Excessive Rainfall Discussion

[Abbreviations and acronyms used in this product]
Geographic boundaries:    Map 1- [Color] [B/W Print Version]      Map 2 - [Color] [B/W Print Version]

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
851 PM EDT Sat Oct 20 2018

Day 1
Valid 01Z Sun Oct 21 2018 - 12Z Sun Oct 21 2018

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.


Day 2
Valid 12Z Sun Oct 21 2018 - 12Z Mon Oct 22 2018


...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.


Day 3
Valid 12Z Mon Oct 22 2018 - 12Z Tue Oct 23 2018


...Coastal Texas...
The inverted surface trough nosing toward Brownsville and the
South Texas coast on Day 2 is forecast by most global models to
begin to lift north along the Texas Coast on Day 3 (Monday and
Monday Night). There may be ongoing convection at the beginning of
the period on the South Texas coast, lingering from Day 2 with an
attendant threat of heavy rainfall and flash flooding.

Through the course of the Day 3 period, the 12Z GFS and ECMWF both
show the nose of a ribbon of enhanced low-level moisture transport
focusing further north up the coast in the presence of PWs around
2 inches. This should lead to additional heavy rainfall along the
remainder of the Texas coast. However, most models also keep even
the marginal instability contained offshore over the Gulf of
Mexico, with a lack of notable height falls or cooling aloft. If
anything, a shortwave ridge remains in place across the region.
This may limit rain rates over land areas in the Day 3 period, and
thus the risk of flash flooding.

However, a fair number of NWP models now show the potential for
over 1 inch of rainfall along the central and northern Texas coast
and this, combined with the deep moisture available, suggest there
should be at least a small risk of flash flooding. Therefore, a
Marginal Risk was maintained along the coast, but trimmed from
areas further inland where instability may be more meager. It was
also trimmed out of southern Louisiana, as most NWP models now
keep the bulk of the rain offshore of Louisiana through the period.


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: 851 PM EDT SAT OCT 20 2018