Large Scale Summary
- Support for troughing continues in the central N Pacific from about 160W-180 (mainly N to NNW of Hawaii), and at 135-150W off the California coast at 34-42N. Look for a transition to hot conditions about mid June.
- Well above average temperatures are suggested in the NOAA/NCEP/CPC guidance for about 18 June through July.
- Heat wave conditions in the Sierra Nevada foothills and west slope areas from El Dorado Co south in June mainly after the 19th. About average rainfall is expected in Sierras from monsoonal thunderstorms (TSTMS). There could be some increased moisture being realized in late June due to troughing near the Southern California coast.
Near Term Forecast
Northern California
- Precipitation Dates in Siskiyou Mountains, and N to Central Sierras are: June 19, 28-30.
- NORCAL Hot/Dry Periods: Transitioning to above average temperatures with a few thunderstorms.
Central California
- N Sierra Precipitation: June 19, 28-30, July 13.
- Central Sierra Precipitation: June 19, 28-30, July 13
- IVTinit analyses are showing an increase in troughing near the coast of Central and S California. Be aware that although tropical moisture is not yet near SOCAL, the Sierras, and Central California, there is a chance that the trough may briefly intensify multiple times during the period 17-30 June. If intensification occurs, producing TSTMS, without rain or sufficient rain, rapidly changing conditions re: fire activity may develop quickly in the Sierras.
- Precipitation: Sierras, initially mild. Transition to above average temperature but near to below average precipitation. Below average precipitation in July (TSTMS focusing on July 12-13th in Sierras.
- HOT DRY PERIODS: Transition to above average temperatures with mainly dry thunderstorms (TSTMS). Re: TSTMSÖsee our special statement above. (rainy periods June 28-30 and July 13).
Southern California
- PRECIPITATION: showers are unlikely coast and valleys, and limited to the higher mountain areas, Mt Laguna and Palomar in San Diego Co, San Jacinto mountains, and the eastern San Bernardino Mountains, and the San Bernardino Co high deserts. Tropical cyclone moisture mayreach into Arizona, then flow will resume from the east, bringing showers into SOCAL mountains and deserts on a few occasions: 28-30 June and around 12-15 July. SOCAL initially cool at coast, then becoming hot as east winds develop aloft.
- MILD PERIODS: Troughs will contribute mild conditions still into early to mid June. We then transition into warm or hot, and minimal rains from TSTM-type conditions (mountains and deserts). Marine layer conditions influence coast. Upper lows and troughs may develop. These will cause changes in wind conditions inversion height variable, but trending lower than average in July due to recurring high pressure aloft.
- DRY/HOT: late June after TSTM periods, and after TSTM periods in July.
Southern California Deserts Outlook.
- SOCAL deserts: With weakening of troughs in general we return to a more normal beginning of hot weather during mid June. We turn more persistently hot as tropical cyclones over S Baja and southern Mexico induce development of hot and recurrent upper high pressure, as we head into July. Shower dates in SOCAL Deserts: 28-30 June and around 12-15 July.
Medium Term Outlook — July 15 - August 15
Northern California
- Below average rainfall. Summer monsoonal showers/TSTMS will appear will be near average in the Sierras but below average for Siskiyouís and coastal mountains.
- For valleys and mountains of Northern California, turning hotter and drier than average during latter part of June, and July through most of August.
Southern California
- Near average precipitation with near to above average temperature. Weíll see a trend toward a shallower and more erratic marine layer influence in July, followed by some hot periods late July and through most of August. Therefore, watch for interspersed hot periods accompanied by shallow coastal marine layer conditions at times during mid June. Hot conditions July and August, with usual cool spells with deeper marine layer as troughs erratically develop.
- Please note that well above normal temperatures are suggested in the NCEP/CPC guidance for 20 June through July.
- Heat wave conditions in the Sierra Nevada foothills and west slope areas from El Dorado Co south in June. Tropical cyclones from Mexico , due to a dry June for monsoonal conditions in western SW Mexico will tend to move westward toward Hawaii in late this month and July. This supports recurrent hot weather and some downslope wind conditions in Sierra west slope alternating with a few TSTM events.
- For the valleys of Southern California: We note from the NCEP monthly guidance that significant hot spells are a possibility in late June-July 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 late June and July. Hottest conditions will usually occur on days when monsoonal clouds are mostly absent. - A normal onset of monsoonal TSTM conditions develops during July and continues through August.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC.