Large Scale Summary
- Support for troughing continues in the central N Pacific from about 135-138W. The main governing agent for this is the MJO with a maximum along about 175W and another about 130W, and the IVTinit feature located 128-138W. The main issue for California is hot weather, especially in SOCAL, and lack of rain.
- The near term precipitation guidance (GFS-model based) shows mostly dry conditions in N and Central California except for rains the first few days of Sep, and middle of Sep (13-17). In principle, dry lightning risk and high fire risk due to breeze and low relative humidity should[Ma1] continue to be monitored in the NWS forecast products issued during Sep for Oregon Cascades, and in the Siskiyou Mountains, NORCAL coastal ranges, mainly Trinity Co, and in the Sierras until significant rains occur.
- Monsoonal moisture and most rain-producing thunderstorms (TSTMS) appear to be limited to western Mexico, Arizona, and SOCAL mountains and deserts, and remain mostly SE and E of the Sierras. Easterly waves and an occasional tropical cyclone in southwest Mexico to southern Baja usually produce most of the rains during this late summer period for SOCAL mountains and deserts, and, if any, for the Central and S Sierras.
- This season, however, the focus of tropical moisture is mainly south and east of SOCAL and southern Sierras. We are entering Sep now, and the pattern favoring flow of moisture from western Mexico into SOCAL and Sierras has returned, with TSTMS mostly occurring in San Bernardino Co mountains and deserts. There is a chance for heavy rains occurring in central and northern Baja California and S Arizona, as tropical cyclone remnants reach a second maximum of occurrence in Sep.
- In September, flow aloft will be strengthening from the west and WSW. Therefore, most of the moisture will tend to come from the southwest, 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. Periods of well above average temperature are suggested in the NOAA/NCEP/CPC
guidance for most of Sep. There is cooling at the California coast in late Sep due to tropical cyclone-related rains and possibly again Sep 28-30. Currently rain appears unlikely for central Sierras/Yosemite NP area during most of Sep. until about 12-16 and 28-30.
Near Term Forecast
- Dry throughout California. Hot in central and S California except immediate coast. There is also the usual risk of late-season TSTMS in mountain areas. There is also still a chance for some cooling as westerlies strengthen in mid Sep, with chance for a few rains developing from Pacific fronts. NORCAL Rain Dates in Siskiyou Mountains and N Sierras: 1-3 Sep, 15-17th, and 25-30 Sep.
- There will continue some risk for 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 west winds, and a chance for rains Sep midmonth.
Central California
- Central Sierra Precipitation: few dry TSTMs Sep 1-4. Possible shift in the pattern to increased west wind, briefly cooler, and a chance for rains Lake Tahoe to Tuolumne Co, and north about Sep 1-4, 13-17 and 19-20.
- CENTRAL CALIFORNIA HOT DRY PERIODS: Above average temperatures occur with a few thunderstorms (TSTMS) in mountains. Showers are suggested for Sep 1-5.
Southern California
- SOCAL PRECIPITATION: Morning fog/low clouds 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 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 end of August and around Sep 1-6. Cooler with increased depth of the marine layer conditions will tend to occur mid Sep.
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. Shower dates near mountains or far SE SOCAL Deserts: 29-31 Aug,in far southern SOCAL mountains and deserts. Windy cooler days occur in the deserts mid and again late Sep.
Medium Term Outlook — September 28-October 31
- 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 130-140W. 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. Spread of rains into NORCAL is not well defined at this point.
- Although an occasional trough or upper low may develop into California from the west, there is little overall change in the pattern or support for troughing into California between September and November.
Northern California
- For valleys and mountains of Northern California: turning cooler with better chance for showers in late Sept and Oct according
to normal behavior for the seasonal change.
Southern California:
- 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. West of SOCAL, the jet stream will slowly be adjusting southward in mid Sep thru Oct.
DISCUSSION OF NOAA/NCEP/CPC MAPS NOV-DEC 2025
- After a dry, probably hot October with Santa Ana winds, beginning about mid Nov, and possibly the end of Oct there is a chance for a rain. We start to see rains extending from the Pacific Northwest (ORWA) south into north portions of NORCAL (Humboldt County, possibly the Bay Area to Monterey) about mid Nov. We should keep an eye on this. The CFSv2 does not show above average precipitation until Dec. in October, the NMME had been
showing a burst of rain around the end of the month, but then the NMME backed away from rains during Nov. Usually there is a good rain around mid November, even if the remainder of the month is dry. - The dry W-SW flow returns in Oct and Nov, with persistent upper high pressure over most of California. There continues to be support for a shift to above average precipitation beginning mid or late Nov (Thanksgiving holiday) through Dec. CFSv2 maps have been showing above average rainfall for Dec 2025. Eventually, the belt of westerlies in the mid to eastern N Pacific still has support to meander far enough south to tap unusual amounts
of moisture in the subtropics 25-38N 130-145W. The timing of this event can be anticipated, albeit loosely by IVTinit. However, once we reach late October we should have a much better idea from our IVTinit verifications and trends out 3-4 weeks on the timing of start of the rainy season. At this time, Dec is still four months away. We need to see trends about October 15 to tell us something more useful for Dec, other than just climatology, for start of the rains. At this time we cannot get too excited about above normal rainfall (and atmospheric rivers).
Figures and Text: Alan Fox, Fox Weather, LLC
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