FXUS66 KSTO 060621
AFDSTO
Area Forecast Discussion
National Weather Service Sacramento CA
1020 PM PST Fri Mar 5 2021
SYNOPSIS...
Rain and mountain snow returns tonight and early tomorrow with
mountain travel delays. Unsettled weather and cooler conditions
likely for at least the first half of next week.
.DISCUSSION
...
The Evening Update: Frontal system has moved onshore onto the N
Coast and will be more progressive as it moves across our CWA after
midnight. Short wave has finished digging, so the trof axis will be
more progressive for the rest of the night, and should move through
our CWA a little after 12z Sat. Skies are expected to rapidly clear
behind the trof based on current satellite trends.
Evaporative and dynamic cooling effects have "slammed/crashed" snow
levels down to around 2-2.5 ft according to the Happy Camp snow
profiler. We have updated the zones this evening to account for
lower snow levels in Shasta Co where Sims/Castella could see some
snow. The Metro Roadcasts suggest the Sims road surfaces threat will
be melting snow, and with road surfaces forecast to be in the low to
mid 40s during the period of heavier precip, little accumulation is
expected on our portion of I-5. However, things will be different
over the higher elevations of the Sierra where we have the WSW in
effect for this incoming precip.
.Previous Discussion...
We are continuing to see quiet weather across our area this
afternoon but that will be changing as we head into the overnight as
an upper level trough approaches the region. Light showers have
already pushed into the NW part of the state and these showers will
continue to slowly push east into the evening with them reaching the
Coastal Range mainly after 10pm/6z. Precip will continue to
overspread the area during the overnight as the trough axis and
associated cold front push through. The heaviest rain and snow will
be between midnight/8z and 6am/14z. We do see some weak low level
instability and this will likely bring some enhanced snow rates over
the Sierra for a short period and we could see snow rates 1-2" /hr.
Snow levels will start out around 5000 feet but will be falling to
3000-4000 feet by 4am/12z. Snow accumulation is looking to be 2-8"
above 5000 feet with locally higher amounts over the Lassen Park
area and a dusting of snow possible down to around 3500 feet. Rain
totals are expected to be between 0.05-0.65" in the Valley with the
lowest totals over the San Joaquin and 0.15-1.0" in the Foothills.
We are seeing increased southerly flow ahead of the trough mainly
from about Chico to Red Bluff and can expected a short period of
increased winds along the front as it sweeps through the area.
Things dry out by the afternoon saturday with broad southwest
flow developing over the region. This will keep the weather quiet
into Sunday with cooler highs falling back to near to just below
average.
The weather pattern will become more unsettled as we move into
next week as long wave troughing sets up over the west. Ensembles
have trended the long wave trough a bit further to the west which
isn`t great news for us in seeing rain or at least higher amounts
of rain. Light showers are still expected on Monday though as a
weak short wave trough embed in the long wave trough moves
through. The best chances for showers will be in the later
afternoon and evening with light showers continuing overnight as
we see weak warm air advection develop. Not a ton of snow is
expected on Monday but snow levels will be low in the 3000 to 4000
foot range. Highs will also be cooler running below average.
-CJM
.EXTENDED DISCUSSION
(Tuesday THROUGH Friday)...
Active pattern will unfold next week across northern California
as a longwave trough axis sets up along 135 deg W. The associated
upper level low located around 600 miles west of Seattle on Monday
morning will gradually wobble south and parallel the West Coast
through the middle of next week. During this time, multiple
shortwave impulses will ripple through the cyclonic flow, and be
aimed at California. This upper low will help reinforce a cool
airmass with below normal temperatures and lower snow levels.
Mountain travel impacts associated with snow are possible as snow
levels are forecast to drop to 3,000 ft (possibly less). Will note
that the National Blend of Models and Weather Prediction Center
Guidance have both recently started to trend a bit drier for early
next week. As always, will continue to keep a close eye on
forecast trends and adjust accordingly. // Rowe
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