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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0817Z Apr 08, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Wed Apr 08 2020

Day 1
Valid 12Z Wed Apr 08 2020 - 12Z Thu Apr 09 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
EASTERN CALIFORNIA...

...Southwest United States...
A persistent closed mid level low off the Central CA coast tracks
across the state into southern NV by the end of the period. Ahead
of the closed low, a moist low level flow feeds moisture and
instability along a frontal boundary, and the moisture and
instability should be sufficient to support convection capable of
excessive rainfall across portions of eastern CA into nearby NV
and western AZ. While there is still some model spread concerning
how quickly the closed low moves eastward, there is generally good
agreement concerning the placement of the moisture and
instability, as well as the placement of the highest QPF. Based on
this, a multi model blend was used to create the WPC QPF and
Excessive Rainfall Outlook (ERO), which included the 00z WRF ARW
and the 00z NAM CONUS Nest.

Shower activity continues rotate around the mid level low, and
regional VWPs showed a 15/25 knot low level south southeast flow
transporting 0.75 inch precipitable water air (per the most recent
blended TPW product) into much of southern CA. The most recent RAP
analysis showed little in the way of instability in the column,
and hourly rainfall rates are generally below 0.25 inches. As the
closed low trundles eastward during the morning hours, steepening
lapse rates should result in marginal instability (generally
around 500 J/KG of MUCAPE) across much of the region. The
increased instability should allow low topped convection to form
near the center of the mid level system, during which time hourly
rainfall rates top 0.25 inches.

Surface low pressure moving across southeast CA into southern NV
during the afternoon hours aids in the increasing low level
convergence over a large area of eastern CA and nearby southwest
NV, mainly after 08/20z. This allows 20 kt low level east
southeast winds focus the 0.75 inch precipitable water air over
far eastern CA in the upslope flow. Model soundings showed very
steep lapse rates, which support SBCAPE values between 500/1000
J/KG, and the combination of moisture and instability could result
in hourly rainfall rates of 0.50 inches during this time.

It is during this time that training becomes an issue, as 850/300
mb mean winds align well with the propagation vectors. The
greatest threat for training appears to be across eastern CA,
along the axis of best low level convergence. Simulated radar from
the 00z WRF ARW/00z NAM CONUS Nest and the most recent HRRRs
support this scenario, generally between 08/15z and 09/00z. Where
training occurs, local 1.50+ inch rainfall amounts are possible.
This area has, for the most part, escaped the heavy rainfall with
the mid level system, and soils should not be as compromised as
further west. That being said, there is a flash flood threat where
the training band sets up, and based on the placement of the QPF
axis and the simulated radar depiction of the training, a Slight
Risk was placed over much of eastern CA outside of the terrain to
the CA/NV border.

Further north into southern NV, the moisture is not as deep, but
500+ J/KG of SBCAPE is expected during the afternoon hours. The
mid levels appear to be fairly dry, so there could be small hail
in some of the storms that develop. Portions of southern NV
received between 0.50/1.00 inches of rainfall in the past two
days, so soils there may be more conducive to isolated flash
flooding. A Marginal Risk was placed here (and far southwest UT)
to cover the threat.

Finally, a Marginal Risk was placed over much of the remainder of
Southern CA (outside of the terrain, where a tight gradient from
locally heavy rain/heavy snow is expected. As the mid level system
comes ashore, the mid level could dry sufficiently to allow storms
to become hail producers, However, given the amount of rainfall
these areas have experienced in the past two days, any storm that
produces 0.50 inches in less than an hour could cause isolated
problems.


...Lower Ohio Valley...
Convection forming ahead of a southward moving frontal boundary
across the OH Valley into the Mid MS Valley late this afternoon
and this evening will support convection capable of producing
heavy to locally excessive rainfall over portions of southeast MO
into KY and possibly western TN. 

Ahead of a cold front stretched the Central Plains into the OH
Valley this afternoon, model soundings showed 1000/2000 J/KG of
SBCAPE in place, mainly after 08/18z. A 20/30 knot low level
southwest flow transports 1.00 inch precipitable water air into
the region, and the combination of moisture and instability should
be sufficient to support the development of convection along and
ahead of the front into the Lower OH Valley, mainly after 08/20z.

Short wave energy tracking from ND into IN/OH during the late
afternoon and evening provides enough lift to allow convection to
organize shortly after developing. During this time, there is a
window (possibly no longer than 3/4 hours somewhere between 09/00z
and 09/06z) where short term training and cell mergers could allow
for hourly rainfall rates to top out between 1.00/1.25 inches,
particularly over the boot heel of MO/western KY and western TN.
After that time, the storms or line of storms developing a deep
enough cold pool to become progressive, racing away after 09/06z.

Three hour flash flood guidance values are generally above 2.50
inches, though there are places where the 14 day rainfall total
have been above normal (following a wet winter season). Short term
training would be required for flash flooding to initiate before
the storms drop southeast. Though the window for flash flooding is
fairly narrow, a Marginal Risk was placed over portions of
southeast MO into western and central KY/TN. This was collaborated
with the local WFOs.


Hayes


Day 2

The Day 2 outlook will be updated by 0830Z.


Day 3

The Day 3 outlook will be updated by 0830Z.


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