Large Scale Summary
- Support for troughing continues in the central N Pacific from about 135-145W, mainly NE of Hawaii. The main issue for California is heat and lack of rain.
- The near term precipitation guidance (GFS-model based) shows mostly dry conditions in Nroth and Central California, focusing on the Sierras, Trinity Co, and the Siskiyou Mountains. In principle, dry lightning risk and high fire risk due to wind and low relative humidity should continue to be monitored in the NWS warning and coordination products issued during Aug and Sep for Oregon Cascades, and in the Siskiyou Mountains, NORCAL coastal ranges mostlyTrinity Co, and in the Sierras until significant rains occur.
- Monsoonal moisture and most rain-producing TSTMS appear to be limited to western Mexico, SOCAL mountains/deserts, and remain mostly south and east of the Sierras. Easterly waves and an occasional tropical cyclone in southwest Mexico to southern Baja usually produce most of the rains during this mid to late summer period for SOCAL mountains and deserts, and for the Central and S Sierras. This season, however, the focus of tropical moisture is mainly south and east of SOCAL and southern Sierras. Although sea surface temperature anomalies are highest in August and September, there is no consistent pattern favoring flow of moisture from western Mexico into SOCAL. Therefore, showers continue widely-scattered in August.
- In September, flow aloft will be strengthening from the west and WSW. Therefore, most of the moisture will tend to come from the west, except during times when high pressure aloft and hot conditions dominate. In the nearer term moisture is either carried NE into Arizona, or NW-WNW-ward out into the subtropical Pacific west of central Baja. We note that periods of well above average temperature are suggested in the NOAA/NCEP/CPC guidance for most of August and Sep, although there is a cooling at the California coast in September.
- Below average rainfall is expected in Sierras from a few thunderstorms (TSTMS} and possibly some frontal passages that occur earlier than shown in the NOAA/NCEP/CPC monthly precipitation anomaly maps (end of this report (pdf only).
- The CFSv2 model suggests rain periods Aug 17-18, 20-22, Sep 7-11, 12-15,17-18,19-21. Currently rain appears unlikely for central Sierras/Yosemite NP area during most of Aug until 12 or 17-18, and 20.
Near Term Forecast
- Hot and dry throughout California except immediate coast. Risk of thunderstorms (TSTMS) to California coastal mountains Humboldt, Trinity, and Mendocino Counties east of the coastal valley areas. These include Ukiah Valley, Larabee and intermediate valleys. In east portion: Sierra west slopes r main hot, but with possible TSTMS in the Sierra crest. There is also a possible significant cooling as westerlies strengthen in mid Sep, with chance of rains developing from Pacific fronts.
- NORCAL
- Rain Dates in Siskiyou Mountains and N Sierras, Aug 17-19, 20-22, Sep 7,11-15, and 19-21. Rain Dates N to Central Sierras are: Rains (from TSTMS) occur Aug 17-18, from Pacific fronts Sep 7,11-15, and 19-21.
- NORCAL Hot/Dry Periods: in NORCAL and Central California due to occasional subtropical lows forming near but W-WSW of SOCAL. There is still some risk of mostly dry thunderstorms (TSTMS). Near term dry TSTMS will tend to be accompanied by gusty winds 15-20 mph, minimal showers, and local cooling during the TSTM periods. Watch for possible shift to cooler, stronger westerly winds, and a chance for rains 12-17 Sep mostly El Dorado Co and Lake Tahoe north.
- Central California
- N Sierra Precipitation: few dry TSTMs Aug 17-18, 20-22. Possible shift in the pattern to increased westerly wnds, much cooler, and a chance for rains Tahoe/El Dorado Co north about Sep, 12-16 and 19-21.
- Central Sierra Precipitation: TSTMS Aug 17-19,south portions. Sep 11-17, 17-18, 19-21.
- CENTRAL CALIFORNIA HOT DRY PERIODS: Above average temperatures occur with a few thunderstorms (TSTMS) in mountains. Sparse showers are suggested for Aug 30-31-Sep 1-3. Sep 3-7. Cooler after 3-7 Sep.
- Southern California
- SOCAL PRECIPITATION: Morning fog will be followed by early burn offs and dry days. Periods of deeper marine layer will occur mainly mid Sep with cooler days, and some morning drizzle possible coast and coastal hills, especially Santa Barbara Co Pt Conception to San Luis Obispo, and also Orange/San Diego Coís.
- SOCAL WARMER OR HOT PERIODS: We return to hot weather inland for SOCAL, but minimal rains from mountain TSTM-type conditions that may still develop in August and around Sep 2-3. Cooler with increased depth of the marine layer conditions will tend to occur mid Sep. Inversion height Ö.lower than average in August due to recurrent high pressure aloft. Transition to deeper marine layer in Sept as indicated above.
- Southern California Deserts
- SOCAL deserts continue hot but changeable as tropical cyclones over S Baja and SW Mexico induce development of hot and recurrent upper high pressure, as we head toward end of August and first week or so of September. Shower dates near mountains or far SE SOCAL Deserts: 19-20 Aug, and about 25th in far southern SOCAL mountains and deserts. Windy cooler days in the deserts mid to late Sep.
Medium Term Outlook: September 15-October 15
- The IVTinit pattern along with the Madden Julian Oscillation (MJO) both are showing a tendency for troughing in the subtropical mid N Pacific between SOCAL and Hawaii, preferably longitudes 135-150W. The storm track is showing signs of a start of gradual southward migration in the mid Pacific west of 135W while upper high pressure will tend to become more intermittent as rainy fronts reach the Pacific Northwest coast, and begin to spread rain into NORCAL coast areas, and coastal mountains/Siskiyouís. Although an occasional trough or upper low may develop into California from the west,
there is little overall change in the pattern as we drift from September to early October. - Northern California Sep 15-Oct 15: For valleys and mountains of Northern California: hotter and drier than average in most of August in first part of September then cooler with better chance for showers in mid to late Sept and Oct.
- South half of California: Sep 15-Oct 15: Near average precipitation with above average temperature. Weíll see a trend toward deeper and more erratic marine layer influence in September. Cool weather will occur during the periods of deeper marine layer. Not much hope for rain yet in SOCAL, but the jet stream will slowly be adjusting southward in mid Sep thru Oct.
Beginning in November
- We start to see rains extending from the Pacific Northwest (ORWA) south into north portions of SOCAL (Santa Barbara County). We should keep an eye on this. However, IVTinit is not robust on this seasonal shift yet, and may not signal it until sometime in Sep. Therefore, for now, we follow the usual pattern, and may not see any rains of interest until the second week or so of November into central and southern California.
- November and December 2025 look interesting, though in the trend towards above average rainfall in California, including SOCAL. . We are currently seeing a possible shift toward above average rainfall from subtropical systems in NORCAL in Nov, with the trend continuing to increase in December. But that is all we can say at this point, still 4 months ahead.
Figures: Alan Fox, Text: Alan Fox, Fox Weather, LLC
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