Excessive Rainfall Discussion

[Abbreviations and acronyms used in this product]
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
654 AM EDT Sun Oct 21 2018

Day 1
Valid 1051Z Sun Oct 21 2018 - 12Z Mon Oct 22 2018


...Far South Coastal Texas...
The low level flow off the western Gulf is expected to veer with
time this period from northeasterly early day 1...to a more
easterly to east southeasterly toward the end of the period.  This
onshore flow will help focus a boundary layer convergence maxima
along the far south Texas coast into coastal northeast Mexico in
the vicinity of a southeast to northwest oriented inverted trof
across this area.  With pw values expected to remain 2.0 to 2.5+
standard deviations above the mean through the period...heavy
rains are possible along the immediate far south Texas coast. The
previous marginal and slight risk areas for this period over far
south Texas were suppressed farther to the south by approximately
30-50 nm to match the model consensus for the area of heavy rain

...Portions of Nevada/California/Arizona/Utah...
Showers and thunderstorms in and near southern NV are thriving to
the northeast of an upper level low located over southern CA.
Precipitable water values are 0.5-1" per recent GPS data (2-4
sigmas above the mean for late October), with surface dew points
currently into the low to mid 50s.  MU CAPE values are 500-1500
J/kg have been pushing northward from western AZ and southeast CA
near and south of a 700 hPa trough moving northward into the area.
 Effective bulk shear of 20-45 kts lies across the area per SPC
mesoanalyses. With the expansion of showers and thunderstorms,
expect precipitable water values to rise another 0.25" and for
rainfall efficiency to slightly increase into the morning hours,
with another round of activity expected this afternoon and evening
as the system moves northward.  Radar estimates have advertised
localized areas of 0.5"+ so far, which could enhance further,
particularly if any mesocyclones form within this environment
(supported by the available bulk shear).  Short periods of
training with an environment with brisk low-level inflow could
also lead to these rain rates/totals.  Hourly rain totals of 0.5"+
appear probable, with local amounts of 1-2" expected as activity
spreads northward ahead of the upper low and 700 hPa trough.  A
marginal risk of excessive rainfall exists across an arid region
which is sensitive to heavy rainfall.


Day 2
Valid 12Z Mon Oct 22 2018 - 12Z Tue Oct 23 2018


...Western Gulf States...
Tropical moisture is expected to remain just offshore of TX and LA
during Day 2. As has been the case the past few model cycles,
there is still some model spread concerning just how much of the
moisture impacts the coast, and whether convection developing in
the moisture plume will cause flash flooding. A low level east
southeast flow aids in pushing 2.00/2.25 inch precipitable water
air toward the TX coast, mainly after 23/00z. Model soundings
showed instability remaining in the marginal range, as lift from
short wave energy rounding the western edge of the shrinking mid
level ridge over the Gulf of Mexico crosses TX during the period.

While most of the 00z model suite showed an axis of heavy rainfall
along an inverted trough pointing toward the TX coast, there
remains a fair amount of spread concerning where this occurs. A
blend of the 00z GFS/ECMWF places the axis of heaviest rainfall
between KPSX and KGLS, with 2.00+ inch rainfall amounts coming
just inland between these two points. A small Slight Risk area was
considered for this area, but given the model spread in the 00z
model suite, and the volatility of the solutions over the past few
cycles, this idea was shelved. This decision was collaborated with

A fairly large Marginal risk was extended across much of coastal
TX for Day 2 to cover the flash flood threat. The risk area was
extended across portions of interior south TX (west to the Rio
Grande River), as moisture from Willa could become involved in the
flow across this area. Most of the 00z guidance supports a locally
heavy rainfall threat here, though the 00z NAM may be too
aggressive bringing the deep moisture into TX during Day 2.


Day 3
Valid 12Z Tue Oct 23 2018 - 12Z Wed Oct 24 2018


...New Mexico...
A long wave trough tracking from CA/NV early on Day 3 interacts
with deeper moisture streaming northward across NM, posing a flash
flood threat. A low to mid level southeast flow ahead of the mid
level trough transports 0.75/1.00 inch precipitable water air
(which is between two and three standard deviations above the
mean) over central and eastern portions of NM, with the best low
level moisture transports occurring in the 23/18z to 24/00z time
frame. The combination of synoptic scale lift ahead of the long
wave trough and deepening moisture supports an expanding area of
moderate to heavy rainfall.

Model soundings showed little in the way of surface based
instability across NM, keeping the best instability across AZ and
westernmost NM. Elevated instability is possible across central
and eastern NM, as model soundings did show some instability above
the 850 mb level. However, instability could end up being the
limiting factor for a more expansive flash flood threat, and the
event may be more of a flood versus flash flood scenario. That
being said, the 00z ECMWF/GFS showed local 1.50+ inch rainfall
amounts across central and eastern NM, which could be difficult to
attain without deeper instability. A Marginal Risk was placed over
this area to account for the low end flash flood threat.

Moisture from Willa could become involved in the lift associated
with the broad long wave trough crossing the Rockies and Southwest
states, which could result in a flash flood threat during Day 3
across portions of interior south TX. There is a fair amount of
spread concerning how quickly the moisture streams into south TX
during Day 3, with the 00z NAM seemingly too fast with the bulk of
the moisture arriving much sooner than the model consensus.

However, each 00z model showed a distinct moisture connection from
Willa. An increasing low level southeast inflow transports
1.50/1.75 inch precipitable water air (which is between two and
three standard deviations above the mean) from the Big Bend to the
Rio Grande Valley between KUVA and KLRD. Model soundings here
showed marginal instability, some of which becomes elevated as it
crosses the Rio Grande River into south TX. A large area of rain
with embedded convection is expected in the moisture plume, and
the potential for training exists as 850-300 mb winds become
better aligned with the propagation vectors toward 24/12z.

An axis of 1.50/2.00 inches of rainfall was placed in the above
mentioned areas, with both the 00z GFS/ECMWF showing locally
higher amounts (but in different locations). The model spread
concerning the timing results in a lower than average forecast
placing of the highest rainfall axis. A Slight Risk was considered
for a portion of interior south TX, but given the uncertainty, and
after collaborating with WFO CRP, a Marginal Risk was placed over
much of central and interior south TX to cover the threat. If
model agreement on the placement and timing of the highest
rainfall amounts increases, a Slight Risk could be introduced in
subsequent forecasts.


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

Last Updated: 654 AM EDT SUN OCT 21 2018