Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 201656

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
955 AM PDT Tue Mar 20 2018



Impressive satellite imagery this morning shows a strong upper low
centered around 32N/138W, slowly moving eastward. To the south, a
strong and very moist plume of precipitable water is being directed
straight towards Pt Conception by a strong upper jet. Currently the
precipitable water plume has 2"+ amounts embedded within a wide
swath of 1.5" amounts, well over 200% of normal for this time of
year. The subtropical origins of this plume will keep freezing
levels higher than typical over much of the area, with snow a threat
only at higher elevations.

For today, expect the upper low to continue moving east towards the
coast, displacing the upper ridge eastward. A s/wv ahead of the main
system will bring light precip to coastal areas of Central and
Northern CA late this morning through afternoon, as well as inland
through the Upper Klamath. A southern component should increase in
intensity late this afternoon and evening, bringing an initial
uptick in precip to the Santa Lucia Mtns south through the Santa
Ynez range. Expect this to continue overnight, focusing moderate
precip over Southern CA and the Central and Southern Sierra. Through
the day on Wednesday, the upper low off the coast continues to move
eastward, slowly opening up into a deep trough whose associated cold
front will intensity precip even further Wednesday night.

Current timing looks like the heaviest precip will hit the SBA
county South coast between 00Z and 12Z Thursday. The NAM directly
the moisture plume the furthest south, aiming near the VTU/SBA
county line, while the GFS directs it more towards the Santa Ynez
Mtns. The EC has the furthest north aim, towards the western coast
of SBA county. Erred towards the further east solutions, which
matched up better with both the ARW and NMM high resolution models.
This results in a more vigorous round of high intensity precip over
the burn areas of Santa Barbara and Ventura counties, which lines up
well with the WFO and WPC thinking. At its peak, models suggest
moisture flux anomalies of 4-5 std deviations above normal.

Precip shifts southward and weakens over SoCal through the day on
Thursday, but increases quite a bit as the front moves inland over
the Central Valley and into the Sierra. Looks like the warm precip
event will bring heavy precip into the Sierra, generally from the
Tahoe region southward, especially focusing on the Central and
Southern end through Thursday evening. The subtropical airmass will
keep freezing levels around the 7000-9000 ft mark over the Sierra
and 9000 ft or higher over So Cal on Thursday.


As the moisture plume exits southern CA late Thu afternoon and
evening, focus for heaviest precip is expected to shift to generally
the southern half of the Sierra associated with a trough aloft and
secondary cold front moving across central CA into central NV.
Although precip even in the Sierra should be tapering off during
this time frame compared to the wetter periods Wed night into
earlier on Thu, still expect moderate amounts in the southern half
of the range to continue through Thu evening before tapering off
later Thu night. Additional light to moderate precip is expected
across NV and southern CA coastal areas as well.  Expect this
activity to wind down later Thu night into early Fri.  Expect
freezing levels in the Sierra to fall from 6000-8000 ft late Thu
afternoon to around 3000-6000 ft by late Thu night.

Meanwhile, expect precip to gradually increase over northern CA Thu
evening into Fri due to isentropic lift ahead of the next frontal
system.  Expect precip to be mainly light to moderate across the
region through the middle of Fri afternoon with locally heavy
amounts closer to Fri afternoon and evening as a cold front moves
into the region.  A lull is expected Sat morning before a follow-up
system quickly spreads additional precip across the north later Sat
into Sun.  Expect freezing levels generally 2000-4000 ft.


Rivers will remain near present levels as another system begins to
impact the area on Tuesday. Many basins across the area could
receive several inches of rainfall over the next 5 days, so expect
rises on rivers throughout CA.

There are several monitor stages currently forecast throughout the
forecast area. These include Michigan Bar and McConnell on the
Cosumnes River, Bear Creek at McKee Road, and Portola on the Middle
Fork of the Feather River.  The Yuba River at Marysville is also
expected to reach Monitor stage.

Several locations along the southern central coast and in southern
California are also approaching monitor stage. Monitor Stage is
forecast for the Sisquoc River at Garey.  Runoff may be enhanced in
areas with recent burn scars.

The forecast on the Sacramento River has increased some this morning
with a much better possibility of weir flow at Tisdale Weir by this
weekend. The current forecast is for nearly 2 feet of overflow.

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. However, it`s difficult to forecast these rain
on snow situations, thus there is a more uncertainty in the runoff
forecast than usual. Flows are not expected to approach monitor
stages in the east Sierra or on the Merced River at this time.

Reservoir inflows are likely to be substantial along the Sierra
range as low level snow melts off with the rain.  Changes to
reservoir releases would have an impact on downstream river levels,
so expect changes throughout this week.

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



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