April 25, 2019 – A few isolated showers possible over the mountains this afternoon and evening and then again early next week, otherwise dry. A slow cooling trend through early next week but daytime highs remaining above normal.
Discussion
Fair skies over the CWA this morning under upper level high pressure ridging over the west coast. Daytime highs Wednesday ran from 15 to 20 degrees above normal and another very warm day is in store for today. The main upper ridge axis is forecast to shift east of the forecast area by this afternoon in advance of a weak trough forecast to approach the Pacific Northwest coast. This will allow for a slight cooling today. A shortwave pivoting out of the offshore trough and over the retreating ridge may bring a few light showers this afternoon and evening over the Shasta county mountains. The upper trough pushes into the Pacific Northwest on Friday with westerly flow aloft bringing a slow cooling trend into the weekend. By Sunday, an upper low approaches the SoCal coast while weak troughing develops over the Pacific Northwest and NorCal. Daytime highs cool to about 10 degrees above normal while PacNW trough brings a slight threat of showers to the Sierra Cascade range Sunday afternoon and evening.
Extended Discussion (Monday through Thursday)
Split mid-latitude flow remains a fixture in the forecast moving into next week. A compact upper low is expected to move across far northern Baja California early Monday with diffluent flow aloft affording a chance for a few showers down in Tuolumne County. While this system quickly advances into the Four Corners on Tuesday, amplifying northern stream flow moves into the Central Great Basin.
The western extent of height falls should graze interior northern California thus bolstering the chance for mountain showers on Monday and Tuesday across the Sierra-Cascade ranges. Forecast soundings show modest instability but a limiting factor is the amount of dry air below the cloud layer leading to enhanced evaporative processes. Impacts across the Valley will be an uptick in the dry, northerly winds. Conditions should improve by mid-week as the upper trough shifts toward the High Plains.
The warmest day of the week will be Monday (upper 70s to lower 80s over the Valley) before cold advection brings numbers closer to climatology. Looking beyond Wednesday, disturbances cutting underneath an upper ridge across Alaska should be monitored. Both reliable ensemble systems (ECMWF/GEFS) show a trough passage over the West Coast late next week.