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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0105Z Apr 09, 2020)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
905 PM EDT Wed Apr 08 2020

Day 1
Valid 01Z Thu Apr 09 2020 - 12Z Thu Apr 09 2020

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
CALIFORNIA DESERT AREAS...

...Southwest United States...
Large closed low over the southern CA coast will slowly edge
eastward overnight as multiple mid-level vorticity centers move
about the mid-level low center. PW values have come down across
southern CA with GPS estimates between 0.4 and 0.6 inches as of
00Z, while higher values near 0.8 inches have shifted east into
southwestern AZ. 00Z radar/satellite imagery showed isolated to
scattered post-frontal convection across southern CA into the
Colorado River Valley.

The 00Z sounding from NKX showed ~200 J/kg of instability beneath
the core of the upper low and spotty values of weak CAPE are
likely spread across portions of southern CA into southern NV and
AZ as of late Wednesday evening. However, a consensus of short
range forecast guidance suggests weakening CAPE magnitude and
coverage through 12Z across the entire southwestern U.S.

Some increase in convective coverage is possible over the next few
hours before loss of daytime heating and diminished CAPE reduces
the number of showers/thunderstorms after 04Z. As low-mid level
vort energy swings around the low center in CA overnight,
increasing low level mass flux convergence will be slow moving
across southern portions of the San Joaquin Valley along with
northeasterly/northerly upslope flow into Kern/King counties. PW
values are forecast to approach 1 inch which shows standardized
anomalies of +2 to +3. While weak instability (if any) will likely
limit 1 hour rates below 0.5 in/hr but 3 hour rainfall totals near
1 inch will be possible. Given above average rainfall across the
region over the past week, a Marginal Risk is warranted.

Farther north into south-central NV, low level convergence will
combine with strong upper level divergence and diffluence within
the left exit region of a 100+ kt jet max located from the
northern Baja Peninsula into western AZ based on RAP data and the
18Z Yuma, AZ sounding. Elongated arcs of repeating showers and
thunderstorms will be possible across portions of south-central NV
overnight but lacking instability continues to be a limiting
factor for flash flooding.

...Ohio Valley...
The Marginal Risk area remains largely unchanged from the 16Z ERO
update. A cluster of strong convection was ongoing at 0030Z across
IN/IL ahead of a strong cold front while convection more scattered
in nature was located farther south, near the Ohio/Mississippi
River junction. Wind profiles suggest the majority of convection
will be progressive in nature, limiting the flash flood threat but
there are a couple of factors to consider. First, portions of WV
received heavy rain over the past 24 hours which has lowered flash
flood guidance values to less than 1 inch in 3 hours. Instability
will be lower with northward extent, but these 3 hour rainfall
values appear possible, even in a progressive pattern. The other
consideration is increasing 850 mb flow across KY and TN overnight
into the 40-50 kt range which may have the effect of increasing
training potential with any convection that can sustain ahead of
the main line. Otherwise cell mergers may combine to produce
localized rainfall in excess of area FFG values.

Otto


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...
...20Z Update...
Increased the overall extent of precipitation across portions of
southern California the mid-level closed low remains stationary
across the region. Models have shown this trend over the past
couple of runs; though the 12Z NAM is a bit too robust with QPF
totals. Initially the thought was the 12Z UKMET too slow with the
moisture/energy swinging around the mid-level low. However, both
the 12Z CMC/EC are supporting this notion. This was hard to ignore
given the amount of precipitation received across coastal regions
of southern CA, especially along the Transverse Mountains. Based
on this assessment and the remaining 12Z guidance, with residual
energy circulating around the low, the overall synoptic scale
forcing will begin to weaken resulting in atmospheric ascent
starting to diminish. However, moisture flux will increase as
evident by the precipitable water values (>0.75 inches) surging
inland aided by southwesterly flow coming off the Pacific Ocean.
This low level flow will become orthogonal to the southern coastal
ranges promoting enhanced rainfall. While rain rates should at
around or below 0.25 inches, could see the potential for upslope
regions to observe higher values also aided by elevated
instability. The differences among the models will manifest in
sharp variations where the best moisture convergence occurs and
thus highest QPF. Given the antecedent conditions and residual
burn scars, local to more scattered flash flood could still occur.
 Thus, felt it prudent to expand the Slight/Marginal Risk areas
which will also account for model variability, expected
precipitation totals, and rain rates.

...Previous Discussion...
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...
...20Z Update...
Most of the guidance has trended higher in coverage and QPF
amounts.  While most of the models are in fairly good agreement
with respect to where convection will initiate and evolve, the 12Z
NAM was a bit farther north compared to consensus.  While the NAM
may be onto something given the light low level flow and 500/700mb
forcing being farther north, it is an outlier at this time. 
Therefore, modified the Marginal to account for the most recent
WPC QPF, which was nudged a bit farther west given the
front/instability axis and overriding mid-level energy.  A Slight
Risk was not deemed necessary at this time given the expected
progression of the convection and fairly decent FFG.

...Previous Discussion...
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/Pagano

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...
...20Z Update...
Increased QPF across portions of far southern CA given some models
have shows a slower and more offshore retrograde of the mid-level
low.  As a result, this would promote continued moisture
convergence along the coastal ranges helping them to ring out more
moisture.  While the 12Z EC has dramatically increased the QPF
across this region, largely due to the mid-level low shifting
offshore to aid in strong moisture flux, the 12Z GFS has
consistently shown the mid-level low a bit farther inland,
reducing the overall QPF footprint. However, there are other
pieces of guidance that are trending higher with QPF, namely the
NAM and UKMET, though the UKMET positioning is questionable given
the slow progression.  Therefore, leaned toward the EC/ECENSmean
and NAM across this region.  If an upward trend continues with the
QPF amounts at future updates, than a Slight Risk may be
warranted.  But for now, given the forecasted amounts and
uncertainty, will maintain the Marginal Risk area. 

...Previous Discussion...
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...
...20Z Update...
Reduced QPF amounts to account for a lot of uncertainty in terms
of if/when convection will initiate along a dryline in
western/central TX aided by energy and moisture ejecting from a
closed low over southern CA. While the typical ingredients will be
present, the overlap and magnitude of moisture, lift and
instability may not. Thus, the Marginal Risk was broadened and not
as focused. 

...Previous Discussion...
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/Pagano




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