LARGE SCALE SUMMARY
- Support for troughing continues in the central N Pacific from about 150-160W (mainly N to NNE of Hawaii), and at 128-145W off the California coast at 35-40N. We suggest showery conditions based on model inputs during May 1-5, NORCAL, late 5-6 Sierra snows. GFS showers mostly rain ORWA/NORCAL 9-10, 13. Cool and showery 10-18 Central and Northern California then dry and mild 19-23. Showers and rains 27-31 May NORCAL and
Central California.and again briefly around 10 June or so. - Watch for some more cool and showery periods in early May (2-5, 7, cool/showery 10-18 May, dry and briefly warmer 19-23rd, showers 27-31 May and possibly early June. We look for a transition to hot conditions about mid June.
Near Term Forecast
- NORTHERN CALIFORNIA: Precipitation Dates in N and Central Sierras are: May 1-6, 7, and 10-18. Cool to near seasonally average temperatures. Cool with occasional low snow levels during following events: North Sierra Precipitation: May 1-6,7, 10-18; Central Sierra Precipitation: May 1-6,7, 10-15
- CENTRAL CALIFORNIA: Precipitation: Sierras on May 2-5, 7, 10-18 and 25-29 again cold or cool. Briefly warmer 18-23 May. Warmer periods May 18-24.
- SOUTHERN CALIFORNIA: PrecipITATION appears to be sparse and disorganized at this time, intermittent in May as marine layer/drizzle events occur. SOCAL had a cold showery event from an erratic upper low on early April 26. We could see another couple of those SOCAL lows and showers about May 7 and possibly again 25-27 and about 30-31, (more likely drizzly cold systems). 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 in the coast and coastal valley areas. A night or two may have isolated or no frost in the colder valleys along the Central Coast (San Luis Obispo Co) if clearings occur for a time overnights/early mornings after the marine drizzle events. May 18-23 after cool periods associated with deep marine layer and drizzle/light rains.
- 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 below average. Shower dates in SOCAL Deserts: Rain sparse in May, with best chance for showers in about May 3-4, 7, 14 and again occurring mainly May 26-27, 30 in a few desert showers.
Medium Term Outlook — May 25 – June 30
- Northern California: Below average rainfall. Positive precipitation anomaly more likely will be at the North Coast Mendocino Co north from coastal drizzle and light rains from weak end-of-season cold front or trough May 25-27. For valleys and mountains of Northern California, turning hotter and drier than average during late spring and first month of summer (10-30 June).
Southern California: Near average precipitation with near average temperature June 10 - 30. 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 to late June. Transition to dry hot conditions by mid June. 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. We suggest heat wave conditions 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 a dry June for monsoonal conditions in western Mexico. For the valleys of Southern California, turning warmer and drier than average at times during June. Significant hot spells are a possibility in June for inland SOCAL citrus and avocado regions. 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 late onset of monsoonal TSTM conditions developing during mid to late July with below average rain amounts for Arizona, SE California, and the Sierras.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC.