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FOUS30 KWBC 132016
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Sat Apr 13 2024

Day 1
Valid 16Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE CENTRAL
CALIFORNIA COAST AND INTO THE UPSLOPE OF THE NORTHERN TO CENTRAL
SIERRA...

...16Z Update...

Only very minor tweaks were needed to the inherited Marginal Risk
areas in California. The upslope of the Sierras Marginal was
adjusted west off the higher terrain where at least a good portion
of the forecast precipitation will be in the form of snow.
Conversely, added a bit more of the adjacent Sacramento Valley to
the Marginal due to antecedent wet soils and expected runoff from
the mountains.

Otherwise, very little has changed about the meteorological setup
described in the previous discussion below.

Wegman

...Previous Discussion...

No changes made to the marginal risk areas along the central
California coast or into the upslope regions of the northern to
central Sierra. There continues is good model agreement on the
slow eastward inland movement of height falls from the strong
closed low off the northern California coast day 1. Simulated hi
res radars continue to depict a well defined area of precip moving
inland along the associated front in the first half of day 1
across the central to southern California coastal regions and
inland into the upslope regions of the northern to central Sierra.
Anomalous PW and 850-700 mb moisture flux values, 1 to 2+ and 1 to
3+ standard deviations above the mean respectively, expected along
the slowly southeastward moving frontal boundary. Hourly rainfall
rates with the lead area of precip expected to max out in the
.25"-50"+ range. HREF 1 hour neighborhood probabilities for .50"+
amounts show relatively small areas moving along the central CA
Coast Range and Transverse Range with the frontal precip band.
There are no HREF probabilities for hourly amounts greater than 1".
This initial precip area will then be followed by a secondary
precip area developing along the central CA coastal range from San
Francisco south to just north of Point Conception as the main
upper vort swings southward along the central California coast.
HREF neighborhood probabilities for .50"+ hourly amounts are lower
with the second precip area compared to the lead area. Most of the
hi res runs show hourly totals in the .10-.25" range along the
central CA Coast Range with the second precip area. Given the
relatively low probabilities for .50"+ hourly amounts, the risk was
kept at marginal with flash flooding remaining isolated.

Oravec


Day 2
Valid 12Z Sun Apr 14 2024 - 12Z Mon Apr 15 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
COASTAL SOUTHERN CALIFORNIA AND THE CENTRAL APPALACHIANS...

...2030Z Update...

...Southern California Coast Ranges...

No changes were needed to the inherited Marginal Risk area for the
southern Santa Lucia and Transverse Ranges. Onshore flow supported
by a vertically stacked low off the coast of central CA will
transport abundant Pacific moisture into the Marginal Risk area
Sunday. Amounts will not be overly impressive as the rainfall
pushes east and weakens with time, but saturated soils in this area
and 1-2 inches of rain should be enough to cause isolated flash
flooding concerns.

...Central Appalachians...

Introduced a Marginal Risk area from northern WV northeast to the
Catskills of NY with this update. A strong cold front will push
southward on Sunday, supported with plenty of moisture from the
saturated soils across much of the Northeast due to recent heavy
rainfall. The front will push south and east with time, so portions
of central NY and northeast PA will see the strong storms first as
MUCAPE values of 1k to 2k are drawn northward ahead of the front
and marginal (PWATs to 1.25 inches) atmospheric moisture advects
into the area.

As the front moves southward, storms will form more to the west
into WV and southwest PA during the afternoon and evening. Soils
here are even more sensitive than areas further northeast, and
while the storms will largely move southeast, perpendicular to the
axis of the front, ideal instability and enough moisture will
allow for the storms to become strong enough to pose a flash
flooding threat. CAMS guidance suggests that for most areas, a
single round of showers and storms is likely. However, given that
nearly all of the rainfall will convert to runoff, and upsloping
may allow some of the storms to "hang up" on the ridges, a higher-
end Marginal threat exists. Any slowing of the storms or increase
in moisture would quickly lift southwest PA into the Slight risk
category, so the area will continue to be closely monitored.

Wegman

...Previous Discussion...

The secondary area of precip dropping southeast along the central
California coast late day 1 will continue to push eastward
across coastal southern California day 2. No significant changes to
the previous small marginal risk area through the Transverse Range
of southern California. Hourly rainfall rates will mostly be in
the .10-.25" range with this secondary area of precip, which is
expected to be fairly progressive. These hourly rates and the
progressive nature of the precip area should keep any runoff issues
as isolated.

Oravec

Day 3
Valid 12Z Mon Apr 15 2024 - 12Z Tue Apr 16 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE PLAINS...

...2030Z Update...

No major changes were made to the inherited Marginal Risk over
portions of the Plains. The parent low in the guidance is moving a
bit faster, so the Marginal was trimmed a couple rows of counties
from the west.

On the southern end of the line across OK and KS, instability
greater than 2,000 J/kg and decent moisture transport along an LLJ
will support severe thunderstorms that will track across those
states. The instability will favor the strongest storms carrying
hail, which will diminish the flooding threat. Further, soils in OK
and KS are a bit drier than normal. Finally, the storms will be
moving eastward at a decent speed, largely perpendicular to the
frontal axis. These factors will all work against any more than an
isolated threat for localized flash flooding where any heavier
rains persist longest and away from any hail cores.

To the north across the Dakotas, a few variables are a bit more
favorable for the development of flash flooding, including a bit
lower instability that will diminish the hail threat in favor of
heavy rain, and the northern comma-head of the developing low
largely staying in place/moving slowly. This will favor a larger
precipitation shield that will have embedded convection as the
instability from the south advects north with the dry slot above
the lower atmospheric moisture. Thus, forecasted rainfall through
12Z Tuesday is higher than further south. The primary factor
working against flash flooding are the dry antecedent conditions
and overall flood resistance of this area. Greenup has yet to begin
to any significant degree in this region, so that will allow more
rainfall to convert to runoff. Nonetheless, with the parent low to
the south still developing, it will take a while into Monday night
for the low to get its act together. This will generally
diminish the overall forecast precipitation footprint across the
northern Plains.

Wegman

...Previous Discussion...

The closed low that is expected to move inland from central
California into the Great Basin day 2 will be ejecting
eastward through the Central Rockies into the Central High Plains
day 3. We continue to depict a broad marginal risk area through
portions of the Plains in a region of strengthening low level
southerly flow and increasing PW values. PW values expected to
increase to 2 to 3 standard deviations above the mean along and
ahead of the strengthening north to south oriented frontal boundary
expected to push eastward into the Plains on Monday. This and and
a broad region of instability along and ahead of this front will
support increasing convection, especially late Monday afternoon
into the early hours of Tuesday. There continues to be a large
spread with qpf details among the models at the day 3 period. We
kept a fairly broad marginal risk area to cover this spread,
although we did narrow the marginal risk area to better fit the
latest spread. Two areas of concern are for a convective line
forming late Monday afternoon into the early hours of Tuesday along
and ahead of the front over the Southern to Central High Plains.
The other area of concern would be with a slow moving comma
head/deformation precip area farther to the north to the north of
the closed low track. There is somewhat better qpf agreement with
the comma head/deformation precip area compared to the convective
frontal max farther to the south. With precip totals generally
below average over the past few weeks across large portions of the
Plains, and current model qpf spread, the risk level was kept at
marginal.

Oravec

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt