Large Scale Summary
- Support for troughing continues in the central N Pacific from about 142-150W (mainly N to NNE of Hawaii). After a few early season monsoonal showers/TSTMs in early July, a hot dry "dome" of upper high pressure appears to persist over California during most or nearly all of August. Summertime hot conditions dominate in July.
- Periods of well above average temperatures are suggested in the NOAA/NCEP/CPC guidance for about 18 June through July away from the coast of California. Coastal areas remain seasonably mild until/unless east winds start to develop later in summer and early fall.
- Heat wave conditions occur in the Sierra Nevada west slope areas from El Dorado Co south during most of July. Below average rainfall is expected in Sierras from thunderstorms (TSTMS). Most TSTMS will be generated by moisture from tropical cyclones affecting the southern Gulf of California and S Baja.
- Rain Dates: Rains (from TSTMS) occur July 1,2,4,5 mainly NORCAL mountains. Other rain dates: July 14-17, mainly central Sierras.
Near Term Forecast July 4 – August 1
- Hot conditions currently occurring in the central US to Ohio Valley may also expand westward in July to bring risk of excessive heat to California valleys and
Sierra west slope. There is also increased risk of excessively high Lower Atmosphere Stability Index (LASI) with possibility of explosive growth of fires a few days before the July 4th holiday and on other dates.
Northern California
- NORCAL Rain Dates in Siskiyou Mountains, and N to Central Sierras are: Rains (from TSTMS) occur July 1,2,4,5.
- NORCAL Hot/Dry Periods: Transition to hot dry conditions 28-30 June in NORCAL and Central California as high pressure aloft over the central US drifts west into California. There is still some risk of mostly dry thunderstorms (TSTMS). TSTMS will tend to be accompanied by strong gusty winds 18-25 mph, minimal showers, and local cooling during the TSTM periods.
- During these periods of mostly dry TSTM activity, watch for lightning strikes into dry brush, and fickle/sudden increases of wind speed.
Central California
- N Sierra Precipitation: July 2,4,5, 14-17.
- Central Sierra Precipitation: July 2,4,5, 14-17.
- HOT DRY PERIODS: Transition to above average temperatures with thunderstorms (TSTMS) in mountains. Sparse showers are suggested for July July 3-4, 14-17. Strong fickle and gusty winds may occur together during the periods indicated for TSTMS and sparse showers. The best chance for significant TSTM events with local brief moderate rains may occur 2,4,5,14-17.
Southern California
- SOCAL PRECIPITATION: Sparse showers are suggested for July 1, 4, 14-16. Strong fickle and gusty winds may occur together during the periods indicated for TSTMS and sparse showers. The best chance for significant TSTM events with local brief moderate rains may occur 2,4,5, 14-16. San Diego Co, San Jacinto mountain areas, and the eastern San Bernardino Mountains are those mountain areas most likely to receive local heavy TSTM events with brief moderate rains. Tropical cyclone moisture may reach into Arizona, then flow will resume from the east, bringing showers into SOCAL mountains and deserts on a few occasions: around 1,4 July with local cooling during rain periods.
- SOCAL WARMER PERIODS: Troughs will help keep the coastal areas less hot than the inland valleys (Escondido, Ramona, Hemet, and of course the eastern and southern DesertsÖall of these will experience some extreme hot spells and overall minimal rains. We then transition into warm or hot, and minimal rains from TSTM-type conditions (mountains and deserts). Marine layer conditions will affect the coast but will irregularly contribute cooling as the marine layer intermittently deepens for a short time. East winds will contribute some robust clearings to the coast with resultant hot days during the second half of July.
- Inversion height - lower than average in mid to late July due to persistent high pressure aloft.
- SOCAL Hot Periods: Following TSTM periods in mid to late July.
Southern California Deserts
- SOCAL deserts turn to more persistently hot as tropical cyclones over S Baja and SW Mexico induce development of hot and persistent upper high pressure, as we head into July.
- Shower dates in SOCAL Deserts: 1,2,4,5 July, 14-17 July.
Medium Term Outlook: August 31 and 1-10 September
- The IVTinit pattern along with the Madden Julian Oscillation (MJO) both are showing a tendency for troughing in the mid Pacific as mentioned, north of Hawaii. In this pattern there is less tendency for troughing into Central and S California. Even N California appears quiet but subject to heat waves as atmospheric rivers try to develop in the central and southern Gulf of Alaska and keep the focus of stable upper high pressure about 36-40N in the western US as well as the ocean area west of California. Troughing and upper lows continue to recur in the mid Pacific, while tropical cyclones contribute additional support for upper high pressure for the West Coast and California. These argue for sustained hot weather in August, developing with fewer than usual breaks in the sustained heat.
- There is not much overall change in the pattern as we drift into August. However, August often sees a turn of the upper winds to SE-ESE, and a more NW-ward drift of tropical moisture up the Gulf of California into SOCAL, California and Arizona. Showers that develop in Arizona are more likely to find their way into SOCAL and Central California/Sierras in August. However, the pattern of monsoonal moisture arriving from the Gulf of California may become persistent.
- Northern California - For valleys and mountains of Northern California: hotter and drier than average in most of August and early September.
- South half of California: 1 ñ 31 August: Near average precipitation with above average temperature. Weíll see a trend toward a shallower and more erratic marine layer influence in August, and into September. Therefore, watch for interspersed hot periods accompanied by shallow coastal marine layer conditions at times. Hottest conditions are currently indicated for August, and erratic cool spells with briefly deeper marine layer as troughs occur.
- 30 Day Precip/Temp maps for August suggest below average rain from monsoonal TSTMS for Western Mexico and S Arizona as well as SOCAL in August, but still continue above average temperatures for the most part in California. This would be interpreted to indicate a hotter than normal August, but with the trend for below average rainfall continuing.
- Well above average temperatures are expected to hang on from August into the first part of September. By mid September coastal temperatures regain some of the cool ocean influence, thanks to the cool ocean and onshore, albeit shallow surface flow. As tropical cyclones meander closer to the coast of Baja and California in August and September, there is a greater support for development of hot east flow. So we will watch for those occurrences in both August and first part of September.
- From the NOAA/CPC/NCEP monthly outlook maps, please note that September continues the trend in recurrently hot dry conditions (warmer than average). The map for October (SOCAL) suggests continued above normal temperatures, presumably from recurrent east winds and Santa Anas for SOCAL.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC
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