30-Day Weather Outlook for July 31, 2017, to August 31, 2017
Summary- The basic pattern consists of high pressure continuing for the SW US and Great Basin/Rockies, with a tendency for troughing to the west of Oregon and NW Calif. Pacific into Oregon. High pressure will steer tropical cyclones NW or WNW past S Baja, and then turn westward towards Hawaii. Upper high pressure contributes hot weather for the near term (first week of Aug), followed by troughing during August 9-10th, most of which focuses in N Calif and Oregon. Beginning August 9th, we enter a succession of troughing and ridging, producing alternately cool and possibly showery periods in the mountains of N Calif/SierraNV, then followed by hot upper high pressure.
The solar eclipse on August 21st currently falls into one of the sunny and dry breaks, then is followed by a period of showers and TSTMS on the 23-27th. Tropical cyclones continue more likely to move WNW or NW, then turn westward without recurving to the N during August. Moisture for TSTMS in the SierraNV continues to be intermittent but is occasionally active thru August. Sea surface temperature anomalies (SSTA) have decreased off S Baja, while warmer than normal SSTAs remain over the region near and west of Baja. This will provide a good energy source for cyclones moving NW-WNW off S Baja, and will help to maintain warmer than normal weather in Calif and the far W US. Usually, cyclones begin to recurve N towards SOCAL bringing moisture and TSTMS into S and central Calif about 1 Sept.
A large area of warm SSTAs will continue from S Calif and N Baja WSW to Hawaii. This gives some support for redevelopment of intermittent subtropical westerlies developing in early season Oct–Nov-Dec, mostly directed into N Calif and Oregon.
El Niño, though weakly present in June and July has become quiescent. The ENSO index appears to remain neutral during the upcoming wet season 2017-18. The SSTA pattern now, if continued thru winter as suggested by NMME, focuses rain into central and N Calif with below normal rainfall in late fall-early winter for SOCAL.
In the Near Term – Aug. 3-17… 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. Coastal eddy conditions have increased probability during August 10-11th, and 15-16th. TSTMS continue a daily occurrence in the mountains – deserts.
Summary – Aug. 18-31… In the southern California avocado growing areas, from San Luis Obispo south, prolonged hot periods occur during Aug 18-31. TSTMS occur mostly in the afternoons-evenings in the mountains and deserts of S Calif. The period of August 23-31th 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 to be subdued, followed by ENSO-neutral conditions thru mid-Fall to early winter. However, sea surface temperature anomalies in the mid-latitude Pacific favor development of a WSW-ENE storm track oriented toward northcentral and N Calif, producing a wet Oct -Dec in the N half of Calif. El Niño Conditions are currently not indicated for Nov-Dec 2017, and above normal rainfall in SOCAL is not indicated. Wetter than normal conditions are more likely for the central and N SierraNV in Oct – Nov – early Dec.
...Alan Fox, Fox Weather LLC...