June 19, 2020 – Hot and dry weather is expected across the region for at least the next week.
Discussion
Other than a few high clouds spilling through the ridge along the West Coast, skies are clear across interior NorCal early this morning. Surface pressure gradients continue to relax and winds are generally light except for some local north to northeast gusts of 15-30 mph in the usual windier foothill locales. Current temps are milder compared to 24 hours ago and range from the upper 30s in the colder mountain valleys, to the 60s and 70s across the Central Valley and foothill thermal belts.
Highs will continue to inch up a few degrees across most of the region today as the upper ridge moves overhead. The exception will be the Delta region as a little more onshore gradient and shallow marine layer will result in localized cooling there.
Little change is expected over the weekend as the ridge flattens and zonal flow develops to our north. This will allow at least a light to moderate Delta Breeze to continue. Warmest readings across the Central Valley are expected to be a little above the century mark.
The onset of hotter weather is expected Monday as the heights aloft build across the region suppressing the marine layer resulting in widespread triple digit heat across the Central Valley. The northern half of the Sacramento Valley will see highs in the 105-110 degree range, and overnight lows will trend warmer with readings mainly in the upper 60s to mid 70s. This will result in increased heat risk.
Extended Discussion (Tuesday through Friday)
A stout upper ridge will generally sit over the Desert Southwest and Four Corners region for much of next week. This supports a several day stretch of hot temperatures with departures of around 10 to 15 degrees from climatology. The core of the heat is expected over the northern Sacramento Valley into the adjacent foothills with daily high temperatures running between 104 and 108 degrees while overnight lows remain in the low to mid 70s. Most daily temperature records should remain safe given unattainable numbers in the mid 110s. While hot, expected highs will stay closer to the 100 degree mark over the southern Sacramento and northern San Joaquin Valleys given occasional Delta influences. It will be interesting to see how this heat wave pans out given mid- level heights maximize around 590-dm which is low for typical summer heat. Looking aloft, 850-mb temperatures are likely to run in the 27-30C range which is approximately 2 standard deviations above average. With deep, well-mixed boundary layers likely, these 850-mb temperatures would equate to near 110 degree readings.
While the deterministic and ensemble guidance are in line with the pattern through the middle of next week, there are some uncertainties regarding potential disruption of this ridge by later in the week. The GFS continues to be more aggressive with an offshore trough which slowly lowers heights relative to the more sheared out ECMWF solution. There is about 10 degrees of spread between these two forecasts which acknowledges this uncertainty. Have stayed on the warmer side as supported by the multi-ensemble based pieces of guidance. Overall, most days will be quite hot with limited cloud cover. Any showers from enhanced monsoonal flow should stay east of the Sierra crest.