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Northern CA Forecast Discussion
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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