Quantitative Precipitation Forecast
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004
FOUS30 KWBC 220043
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
743 PM EST Thu Nov 21 2024

Day 1
Valid 01Z Fri Nov 22 2024 - 12Z Fri Nov 22 2024

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

...01Z Update...

In coordination with MTR/Monterey, CA forecast office, the High
risk area was expanded southward with this update to account for
the heavy rains that have impacted much of Sonoma County.

The plume of rainfall impacting much of northern California has set
up a little bit south of where most of the HiRes guidance has
suggested the heaviest rain would be. This has somewhat reduced the
rainfall amounts in extreme northwest California and southwest
Oregon, but consequently increased them today just north of the Bay
area. Ongoing flooding across Sonoma County has closed roads along
the Russian River west of Santa Rosa.

Through tonight, a cold front associated with an approaching
deepening low will both increase rainfall rates ahead of the front
in the atmospheric river plume, but also pull the heaviest rain
north and more parallel to the coast, to include far northwestern
California and southwest Oregon. Thus, despite the southward
adjustment today, the northern portions of the risk areas should
still pick up a prolonged period of heavy rain with rates to 1 inch
per hour through tonight.

Wegman

Day 2
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

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

...20Z update...

As a new surface low deepens ~300 miles west of the Oregon coast
early Friday morning, the attendant cold front and associated
frontal band of heavy rain is expected to be near or just inland
of the coastal CA/OR border. While the threat for rain rates
greater than 0.5 in/hr will be quickly lowering for southwestern
Oregon into northwestern CA, antecedent rainfall since Tuesday and
localized potential for SW to NE oriented bands could support
runoff from additional heavy rain with peak rain rates near 0.5
in/hr well into the period.

Farther south, very little change was needed for the Moderate Risk
in the northern Sierra Nevada and what constitutes a higher end
Slight Risk from Humboldt County to just north of San Pablo Bay.
Antecedent rains combined with rain rates of 0.5 to near 1 inch per
hour will likely result in numerous flooding concerns, especially
across burn scar locations.

Otto

...previous discussion follows...


The long duration atmospheric river begins to become more
progressive and wane in intensity Friday as a cold front finally
pushes the moisture axis southward. However still expecting rather
widespread 0.25" per hour rates with this last push of rainfall,
with localized 0.5" per hour amounts likely. These higher rates
should only last for a couple hours during the morning over coastal
areas, but should persist a bit longer in the Sierra Nevada.
Overall expecting 1-3" of additional rain over coastal areas, with
3-6" over the northern Sierra Nevada. Warmer temperatures will keep
snow levels pretty high such that even into the terrain areas,
most of the precipitation is expected to fall as rain (aside from
the highest peaks).

By Friday conditions over much of the area will be saturated, and
this last push of elevated rainfall rates on top of saturated ground
will likely result in a continued flood threat...with additional
landslides likely and a continued risk of burn scar flash flooding.
Only minor changes were made to the inherited ERO areas. A MDT risk
remains for the northern Sierra Nevada where the heavier rainfall
will persist longer into Friday resulting in the aforementioned
higher additional rainfall totals. A flooding risk is likely to be
ongoing over coastal areas as well Friday morning, but should be a
shorter lived threat with the higher rainfall rates coming to an end
quicker...thus will continue to carry a Slight risk here.

Chenard

Day 3
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

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

...20Z update...

Similar to the D2 outlook, the cold front will be near the northern
end of the Marginal Risk area Saturday morning in the northern
Sierra Nevada, advancing toward the southeast. The associated plume
of IVT will be quickly weakening and moving south, but as discussed
in the previous discussion, antecedent rainfall may leave a small
area of the Sierra Nevada vulnerable to localized flash flooding
from an additional 1-2 inches. Instability is forecast to be low
during the daytime hours and fairly shallow. When steeper mid-
level lapse rates arrive beyond 00Z, the loss of daytime heating
should further reduce any instability present.

Will leave the Marginal Risk in place for now, but as the forecast
window draws nearer and hires model guidance becomes available, the
need for continuing the Marginal Risk can be reassessed.

Otto

...previous discussion follows...


At 12z Saturday the cold front and associated moisture plume will be
dropping south across the central Sierra Nevada. Rainfall intensity
will be dropping fairly quickly by this time and the prolonged
atmospheric river event will finally be coming to an end. We will
continue to carry a small Marginal risk over portions of the Sierra
Nevada where an additional 1-2" of rain is possible Saturday
morning. Typically rainfall of this magnitude would not be a
concern. However, much of this risk area will have seen at least 3-
7" of rain total from the event, so even the modest additional
rainfall Saturday morning could result in a flood threat, and thus
continuing with a Marginal risk for now seems best. Although the
risk is certainly lower than previous days, and quite possible this
risk could be removed on future updates if additional rain after 12z
Saturday trends down.

Chenard

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