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AGUS76 KRSA 091451

HMDRSA



Hydrometeorological Discussion

National Weather Service / California Nevada RFC / Sacramento CA

750 AM PDT Tue Apr 9 2024



...WARM AND DRY THE NEXT FEW DAYS...

...SHIFT TO COOLER AND UNSETTLED FOR FRIDAY INTO THE WEEKEND...



.METEOROLOGICAL CONDITIONS (SHORT TERM: TUE AM - FRI AM)...



A weak s/wv trof is moving across BC and the Pacific Northwest this 

morning...while an upr ridge aligned just off the west coast will 

make its way inland later today into early Wednesday and strengthen. 

This will bring dry conditions and warmer above normal temperatures 

to the region...peaking either Wednesday or Thursday anywhere from 

about plus 5- to plus 15-degF over seasonal norms. On Thursday...the 

upr ridge will get displaced downstream of the area across the Rocky 

Mountain states...while a deep s/wv trof begins to drop down the 

coastal waters of the west coast and a weak disturbance along 30N 

moves within 130W. Models are hinting at the possibility of some 

rather scattered showers developing near the crest of the southern 

Sierra...and near or just above Shasta Dam during the afternoon and 

evening hours on Thursday as the pattern begins to evolve. 





.METEOROLOGICAL CONDITIONS (LONG TERM: FRI AM - MON AM)...



A large upper low is forecast to approach CA from the nw at the end 

of the week bringing precip back to the region through the coming 

weekend. The det GFS/ECMWF are closer together than yesterday on the 

path and timing bringing initial showers ahead of the system into 

nrn CA Friday morning/afternoon with the core of the low west of the 

Bay Area by early Saturday morning. Models then carry the low south 

along the coast the rest of the day spreading precip across central 

CA and into srn CA. The GFS/ECMWF then have the low west of Monterey 

County by early Sunday continuing a slow southerly pace offshore 

reaching Point Conception in the afternoon before finally heading 

inland across srn CA in the evening. The ECMWF continues to track 

the low closer to the coast than the GFS and is still faster timing 

wise regarding its inland progression. At the end of the period, the 

ECMWF has the core of the low along the CA/AZ border while the GFS 

has it stretched across the srn CA coast near San Diego. 



There is additional uncertainty when factoring in the ensembles, 

which are very different on both the timing and path of the low 

compared to the det runs. The ensembles carry precip inland more 

quickly than the det GFS/ECMWF and also have a variety of solutions 

regarding where and when the low begins to move eastward through CA. 

The 500 mb height clusters are split into 5 groupings each with the 

core of the low taking a different path. The det runs are closest to 

cluster 2, which contains 22% of the GFS/ECMWF/CMC ensemble members. 

This cluster has the upper low around Point Conception at 00z Mon 

similar to the det GFS/ECMWF. Cluster 1, which contains 43% of the 

ensembles, has the core of the low over nrn/cntrl CA while clusters 

3 and 4 (18%/17%) already have the system beginning to move into the 

Four Corners. This disagreement on the timing and path of the low is 

resulting in uncertainty around the QPF. This uncertainty is highest 

around coastal areas as it will depend if the low ends up moving 

inland through nrn CA or cntrl/srn CA and when. Looking at the 48 hr 

QPF clusters ending 00z Mon, this is well illustrated. The clusters 

are split into 4 groups at 37%, 28%, 22%, and 13%. Clusters 3 and 4 

show the result of a more southerly track predicting higher QPF on 

the order of 1-3" over the central coast through the western 

Transverse while cluster 1 is closer to 0.50-1.50" focusing precip 

across nrn coastal CA. 



The disagreement between the ensembles and the det runs have made 

for a difficult forecast as the NBM leans towards the ensembles. The 

official QPF was a blend of morning WPC guidance (timing/locations 

more similar to the det runs) and the latest NBM. The highest QPF is 

expected over the central coast mountains, western Transverse Range, 

and the Sierra. QPF 12z Fri-12z Mon: 0.75-1.50" central coast (1-2" 

mountains), 0.50-1.50" Sierra, 0.30-0.80" north coast/soCal/valleys, 

and 0.10-0.30" for far southern coastal CA. Little to no precip over 

se CA.



Freezing levels will lower Friday from nw to se  down to 6.5-9 kft 

north of I-80 and 8-11 kft to the south in the afternoon. Lower 

freezing levels will spread along the coast as the low passes down 

to 4-6.5 kft west of I-5 and 6.5-10.5 kft to the east Saturday 

morning. These levels will also progress inland the rest of Saturday 

with most of CA down to 4-7 kft Sunday morning and up to 7-10 kft 

towards far nrn/srn CA. By the end of the period the region is 

expected to see freezing levels between 5.5-8.5 kft.





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







Kozlowski/AS



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