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AGUS76 KRSA 171643

HMDRSA



Hydrometeorological Discussion

National Weather Service / California Nevada RFC / Sacramento CA

830 AM PST Sat Feb 17 2024



...A PAIR OF SYSTEMS TO BRING PERIODS OF WIDESPREAD PRECIPITATION TO 

THE REGION THROUGH TUESDAY...

...ISOLATED THUNDERSTORMS IN THE AFTN/EVE THRU AT LEAST MON OVER 

PARTS OF NRN CA...



.METEOROLOGICAL CONDITIONS (SAT AM - FRI AM)...



The first storm system has arrived this morning as blended TPW 

imagery shows a moisture plume with a 1-1.50" PW core stretched from 

Cape Mendocino through the Monterey Bay and all the way back to the 

Hawaiian Islands. This moisture is riding the back edge of a ridge 

and the southern end of a large surface/upper low over the Gulf of 

Alaska. Radar is recording precip streaming into the Bay Area with 

the moisture plume and through to the northern Sierra with a second 

band of precip along the north coast. Lightning detection has also 

picked up several strikes west of northern CA just behind the main 

cold front. an upper trough will also approach northern CA the rest 

of this morning as the moisture plume heads to the south. The trough 

will become a more defined shortwave by late this afternoon reaching 

just offshore of the north coast as the moisture plume arrives at 

Point Conception. This will keep precipitation going across northern 

and central CA the rest of today as well as provide chances of 

thunderstorms this afternoon/evening. HREF exceedance probabilities 

show a 20-50% chance of 0.50"+/hr rain rates along the norCal coast 

later today. The upper trough will lift through nw CA tonight into 

OR as the moisture plume focuses itself across soCal. Highest 

precipitation amounts from this first system are expected across the 

northern CA coast (especially Cape Mendocino/Kings Range: 2-4"), the 

central coast mountains, Shasta, and the northern/central Sierra at 

1-2.50". 



The second system will approach Sunday from the west as a 

surface/upper low. The low will begin to pull moisture from today's 

plume back northward into central CA Sunday morning. Precipitation 

intensity is expected to pick up along the coast by the afternoon as 

the low continues to approach. The presence of the large low and 

moisture will also allow for thunderstorm chances again across 

northern CA and into the Sac Valley in the afternoon/evening. HREF 

exceedance probabilities are predicting 30-70+% chances of 0.50"+/hr 

rainfall rates across the Sierra, Shasta, and central coast 

mountains in the afternoon/evening and a 10% chance of 1"/hr rates 

in isolated spots. More widespread moderate/heavy precip is then 

forecast overnight as the low begins to shift northeastward and 

continues to draw moisture across the state from offshore.



Monday morning, the low is forecast to reach the 130W line still 

offshore as the front edge of its trough begins to move over CA. The 

moisture plume will then begin to shift back south of Point 

Conception. These factors should keep precip going across the state 

Monday morning with the highest amounts focused over the western 

Transverse Range, Big Sur Coast, and Sierra. An additional surge of 

precip over the northern CA coast, mainly from Sonoma to Cape 

Mendocino, is expected later Monday morning and afternoon as the low 

moves closer. Models do have parts of the southern Sierra/Santa 

Lucias/western Transverse in the left exit region of an upper jet 

for much of Monday, but the main moisture plume does not seem to 

coincide at that time instead concentrating over far southern CA. 

The surface/upper low will remain just offshore of the north coast 

for much of Monday and along with lingering moisture will result in 

additional thunderstorm chances for northern/central CA, 

particularly over the Sacramento Valley where SPC has a marginal 

risk of severe thunderstorms.



