FOUS30 KWBC 182006
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Mon Nov 18 2024

Day 1
Valid 16Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST
LOUISIANA INTO SOUTHERN MISSISSIPPI...

16Z Update...

Only changes of note for this update were to trim back the Marginal
Risk area across the central Plains to account for where the
rainfall has ended or is about to come to an end. Elsewhere, only
minor adjustments were made to the Marginal and Slight Risk areas
near the central Gulf Coast where the latest hires model guidance
continues to support a heavy and locally excessive rainfall threat.

Orrison


Previous Discussion...

...Central Gulf Coast...

An axis of much above average tropical PW values expected to be
drawn rapidly northeastward from the remnants of Sara, across the
central Gulf of Mexico and into the Central Gulf coast ahead of the
negatively tilted closed low ejecting northeastward from the
Southern Plains into the Upper Mississippi Valley day 1. These
anomalous tropical PW values, 3 to 4+ standard deviations above the
mean will enhance convection along and ahead of the cold front
pushing eastward across the Gulf coast, primarily during the second
half of day 1 from southeast Louisiana into southern Mississippi.
A slight risk area was maintained where the latest HREF
neighborhood probabilities are highest for 2 and 3"+ amounts. The
simulated hi res radars are in good agreement in showing potential
for a period of training of cells in this anomalous PW axis in the
0000-1200 UTC Tuesday time period across the slight risk area of
southeast LA into southern MS. In areas of training, hourly
rainfall amounts of 1-2"+ possible, resulting in flash flooding,
especially in the urbanized areas of New Orleans.

...Central Plains...

Across the Southern to Central Plains toward the Upper MS Valley...there
is fairly good agreement on an axis of heavy rain from northern
OK, across much of KS, eastern NE, northwest MO, southeast SD, much
of IA, southern MN into southwest WI in a region of strong
isentropic lift ahead of the deepening surface low moving quickly
northeastward across these areas. While widespread heavy amounts
are likely, hourly rates will are mostly depicted as .25-50"+ in
the latest hi res guidance. This should keep the threat of any
runoff issues at marginal. The previous marginal risk area was
trimmed on the eastern end, taking out much of IA, MO and eastern
OK. The current marginal risk coincides well with the axis of the
highest HREF 2"+ day 1 probabilities.

Oravec

Day 2
Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL GULF COAST AND FOR PORTIONS OF NORTHWEST CALIFORNIA AND
SOUTHWEST OREGON...

20z Update: A Slight risk was added to coastal northwest CA into
southwest OR. An Atmospheric River will begin impacting this area
Tuesday night, and we should see the potential for some intense
rainfall rates as the cold front moves ashore. Several 12z HREF
members indicate a well defined narrow band of low topped=20
convection along this front that will be capable of producing=20
heavy rainfall rates along coastal areas. Both the HREF and REFS=20
show a high probability of 0.5" per hour rainfall as the front=20
moves inland. The convective intensity should weaken as it moves=20
inland, however some of that will be offset by orographic=20
enhancement...so while 1" per hour probabilities drop off over=20
land compared to over water, there are still some low end=20
probabilities. Rainfall through 12z Wed will generally be in the=20
2-5" range over the Slight risk, which combined with the high rate
potential suggests a Slight risk upgrade is warranted as some=20
flooding concerns could arise Tuesday night.

Only minor changes made to the Slight risk over the central Gulf=20
Coast as that looks on track.

Chenard

...Previous Discussion...

The convection that is expected to enhance along the Central Gulf
coast late day 1, will continue into day 2 as the surface front
continues to push east along the Gulf Coast. The tropical PW
values entrained ahead of this front day 1, will continue to push
across central to eastern Gulf coast day 2. Training of cells in a
southwest to northeasterly direction likely to continue Tuesday
morning from southeast LA into far southern MS, far southern AL and
the western FL Panhandle. There is some timing differences as to
whether heavy rains will continue in the vicinity of New Orleans
early Tuesday, or have pressed just to the east. The New Orleans
metro area was kept in the slight risk given potential for an
overlap of day 1 and day 2 heavy rain areas. The latest HREF 12
hour neighborhood probabilities ending 00Z Wed show the highest
probabilities for 2 and 3"+ amounts remaining across far southeast
LA, far southern MS, far southern AL and the western FL Panhandle.
This was the region depicted in a slight risk for day 2, with the
previous risk area trimmed on the northern end across south central
AL after collaboration with WFO MOB. The slight risk area
corresponds with the hi res consensus for the greatest hourly
totals of 1-2"+ in regions of training.

...Coastal Washington, Oregon and far northwest California...

Models are in good agreement with the rapid cyclogenesis event
across the northeast Pacific off the Pacific Northwest coast. The
low level southerly flow along and ahead of the associated cold
front will become very anomalous Tuesday night into early
Wednesday, with 850-700 mb moisture flux anomalies of 3-4 standard
deviations above the mean impacting the Pac NW/far northwest CA
coast. A well defined area of frontal precip likely to push inland
across coastal Pac NW/far northwest CA after 0000 UTC. Hourly
rainfall rates in the .25-50"+ range possible, especially across
southwest Oregon into far northwest CA where the strong southerly
low level flow will have a better upslope component. For areas of
northwest California into far southwest Oregon, this will be the
beginning of a multi day Atmospheric River event. No changes made
to the marginal risk area from the previous issuance.


Oravec

Day 3
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...

20z Update: Upgraded portions of northwest CA to a MDT risk with
this update. This is generally for the same area that was already
under a day 4 MDT risk. The stalled Atmospheric River will result
in prolonged moderate to at times heavy rainfall across the area
Wednesday into Wednesday night. Most of the MDT risk area will see
4-8" of rain from the event through 12z Thursday, with some of the
more orographically favored peaks around 10". While not persistent
through the entire period, rainfall rates will likely periodically
exceed 0.5" in an hour. The flooding threat will increase through=20
the period as conditions become saturated and stream/river levels=20
rise. Locally significant impacts will be possible, with the event
continuing into day 4 (Thursday and Thursday night) as well.

Chenard

...Previous Discussion...

The multi day atmospheric river event the began late day 2 for far
southwest Oregon into northwest California will continue day 3. The
rapidly deepening day 2 low over the northeast Pacific will anchor
a nearly stationary upper vortex off the Pac NW coast day 3. Strong
persistent deep layered west southwest flow on the south side of
this vortex will impact northwest California into far southwest
Oregon day 3. with IVT values of 500-800 km/m-1 s-2. and 850-700 mb
moisture flux anomalies of 2-4 standard deviations above the mean.
There is fairly good model agreement with heavy precip totals of
3-5" primarily over northwest California, with isolated totals of
6-8" possible. No significant changes made to the previous slight
risk area.

Oravec

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!457bTluHyvd6iKvEa6pNaQJ1vu4h9sJdBxA8cudbUgVl=
s7CWqu5ER8ABT0Yl5jgCD2n92euj85BFKee9yZxLwjRSipM$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!457bTluHyvd6iKvEa6pNaQJ1vu4h9sJdBxA8cudbUgVl=
s7CWqu5ER8ABT0Yl5jgCD2n92euj85BFKee9yZxLiXDmo7A$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!457bTluHyvd6iKvEa6pNaQJ1vu4h9sJdBxA8cudbUgVl=
s7CWqu5ER8ABT0Yl5jgCD2n92euj85BFKee9yZxLz2H0ZBs$=20

=3D =3D =3D
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