30-Day Weather Outlook for July 24, 2017, to August 24, 2017

  • Jul 26, 2017

Summary- The basic pattern consists of high pressure over the SW US and Great Basin/Rockies, with SW flow from the Pacific into Oregon. High pressure continues thru August 1 and possibly thru August 9 as tropical cyclones move NW off the Baja coast, then turn westward. Upper high pressure rebuilds July 28th with hot weather. Numerical guidance (GFS) tries to bring tropical moisture into Southcentral Calif and the SierraNV from a tropical cyclone remnant about August 1-3. If this occurred it would be a major rain event. However, it is more likely and in line with typical behavior of tropical cyclones, for a turn to the west, without recurvature into the Calif coast. Usually storms begin to recurve towards Calif bringing moisture onshore after September 1. Moisture for TSTMS in the SierraNV continues to be limited but increases at times during first half of August, as is normal. A pool of warmer sea surface from SW Mexico to near Hawaii will maintain an environment favorable for tropical cyclones, and their NW movements towards SOCAL. A large area of warm SSTA’s will continue to extend W from S Calif and N Baja WSW to include Hawaii. This gives some support for redevelopment of intermittent subtropical westerlies developing in Oct – Nov, mostly directed into N Calif and Oregon. Monsoonal moisture will continue to be a factor to produce mountain TSTMS thru Sept. El Niño is currently quiescent, with no consistent trend. Some redevelopment of El Niño is possible in midwinter 2018, with the latest NMME suggesting a wet January 2018.

In the Near Term – July 29 – July 31… In the southern California avocado growing areas from Santa Barbara County to San Diego County, it continues to be hot inland with persistently warmer than normal conditions. The typical summertime coastal fog pattern becomes thin or interrupted. TSTMS become more of a daily occurrence in the mountains – deserts.

Summary – Aug. 13 – 30… In the southern California avocado growing areas, from San Luis Obispo south, prolonged hot periods occur during Aug. TSTMS in mostly afternoons-evenings in the mountains and deserts of S Calif. The period of August 23-31 has potential for an increase in TSTMS again in SOCAL mountains and deserts.

Seasonal Outlook / El Niño Update...September 1 – November 30, 2017… As normally occurs, there will be some monsoonal SE flow episodes with TSTMS in the mountains and deserts of SOCAL, and some early rains from cutoff lows occurring in central coast and central Calif during Oct and Nov.

El Niño continues subdued thru mid or late Fall. However, sea surface temperature anomalies in the mid-latitude Pacific favor development of a WSW-ENE storm track oriented toward central and N Calif, producing a wet Oct and Nov in the N half of Calif. El Niño, if it redevelops, now appears more likely to have too much competition from other factors to encourage above normal rainfall in SOCAL. Wetter than normal conditions are more likely for central Calif and the central and N SierraNV, mainly in Oct – Nov.

For the late Fall to early Winter, i.e. Nov – Feb… we start with a wet and mild pattern in Nov, then below normal rainfall in Dec, but a possible return to wetter than normal for most central and N Calif in Jan. This is followed by a dry and warm Feb, per the NMME guidance. CFSv2 guidance shows drier than normal for Dec, but we place more confidence in the ensemble (NMME) guidance at this time for the longer range.

...Alan Fox, Fox Weather LLC...

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