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AGUS76 KRSA 182028

HMDRSA



Hydrometeorological Discussion

National Weather Service / California Nevada RFC / Sacramento CA

130 PM PDT Mon Mar 18 2024



...UPPER LOW WILL BRING SCATTERED SHOWERS AND ISOLATED THUNDERSTORMS 

TODAY AND TUESDAY MAINLY OVER SRN SIERRA AND SRN CA AND SRN NV...

...PRECIP SPREADS OVER THE REGION FOR THE END OF THE WEEK INTO THE 

WEEKEND...



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



Little change to the afternoon forecast with the biggest increase of 

a few hundredths over Srn CA with showers and thunderstorms this 

afternoon. Scattered Showers and thunderstorms over Srn Ca and Srn 

NV and along the Srn Sierra as they wrap around the upper low 

centered near the CA/AZ border this afternoon and evening.  The 

upper low moves a little westward this afternoon then shift backs 

east into Arizona tonight into Tuesday and off into New Mexico 

Wednesday. Meanwhile ridge of high pressure continues with warm and 

dry conditions over Pac NW and into Nrn CA and Nrn Nv today and 

shifts south into Tuesday and east on Wednesday as an upper level 

low approaches the Pac NW coast. Scattered showers and thunderstorms 

today over Srn CA and Srn NV and along the Srn Sierra. HREF shows a 

probability of 20-40% of half an inch or more of precip in an hour 

this afternoon into early evening over Srn CA, especially over SW 

San Bernardino and Orange and Western Riverside and San Diego 

Counties this afternoon into early evening.  Precipitation amounts 

will be variable with the convection but generally a hundredth or 

two to around a quarter of an inch will local amounts to around a 

half of an inch possible. Showers lingering over Srn Ca (mainly over 

higher terrain) and Srn NV and spreading north in the Sierra and 

over the Shasta Basin on Tuesday and tapering off down south on 

Wednesday as the pattern begins to shift. Precipitation amounts will 

be variable with the convection but generally a hundredth or two to 

around a tenth of an inch will local amounts up to a half of an inch 

possible. 12Z HREF shows a 10-20% probability of amounts a half an 

inch or more an hour over the Srn Sierra Tuesday afternoon. Freezing 

levels around 8000 ft and higher through Wednesday. 



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



Forecast remains on track for a surface/upper low to impact the 

region late this week and into the coming weekend. There are still 

timing differences between the models on when initial precip from 

the system will reach the nrn CA coast. The ECMWF remains the faster 

model with the 12z det ECMWF bringing in showers early Friday 

morning while the 12z det GFS holds off until the afternoon. There 

also remains a decent spread on timing amongst the ensembles 

anywhere from Thursday afternoon to Friday evening. Either way the 

low is expected to result in precipitation across nrn/cntrl CA, srn 

OR, and nrn NV at the end of the week as the low approaches from the 

nw. The ECMWF also remains quicker at taking the system inland early 

Saturday compared to the GFS which has the core of the low still 

offshore at the end of the extended. Both solutions result in a 

troughing pattern across most of the west coast and allows for 

scattered precip across the region. In addition to the timing 

differences, the ECMWF is overall wetter across CA compared to the 

GFS even in the ensemble members. The ensemble spread in the GFS at 

Arcata ending Sunday morning is anywhere from 0.20-1.40" while the 

ECMWF spread is even wider at 0.10" to over 2.50". This makes for 

some uncertainty in the QPF for the long term.



The afternoon forecast was a combination of the latest WPC guidance 

and the 12z GFS/ECMWF along with the previous forecast. This 

resulted in a 0.25-0.50" decrease across the southern Sierra and a 

0.10-0.30" increase over nrn CA. QPF for the extended: 1-2.75" north 

coast/Shasta, 1-3" Sierra, 0.40-1.30" Bay Area/central coast, 0.10-

0.75" down the valleys (higher towards the foothills), and a few 

hundredths to a couple tenths along coastal soCal. Freezing levels 7-

9 kft north of I-80 into Friday and 9-10.5 kft to the south. The 

approaching low will lower freezing levels nw to se down to 4-5.5 

kft north of I-80 and 5.5-11 kft to the south early Saturday. This 

downward trend will continue the rest of the long term reaching 3.5-

4.5 kft north of I-80 and 4-7.5 kft to the south early Sunday.



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







Osborne/AS



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