June 18, 2020 – Fire weather concerns linger into the early evening hours for portions of interior Northern California. Lighter wind expected Friday. Dry with above normal temperatures expected this weekend. Widespread triple digit heat forecast in the Central Valley next week.


Satellite imagery this afternoon shows the eastern Pacific ridge sitting offshore, while a broad trough sits over the intermountain west. This has kept fire weather concerns in the forecast for portions of the Sacramento Valley. While overall winds are expected to decrease this afternoon, there’s still a chance we could see with gusts up to 25-30 mph through the early evening hours. These winds, combined with the poor daytime humidity, has allowed for the Red Flag Warning to remain in effect until 8PM. Locally breezy wind continue tonight into tomorrow morning over portions of the north and northeast foothills and mountains. Humidity values overall look to be moderate, but for those locally breezy areas, recovery could be poor.

Tomorrow, the pressure gradient over NorCal weakens leading to lighter wind. Heights and thicknesses continue to trend upwards meaning we’ll see an even warmer day. Triple digit heat will be possible for portions of the Northern Sacramento Valley, while areas south peak in the mid to upper 90s. Most areas in the forecast area will be about 5 to 11 degrees above their seasonal average.

By Saturday, the high pressure is forecast to shift to our east, with the flow over much of the PacNW becoming more zonal. We’ll see increased onshore flow with max temps near or slightly lower than Friday. Desert SW High then builds NW over CA Sunday into Monday. Triple digit heat forecast over the northern half of the Sacramento Valley Sunday, becoming widespread over most of the Central Valley Monday.

Extended Discussion (Monday through Thursday)

Today will likely be the last day the max temp will be under 100 degrees in RDD, period! Well not forever, but at least through the EFP, and beyond, according to the Climate Prediction Center (CPC) as excessive heat is noted over the Central Valley next Friday and Saturday (June 26/27), as well. So get ready for a prolonged heat wave, but not necessarily a record breaking heat wave for our main Climo sites, although SAC (Executive AP) has a 14% chance of tying the record max (106) on Thu, 6/25 according to our NBM Viewer. (The records during this period at the main Climo sites are all >105-109 from south to north, and some up north >110, mostly unattainable in this upcoming episode, i.e., Redding and Red Bluff were 117 degrees in 2006 for 6/25.) The limiting factor for record heat may be the relatively lower 5H heights, 585-590 dam heights and +1 to 2 standardized height anomalies, compared to the more anomalous 594+ dam 5H heights we see in mid summer.

Ridging over the Ern Pacific will be shifting inland, into the Desert Southwest and Four Corners region, with the northern extension of the ridge axis forecast to sit over northern California supporting an extended period of heat. 850 mbs temps are forecast to range in the mid to upper 20s degrees C during this time, supporting Valley high temperatures in the upper 90s and low 100s although there is some room for readings above 105-108 degrees as noted by the ensemble spread. Thu (25th) looks to be the hottest day, but max temps during this time will generally run some 8-15 degrees above normal. Thermal belt areas (within the temperature inversion) will remain quite warm, maybe even unpleasant through much of the evening in this pattern. Little relief is expected, except maybe a little cooling from the marine/Delta influence, especially as the heat builds in the Valley causing a sea-land temp gradient, or from a synoptic influence from the approaching cut off low per the GFS later in the period.

The only true impedance to this ridge is an offshore trough which does look quite sheared in nature, or weak cut-off low migrating towards the central CA coast. Other than a few additional high clouds, perhaps more so over the higher terrain, the slim chances of convection appear are problematic and appear to be limited to the Sierra Crest and into Srn ORE.