Excessive Rainfall Discussion

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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Sun Sep 20 2020

Day 1
Valid 16Z Sun Sep 20 2020 - 12Z Mon Sep 21 2020


1600 UTC update

No changes made to the previous Excessive Rainfall Outlook.


0900 UTC discussion

...Middle/Upper Texas Coast...
Tropical Storm Beta will continue to approach the middle Texas
coast through Sunday night, following the 03Z NHC advisory,
bringing what could be a tight gradient in rainfall very close to
the shore. As of 07Z this morning, infrared satellite imagery
shows bursts of convection, displaced northeast of the center (due
to southwesterly shear), as well as deeper convection in a
northwest to southeast arc, off of the southeastern Louisiana
coast. As the center of Beta gets closer to the Gulf Coast,
precipitable water values will surge upward across much of the
middle and upper Texas coasts with values reaching into the 2.25
to 2.5 inch range by 12Z/21. 850 mb winds are expected to veer
from northeasterly to southeasterly during the period with speeds
of at least 30 kt by 12Z Monday along the middle/upper Texas
coastline. With low level winds in excess of deeper layer mean
winds, the potential for training axes of heavy rain with rates of
at least 1-2 in/hr increases along with the potential for flash
flooding. Forecasts of instability are relatively low as one moves
inland from the coast which should tend to limit rates across
inland locations until during the day on Monday.

...Southeast Louisiana...
As Beta nears the Texas coast, the model consensus shows dry air
wrapping in to the south of the center, and then east of the
center by 12Z Monday. East of the dry intrusion, a low level
confluent axis sets up with deep moisture (precipitable water
values peaking in the 2.25 to 2.5 inch range). Infrared satellite
imagery already supports a secondary area of deeper convection
south of the Louisiana coast and heavy rain will likely continue
to edge closer to the coast during the day today with the
convective axis shifting westward Sunday night and possibly
re-orienting as Beta continues to move to the west.

...Eastern Florida Peninsula...
East-northeasterly low level flow will remain across much of the
eastern Florida peninsula during the day today and tonight. The
nose of stronger 850 mb winds (30 kt at MLB as of early Sunday
morning) and axis of high precipitable water values (2.25 to 2.5
inches) is expected to translate southward during the day as high
pressure continues to build south down the East Coast. 850 mb wind
speeds are forecast to weaken and broaden near and beyond 00Z
along the eastern Peninsula and deep-layer mean winds (storm
motion proxy) are expected to be slightly stronger than what was
seen during the day on Saturday. Therefore, while the threat for
locally heavy rain will continue today, the focus does not appear
as clear as Saturday's event. Heavy rain, in excess of 5 inches,
may be limited to one or two rather small areas. The rainfall rate
probabilities for 3+ in/hr and 5 inches in 24 hours (ending 12Z
Monday) from the 00Z HREF are less than 30 percent (compared to
the 80-90 percent forecast from Friday night for the 24 hour
period ending 12Z Sunday).


Day 2
Valid 12Z Mon Sep 21 2020 - 12Z Tue Sep 22 2020


...2130 UTC Excessive Rainfall Outlook Discussion...
Given a fairly consistent track for Beta from NHC, the
modifications needed to the QPF were fairly minor early in the Day
2 Excessive Rainfall Outlook as the storm slowly approaches the
coast.  By 22/00Z, the storm was forecast to be right along the
Middle Texas Coast when it slows it westward motion and begins a
turn towards the northeast.  The resulting slow storm motion
leaves the Upper Texas coast in peristent onshore flow and
rainbands.  At this point, still think a Moderate Risk is
appropriate given the combined rainfall forecast for Day 1 and Day
2.  One big concern is a burst of convection on the west side of
Beta during Sunday afternoon...which may ultimately result in a
discontinuious jump in the center of Beta and what implications
this has on the track/timing of the system.  It may require
significant shift in QPF with a corresponding shift in the concern
for excessive rainfall.


...0830 UTC Excessive Rainfall Outlook Discussion...

The main flash flood threat remains tied directly to Beta, and the
WPC QPF and Excessive Rainfall Forecast were based on the most
recent NHC track, which is close to the 00z model suite consensus.

There is better consensus on a landfall for the system along the
Middle TX coast later in the period. Along and ahead of the
system, banded convection affects the Mid and Upper TX coast,
producing hourly rainfall rates of 2.00/3.00 inches possible
(within 2.25/2.50 inch precipitable water moisture plume). Banding
comes ashore across Upper TX coast into coastal southwest LA
later, as the low level inflow lags the center to some degree. 
Much of the 00z model guidance (including high resolution
guidance) indicated a general 4.00/8.00 inch rainfall near the
landfall location, with the 00z NAM CONUS Nest showing local 10.00
inch amounts. These higher end amounts are possible where training
occurs as Beta begins to slow.

There was a near unanimous signal from the 00z guidance suite that
the moisture from Beta gets inland, and begins to slowly erode, as
drier air starts to become involved with Beta from the north and
west, particularly after 22/00z. Beta is expected to feel the
influence of a mid and upper level trough crossing the Southern
Plains, which is expected to turn Beta to the northeast and east
during Day 2. This turn could limit the northern extent of the
higher rainfall amounts.

With much of the guidance showing more of an easterly landfall
position, the highest rainfall amounts with Beta were spread
further along the Mid and Upper TX coast. Based on that, and the
expected turn with Beta, a Moderate Risk was extended across much
of this area (which was collaborated with WFOs CRP/HGX). Should
there be track changes further up the TX coast, the Moderate Risk
could move a bit to accommodate. 

Further north across north TX, there could be an enhanced area of
rainfall as moisture in the deep easterly flow north of Beta
interacts with lift in the right entrance region of a 50 knot jet
streak tracking across the Southern Plains. Model soundings show
almost no instability in the column, and it appears as though
frontogenetic forcing in the absence of elevated instability
becomes the main driver of banded heavy rainfall. A Slight Risk
was considered for this area, as some of the high resolution
guidance showed 4.00+ inch rainfall amounts. Since instability is
expected to be lacking, the area was included in a larger Marginal
Risk (which was collaborated with WFO FWD).


Day 3
Valid 12Z Tue Sep 22 2020 - 12Z Wed Sep 23 2020


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

Last Updated: 425 PM EDT SUN SEP 20 2020