Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 041746
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Sun Dec 4 2016

...PRECIP EXPECTED OVER NORTHERN AREAS THROUGH TUESDAY...
...POTENTIAL FOR HEAVIER RAINS OVER NORTHERN CA THURSDAY/FRIDAY...

.24 HOUR PRECIPITATION TOTALS (ENDING DEC 4 AT 400 AM PST)...

No precipitation fell over the last 24-hrs, with the exception of a
few hundredths over the W slopes of the Srn OR Cascades and the
Smith Basin.


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

Light showers are moving into the OR/CA border this morning as a
weak s/wv moves in from the Pac NW, with a few hundredths of an
inch picked up at gages so far. Blended TPW product is reading around
0.9" precipitable water just off the Nrn CA coast. Light precip will
move through the Smith and Upper Klamath basins today, but with weak
dynamics and not much in the way of good moisture transport, do not
expect significant totals. Expect precip to wrap up by Monday
morning. Freezing levels are running around 7800-ft according to the
12Z Medford sounding this morning, but should lower to near 4000-ft
by Monday morning as the cold air streams in.

Second wave of precip is expected to move in early Monday evening,
as the upper trough drops out of the Pacific NW towards the Great
Basin. This will be mostly driven by dynamics associated with the
upper jet moving overhead, and with meager moisture (PWAT around
0.75" at most), less than a quarter of an inch of precip is expected
overall. Freezing levels during this time will remain quite low,
around 3000-ft near the OR/CA border.

Ridging develops behind the upper trough, bringing dry conditions to
the region for a brief period Tuesday night into Wednesday morning.
Meanwhile, an upper low with a very strong associated surface low is
expected to move towards the Pac NW, drawing significant moisture
plume towards the Northern CA coast. Models differ quite a bit with
timing and intensity so far, but the GFS and Canadian models seem to
latch onto warm frontal precip beginning over the North Coast
Wednesday evening. The Nam is the slowest, and the EC is a middle
ground for timing. Leaned the current QPF more towards the quicker
GFS/Canadian, consistent with the National Model Blend. Expect light
to locally moderate precip to move through much of Northern CA
through Thursday morning as the warm front passes through. The cold
front is expected to approach later Thursday morning, and with the
moisture plume taking aim at the coast from Cape Mendocino to the SF
Bay, expect heaviest precip there. Heavy precip should move inland
by the afternoon, with ample moisture and strong upslope flow
expected to create orographically enhanced precip over the Northern
and Central Sierra through the evening and overnight period.

By Friday morning, some light showers are expected to remain over
the coast, as well as light to moderate precip along the Northern
and Central Sierra, and Northern NV due to lingering moisture and
westerly flow. The majority of precip should wrap up by early
Saturday. Because the moisture tap for this system has a more
subtropical nature, freezing levels are actually expected to rise
during the event. Expect them to start out around 7000-ft over the
Sierra Wednesday night, then rise to 9000-ft on Thursday.


.HYDROLOGICAL CONDITIONS...

Rivers across the area will slowly recede towards baseflow over the
next few days. Moderate rises will occur across the area late next
week as the next system move into the northern half of CA. All
forecast points are expected to remain below monitor levels the next
5 days.


More information on the CNRFC website at http://www.cnrfc.noaa.gov

JM/MI

$$



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