< Day 2 Outlook
WPC Day 3 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 2017 UTC Sun Sep 20, 2020
Valid: 12 UTC Sep 22, 2020 - 12 UTC Sep 23, 2020
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Sun Sep 20 2020
Valid 12Z Tue Sep 22 2020 - 12Z Wed Sep 23 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR SOUTHEAST
TEXAS ACROSS SOUTHERN LOUISIANA INTO FAR SOUTHWEST MISSISSIPPI...
..2130 UTC Excessive Rainfall Outlook Discussion...
Much of the 20/12Z numerical guidance still showed Beta weakening
in response to the system ingesting dry air combined with
southwesterly shear. The focus for heavy rain begins to shift to
the north side of the system as the system evolves into a
depression and perhaps a remnant low by the end of the period.
For the moment, the previously-issued Slight Risk and the
surrounding Marginal Risk areas cover the potential for excessive
rainfall associated with the storm center early in the period and
with the area of heavy rain in/near the Arklatex region during the
latter half of the period. Placement of the QPF and the
associated risk of excessive rainfall may shift...especially as
Beta continues to become better organized and the exact track and
timing become better defined. For the moment, the uncertainty
precludes the confidence to upgrade to a Moderate Risk area but
remains a possibility for later outlooks.
...0830 UTC Excessive Rainfall Outlook Discussion...
...Southern Plains into the Lower Mississippi Valley...
The main flash flood threat could transition from Beta to inflow
banded convection across portions of southeast LA into far
southwest MS during Day 3. The WPC QPF and Excessive Rainfall
Outlook were based on the most recent NHC, as well as the 00z
Much of the 00z regional/global guidance showed Beta weakening as
dry air entraining into the system from the west and increasing
southwest shear begins to take their toll on the system, slowly
eroding the deeper moisture associated with it. Low level inflow
to the east of Beta is expected to focus the highest rainfall
across far southeast TX (including much of the upper TX coast),
where 2.25 inch precipitable water in the southeast low level
inflow could allow bands of convection to produce local totals
near 4.00 inches. There is some spread in the 00z guidance where
this occurs, and this may be tied to the positioning of the
instability gradient near the coast. However, there is enough
moisture in the column to support a Slight Risk here for Day 3.
The greater flash flood threat could transition from Beta to
portions of southeast LA and far southwest MS, as there is an
increasing model signal for banded convection to develop in the
2.25 inch precipitable water plume in the low to mid level south
southeast flow. There is some model spread with respect to the
placement of the band, as the low level convergent flow varies
longitudinally within the 00z model guidance suite. A blend of the
00z ECMWF/GFS was used to construct the placement of the inflow
band across southeast LA, though the 00z ECMWF QPF amounts seem a
bit high with QPF amounts at this juncture.
A small Moderate Risk was considered here, but the spread in the
placement of the axis of highest QPF raises just enough doubt for
the confidence needed for a Day 3 Moderate (and this decision was
collaborated with WFO LIX). Should agreement in the placement of
the rainfall maximum with the banded convection increase with
later model runs, a Moderate Risk is certainly possible in
subsequent forecasts. Instead, the Slight Risk from southeast TX
and southwest LA was extended over this area to cover the threat.
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt