Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 181654

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
955 AM PDT Sun Mar 18 2018



A few light showers persist over NE NV this morning as an upper
trough moves east out of NV into the Great Basin.  Showers should
wrap up by early afternoon.  A weak wave aloft could provide a few
light showers near the Shasta drainage this evening, otherwise dry
conditions are expected across the region into Mon night.

Models continue to point to a wet period Tue through at least Fri
night as a deep tropical moisture plume reaches the central and
southern CA coast.  Model agreement has improved through Wed
morning, but still degrades somewhat beyond that point.  Overall
timing with heaviest amounts is a bit quicker than yesterday`s
forecast.  Through Wed morning, multiple models were blended
together as a starting point for the QPF.  Wed afternoon through the
remainder of the forecast period, the ECMWF alone was used as the
starting point and the anchor of timing for the system.  The GEM was
reasonably close in timing, while GFS was faster, although the GFS
is trending slower, with the 06 UTC run noticeably slower than the
00 UTC run.

Expect precip to spread across CA and southern OR into western NV
quickly on Tue as the moisture plume reaches the area.  By Tue
afternoon, expect PW values of around one inch or higher from near
Monterey Bay all the way to near the CA/Mexico border, with peak
values in the core near 1.3-1.4".  With weak surface low pressure
off the northern CA coast, expect H85 flow in the core of the
moisture plume to be SW around around 20-40 kts, leading to initial
moderate to locally heavy precip rates in the Santa Lucia Mtns to
the Santa Ynez Mtns in southern CA.  Light to moderate amounts are
expected elsewhere.  Freezing levels should start off around 5000-
8000 ft in the northern half of the area and 8000-11000 ft in the
southern half.

Through Tue night into Wed, expect a persistent pattern with best
moisture flux into the coastal areas from the Santa Lucia through
the Santa Ynez Mtns with weak to moderate low-level flow.  By Wed
evening, expect an approaching surface low and shortwave aloft to
begin enhancing precip rates along the plume as it begins to make
better forward progress.  Wed night through Thu should have
opportunities for heavier amounts in the Sierra and southern CA (and
briefly the Shasta drainage and north coast) as the main precip band
pushes through the southern part of the state and as the upper
trough moves inland.  Expect lighter precip to affect the remainder
of the region.  Another shortwave aloft should bring the potential
for additional precip in northern areas Fri through Fri night.
Freezing levels should begin to fall around Wed night, and by Thu
night should reach 2000-6000 ft for the northern two thirds of the
region.  Areas further south should see levels around 6000-8000 ft.


Rivers will continue to recede the next 2 days before another system
begins to impact the area on Tuesday. Many basins across the area
will receive several inches of rainfall through next weekend, so
expect rises on rivers throughout CA.

Currently, only Michigan Bar is forecast to reach monitor stage over
the next 5 days. That could change later this week, particularly
along the central coast and southern CA where some of the heaviest
rainfall is expected. Runoff in some area will be enhanced from
recent burn scars.

There will be rises on streams in the Sierra as well, but current
thinking is that runoff will be somewhat limited by the fresh snow
received last week. Flows are not expected to approach monitor
stages in the east Sierra or on the Merced at this time. Heavy
rainfall is also expected below Shasta, where local tributaries
could contribute to weir flow at Tisdale Weir by next weekend.

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



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