Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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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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