Large Scale Summary
Support for troughing continues in the central N Pacific from about 150-160W (mainly N to NNE of Hawaii), and at 128-140W off the California coast at 35-45N. Showery conditions based on model inputs during May 6 Sierra snows. GFS showers mostly rain ORWA/NORCAL on 12-early13 north coast of California, 17-18, 21-23, 26-31, and June 8-9. We look for a transition to hot conditions about mid June.
Near Term Forecast — May 15 - June 15
Northern California: Precipitation dates in N coast and N-Central Sierras are May 12-13, 17, 26-31 May, and June 8-10. Mild/Dry Periods: Cool to near seasonally average temperatures near the rainy periods, but warm sunny break 18-19 May. Cool with occasional low snow levels during May 12-14, 26-31.
Central California: N Sierra precipitation May 12-13, 17, 26-31, and June 8-9. Central Sierra precipitation May 12-13, 18, 23, 28-31. Warmer 15-16 May.
Southern California: Precipitation May 12-13 mountains, otherwise sparse and disorganized at this time. Move into more of a spring marine morning drizzle pattern in May and early June. Mid May appears to have a greater chance for deep marine layer events, so that the hot conditions are mainly intermittent and inland, while the cool periods are at the coast and coastal valley areas. Colder nights San Luis Obispo Co valleys if clearings occur for a time overnights/early mornings after the marine drizzle periods. Dry/warm events: May 15-21 after cool periods associated with showers in mountains, deep coastal marine layer and drizzle.
Southern California Deserts: A little cooler than average during recurrent development of troughs and cold fronts from the Great Basin (Nevada/Utah) into Arizona (persistent northwest flow pattern for S California).Turning warmer than average for a brief period in mid May. Rainfall amounts mostly near average. The dry periods will tend to be balanced by the wet ones. Shower dates in SOCAL Deserts: Rain sparse in May, with best chance for showers in about May 12-14 and again occurring mainly May 27-29, 30-31 in few desert showers.
Medium Term Outlook — June 15 – July 15
- Northern California: Near average rainfall. Positive precipitation anomaly more likely will be at the North Coast Mendocino Co north, Sierras rain/snow from end-of-season cold fronts or troughs May 27-31. For valleys and mountains of Northern California, turning hotter and drier than average during first month of summer (10-30 June). Continuing hot and drier than average in July.
- Southern California: Near average precipitation with near average temperature. More persistent marine layer conditions during May will tend toward a shallower and more erratic marine layer influence. Therefore, watch for interspersed hot periods accompanied by shallow coastal marine layer conditions at times during mid June. Transition to dry hot conditions by mid June and going into late June as tropical cyclone conditions start to develop off southern Baja. 30 Day Precip/Temp maps for June and July suggest a return to minimal monsoonal TSTMS but continue above average temperatures. Please note that well above normal temperatures are suggested in the NCEP/CPC guidance for June. Heat wave conditions possible in the Sierra Nevada foothills and west slope areas from El Dorado Co south in June. Below average rainfall is expected in Sierras from monsoonal thunderstorms (TSTMS), due to dry conditions for monsoonal thunderstorms in western Mexico. For the valleys of Southern California, turning warmer and drier than average at times during June. We note from the NCEP monthly guidance included next page that significant hot spells are a possibility in June for inland SOCAL citrus and avocado regions if or as tropical cyclones develop near southern tip of Baja. There is risk of highs reaching to near 105 in the Ramona-Temecula areas and 95-100 in Fallbrook during mid to late June. A very late onset of monsoonal TSTM conditions develops during late July with below average rain amounts for Arizona, SE California, and the Sierras, and below normal monsoonal
TSTM activity for Jalisco,Sonora,Sinaloa states, and even Michoacan on the south coast.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC.