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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0034Z Feb 21, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
733 PM EST Tue Feb 20 2024

Day 1
Valid 01Z Wed Feb 21 2024 - 12Z Wed Feb 21 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOS ANGELES METRO AREA...

...01Z Update...
The on-shore flow within the core of the atmospheric river has
generally been 20 kt to 25 kts as the axis has been making its way
southward in southern California...leading to a relative lull in
rainfall rates from this afternoon into the evening hours with
MRMS showing rates generally at or below a quarter of an inch per
hour.  Latest runs of the HRRR and the higher resolution guidance
from the 20/12Z and 20/18Z runs...however...show the potential for
rainfall rates to become re-invigerated later tonight as mid-level
temperatures drop/mid-level lapse rates steepen in response to the
trough axis coming inland. After several runs of the HRRR had
backed off on amounts...the 21/22Z HRRR once again started showing
potential for an additional 0.5+ inch amounts within the eastern
portion of the Moderate Risk area developing around 07Z.  Given
the presentation on satellite...the 22Z HRRR seemed more viable of
a solution and only a minor realignment was made to the outlook
areas.  Overall...though...forecast reasoning changed little since
the 20/16Z outlook.  \

Bann


...16Z Update...
The previous forecast reasoning still holds, with the heaviest
rainfall forecast along the Transverse Ranges in southern
California where onshore flow will be orthogonal to the mountain
face. The Moderate Risk covering this area was expanded southwards
a small amount, just touching the northern San Diego metro area,
to account for a southward trend in hi-res model QPF. HREF EAS
probabilities of at least one inch of rain remain high (60-90%)
along the Transverse Ranges, but have lowered over the Los Angeles
and northern San Diego metro areas due to increased forecast
spread among the hi-res guidance. Even though spread has
increased, enough of the model guidance continues to suggest
heavier QPF over the metro areas that the Moderate Risk is still
justified. Elsewhere, only minor adjustments were made to the
Slight and Marginal Risk areas. The Slight Risk includes most of
coastal California and much of the Central Valley where
thunderstorms with locally heavy rainfall will be possible this
afternoon. The Marginal Risk area extends from the Southern Oregon
Coast into the northern California Coast, down through the San
Joaquin Valley, and into the desert regions of southern California
and Nevada, including the Las Vegas metro area.

Dolan

...Previous Discussion...

Rain associated with the atmospheric river and surface front will
continue spreading across California, south to the Mexico, and
northeastward from there into southern Nevada and the Las Vegas
area during the morning hours. As the day progresses the rain will
lessen for portions of northern and central California while
continuing across southern California into midday and adjacent
southern Nevada/northwest Arizona through the afternoon. As this
will be the second half of the next bout of heavy rainfall which
starts late this evening, flash flooding that begins tonight will
continue to worsen through the morning hours as the rain
continues. A 150 kt southwesterly jet will be flowing in parallel
with the rainfall, keeping the overall atmospheric flow
unidirectional. Thus, while individual showers and any embedded
storms will be moving very quickly, training will be constant. In
the corridor of the Atmospheric River, expect steady rainfall for
most areas.

A relative lull in the rain is expected during the afternoon hours
before a more robust round of widespread rainfall with embedded
showers and a few storms track along all of the central and
southern CA coast. The swift motion of the shortwave trough and
accompanied strong upper level jet will be supportive of yielding
rates of 1 inch/hr in some of the embedded cells while elsewhere
hour rates will be int he 0.25 to 0.50 inch/hr range. These
accumulations over the water logged L.A. basin will further
amplify the impact of flooding across the area. A Moderate Risk
area was adjusted and maintained for parts of Los Angeles, San
Bernardino, Orange, Riverside and San Diego Counties to reflect
the current threat level for excessive rainfall and flooding
impacts.

