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AGUS76 KRSA 011432

HMDRSA



Hydrometeorological Discussion

National Weather Service / California Nevada RFC / Sacramento CA

730 AM PDT Mon Apr 1 2024



...DRY CONDITIONS TO START THE WEEK WILL TRANSITION TO COOLER AND 

WETTER BY THE MIDDLE OF THE WEEK...



.METEOROLOGICAL CONDITIONS (SHORT TERM: MON AM - THU AM)...



As the base of the positively tilted upr trof moves across southern 

CA this morning and afternoon...a few showers will be possible 

mainly over the higher terrain from southern San Bernardino county 

down across western Riverside and San Diego counties. Upstream of 

this feature...an upr ridge over the eastern Pacific is nudging over 

western Canada...the Pacific Northwest and northern CA. This is 

expected to generate dry north to northeast flow across the region 

through tomorrow...before the next upstream system moving over the 

northern Pacific reaches the west coast on Wednesday and digs 

southward. This will spread precip across the upper Klamath 

River...the northeast CA Plateau...portions of the Sierra down to 

approx the Merced River basin...and northwest NV. Totals are 

expected to run anywhere from 0.10- to 0.50-inch with local amounts 

approaching 1.00-inch for portions of the upper Klamath River basin 

and northeast CA. Freezing levels with the area of high pressure 

will peak on Tuesday anywhere from about 10000- to 12000-feet...and 

then drop considerably as the cooler airmass ushers in with the 

polar upr trof. By early Thursday freezing levels across the upper 

Klamath River basin and northern CA along with portions of central 

CA will plummet to 2000- to 3500-feet. Ahead of the frontal 

boundary...these will still be in the range of 5000- to 11500-feet 

from northwest to southeast...but will drop in the extended period 

as the cold front clears the region.





.METEOROLOGICAL CONDITIONS (LONG TERM: THU AM - SUN AM)...



An upper low will be stretched just offshore of the PacNW with 

troughing beginning to move into nrn CA Thursday morning. This will 

bring showers to nrn CA/NV and srn OR before spreading further 

across the region the rest of Thursday as the system begins to push 

inland. The low will slowly progress eastward the rest of the work 

week bringing scattered precip to much of the region. This system 

does not have an abundance of moisture with models showing roughly 

0.25-0.50" PW with the low. This should limit potentially for 

heavier precipitation. 



Beyond Friday, there is some uncertainty on the progression of the 

low. There are differences in the timing between the GFS/ECMWF into 

the weekend. The GFS quickly moves the core of the low out of the 

area into ID/WY by early Saturday morning with a transitional ridge 

between the low and the next shortwave just offshore of OR. The 

ECMWF still has the bulk of the system over CA/NV with the core of 

the low in ne NV. These differences extend to the ensembles as well. 

The QPF 500mb height clusters are split in to 4 groups each showing 

the low in a different position Saturday afternoon. At that time, 

the det GFS already has another upper low reaching OR with troughing 

off the nrn CA coast inviting additional showers into nrn CA. 

Meanwhile the ECMWF only then has the core of the low over eastern 

ID with ridging beginning to push into OR/nrn CA ahead of an 

offshore shortwave. The ensemble clusters are grouped based on these 

differences with clusters 3 and 4, containing roughly half of the 

GFS members, predicting faster movement of the system and clusters 1 

and 2, containing 34% and 40% of the ECMWF and 45%/35% of the CMC, 

predicting a slower moving system. Either way, it looks like showers 

will remain a possibility on Saturday but the aerial extent of those 

chances is uncertain. These timing differences continue overnight as 

the GFS remains further ahead on the overall pattern than the ECMWF. 



To summarize, a large upper low will move through the region late 

this week into the weekend bringing additional precipitation to 

CA/NV but uncertainty on how quickly the system passes overhead. The 

forecast was a combination of WPC and the latest NBM. Highest precip 

amounts are expected across the Sierra, the central/srn CA coastal 

mountains, and the mountains over nrn NV. QPF 12z Thurs-12z Sun: 

0.50-1.25" over the Sierra and the eastern Transverse Range/San 

Diego mountains, 0.30-0.75" Santa Cruz/Santa Lucia mountains, 0.75-

1.50" nrn NV mountains, 0.25-0.50" coastal soCal, and 0.10-0.50" for 

much of the rest of the region aside from se CA which is expected to 

see little to no precip.



The approaching system will lower freezing levels from nw to se on 

Thursday starting at 2-5 kft north of I-80 and 3-11.5 kft to the 

south. Lowering freezing levels will spread across the region the 

rest of Thursday down to 2-3.5 kft north of Point Conception and 2.5-

5.5 kft to the south. Some fluctuation but generally predicting 

levels between 2.5-5 kft through Saturday morning. Some uncertainty 

after that as levels will depend on how quickly the system exits and 

the ridge moves in.





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







Kozlowski/AS



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