230 FXUS66 KSTO 210617 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 1115 PM PDT Fri Aug 20 2021 .SYNOPSIS... Smoky conditions continue to impact northern California this afternoon. Expect breezy conditions to develop on Saturday, leading to localized critical fire weather conditions around the Caldor Fire. Smoke will gradually improve over the Valley through the weekend. && .DISCUSSION... The Evening Update: Stronger Delta Breeze and upvalley flow this evening than last leading to slightly cooler temps than 24 hrs ago. Upper low along the WA/OR coast this evening has restored the Delta Breeze this evening leading to some improved VSBYS, albeit some low CIGS still remain. This feature is forecast to move SEwd across OR on Sat and into ID by Sat evening is likely to maintain a strong enough Delta Breeze to allow for better VSBYS than today, albeit the smoke layer may be difficult to eradicate, and will likely be pushed Ewd and up the W Slope of the Sierra. Minor cooling expected on Sat with the lowering 5H heights and synoptic cooling, in addition to the Delta Breeze. Some gusty ridge winds also likely over the Sierra as the trof moves across Norcal. Weak troffing will linger into Sun with decreasing winds as Sat's trof moves away. JHM .Previous Discussion... Smoky skies continue to blanket a good portion of northern California this afternoon as a result of the numerous area wildfires. Main short-term challenge for the day will be high temperature forecasts as smoke will prevent full-potential heating for many communities. Northern Sacramento Valley is running anywhere from 6 to 11 deg F cooler compared to this time 24 hours ago, whereas the southern Sacramento and northern San Joaquin Valleys are running +/- 4 deg F from yesterday. Forecast highs for this afternoon range from the middle 80s to near 90 in the Delta to Sacramento City, up to the low/middle 90s in northern Sacramento Valley. Did go ahead and shave several degrees from the the forecast region-wide, but even that may not be enough. Dry, northwest flow aloft remains overhead as an upper level low quickly ejects east from the Rockies. Embedded in this flow is a shortwave that will deepen near Washington State over the next 24 hours. This will help promote increased onshore flow to central and northern California over the next several days. Expect gradual day-to-day improvement with the smoke and air quality as this south to west surface flow develops. Initially, improved air would be realized in/around the Delta, then expand upvalley through the day on Saturday and Sunday. While the wind will bring air quality benefits to the region, it will unfortunately reintroduce fire weather concerns in the mountains and foothills. These breezy onshore winds will lead to an afternoon and evening of critical fire weather conditions, particularly in/around the Caldor Fire. Consequently, a Red Flag Warning has been issued for the area in/around the Caldor Fire from 11 AM PDT to 8 PM PDT Saturday as there will be potential for rapid spread of existing fire. Wind gusts tomorrow afternoon around the fire will generally be 20 to 30 mph with localized gusts to 40 mph, then diminish after sunset. Windy conditions are possible again on Sunday afternoon, though at this point do not appear to be as strong as Saturday. Troughing will remain over the Pacific Northwest into the start of next week as 500 mb heights begin to rise over the Desert Southwest. Valley high temperatures are generally expected to be with about 5 deg F of normal. // Rowe && .EXTENDED DISCUSSION (Tuesday THROUGH Friday)... Cluster analysis of the ensembles favors a broad upper level ridge over the Desert Southwest with troughing over the Pacific Northwest middle next week. Would anticipate gradual day-to-day warming, particularly if the trough's influence remains to our north and the ridge becomes our dominate synoptic feature. Dry conditions are forecast to prevail as the National Blend of Models keeps probability of precipitation less than 5 percent region- wide through next week. && .AVIATION... Smoke from area wildfires will continue to impact interior Northern California over the next 24 hrs. Periods of MVFR visbys due to FU/HZ though the afternoon at terminals, with VFR likely to return to Sac terminals southward after 03Z due to onshore winds. Wind gusts up to 30 knots possible in the vicinity of the Delta after 00Z. Elsewhere, winds generally less than 15 knots. Breezy southwest winds expected over the Sierra and southern Cascades after 18Z Saturday with local gusts from 20-35 knots expected. && .STO WATCHES/WARNINGS/ADVISORIES... Red Flag Warning from 11 AM to 8 PM PDT Saturday for Northern Sierra Including the Tahoe and ElDorado NF/S West of the Sierra Crest. && $$