June 22, 2018 – Hot and dry weather through the weekend. Temperatures returning to near normal next week. Heat Advisory from noon today to 8 PM Sunday. Red Flag Warning from midnight tonight to 8 PM Sunday.
Discussion
Beginning of a relatively modest heat wave over NorCal today and through the weekend as 5H heights rise to around 590 dam, then lower slightly on Sat as low pressure passes to the N, and then rise again on Sun behind the low pressure system. This is a relatively modest heat wave in looking at the +1 temp anomaly, as the +2 anomaly generally stays to our W, and a 5-10 year return interval. This will be the 2nd time this month the max temps at the major climo sites will exceed triple digits since June 12-13 when most of the readings reached the low 100s. Max temps during this heat wave are expected to peak out between 105-110, with the hottest expected in the Nrn Sac Vly, RBL/RDD.
As the low pressure system passes to our N and NE later tonight and Saturday, northerly pressure gradients are forecast to increase to 11-12 mbs at times, through 06 Sun, before weakening on Sun, and turning onshore/up valley by the end of the weekend. The synoptic pattern is conducive to the development of a northerly barrier jet in the Sacramento Valley. Staudenmaier (1994) and Mathews & Juskie (2005) have documented this phenomenon (in WRH TA-Lites) and its impact on wind advisories/red flag warning considerations. The one noticeable difference is the more easterly track of the digging low pressure system (into the central Rockies) in contrast to autumn when the low pressure systems have a tendency to dig into the Great Basin, farther W than this current system. One of the effects of this northerly wind is the adiabatic warming/drying of the katabatic winds which elevate temps and desiccate humidities both overnight and during the day.
One potential stumbling block is the timing of the return of the Delta Breeze and its impact on max temps on Sun. With the upper ridge axis building/moving inland Sat nite/Sun, the warmest air is expected to be over the CWA and a very warm night/and hot day are in store for most of the CWA. The heating of the north end of the Sac Valley should create a large temp/density gradient between the land and the ocean, resulting in a strong onshore pressure gradient by Sun afternoon or evening. This should cause at least a moderately strong Delta Breeze which should cool the Delta influence areas quite rapidly. Sun could see a wide range of temps between the Carquinez Strait and the Ern Sacramento Suburbs and foothills. Since the marine layer will be relatively shallow (“squashed”) by large scale subsidence, the cooling effects may be limited away from the immediate Delta Areas and temps in the upper 90s to around 100 are forecast for SAC/DTS on Sun. However, the Delta area should be much cooler and have removed zone 18 from the Heat Advisory on Sun.
Seasonable weather returns to the CWA on Mon as another Pacific NW trof provides onshore flow and some cooling.
Extended discussion (Tuesday through Friday)
Minor model differences through the extended forecast period but general consensus is towards broad upper troughing. This will help to keep temperatures only slightly above normal. Dry weather expected through next week.