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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1927Z Sep 27, 2021)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Mon Sep 27 2021

Day 1
Valid 16Z Mon Sep 27 2021 - 12Z Tue Sep 28 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST U.S....

16Z update... Some of the latest guidance is showing the QPF near
the Flagstaff metro a bit further west so the Marginal Risk area
was expanded to reflect this trend. Overall, the coverage area
still reflects the elevated threat for localized heavy rain and/or
flash flooding.

Campbell

Previous discussion...

...Southwest U.S/Southern Great Basin....
The inherited Marginal risk was generally left unchanged across
portions of the Southwest. Scattered convection is again likely
near the closed mid level low, and like the past couple days, a
few localized instances of flash flooding are possible. The 00z
HREF does indicate a possible focus for higher totals over
portions of Yavapai and Coconino counties of northern AZ...where
neighborhood probabilities of 1"+ an hour get into the 40-60%
range. Thus the flood risk may be a bit higher here than elsewhere
within the Marginal risk...although given the small areal extent
of these higher probs and some inherent uncertainty on exactly
where an axis of higher rains may actually develop...not seeing
enough to go any higher than the Marginal risk.

Chenard


Day 2
Valid 12Z Tue Sep 28 2021 - 12Z Wed Sep 29 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FOUR CORNERS REGION AND OVER PORTIONS OF THE TEXAS HILL
COUNTRY AND SOUTH TEXAS...

21Z update... In coordination with the local forecast offices in
San Angelo, San Antonio and Corpus Christi it was decided to hoist
a Marginal Risk for a majority of the Hill Country and a few of
the surrounding areas. Although it has been drier of late and the
FFG is higher, local hourly rainfall rates of 1 to 2.5+ inch/hour
is present in some of the hi-res guidance over the Hill Country.
These rates could lead to localized flooding and rapid runoff,
thus the need for a Marginal Risk for excessive rainfall.

Campbell

Previous Discussion...

...Colorado/New Mexico/Arizona...
The inherited Marginal risk was generally left unchanged across
portions of eastern AZ, western NM and southwest CO. By this time
the closed low that has been overhead should have opened up and
pushed off to the east. However PWs should remain 1-2 SD above
average, and decent instability looks to persist. Guidance
indicates another day of isolated to scattered convection, and
given the instability/moisture parameters in place...it is
possible that a localized flash flood risk remains. Overall would
say that the trend has been a bit lower with QPF
magnitudes/coverage...and possible we may be able to remove the
Marginal risk altogether at a future update. But for the moment
think keeping things status quo and continuing to monitor is the
best option.

...Texas and Louisiana...
Locally heavy rainfall could evolve over portions of TX and LA
Tuesday into Tuesday night, but dry soil conditions (over TX) and
average streams flows are resulting in high flash flood guidance
values...thus not confident enough at the moment to introduce any
risk areas. The 00z HREF members bring locally heavy convection
into LA early Tuesday, but even they show little to no exceedance
of FFG, and the global models are not as aggressive in this
scenario anyways. Over TX, we should see convective development
along a frontal boundary passing through the middle of the state,
along with the potential of additional activity closer to the
coast. Plenty of instability is forecast, and PWs should end up
1-1.5 SDs above average...so not bad, but not extreme either.
Nonetheless the PW/CAPE combos should be enough to support some
locally heavy rainfall rates. Just a matter of how organized and
persistent this heavier convection is. Nothing too well defined in
the lower/mid levels to organize activity...although a 250mb
subtropical jet may provide some upper level divergence over the
area. Model spread remains rather high, and not seeing a strong
consolidated QPF signal. Given these factors, combined with the
high FFG, think we can hold off on any risk areas at this
time...with the understanding that a Marginal risk or two could be
introduced a we get closer in time and CAMs come more into play
helping diagnose the risk a bit better.

Chenard

Day 3
Valid 12Z Wed Sep 29 2021 - 12Z Thu Sep 30 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL PLAINS...

21Z update... The models continue to target the axis of higher QPF
from north-central Kansas to the southeast corner of South Dakota,
with the trends filling more over central Nebraska. With this bump
up in QPF amounts/footprint the western edge of the Marginal Risk
area was extended westward into central Nebraska. Generally, still
expecting 1 to 2 inches with isolated areas of 3+ possible.

Campbell

Previous discussion...

...Central Plains...
A period of southwest to northeast training convection is possible
across portions of northern KS into central/eastern NE Wednesday
into Wednesday night. Should have a well defined surface front
across the area, with shortwave energy overrunning the front,
along with decent upper level divergence in the right entrance
region of a 250mb jet. Instability around 2000 J/kg and PWs up
over the 90th percentile indicate an environment favorable for
heavy rainfall rates. Thus confidence is increasing that we will
see a nice swath of 1-2" areal averaged rains...with embedded
narrower swaths of 3"+ amounts. Both of our automated ERO
algorithms...one based on our PQPF and the other machine
learning...also indicate a flash flood risk with this activity.
One limiting factor will be the very dry soil conditions over the
region. This will most likely keep any flash flooding pretty
localized in nature...thus while we will introduce a Marginal
risk...not currently thinking the risk is any higher than that.

Our model preference at the moment is something close to a 00z
EC/UK/CMC REG blend...with the GFS appearing too progressive and
the NAM too far north with its convective focus.


...Southern Plains into Lower MS Valley...
Similar to what was discussed on day 2...areas of locally heavy
rain will again be possible on day 3...this time expanding in
coverage a bit to include more of the Southern Plains and Lower MS
Valley. Again it remains a bit unclear how organized heavier
convection will be...and exactly where it will focus. The better
mid/upper forcing is to the north and PWs, while above average,
are not currently forecast to get to well above average levels.
However we will still have the subtropical jet around and plenty
of instability and sufficient moisture for heavy rainfall...so the
risk of flooding is non zero. Will hold off on any Marginal risk
areas given the very high FFG over the region and
uncertainty...but will continue to monitor and may potentially
need to introduce areas in future updates.

Chenard


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