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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0804Z Sep 27, 2021)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Mon Sep 27 2021

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

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

...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...

...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

The Day 3 outlook will be updated by 0830Z.


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