Models have the moisture plume stalling across far southern CA 

Monday afternoon and barely moving for the next 24 hours finally 

making its exit Tuesday evening/overnight. Meanwhile the core of the 

surface and upper low will move northward towards the PacNW as 

another shortwave spins off the western side of the low before 

beginning to separate and head southeastward well west of Point 

Conception. This will result in an elongated troughing pattern 

offshore of the west coast later Tuesday afternoon. Lingering 

moisture and the upper dynamics will keep scattered showers going 

across much of the region Tuesday. Favorable flow will result in 

enhanced precip over the mountains areas again with lesser amounts 

elsewhere. The southern edge of the trough is then expected to head 

eastward towards land Tuesday night. The GFS is quicker at bringing 

the trough inland and across soCal than the ECMWF. The GFS has the 

trough axis pass through soCal Wednesday morning while the ECMWF 

still has it offshore. This is resulting in the GFS forecasting 

heavier precip across soCal Wednesday morning than the ECMWF (nearly 

1" vs a few hundredths). Either way the trough looks to exit the 

region by Wednesday evening leaving only light scattered showers in 

its wake. Behind that, ridging will build in for the rest of the 

week allowing dry conditions to return the rest of the period.



To summarize, two systems are forecast to impact the region this 

weekend into early next week. The first a front/upper trough moving 

through northern CA and into OR today/tonight along with a moisture 

plume moving south along the coast bringing precip to 

northern/central CA. The second as a large surface/upper low slowly 

approaching from the west and eventually heading northward towards 

WA/OR. This second low will draw back tropical moisture from 

offshore of soCal on Sunday resulting in widespread moderate to 

locally heavy precip late Sunday through Tuesday. There is still 

uncertainty in the QPF, particularly with the second system. 

Disagreement remains mostly regarding how much heavy precip will 

spread across nw CA and how much precip will fall across area 

mountains. It always difficult to predict how much orographics will 

enhance precip, but additionally the movement of the low and 

moisture plume will also play a significant roll. Models often 

struggle with these offshore lows and moisture plumes which makes 

forecasting QPF difficult. 24 hour ensemble spread at Arcata remains 

over 2" in both the GFS and ECMWF. The 24 hour spread in the 

Transverse Range is over 4". 



The other issue is Wednesday. The differences between the GFS/ECMWF 

regarding the movement of the upper trough carry into the ensembles 

as well. The 24 hour QPF clusters ending Weds afternoon illustrate 

this well. They're split into 3 groups at 60%, 24%, and 15%. Cluster 

1 is mostly dry across soCal, but contains 78% of the ECMWF 

ensembles. The GFS ensembles are split into thirds across each 

cluster group. The wettest is cluster 3 which predicts another 1-2" 

across coastal soCal and nearly 3" in the Transverse range. The 

official QPF is on the dryer side given the CMC ensembles are about 

55% dry as well in line with the ECMWF, but it is worth pointing out 

that wetter conditions are possible.



The official QPF was a blend of WPC and the latest NBM and the 

previous forecast. Heaviest precip is forecast along the CA coast 

from LA County northward, across the coastal mountains, Shasta, and 

the Sierra. QPF 18z Sun-00z Thurs: 1.50-3+" CA coast LA northward 

(3.50-6" mountains locally to 7+ Transverse/Big Sur), 4-8.5" 

northern/central Sierra, 2.5-6" southern Sierra, 4-7" Shasta, 1.5-3" 

Bay Area/central Coast/valleys, and 0.50-1.25" far southern coastal 

soCal.



Freezing levels today 5.5-7 kft north of I-80 and 6.5-9 kft to the 

south to Point Conception. Levels will lower overnight reaching 4.5-

7 kft north of I-80 and 6.5-8 kft to the south. Higher freezing 

levels will spread northward overnight Sunday as tropical moisture 

gets drawn back northward up to 6-9 kft north of Point Conception. 

Monday morning as the low gets closer nw CA will cool down to 5-6 

kft up to 7.5 kft into the central Sierra. The rest of Monday and 

Tuesday freezing levels generally 4.5-7.5 kft and 7.5-9.5 kft over 

soCal. The trough moving through will lower freezing levels again 

Wednesday to 4.5-5.5 kft north of I-80 and 5-7 kft to the south.





QPF graphics are available at www.cnrfc.noaa.gov/qpf.php







AS



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