Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Sat Oct 20 2018
Valid 1654Z Sat Oct 20 2018 - 12Z Sun Oct 21 2018
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER FAR SOUTH
TEXAS & NEAR LAS VEGAS...
...Far South Texas...
A surface frontal boundary is expected to remain stationary
offshore of the western Gulf coast this period. Showers should
remain active in the vicinity of the front...with model consensus
for the bulk of the activity and the heaviest rainfall amounts
remaining offshore of the South Texas coast where instability will
be the greatest. The western edge of this activity and
potentially heavy rains may affect far south Texas...especially
along the immediate coastal areas...later today and tonight. The
risk area was confined just to far South Texas and kept at only a
marginal given the model consensus for the bulk of the activity
...Southern Nevada & Vicinity...
An upper low/trough moving into the region combined with
increasing moisture on the heels of 20-30 kt low-level south to
southeasterly flow at 850/700 hPa (exceeding the mean 850-400 hPa
wind by close to 50%) is expected to bring moisture up to a level
approaching that seen during the summer monsoon tonight.
Precipitable water values should rise into the 0.75-1.25" range by
then with MU CAPE values of 1000+ J/kg expected by the end of the
period. Considering the upper level disturbance moving eastward,
the MU CAPE is likely to be tapped. Of concern would be any
convection that attempts to align into short bands within the
sensitive desert areas in and around Las Vegas, where hourly rain
totals of 0.50"+ are problematic. Considering the ingredients
expected to be in place, introduced a marginal risk area in this
Valid 12Z Sun Oct 21 2018 - 12Z Mon Oct 22 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER DEEP SOUTH
...Deep South Texas...
Despite a large surface high sinking into the Southern Plains and
Lower Mississippi Valley, sustained pressure falls off the South
Texas coast should develop an inverted surface trough (despite
regionally increasing pressures) nosing up toward the immediate
vicinity of Brownsville, TX. These trends should have the effect
of creating increasingly cyclonic flow (with a ribbon of onshore
low-level flow just north of the surface trough) and increasing
wind speeds as the pressure gradient tightens. It is in the ribbon
of onshore flow that heavy rainfall appears likely with enhanced
low-level moisture transport, surface speed convergence along the
coast, and deep moisture (PW above 2.2 inches). Convective rain
bands with heavy rain are most likely to be sustained in this area.
However, there is some uncertainty as to the exact position of
these features. The 12Z suite of hi-res models tend to focus the
heaviest rainfall south of the US-Mexico border, while the global
models focus it closer to Brownsville. The WPC forecast generally
followed closest to the operational ECMWF, which does allow for a
threat of heavy rainfall in the Lower Rio Grande Valley right in
the McAllen to Brownsville area. Given the positional uncertainty,
only small risk areas were included in far southern Texas.
However, a focused area of heavy rainfall and possibly flash
flooding does appear likely just north of the cusp of the surface
trough. This would be focused within 50 miles of the coast in the
instability gradient, and MUCAPE around 500 j/kg with very high
PWs (over 2.2 inches) would support rain rates as high as 2-3
in/hr in some of the convective rain bands. Therefore, a small
Slight Risk was added for the immediate Brownsville area in
coordination with WFO BRO.
...Desert Southwest and Great Basin...
A closed mid-upper level low will begin to eject from the
California coast into the Great Basin on Sunday and Sunday Night.
As height falls spread over the region, mid-level lapse rates
should steepen, in conjunction with increasing low-level moisture,
to generate some convective instability. Along a NW-SE axis from
central Nevada into central Arizona, there will be a trade-off
between stronger forcing and steeper mid-level lapse rates focused
along the mid-level low track in Nevada, and higher low-level
mixing ratios and thus stronger CAPE in Arizona. 12Z hi-res models
tend to focus the more widespread convection and heavier QPF in
Nevada (streaks of 1-2 inch amounts), but more limited low-level
moisture and instability further to the north may limit rain
rates. The 12Z HREF does show small (less than 10 percent)
probabilities of 1 in/hr rain rates, again mostly in Nevada. To
account for the potential for isolated heavy rain rates, a
Marginal Risk was maintained in the region, and due to a variety
of uncertainties, the risk area was kept relatively broad. A lack
of overlap between the stronger (MUCAPE 500+ j/kg) instability and
PW values in excess of 0.75 inches with the stronger forcing
prevented any higher excessive rainfall probabilities.
The Day 3 outlook will be updated by 2030Z.
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