Hydrometeorological Discussion
Issued by NWS California-Nevada RFC

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AGUS76 KRSA 221703

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
930 AM PST Thu Feb 22 2018



Next system diving in from the north is beginning to carve out
another upr trof across the western US. This system is a bit
stronger than the previous system from yesterday...and is taking a
trajectory closer to the region with a ribbon of mid-level vorticity
moving across CA into southern NV. Radar imagery is indicating a
band of precip dropping southward across the I-80 corridor from the
Sierra eastward across the state of NV...all in the form of snow.
Also...an area of showers is shifting across northwest CA moving
toward the southeast.

Precip is expected to drop south across the region and affect most
areas with at least scattered light precip. The one exception is the
southeast CA deserts. Best amounts continue to be focused along much
of the Sierra...primarily from the Yuba River basin south through
the upper San Joaquin River basin. Totals in these areas will range
from 0.25- to 0.50-inch. Elsewhere...amounts will be scattered and
generally remain below 0.25-inch.

This system will exit to the east tomorrow with northwesterly flow
remaining across the region as a brief s/wv ridge slides over the
area for dry conditions later Friday afternoon into the evening
hours. However...the next system to impact the region will arrive
from the northwest on Saturday...but take a more inland track as
compared to the system today. The s/wv trof will move across the
Pacific Northwest and then dive southeast toward the Great Basin.
This will bring the best opportunity for light precip across the
CA/OR border area inland across the northern two-thirds of NV. The
Sierra...except for extreme northern portions of the range...will
miss out on precip with this system. As will areas south and west of
the Sierra.

Freezing levels with the first system will bottom out near sea level
across northern areas...2000-feet for central areas...and close to
3500-feet for southern CA. These will rebound later Friday...and
with the Saturday system look for less of a drop given the
trajectory of the s/wv trof. By Saturday afternoon/evening...look
for 2000-feet northern areas...3500- to 5500-feet for central
areas...and 7000- to 8500-feet across southern CA.


The extended range features a large trough or potentially cut-off
low that is expected to move over the region early next week.  Model
guidance starts off in decent agreement and steadily loses agreement
through the period.  The GFS is most progressive with the system and
has the trough exiting the region by Tue evening, while the ECMWF
shows a cut-off low forming and lingering over the region through
Wed evening.  The GEM solution is certainly in between the other two
and shows a cut-off low forming but tracking across the area more
quickly than the ECMWF shows.  The GFS shows another system
impacting the north at the end of the forecast period, while the
other models are dry.

The low-confidence CNRFC QPF follows a compromise between GFS and
ECMWF solutions and shows mainly light precip spreading from the
north Sun morning down into the Sierra and central CA as well as
northern NV by late Sun into early Mon.  Light precip should reach
southern CA/NV by Mon night and wind down across the entire region
by later Tue night. Brief moderate amounts are expected in the Smith
basin and southern OR Cascades Sun evening and overnight.  For now,
we omitted the late Tue night precip in the far north that only the
GFS is showing.  As far as freezing levels are concerned, expect
levels generally 2000-4000 ft except around 5000 ft in southern


Cold temperatures combined with little to no precipitation are
resulting in low runoff conditions. All forecast locations within
the CNRFC area are forecast to remain below monitor levels for the
next five days.

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



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