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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0953Z Apr 08, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
553 AM EDT Wed Apr 08 2020

Day 1
Valid 0949Z 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.


...Ohio Valley...
10z update...
The Marginal Risk was extended into southern OH and WV to account
for early morning activity forming/persisting along the weakening
cold pool extending into this region. Upstream development could
train over areas with one and three hour flash flood guidance as
low as an inch, resulting in at least an isolated flash flood
threat this morning.

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

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

...Southern California...
The strong closed low moving slowly northeast day 1 across
southern California will become stationary day 2 and rotate back
toward the south across Southern California.  The 0000 UTC NAM and
GFS have both trended farther to the west with the strong closed
low as it drops southward during the second half of day 2.  This
is resulting in an uptick in its precip totals from the past few
runs over Southern California.  As the closed low sinks
southward..west southwest to westerly low level onshore flow will
persist across far southwest California.  Persistent hourly
rainfall rates of .10-25" possible...with upslope enhancement from
the eastern portions of the Transverse Range through the Peninsula
Range.  Across these areas a slight risk has been introduced for
locally heavy totals resulting in additional runoff issues,
especially across any burn scar regions.

...Southeast Texas...
The latest 0000 UTC model guidance is showing an axis of moderate
to heavy precip totals Thursday along and ahead or a  strong cold
front will be pressing southward Thursday over the Southern
Plains/Lower Mississippi Valley region Thursday.   Strong boundary
layer convergence  and favorable right entrance region jet
dynamics will enhance lift in the axis of pw values expected to be
in the 1.75-2.00" range(1.5 to 2+ standard deviations above the
mean.  There is still a lot of spread among the 0000 utc guidance,
including the hi res arw, nmmb and arw2 for the 1200 utc Thu to
0000 utc Fri time frame.   While stream flows are average to below
average across this area as per the National Water Model, there is
potential for hourly rates of 1-2" that may result in localized
runoff issues, especially in urbanized regions of San Antonio to
Houston

Oravec

Day 3
Valid 12Z Fri Apr 10 2020 - 12Z Sat Apr 11 2020

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER FAR
SOUTHWEST CALIFORNIA AND CENTRAL TEXAS...

...Far southwest California...
As has been the case over the past few days, there are timing
differences on day 3 with how progressive to the east the strong
Southern California closed low will be.  While the GFS and NAM
have trended toward some of the earlier slower run  solutions
during the day 2 period,  they are on the faster end day 3,
especially the GFS.  The slower ECMWF, UKMET and CMC have more
precip lingering over far southwest California day 3.  These
slower models have performed better with the overall slower trend
with this system and given that this strong closed low will remain
separated from the northern stream flow, the slower solutions
still look good.  Kept a small marginal risk area for day 3 over
far southwest California, in SGX's CWA given potential for
additional moderate to heavy amounts on top of what may fall in
previous days.  The NAM and GFS have heavier precip totals farther
east into portions of southern Arizona given their more
progressive solutions with the strong closed low.  However, their
fast bias with this system would suggest the better chance of
moderate to heavy precip is farther west. For this reason, no risk
areas were denoted for the NAM or GFS precip day 3 across AZ.


...Central Texas...
A warm front will be retreating northeastward across the Southern
Plains Friday night into early Saturday.  Strengthening low level
southerly to south southwesterly low level flow will overrun this
boundary, supporting potential elevated convection along and north
of this front.  The marginal risk area was draw toward the ECMWF,
UKMET and in house qpf ensemble solutions, with an axis of
potentially heavy rains from eastern portions of MAF's,central
portions of SJT's and southwest portions of FWD's CWAs  It has
been dry across these regions over the past 2 weeks with mostly
below average stream flows as per the National Water Model.  For
this reason, only a marginal risk denoted.
 
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