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Northern CA Forecast Discussion
FXUS66 KSTO 142122 AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
222 PM PDT Fri May 14 2021

SYNOPSIS...
Cooling trend continues today and into the weekend, with warmer temperatures Sunday and Monday. Scattered showers and thunderstorms are possible today and into the weekend, mainly over the mountains and northern Sacramento Valley as a low pressure system moves over the region. Some showers may spread into the southern Sacramento Valley Saturday night.


.DISCUSSION

... GOES-West visible imagery reveals some passing high clouds over the northern third of California this afternoon, as well as some convective cumulus starting to bubble up in the higher elevations. Temperatures are generally running several degrees cooler compared to 24 hours ago, and expect this to similarly translate over to this afternoon`s highs in the 80s to near 90 deg F. Delta breeze is pumping in Bay-cooled air to Solano county where some communities may struggle to get out of the 70s today. Could see some marine stratus try to push into the Delta overnight and on early Friday. Over the next several days, the main weather concern will be the potential for showers and thunderstorms. An upper level trough will drop south from the Pacific Northwest over the next 24 hours, becoming a closed low over the Golden State by tomorrow. For the short-term from now through around midnight, the best opportunity for showers and isolated thunderstorms will be confined to far northern California, including the areas around Trinity, Western Shasta, and Western Tehama counties. The best chance will be farther north around the Oregon border. Some non-zero thunder probabilities spill into the northern Sacramento Valley; the National Blend of Models gives Redding around a 20 percent chance of thunder through about midnight. On Saturday, shower and thunderstorm chances will expand south and east over the Sierra as the upper low moves inland. Additionally, precipitable water values will increase upward to around an inch, and combined with plenty of CAPE for California standards (approx 500-1000 J/kg), do expect showers and thunderstorms to be more numerous compared to today. Decaying convection will likely linger into the evening hours and beyond sunset, perhaps bringing showers to portions of the southern Sacramento Valley with the east/southeast flow. Main threats associated with these storms will be locally brief heavy rainfall (more on Saturday than Friday), some gusty winds, small hail, and lightning. Fuels remain at record dryness for this time of year with at least one location (Stonyford/Western Sac Valley) approaching the 90th percentile on the Energy Release Component (ERCs) charts. Best chance for dry lightning will be today, but that threat will mostly be to our north in Siskiyou county. As more moisture makes its way into California on Saturday, the showers and thunderstorms in the Sierra will become "wetter", and will mitigate (to an extent) dry lightning. That all said, even with "wetter" showers/thunderstorms, small fire ignitions will be very possible, especially if lightning strays away from the cloud base. Shower and thunderstorm chances will be more limited to the Sierra Nevada and foothills on Sunday as the upper low exits toward the Great Basin/Desert Southwest. Some ridging across northern California will allow temperatures to rebound to the upper 80s/low 90s around Redding by Sunday or Monday afternoon. // Rowe


.EXTENDED DISCUSSION

(Tuesday THROUGH Friday)... Ensemble guidance advertises the arrival of another trough to the California coast around Tuesday/Wednesday. Given the uncertainty with the timing/strength of this trough, official forecast follows with the National Blend of Models. This includes the chance of showers over portions of Shasta, Tehama, and Butte counties on Wednesday. Looking at the latest Climate Prediction Center`s 6-10 day outlook -- valid May 20 to 24 -- below normal temperatures are slightly favored for much of the West Coast.








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