April 24, 2018 – Mainly dry and warm this week except for a chance of a few showers or thunderstorms over the mountains each day.

Discussion

Mostly clear skies across interior NorCal early this morning. Current temperatures range from the 30s in the mountain valleys to the 50s to lower 60s in the Central Valley.

Ridge of high pressure will shift slightly eastward the next few days as a large scale upper level trough edges closer to the coast. This trough will bring enough instability for a few afternoon and early evening showers and thunderstorms along the Sierra crest. Warm weather will continue with above average temperatures, then readings will start a gradual cool down across the forecast area Thursday.

Still some uncertainty on where the low will move ashore late in the week. However, the cool down will accelerate Friday with a chance of showers and thunderstorms mainly across the mountains.

Extended discussion (Saturday through Tuesday)

Extended models consistent in idea of large upper low over the western U.S but differ on details. Either GFS or ECMWF would indicate a threat of showers over most of the forecast area especially during the afternoon hours. Stability progs indicate some instability over the northern mountains and Sierra Cascade crest on Saturday so limited thunderstorm threat to these areas.

Instability spreads westward on Sunday so may see some isolated thunderstorms dropping down into the northern and central Sacramento valley. Cloud cover and cooler airmass associated with the low will bring cooler temperatures over the weekend down to a little below normal for this time of year.

Models diverge early next week so confidence in forecast lowers but keeping a threat of showers over the Sierra Cascade range. Upper low should begin to shift inland by next Tuesday whichever model verifies making Tuesday the last day of any shower or T-storm threat before upper ridging builds over the west coast the middle of next week.

Daytime highs start a warming trend with a slight warm up to near normal on Monday then climb to several degrees above normal on Tuesday as upper ridging starts to build in. At this point the second half of next week looks dry.