Large Scale Summary
- SupSupport for troughing continues in the eastern N Pacific 128-130W and possibly also 130-138W. The main governing agent for this is the MJO with a maximum along about 170W and another about 130W, and the IVTinit feature located 126-140W. The main issue for California is hot weather, especially in SOCAL, and the current lack of rain.
- The near term precipitation guidance (GFS-model based) shows showery conditions in ORWA, NORCAL and Sierras for Sep (mainly 8th and 10-16th). However, according to our latest GFS model guidance, the period 19-22 Sep consists of a hurricane near the Baja California coast near Laguna San Ignacio on the 21st. Slow northward movement is expected with eventual recurving probably to the NW and remaining SW of San Diego. Rain from this system will most likely affect the mountain areas San Diego Co southward. In principle, dry lightning risk and high fire risk due to breeze and low relative humidity should continue to be monitored until significant rains occur over wide areas of the ORWA and NORCAL.
- 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 have entered 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.
- This September, we have some unusually warm sea surface temperature anomalies (SSTA). In the far eastern N Pacific west of California and ENE of Hawaii, the SSTA's are 4.5 deg F above average to as much as 7.0 degF above. (2.5 to 3.9 C above average).
- To put this in perspective, this means that at 30N 140W in Figure 3 (Sep 1 SSTA map, the sea surface temperatures at estimated 76F are almost warm enough to support a tropical cyclone (minimum SST of 78F).
- In September, flow aloft will tend to strengthen 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. We note that periods of well above average temperature are suggested in the NOAA/NCEP/CPC outlook in most of Sep.
- The combination of above average WSW to SW flow from 135W to the Central California coast and the current ëpoolí of unusually warm sea surface west (upstream)from California argues for unusual warmth (day and night) plus an increase in support for rains as winter approaches.
- The current tendency for troughing west of California will tend to decrease the number and strength of Santa Ana events that decrease the seasonal cooling of the sea surface related to upwelling off the California coast, normally related to the Santa Anas.
- Rain Dates: 10-11, 14-15, Sep NORCAL coast to mountains, 19-20, 22 central-S Sierras, 27-30, Oct 5, 14-15.
Near Term Forecast
- Less warm throughout N California. The cool spells focus on mostly 11-12,14,17-20, and 27-30 and about Oct 5. Another hot spell is possible Oct 7-12. Those mentioned rainy periods would also have the usual risk of TSTMS as often occurs in late season fronts in mountain and nearby desert areas. There is also still a chance for some cooling as westerlies strengthen in mid Sep, with chance for some rains developing from Pacific fronts and tropical cyclone remnants.
Forecast for Central California
- Central Sierra Precipitation: few dry TSTMs Sep 10-12,14,17-21. Possible shift in the pattern to increased west flow, briefly cooler, and a chance for rains Lake Tahoe to Tuolumne Co, and north about 8-12 and 14th, then again 17-20.
- CENTRAL CALIFORNIA HOT DRY PERIODS: Above average temperatures occur with a few thunderstorms (TSTMS) in mountains. Showers are suggested for Sep 10-12,14,19-23.
Forecast for S California
- SOCAL PRECIPITATION: Morning fog/low clouds will be followed by periods of deeper marine layer mainly mid Sep with cooler days, and some morning drizzle possible coast and coastal hills. Tropical rains are possible 19-23 Sep in SOCAL mountains.
- 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 October. Cooler with increased depth of the marine layer conditions will tend to occur mid Sep but with warmer conditions and periods of shallow marine layer in Oct as high pressure builds a little. The trend after mid Oct will be for resumption of frontal systems developing between California and mid Pacific near 145W.
Southern California Deserts Outlook
- SOCAL deserts continue seasonably warm or hot, changeable as tropical cyclones develop over S Baja and SW Mexico during this currently active season. Tropical cyclones induce development of alternatively TSTMS, then returns to recurrent upper high pressure with hot days. However, the days are shorter, with lower solar angle, so the heat is less intense.
- Shower dates near mountains or E SOCAL Deserts and mountains, 14-15,and 19-23 Sep. Windy cooler days occur in the deserts mid Sep and again and again about 3rd week of Sep.
Medium Term Outlook — Oct 11 -Nov 11
- 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 126-140W. The storm track is showing signs of a gradual southward migration in the mid Pacific west of 123W while upper high pressure will tend to become more intermittent as rainy fronts reach the Pacific Northwest coast. Spread of rains into NORCAL and Central California are not well defined at this point, but should start to accelerate in late Oct and first part of Nov. Although an occasional trough or upper low may develop into California from the west, there is some return of the hot dry pattern and alternatively for troughing into California between October to early November.
- Northern California For valleys and mountains of Northern California: Drier and warmer than average in most of Oct. West of NORCAL, the jet
stream will slowly adjust southward in Oct. - South half of California: West of SOCAL, the jet stream will slowly adjust southward in Oct. Tropical cyclone activity over Baja will tend to remain S and E of SOCAL.
- DISCUSSION OF NOAA/NCEP/CPC MAPS NOV-DEC 2025. A mostly dry and probably hot October should yield to a rainy start in Nov
beginning with rains about Oct 30-31 and again Nov 10-20 as fronts and troughs begin a regular pattern of movement from the west into California. - Eventually, the belt of westerlies in the mid to eastern N Pacific has support to meander far enough south to tap unusual amounts of moisture from the subtropical Pacific Ocean at 25-35N 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 timing of start of the rainy season. From the NOAA/CPC outlook maps Dec 2025 appears robust for rains throughout California and most of Baja. This argues for early onset of atmospheric river patterns into California, as well as robust upper lows in the Baja California region.