FOUS30 KWBC 190828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
328 AM EST Tue Nov 19 2024

Day 1
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...

...Central Gulf Coast...

Latest satellite imagery is showing enhancing convection to the 
south of southeast LA associated with the anomalous tropical 
moisture surge ahead of the cold front moving east across the 
Central Gulf Coast. Trends in the latest hi res guidance suggest 
this offshore enhancing convection will become the heaviest
frontal/pre-frontal rains as organized convection currently along 
the front farther to the north begin to weaken after 1200 UTC. Hi 
res guidance is in fairly good agreement that the heaviest rains 
with the offshore enhancing convection will remain just to the 
south of far southern MS, far southern AL and the western FL 
Panhandle, but still affecting far southeast LA. HREF neighborhood
probabilities show that the axis of the highest probabilities for 2
and 3"+ amounts will remain offshore with the northern edge of 
these higher probabilities affecting areas from far southeast LA 
and along the immediate far southern MS, far southern AL and 
western FL Panhandle. Changes to the previous outlook were to trim 
the northern extent of the slight risk area by approximately 20 nm
and to trim the western extent of the slight risk by 
approximately 40nm to account for the hi res trends. At the 
moment, the consensus on timing of convection supports removing the
New Orleans metro area from the slight risk, with the heaviest 
rains likely to the east and southeast prior to 1200 UTC Tuesday. 

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

Latest models continue to be in good agreement on the rapid 
cyclogenesis event across the northeast Pacific off the Pacific 
Northwest coast, signaling the beginning of a multi day Atmospheric
River event for northwest CA into far southwest OR. Anomalous low 
level southerly flow expected along and ahead of the associated 
cold front Tuesday night into early Wednesday, with 850-700 mb 
moisture flux anomalies of 3-4 standard deviations above the mean 
impacting the coastal Pac NW, south into 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 Wednesday. The 
strong southerly low level flow will be mostly parallel to the 
terrain along the Pac NW coast, limiting hourly rainfall rates to 
.10-.25"+. Greater hourly rates likely across far southwest Oregon 
into coastal NW CA where a more significant upslope component to 
the low level flow is likely. Hi res guidance is fairly similar in 
showing hourly rates of .25-.50"+ in the 0600 to 1200 UTC Wednesday
period across far southwest OR into far NW CA. HREF .50"/hr 
probabilities subsequently are high across this area, with 1"/hr 
probabilities much lower and primarily offshore. No changes made to
the previous slight risk area, which corresponds well to the above
mentioned axis of high .50"+/hr HREF neighborhood probabilities. 

Oravec


Day 2
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

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

The multi day atmospheric river event the began late day 1 for far
southwest Oregon into northwest California will continue day 2,
primarily across northwest California. The rapidly deepening low 
over the northeast Pacific day 1 will anchor a nearly stationary 
upper vortex off the Pac NW coast day 2. Strong persistent deep 
layered west southwest flow on the south side of this vortex will 
impact northwest California day 2, 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. The previous 
moderate risk area was adjusted slightly southward by approximately
10-40 nm to account for model max axis consensus. Hourly rainfall 
amounts of .25-.50"+ likely through the day 2 time period across 
northwest CA with increasing flooding risks with time as soils 
become saturated and streams and river levels rise. The HREF 1 hour
probabilities for .50"+ totals for the first half of day 2 when 
the data is available, remain high across northwest CA. 

Oravec

Day 3
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 221 2024

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

The deep northeast Pacific Vortex remains nearly stationary day 3
off the Pacific Northwest coast, supporting the continuation of the
multi day Atmospheric River event for northwest CA into far
southwest OR. Anomalous deep layered southwesterly flow will
continue on the south side of this vortex. 850 to 700 mb moisture 
flux anomalies will continue to be 2 to 4+ standard deviations 
above the mean with IVT values also maintained in the 500 to 800 km/m-1
s-2 range across northwest CA into far southwest OR.  There is a 
likelihood of very heavy rains again falling day 3 over similar 
regions from the day 2 period. Hourly rates of .25-.50"+ likely to 
continue at times supporting additional 24 hour rainfall amounts 
of 3 to 6 inches and isolated additional 24 hour totals of 6-8" 
across northwest CA into southwest OR. Storm total amounts of 12 to
16+ inches possible across portions of northwest California. Given
this day 2-3 heavy rain overlap potential, a small high risk area 
was introduced over northwest CA, across the west central portions 
of the EKA's CWA. Flood risks will continue to increase day 3 along
with rock and land slides as soils become saturated and streams 
and rivers continue to rise and overflow. There will be a 
northward shift back into far southwest Oregon day 3 for the heavy 
rain potential as the upper flow back and heights rise. The rising 
heights will also keep much of the day 3 precip as rain, increasing
overall runoff. 


Oravec
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