Despite lesser amounts of rain further north west up the coast
leaving the area in the Slight, the widespread rain from as far
north as Monterey all the way to the Mexican border should still
make for widespread impacts from new flash flooding. For central
and northern CA, the heaviest rain is expected to occur roughly
during the same time period southern CA is having a break,
generally from 18Z through 05Z. The same shortwave trough that
will enhance rainfall overnight Tuesday night in southern CA will
hit northern areas first, with local rates to 3/4 inch per hour.
This area has had multiple days of rainfall, so here too all new
rain will convert to runoff and worsen ongoing flash flooding. No
significant changes were made to the ongoing Slight Risk area. The
rain will be heavy at times and moving slower than further south,
but the axis of heaviest rain will be oriented south to north, as
the plume of moisture advances east, which will overall shorten
the duration any one area is getting rain. Thus, the Slight Risk
remains in effect.

The Marginal Risk area encompassing the lee of the San Gabriel
Mountains northeast through the rest of San Bernardino County and
into southern Nevada was maintained. Upper level lift will support
light rain continuing northeastward in a plume from the L.A.
Basin, and the desert conditions may not support the long-duration
of generally light rain, so flood prone arroyos may flash flood on
isolated occasions through the High Desert.

Campbell/Wegman



Day 2
Valid 12Z Wed Feb 21 2024 - 12Z Thu Feb 22 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHERN
CALIFORNIA...

...21Z Update...

The afternoon forecast maintained continuity with only a Marginal
Risk area over portions of Southern California. The slow-moving
frontal system that produced rain in this region over the previous
few days will be moving inland, and the heavy rainfall threat will
decrease quickly Wednesday morning as the cold front pushes south
and east of California. Additional rainfall totals look to remain
fairly low, but the soils will be saturated so any additional
rainfall could pose a flooding threat.

Dolan

...Previous Discussion...

The last of the heavy rain will be during the morning hours as the
strong, fast-moving shortwave sweeps through from west to east.
New accumulations will generally be less than 0.50 inch before
there will be a sharp cut off of the rain and clearing of skies
across southern California. Given the highly saturated soils and
new accumulations expected, a Marginal Risk for excessive rainfall
and flash flooding remains in effect. The latest trends in the QPF
and placement support a small northward/eastward expansion for San
Bernardino, Riverside and San Diego Counties. With the rainfall
ending very early in the period this will likely be dropped with
the mid-morning update, finally ending this multi-day atmospheric
river event. Expect dry conditions throughout California for the
rest of the day Wednesday.

Campbell




Day 3
Valid 12Z Thu Feb 22 2024 - 12Z Fri Feb 23 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE OHIO VALLEY...

...21Z Update...

Latest model guidance suggests a northward and westward trend in
the QPF footprint, so the Marginal Risk area in the Ohio Valley
was expanded north to include the Pittsburgh, Cleveland, and
Indianapolis metro areas, and west to include nearly all of
southern Indiana and part of western Kentucky. The ingredients
will be in place for more efficient rainfall rates to develop
during the morning/afternoon convection pattern evolving with the
approach of a progressive mid-level shortwave. The GFS shows
MUCAPE reaching 700-1000 J/kg with steep mid-level lapse rates and
anomalous moisture in place, and there will be upper level support
for developing showers and storms as a secondary shortwave moves
over the region. Rainfall rates could approach 0.5-1" per hour,
which nears the lower threshold of the 1 hr FFGs in the Ohio
Valley.

Dolan

...Previous Discussion...

The environment is setting up to support locally heavy rain during
this period within a moisture plume in a warm air advection
regime. With some instability in the area and an upper level
trough nosing in to increase forcing for ascent, rainfall
intensity of 0.5 to 1 inch/hr may realize during this period,
along with training of thunderstorms. Local 1hr/3hr flash flood
guidance is as low as 1/1.5 inches over a majority of the Ohio
Valley. A Marginal Risk for excessive rainfall remains in effect
for the region given the elevated threat for flooding concerns.

Campbell


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