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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0055Z Sep 27, 2021)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
855 PM EDT Sun Sep 26 2021

Day 1
Valid 01Z Mon Sep 27 2021 - 12Z Mon Sep 27 2021

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

...01Z Update...
No modifications were made to the Marginal Risk area in the
Southwest as showers and thunderstorms continue, mainly across AZ,
NM and into far western TX.  This activity, which is associated
with an upper level low, should diminish in coverage through the
next few hours with loss of daytime heating and instability. 
Though, periods of heavier showers will likely continue under the
main cut-off low in AZ with the potential for training through the
overnight.

Pagano

Previous Discussion...

...Southwest U.S./Southern Great Basin...
Maintained a Marginal Risk area with only modest modifications
based on latest guidance.  The overall pattern still focuses on a
mid- and upper-level system making its way east/northeast during
the period which supports showers and thunderstorms...especially
from mid afternoon into the evening due to steep lapse rates
resulting from maximum heating of the day in low levels and cold
temperatures aloft.  Forecast soundings still show unsaturated
sub-cloud layers which may counteract the rainfall rates
initially...and there should be areas of CIN which limits areal
coverage away from the terrain. Several days ago...the NCEP
guidance and ECMWF were in agreement that the cold temperatures
aloft would result in CAPE values approaching 2000 J per kg over
the Mogollon Rim.  With the event now being closer in time, the
models are showing the coldest temperature aloft staying farther
south that earlier forecast. This could spell lighter rainfall
rates which would work against flash flooding but may still result
in isolated run off problems in the event of multiple rounds. 

Bann



Day 2
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....

21Z update... Very minimal adjustments made to the Marginal Risk
area that was already in effect to reflect the latest WPC QPF.

Campbell

Previous Discussion...

...Southwest U.S/Southern Great Basin....
Scattered convection will persist across portions of the Southwest
while spreading into portions of southern Colorado. The latest
guidance continues to suggest an area from southwest Colorado to
north-central New Mexico could have localized higher amounts
especially in the areas of higher terrain. The models continue to
show an axis of one-quarter to two-thirds of an inch in and near
the Mogollon Rim in northern Arizona.  But many of the same
concerns expressed in the Day 1 period...where the steepest lapse
rates/best instability/highest rainfall rates set up and the
impacts of any cloud cover from the previous evening...linger in
this cycle. Maintained the Marginal for the time being.

Bann


Day 3
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...

21Z update... The southern bounds of the Marginal Risk area was
expanded a smidge to southward and the northeast edge was expanded
eastward toward south-central Colorado. These adjustments
reflected changes made in the new WPC QPF.

Campbell

Previous discussion...

...Colorado/New Mexico/Arizona...
The mid- and upper-level system slowly making its way eastward
will still be accompanied by mainly late day and evening showers
and thunderstorms.  Present indications are that the best overlap
of deeper moisture (precipitable water values between 0.75 inches
and 1.00 inches) and the best instability (where 500 mb
temperatures are forecast to be on the order of -10 degrees C) are
expected to overlap.  With the upper low weakening and
temperatures aloft beginning to moderate, not sure how much of a
flash flood risk remains as Day 3 progresses but at least a few
ingredients remain in place so maintained the Marginal Risk area.

Bann



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