution model (led by
the 00z NAM CONUS Nest/WRF ARW) that portions of OH into northern
WV and far southwest PA could see another 1.00 inch of rainfall as
the storms move through. Combined with the morning convection,
some locations could see local 1.50+ inch rainfall amounts before
the convection exits later tonight.

Three hour flash flood guidance values are as low as 1.00/1.50
inches over portions of northern WV and and southwest PA (where 14
day rainfall amounts have been 200 percent of normal). Should the
above QPF amounts materialize, these areas could be more
vulnerable to flash flooding, even if the storms remain
progressive. Based on the above, the Marginal Risk was extended
from northern IN into WV and southwest PA to cover the threat. It
is possible, if the initial area of convection survives to produce
heavy rain over northern WV this morning, that a small Slight Risk
could be needed here for the second convection round.


...Lower Mississippi Valley/Lower Tennessee Valley...
Convection forming in the right entrance region of a jet streak
embedded in the persistent subtropical jet could help support
developing convection across portions of southern AR across MS
into western AL this morning through about mid afternoon. The
convection is expected to form on a low level convergence zone
extent across these areas this morning (likely by 07/14z or so).
Instability is expected to transition from elevated to surface
based once the low level CIN is erased, and storms should have
access to 1000/1500 J/KG of MLCAPE focused on the low level
boundary.

A deep pool of moisture is in place across the western Gulf of
Mexico, and a low level southwest flow transports 1.75/2.00 inch
precipitable water air (which is between two and three standard
deviations above the mean) along the low level boundary. Given the
deep moisture in place, storms could produce hourly rainfall rates
near 1.50 inches (and the 07/00z HREF probabilities suggest a
30/50 percent chance of hourly rainfall rates of 2.00 inches with
the developing convection). These rates appear plausible,
especially if training occurs early in the development of the
storms.

There is a solid model signal for local 2.00+ inch rainfall
amounts extending from southern AR into far west central AL,
particularly where training occurs. As the jet streak exits during
the afternoon, the convection should tend to weak and drop
southeast, as it becomes more outflow dominated. Three hour flash
flood guidance values here after generally 2.50 inches or higher,
so training would be needed for flash flooding to initiate. Given
that the ingredients are expected to be in place for heavy rain, a
Marginal Risk was stretched across portions of the Lower MS Valley
into the Lower TN Valley to cover the threat.


Hayes


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHWESTERN UNITED STATES...

...Southeast California into far western Arizona...
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.

Oravec


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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTHERN
CALIFORNIA...


...Southern California...
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.

Oravec




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