September 22, 2019 – Dry and mild weather through the weekend with a few showers possible over the mountains later today. Breezy northerly winds early next week will lead to elevated fire weather concerns. Above normal temperatures are likely Tuesday and Wednesday while a significant cool down and increasing precipitation chances return late in the week.
Discussion
The transient upper ridge has long exited the region as it tracks toward the Intermountain West. The southern extension of a modest upper trough is currently nearing the coast which has spread some high clouds into western sections of Shasta and Tehama counties. While scattered showers are possible over the Shasta county mountains into the far northern Sierra, nearly average moisture will keep rainfall amounts generally below 0.10 inches. Current blended TPW imagery supports this idea while keeping the better moisture support offshore. Overall, most Valley locations will stay in the mid/upper 80s although a few 90 degree readings are possible toward Stockton and Modesto.
As this system dips down toward western Nevada, models develop an upper low forming an “Inside Slider” setup. As is common in these regimes, dry northerly flow will ensue with gusty winds likely on Monday. Model surface pressure gradients between KMFR and KSAC depict 10 mb differences which supports the forecast winds. Decreasing relative humidities along with poor overnight recoveries will be the norm early next week with a Fire Weather Watch in place from Monday afternoon through Wednesday morning. The affected region includes sections of the Sacramento Valley, adjacent foothills, and northern Coastal Range.
In addition to the elevated fire weather concerns, temperatures should be quite warm on Tuesday and Wednesday with Valley highs in the mid/upper 90s while mountain locations sit in the 70s. Relative to climatology, this equates to being 5 to 10 degrees above average. With expected 850-mb temperatures in the 22-25 C range, a well mixed boundary layer could support an additional degree or two in the forecast. The overnight warmth will actually be even more impressive in terms of anomalies. As north/northeasterly flow events often do, downsloping winds will keep readings quite mild, particularly in foothill locations with lows near 70 degrees. Wednesday should mark a close to the brief bout of heat with a pattern change thereafter.
Extended Discussion (Thursday through Sunday)
While the pattern change is slightly delayed compared to yesterday`s guidance, confidence remains rather high in much cooler and unsettled conditions arriving by Friday. Anomalously deep troughing will settle over the western U.S. supporting multi- day temperature drops into the 20 to 25 degree range. The highly amplified setup over North America has been rather well advertised as a powerful ridge across the Gulf of Alaska forces a deep trough to position itself over the West. During the Friday- Sunday (Sep 27-29) timeframe, 1000-500 mb thicknesses potentially drop into the 540-545 dm range which is seasonably low. Dropping temperatures in the column will lower snow levels considerably, potentially reaching the 5,500 to 6,500 foot range at times over the weekend. With the current forecast track favorable for showers over the region, will need to monitor the potential for snowfall accumulations. Additionally, it should feel quite Fall like with Valley locations struggling to escape the upper 60s while 30s and 40s would be more commonplace across the mountains. At this juncture, ensemble means (ECMWF/GEFS/CMC) are in remarkable agreement which is not terribly surprising given the level of amplification in the global pattern. However, details will change, particularly as individual shortwaves become better resolved. The system responsible for this pattern shift actually resides near Siberia at this time.