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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 1709Z Oct 19, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
108 PM EDT Fri Oct 19 2018

Day 1
Valid 1706Z Fri Oct 19 2018 - 12Z Sat Oct 20 2018

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN TEXAS & THE ARKLATEX...

...Central to Northeast Texas...
Southerly to southwesterly low level flow will continue to overrun
the low level cold air that remains entrenched across the Southern
Plains.  This will continue to support a continued wet pattern
over portions of the Southern Plains day 1 where precipitable
water values (PWs) will remain much above average...2.0 to 2.5+
standard deviations above the mean.  Model consensus is that the
area of rain in Texas and then extending northeastward into
eastern Oklahoma will shift to the east-northeast across northeast
TX, southeast OK, and AR with time as shortwaves along the
southern and subtropical jets urge some forward progression. 
Addition precipitation likely to form on the trailing end of this
east northeastward moving rain area over northeast Texas in a
region of favorable right entrance region jet dynamics.  MU CAPE
values of 100-500 J/kg should allow for 0.50"+ rainfall rates from
time to time, which would be problematic over saturated areas
between Dallas and San Antonio.  The eastward shift of the
heaviest precip axis away from where the flash flood guidance
values have been very low warrants a marginal risk this period
over this region.  An eastward shift was made per the 12z guidance
suite. 

..Central TX coast...
The marginal risk area over to central to northeast TX remains
along the central Texas coastal region.  Instability is greater
here with ML CAPE values ~1000 J/kg with thunderstorms trying to
stream in off the Gulf of Mexico.  Rainfall rates have been
somewhat greater here where PWs remain much above average...2.0 to
2.5+ standard deviations above the mean.  Hourly rates of 1-2" are
possible...but with flash flood guidance values modestly
high...believe the threat of runoff issues will be minimal.

Roth/Oravec


Day 2
Valid 12Z Sat Oct 20 2018 - 12Z Sun Oct 21 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH TEXAS...

...Texas...
The deepening flow around surface high pressure over the mid MS
Valley funnels tropical moisture across portions of central and
south TX during Day 2. A low level easterly flow transports 2.25
inch precipitable water air (which is between two and three
standard deviations above the mean) over south TX, along and over
a quasi stationary boundary extending over the nearby western Gulf
of Mexico. Model soundings showed generally less than 1000 J/KG of
MUCAPE, which might suggest that instability could be a limiting
factor for convection. However, the soundings also showed a moist
column through 300 mb, which promotes warm rain processes in the
developing convection. This could offset the less robust
instability profiles expected near the coast.

There is a multi model signal for 1.00 to 3.00 inches of rainfall
over far south TX (with the 00z ECMWF showing rainfall maxima over
4.00 inches). Mean winds in the 850-300 mb layer and propagation
vectors are both expected to remain below 10 knots, which should
lead to cell mergers, as well as short term training. Given the
deep moisture in place, these rainfall amounts seem plausible.
Even with three hour flash flood guidance values are generally
above 2.50 inches across much of south TX, the potential for
training in the deep tropical moisture support a Slight Risk here
for Day 2.

Further north across portions of west and central TX, the
influence from the frontal boundary is less than closer to the
coast, but deep moisture and marginal instability could support
low topped convection during Day 2. These areas received as much
as 600 percent of normal rainfall in the past week (which is
reflected in the three hour flash, which is as low as 1.00/1.50
inches), and additional rainfall could result in a flash flood
threat. There is not a strong model signal for rainfall amounts
this high, but given the wet antecedent conditions, portions of
west and central TX were placed in a Marginal Risk for Day 2.

...Southwest/Great Basin...
Moisture funneled northward ahead of a closed mid level low off
the CA coast, coupled with marginal instability due to steepening
lapse rates, could produce convection capable of heavy to
excessive rainfall across portions of far southern NV and
northwest AZ during Day 2. Precipitable water values near 0.75
inches (which is between two and three standard deviations above
the mean) during the second half of Day 2 spread over these areas,
especially after 20/00z. Model soundings showed MUCAPE values near
500 J/KG, which should be sufficient to support low topped
convection.

The 850-300 mb mean wind and propagation vectors are expected to
be less than 10 knots, and this should result in cell motions less
than 10 knots as well. Short term training is possible in this
environment, and local rainfall amounts of an inch are possible
(though the 00z GFS rainfall amounts may be too high, given the
moisture in the profile). Based on this, and after collaborating
with WFO VEF, a Marginal Risk was placed over portions of far
southern NV and northwest AZ for Day 2.

Hayes


Day 3
Valid 12Z Sun Oct 21 2018 - 12Z Mon Oct 22 2018

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTH TEXAS...

...Texas...
Deep moisture and instability along and over a frontal boundary
just off the southern and central TX coast will feed convection
capable of producing excessive rainfall during Day 3. The synoptic
setup is similar to that of day 2, as a low level 20 to 25 knot
low level east southeast flow focuses 2.25 inch precipitable water
air on the front and just over it along the far south TX coast.
Model soundings again showed generally 500-1000 J/KG of MUCAPE in
place, but a moist column (supporting warm rain processes) could
offset the lack of deeper instability. Developing/ongoing
convection takes advantage of the tropical moisture plume and
increasing low level inflow to produce an axis of heavy rainfall
generally south of KCRP to near KBRO. In addition, short wave
energy rounding the western edge of the mid level ridge over the
Gulf of Mexico could provide synoptic scale ascent for organized
convection, especially after 22/00z.

There is an increasing model signal for 2.00/3.00 inches of basin
averaged rainfall over this area, with the 00z GFS showing over
6.00 inches just south of KCRP. While the 00z GFS amounts may be
high, the potential for training (due to cell motions under 10
knots) could produce local rainfall amounts in excess of 4.00
inches. Given the deep moisture in place, and the strong model
signal for heavy rainfall, a Slight Risk was placed over portions
of far south TX (which was collaborated with WFOs CRP/BRO). It
should be noted that the Day 3 Slight Risk was placed in
approximately the same place as the Day 2 Slight Risk. If the same
areas do indeed receive heavy rainfall both days, a Moderate Risk
could be needed for Day 3 in later forecasts.

...Southwest/Great Basin...
The closed mid level low crossing CA into western NV will provide
steepening lapse rates in conjunction with deep moisture to feed
low topped convection capable of producing heavy to locally
excessive rainfall during Day 3. Model soundings showed generally
less than 500 J/KG in place over portions of far southern NV into
northwest AZ and far southwest UT, peaking near 22/00z. However, a
moistening column in the presence of 0.75 inch precipitable water
air could at least partially offset the lack of deeper moisture to
produce low topped convection. As was the case during Day 2, weak
850-300 mb mean winds and propagation vectors foster an
environment with cell mergers and short term training.

While the 00z GFS may be overdone with its maximum rainfall
amounts over northwest AZ and far southern NV (with amounts
approaching 2.00 inches), local 1.00 inch amounts are possible
where cells merge or train ahead of the closed mid level system.
These rainfall amounts may be sufficient for local runoff issues,
and possible isolated flash flooding. With this in mind, and after
collaborating with WFO VEF, a Marginal Risk was placed here for
Day 3.

Hayes


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