September 17, 2019 – Dry conditions with warmer temperatures expected today. Another weather system is forecast to bring showers and a few thunderstorms Tuesday night and Wednesday, particularly over the mountains.
Conditions will dry out with warm temperatures returning late this week and continuing into the weekend.
Discussion
A progressive Pacific pattern has ushered yesterday’s system into the Central Great Basin. Brief height rises via a shortwave ridge will afford a temporary period of dry weather. 24-hour mid-level height comparisons show rises in upwards of 12 dm which offers warmer temperatures today. Valley highs should range from the upper 70s in the northern Sacramento Valley to low 80s for points southward. While only a few degrees warmer than yesterday, much of the day should be mostly sunny with high clouds arriving late.
The second Gulf of Alaska system in the chain will arrive late Tuesday and continue to impact the region Wednesday, possibly extending into Thursday morning over the high Sierra. Broad ascent ahead of the trough passage will spread light to moderate showers from north to south across interior northern California. The 06Z NAM continues to show ample moisture advection per PWATs in the 1 to 1.25 inch range along the cold front. Relative to climatology, this is roughly in the 90th to 95th percentile.
Cold frontal showers will spread through the region, while instability driven activity falls in the wake underneath the cool, unstable upper low. Model CAPE forecasts and simulated reflectivities show a bulk of the latter activity would be over the northern Sacramento Valley. Overall, the highest rainfall amounts are forecast over the southern Cascades and Sierra with 0.25 to 0.50 inches possible.
Unsettled weather should exit the region on Thursday with skies clearing out from west to east. With the mean trough centered over Nevada, meridional height gradients set up supporting a period of dry, northerly flow over the Valley on Thursday and continuing into Friday. Wind fields do not appear particularly strong although speeds could reach 15 mph or so on Thursday afternoon/evening. A continued moderation in the temperatures will accompany this shift in the winds with highs in the mid 80s on Friday. This still sits around 2 to 5 degrees below average in most locations.
Extended Discussion (Saturday through Tuesday)
A warming trend will continue into the weekend as an offshore ridge tracks across the West Coast. Consequently, Valley highs are forecast to reach the upper 80s to low 90s, while upper 50s to mid 70s will be more commonplace across mountain locales. While conditions remain warm over the weekend, a shift in the pattern is noted in the ensemble guidance.
By late Sunday, a digging trough sets its sights on the western U.S. with a position rather up in the air. Ensemble spaghetti plots shown by the 00Z GEFS indicate a great deal of west-east spread. The path the resultant upper low/trough takes will dictate whether the region sees another northerly flow event or perhaps more precipitation.
The past few runs of the GFS favor a track east of the state into Nevada while the 00Z ECMWF takes the system overhead. It remains difficult to trust any individual model given the continued fluctuating tracks. Definitely worth keeping an eye on given the degree of troughing suggested by the guidance.