Large Scale Summary
- Both Hurricanes Genevieve and Fausto sidestepped past SOCAL. The next rain system to affect California is August 28 — 1 September.
- Rain Dates as indicated in the current forecast solution for August 22, 25-26, and another on August 28-30-31-Sep 1. Hot dry downslope SE winds are possible at times in mid to late Aug between the rainy periods (TSTMS).
- There appears to be a continued development of late August to September rains into W Mexico/Baja, Arizona and eventually in California, mostly mountain and desert areas.
- Regarding the late summer season and transition to autumn, we remain close to the guidance from NOAA/NCEP/ CPC/NWS. An increase in thunderstorm activity is reasonable as we transition into September, and consider the overall pattern. We do expect some fairly widespread early season rains in Sep. Depending on how much rain occurs, the risk for wildland fires will be affected. Typically, tropical cyclones affecting SOCAL begin with hot Santa Ana-like dry conditions, then gradually transition to wet, even west of the mountain crests and to the SOCAL to San Luis Obispo coast.
It would be constructive to consider a few bursts to potentially aggressive periods for wildland fires in California as we go from a possibly wet late Aug-Sept and again in Oct. Certainly lightning and wind are important influencers of the autumn fire weather season. So, even if some wet periods occur, presence of lightning/TSTMS and gusty/fickle winds are certainly risk factors to consider, even in a wet fall. - Bands of heavy rain associated with upper lows and rains, can easily develop into severe weather situations, merging other nearby rain-bands, and create their own problems regarding fire development.
CENTRAL CALIFORNIA
- PRECIPITATION DATES: Rain occurs about 22 Aug if another tropical cyclone event occurs. Timing of rain events is difficult this time of year for two to three months in the future. We will continue to opt for some possible tropical cyclone associated rains in late Aug and Sep, in agreement with NOAA. Systems are convective, with small areal coverage. Upper lows occur, but depending on current flow regimes, the flow of moisture from the tropics into SOCAL becomes intermittent at times. These tend to result in small cumulus cells, albeit localized moderate or heavy showers, but still some limited moisture for precipitation and overall moistening of fuels. If moisture content increases in tropical airmasses affecting Southern California, the probability of clusters of thunderstorms also may increase.
SOUTHERN CALIFORNIA
- PRECIPITATION DATES: SOUTHERN CALIFORNIA SAN BDNO/LA CO MTNS/ but no rain of consequence. Aug 25-28. Aug 28-31 and Sep 1 showers possible.
Heavy rains focus into Arizona mountains/Mogollon area, and Salt Watershed during August. - SOUTHERN CALIFORNIA DESERTS: Precipitation dates will tend to focus into most of August when the moisture reaches north into Arizona and SE California deserts from Mexico.
- Central Sierra Nevada: TSTMS will occur as moisture occasionally comes north into SE California/San Bernardino Co, and into the Central Sierras mainly 10-12 Aug. Other rains occur about 22, 25, 26-27,30 to 1 Sep.
Outlook September- October
- The monsoonal pattern continues for a little while in Sept, then diminishes a bit in October as the seasonal pattern shifts from tropical late Sep to fall (cutoff lows). The focus of heaviest rains occurs in late Nov-Dec and Jan 2027. NORCAL tends to be on-off with rains from fronts and TSTMS, depending on low elevation flow direction. The main rains appear to settle in about late Nov and Dec. The January 2027 Outlook maps from NOAA still need to be taken seriously, as far as rain is concerned.
- Interestingly, we're seeing many moist warm oceanic mesoscale eddy patterns developing, plus weíre seeing some continuation of unusually warm SSTAs (Synoptic Anomalous Warm Ocean Surface-SAWOS) out at 40N 140-155W, still. This would tend to continue support for a persistent and unusually warm SSTA influence in the Central Pacific as we transition into fall during Sep before the break in Oct.
- Best strategy for Sep-Oct is to watch the moisture trends, by watching our IVTinit maps, issued daily. It is reasonable to suggest that the warm oceanic eddies will continue to keep the Baja coast unusually warm and humid. We will watch the mid-North Pacific moisture and heat anomaly currently suggested by the persistent SAWOS pattern at 140-155W, and 38-47N.
- As summer transitions to mid-fall (late Oct), we should expect more interaction from tropical (monsoonal) moisture with flow around cutoff lows and their associated fronts. El Nino is present in the equatorial region (Nino 3.4 from about 130W to Hawaii at 160W). The latest trends from CFSv2 are suggesting more of the usual tropical cyclones we have seen thus far off S Mexico and south of Baja, but with warm upper high pressure west of SOCAL, and southwest of Central California.
Bottom line
- Watch for the shift to the winter season, and development of westerlies when they finally start reaching to the coast. However, weíll need to keep an eye on those warm mesoscale ocean eddies near and upstream to the W and WSW from Central and Southern California. There appears to be support for a lot of warm water left over from the summer months (including the mid Pacific SAWOS zone of warm water). This should be ready to spring into action, with some abundant input of energy and nearby moisture to the west of Central and Southern California when the winter-type westerlies finally begin to increase.
- The current synopsis is rains in late summer-early fall seasons (August-September. October and November currently appear to have a momentary break in flow from the tropics occurring while the pattern shifts from a mainly late summer/monsoonal pattern to an early winter one. The NOAA/NCEP/CPC/NWS maps are tending currently to show the break in precipitation between seasonal rains in Oct to early Nov 2026. This would be followed by an up-tick in rainfall starting in early Dec, and continue into January 2027.
- We are certainly keeping an eye on the Nov-Dec, and finally Jan 2027, time frame here, for moisture, and cyclogenesis potential, with the warm water hanging around in Baja, and Southern California southwestward. The shift to colder conditions more typical of winter appear to not occur until end of Dec 2026 and in Jan 2027. Both rainfall and temperature conditions are shown in this forecast. One thing to keep in mind, the January 2027 outlook map has quite a lot of cold air showing up with the rains. In January, that argues for snow. In this case, with the amount of precipitation expected, there is support for plenty of snow, both in Dec 2026 and Jan 2027. If NOAA ís forecasts are close to reality for SOCAL, we could see a lot of inclement, dangerous, and possibly severe winter weather conditions in Nov - Jan 2027.
Figures and Text: Alan Fox, Fox Weather, LLC
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