Periods of minor to moderate HeatRisk this week with locally elevated fire conditions and breezy north winds across the northern and central Sacramento Valley Thursday. Onshore flow returns late this weekend into early next week.
Discussion
Relatively quiet night across interior NorCal, with light breezes being observed across the Valley. GOES 18 Satellite reveals some stratus forming in the Bay/Delta, with some intrusion possible, mainly for the Delta areas, through the night and into the morning hours. As we move through the morning today, we can expect similar temperatures to what we experienced on Memorial Day. An upper level trough in the PacNW will maintain weak onshore flow over the area today, with areas near the Delta experiencing winds of around 10-15 mph, with gusts of 25 mph possible. Strongest winds are expected in the afternoon hours.
The trough axis will begin to slide over NorCal later today weak upper level ridging will build in the Eastern Pacific, which will introduce breezy northerly winds, warm and dry conditions beginning Wednesday. North winds of around 10-20 mph with gusts up to 25 mph are possible, mainly along the I-5 corridor. As the trough moves further east, and ridging continues to build in the Eastern Pacific, the pressure gradient will tighten some on Thursday which will allow for stronger northerly winds to develop. North winds of 15 to 25 mph, with gusts 30 mph or more. The National Blend of Models (NBM) suggests probabilities of exceeding 40 mph gusts are around 25-50%, again mainly along the I-5 corridor.
Along with the breezy conditions, dry air will enter the area, with relative humidity (RH) values forecast to be in around 10-20% across the entire Valley. Combined with breezy northerly winds and dry air, elevated fire weather conditions are anticipated Thursday.
The change in our upper level pattern will lead to warmer temperatures by midweek. Thursday will see high temps in the low to mid 90s with 80s and 70s in the higher elevations.
Extended Discussion (Saturday through Tuesday)
Friday will be the warmest, with NBM probabilities suggesting a 20-60% chance that areas in the central and northern Sacramento Valley may break 100 degrees, introducing widespread areas of Moderate HeatRisk. Breezy and dry conditions will persist on Friday as well, with low RH values once again in the 10-20% range for the Valley, allow wind gusts will be generally below 20 mph.
On Saturday, we will see a slight lowering of daytime high temperatures, as an offshore trough will flatten the heights and promote more onshore flow to the area once again. Generally, high temperatures will be mid 80s to mid 90s across the Valley, with the cooler high temperatures in the Delta. Similar temperatures and conditions are expected on Sunday.
As we move into the next week, the upper level pattern will become more zonal, with ensembles and Cluster Analysis in relatively good agreement of this pattern manifesting. This pattern will allow for afternoon high temperatures to once again climb into the mid and upper 90s for the Valley, with mid 70s to mid 80s for the higher elevations. Widespread moderate HeatRisk on Tuesday, with isolated areas of Major HeatRisk are currently forecast for the central and northern Sacramento Valley.