April 20, 2018 – Dry and warm today through early next week with just a chance of showers or thunderstorms over the Shasta County mountains on Sunday. Isolated showers or thunderstorms over the mountains mid week.
Fair skies over the entire CWA this morning as upper level high pressure develops over the west coast. A light northerly surface gradient has developed and most locations seeing light north winds. These north winds and warmer airmass bringing warmer temperatures this morning on the order of several degrees. 1000 to 500mb thickness increases by 12 dm today over Thursday with 850 mb temps climbing several degrees.
As a result, expect daytime highs today to jump a good 10 degrees or more over Thursday. Even more warming is expected on Saturday as the overall airmass continues to warm. Highs on Saturday will reach into the 80s in the valley with mid to upper 80s forecast for the northern Sacramento valley.
Models have been consistent in weakening the west coast ridge a bit on Sunday as a weak upper trough moves through the Pacific northwest. Both ECMWF and GFS continue to paint some light precipitation over the northern mountains in the afternoon. Stability progs indicate enough instability for a slight threat of thunderstorms and have included them in the latest forecast grids. For the remainder of the forecast area Sunday will only see minor changes. Upper ridge rebuilds on Monday bringing a bit more warming with Monday seeing some of the highest temperatures of the season so far.
Extended discussion (Tuesday through Friday)
Upper level ridging will bring high temperatures about 10 degrees above normal Tuesday and Wednesday, which means low to mid 80s for the Valley and Delta. A closed low off the coast will gradually approach, bringing moist southwest flow. Some instability showers and thunderstorms in upslope flow are possible over the northern Sierra, the mountains of western Plumas County and eastern Shasta County afternoons/evenings Tuesday and Wednesday. These may become more widespread Thursday and Friday, as the cool core of the low is currently projected to move inland. Temperatures are also forecast to drop to more seasonable levels. Model uncertainty on timing and placement of the low remains high, though.