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AGUS76 KRSA 201434

HMDRSA



Hydrometeorological Discussion

National Weather Service / California Nevada RFC / Sacramento CA

730 AM PDT Sat Apr 20 2024



...WARM AND GENERALLY DRY CONDITIONS THROUGH MON ASIDE FROM A FEW 

STRAY SHOWERS OVER NRN CA AND THE SIERRA...

...GRADUAL COOLING AND THE RETURN OF PRECIP CHANCES MID TO LATE WEEK 

AS A LOW APPROACHES THE CA COAST...



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



High pressure is currently situated over the west coast bringing 

mostly dry conditions this morning. An elongated upper low is 

stretched across the Gulf of Alaska with a surface low offshore of 

BC and a cold front outstretched to the west. Troughing will move 

into WA this afternoon after the cold front arrives along the PacNW 

coast. This may result a few light showers over parts of nrn CA. The 

upper trough will pass through BC the rest of today and overnight as 

high pressure builds over the eastern Pacific. This will act to warm 

temperatures across the region through Monday with afternoon highs 

rising to 5-15 deg F above normal away from the coast. The next 

system then approaches from the west as an upper low.



Uncertainty remains in the forecast for the rest of the week as 

models continue to disagree on the fate of the low. The ECMWF and 

CMC are still much slower on the arrival of the system compared to 

the GFS. Both models have a weak surface low setting up just nw of 

the Bay Area Tuesday that may result in some light scattered showers 

across nrn CA. Beyond that, the GFS speeds up the movement of the 

upper low opening it into a trough offshore of srn CA mid Wednesday 

morning while the ECMWF still has the system over 130W. The GFS then 

takes the system through srn CA Weds afternoon before exiting into 

AZ overnight into Thursday morning. The ECMWF only then has the 

trough beginning to move inland across soCal Thursday morning. This 

is also creating differences in the arrival, amounts, and extent of 

precipitation across CA Wednesday into Thursday. The ensembles are 

divided along similar lines with disagreements on timing based on 

the parent model. Bottom line, showers are expected across nrn CA 

and NV mid to late this week due to an upper low moving through srn 

CA. 



After the aforementioned system exits to the east, models show a 

surface low dropping in from the Gulf of Alaska with a cold front 

offshore of the PacNW and nrn CA. The GFS is ahead with the 

progression of this system compared to the ECMWF and shows a more 

organized surface low with higher precip amounts. From 18z Thurs to 

00z Fri the GFS has just under 0.50" over the Smith Basin while the 

ECMWF still has precip offshore and only at a few hundredths. These 

differences carry over into early Friday as well. By the end of the 

extended, the GFS has over an inch of precip over the Smith Basin 

for the 24 hrs ending 12z Friday while the ECMWF has only a few 

hundredths and the CMC a quarter of an inch. 



All this to say uncertainty in the forecast for mid to late next 

week. Expecting mainly dry conditions and above normal temperatures 

through Monday before gradually cooling from west to east and a 

return of precip chances for mid to late week. The QPF was a blend 

of the morning WPC guidance and the latest NBM. QPF is highest over 

nw CA and the srn OR Cascades at 0.25-0.50" or so with lesser 

amounts elsewhere across northern CA/NV and down the Sierra. 

Freezing levels 9-13 kft N to S this morning with higher freezing 

levels spreading northward throughout the rest of the weekend. 

Freezing levels to then lower mid week down to 8-10.5 kft then 6.5-

10.5 kft by Thursday depending again on the timing of the next low. 

Freezing levels to reach a min Friday morning of 5-7 kft north of I-

80.





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







AS



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