Large Scale Summary
- For the period in July rain dates: 14th of July, and about 9-10 August. Most of these are showery, perhaps a thunderstorm (TSTM), and of course the coastal drizzle that affects the windward coastal hills by the ocean.
- Showers increase in the tropical southern Mexico coast region (Jalisco to Oaxaca states during July and head north through Sonora into Arizona at several intervals
from 14 July onward into mid August, with more rains Sierra Crest. - There appears to be a continued development of August and September rains into W Mexico/Baja, Arizona and eventually in California.
Special Guidance Summary July 14-Aug 20
- July 14-Aug 20: The usual occurrences of breeze and a few showers and TSTMS are suggested for most of July as a risk factor for Fire Weather.
- July should be targeted as a month of scattered afternoon-evening TSTMS in the Central Sierras, and local gusty winds in the Central Sierra foothills.
- Late July precipitation will tend to be focused in the higher terrain areas of the Central Sierra during 20-21 July, and again late in the month to around 1 Aug, mostly occurring in the Yosemite-Tahoe, Donner Pass to Plumas Co areas.
- August - about average occurrence of TSTMS and lightning from strong convective systems that cover areas in the Sierra crest and SE slopes. These can be fire producers for mostly late July to mid-August. Rainfall character will need to be assessed in the daily forecasts. Enough rain to cover forest lands can have a part in diminishing ignition, but we should not depend on TSTMS being sufficiently wet, or precipitation to be sufficiently widespread to diminish ignition.September would be the more likely month to see squall lines and frontal systems that are better organized, and produce more widespread rain. But even with localized heavy rains, amounts of significant rainfall, tend to be small, and heavy rains tend to focus in higher terrain/Sierra crest. The September events that produce widespread rain are tropical cyclone remnants that are most likely to contain widespread rain with high relative humidity. Interaction of these tropical cyclone remnants with frontal bands can add challenges to forecasting early fall rains.
- October will tend to be similar to September but with smaller rain-cell size. Another chance for heavy rains with larger cell sizes may become more likely, but that is more likely in November.
- 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: Timing of rain events is difficult this time of year for two to three months in the future. 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 can become limited at times. These tend to result in small cumulus cells, albeit localized heavy rain, 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: MOUNTAINS 14-15 July, 29-31 July, and Aug 10-15, and Aug 18-22.
- SOUTHERN CALIFORNIA SAN BDNO/LA CO MTNS/ AND TEHACHAPI MTNS 14 to 15 July. Dry and hot, alternating with breezy with chance for a dry TSTM. For 20-22nd TSTMS with areas of moderate rains/small rain-cells, and still risk of dry lightning. A little cooler as troughs occur with deeper moist layer at times. SE flow
with tropical moisture and TSTMs focus in western Mexico and into SOCAL/San Bernardino/Riverside Coís. For now, the TSTM action appears to remain mostly
NW Mexico/Sonora, with TSTMS drifting N along the western lower deserts and Mt Laguna (San Diego Co) to E San Bernardino Mountains and Forest Falls to Big Bear Lake. Heavy rains focus into Arizona mountains/Mogollon area, and Salt Watershed during August.
SOUTHERN CALIFORNIA DESERTS
- Precipitation dates will tend to focus into late July and most of August when the moisture reaches north into Arizona and SE California deserts from Mexico.
VENTURA COUNTY
- MOUNTAIN PRECIPITATION: RAIN SHOWERS/TSTMS will occur as moisture reaches northwestward from Mexico and Arizona into the San Gabriel Mountains and Ventura County mountains.
Outlook mid-August - mid October
- The monsoonal pattern continues for a while in Sept, then diminishes a bit in October as the seasonal pattern shifts. The focus of heaviest rains occurs in Nov-Dec
and Jan 2027. NORCAL tends to be on-off with rains from fronts and TSTMS, depending on wind-flow direction. The big rains appear to settle in about late Nov and Dec. The January 2027 Outlook maps from NOAA look "apocalyptic" as far as rain is concerned. CFSv2 is showing all of California in a widespread rain pattern, typical of those ARK-Storm training scenarios. In fact, the Jan 2027 outlook map for precipitation looks like somebody just decided to "plaster" an ARK-Storm scenario onto the forecast. We know that did not really happen, but it is amusing. Interestingly, weíre seeing many moist warm oceanic mesoscale eddy patterns developing, plus weíre seeing some continuation of unusually warm SSTAs 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 ease slowly into fall during late Aug and early Sep. Western Mexico/Sonora/to Mazatl·n areas next to the Gulf of California
have support for moisture and tropical cyclones, but not much to get excited about, rain-wise. There is minimal suggestion ofmoisture increase during Aug-Sep, except for W Mexico and Sonora, into Arizona. Best strategy is to watch the moisture trends, by watching IVTinit. 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 IVTinit pattern at 140-155W, and 35-45N. 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 more of the warm upper high pressure west of SOCAL, and 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 SW from Southern California. There appears to be support for a lot of warm water left over from the summer months, 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 the Baja, and Southern California southwestward.
Figures and Text: Alan Fox, Fox Weather, LLC
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