AGUS76 KRSA 181725
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Thu Jan 18 2018

...COLD FRONT SWEEPS ACROSS THE REGION TODAY INTO FRIDAY...
...ADDITIONAL SYSTEMS TO AFFECT NRN CA SUNDAY AND TUESDAY...

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

The latest cold front is beginning to edge its way onshore across 
far northwest CA this morning with the best precip aligned from the 
upper Klamath River basin back southwest through the Trinity 
mountains and the Eel River basin near the coast. Blended TPW 
imagery shows the moisture plume entrained just ahead of the 
boundary currently intersecting the coast just north of the Golden 
Gate with values hovering right near 1.00-inch. Trimble IWV sensors 
between Point Arena and Bodega Bay validate this available 
moisture...which has advected inland toward the central Sacramento 
Valley near Chico. Since 18/12Z...precip gages indicate the best 
totals from near Cape Mendocino inland across the Trinity mountains 
and the western portion of the Shasta Lake drainage with 0.25- to 
0.50-inch.

For the past several days...models have had reasonable agreement in 
terms of timing the system and the intensity of the system across 
the region...and that has continued with the overnight model runs. 
With the overall upr trof over the northeast Pacific nudging closer 
to the west coast today...the cold front will start to increase its 
progress inland and to the south. Precip is expected to reach down 
toward the I-80 corridor this afternoon with the best amounts on 
southwest facing slopes between the eastern half of the Shasta Lake 
drainage down toward the Feather River basin. 

Through this evening...precip will shift down the Sierra with the 
best totals between the Yuba River basin through the Stanislaus 
River basin. Then into the overnight...the moisture plume will begin 
to narrow with available moisture dropping of a bit. Precip amounts 
over the central portion of the state will be less than counterparts 
across the northern third of the area. With the cold front and 
moisture plume reaching down to southern CA...scattered light precip 
is also likely in this area. But with flow quickly turning 
northwesterly behind the cold front...the best amounts may be on 
north facing slopes from Santa Barbara...Ventura and Los Angeles 
counties.

For Friday...the upr trof will slide across the region. Although 
moisture will drop off in the cooler airmass...there could be some 
scattered light precip with lowered stability.

Freezing levels ahead of the cold front will be at or above approx 
8000-feet...but with plenty of cold air swinging toward the region 
behind the boundary...these will drop considerably. Freezing levels 
by early Saturday will be down to 2000-feet near the CA/OR 
border...3000- to 3500-feet along the I-80 corridor down into 
central CA and NV...and 4000- to 5000-feet for southern CA and far 
southern NV.

The axis of the mid-level system will shift east of the area late 
Saturday morning...and scattered precip will be on the decrease. 
Another transitory positively tilted upr ridge will slide across the 
area later Saturday into Sunday. This will keep the next system 
offshore until early on Sunday. Light precip will reach the north 
coast from Cape Mendocino northward by 21/12Z. 


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

The next system begins to move in Sunday morning, as Northern CA 
receives the tail end of a cold front moving through the Pac NW. 
While the majority of the frontal energy remains to the north of the 
area, enough makes it south into NrnCA on Sunday to bring moderate 
to heavy precip through the day. Around 0.75-0.9" pwat plume takes 
aim at the North Coast Sunday morning, providing ample fuel for the 
system. Expect precip to increase through the morning hours from 
Cape Mendocino northward, then spreading as far south as the 
Monterey Bay by afternoon. The afternoon hours should also see 
precip moving inland across the Sacramento Valley into the Northern 
Sierra. Expect the frontal energy to dissipate overnight, with 
scattered light showers still possible as the front weakens, leaving 
conditions mostly dry by late Monday morning.

Freezing levels do not look to be as low as initially thought with 
this system, generally remaining 3500-4000 ft near the OR/CA border 
and over the Northern Sierra, with freezing levels closer to 4500-
5000 ft along the coast. Expect warming overnight into Monday, with 
freezing levels coming up another 1000-ft or so. 

Monday should be mostly dry across the region, with the trailing 
showers from Sunday's frontal passage exiting the region. Meanwhile, 
another s/wv is expected to approach the NrnCA coast and produce a 
shot of precip. Another moisture plume with amounts around 0.75-1" 
takes aim at the North Coast, and expect precip to start up again 
Tuesday morning. This system gains better upper level support as an 
upper trough drops out of the gulf of Alaska through the day on 
Tuesday, keeping moderate to heavy at times precip fueled through 
early Wednesday. Precip is expected to remain generally north of the 
I-80 corridor through the end of the forecast period. Freezing 
levels will generally be higher than the earlier system, sitting 
around 6000-7000 ft through Wednesday morning.


.HYDROLOGICAL CONDITIONS...

Over the next 5 days modest increases in river flows in the northern 
half of california will result from the incorming storm systems. No 
locations are forecast to reach monitor levels. 

Additional information is provided at the CNRFC website at 
http://www.cnrfc.noaa.gov


DRK/JM/BMM

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