Large Scale Summary
- For the period late June ñ beginning of July rain dates 30 June, in NORCAL mountains, and to central Sierras, are sparse through most of the month.
July rains are mostly 30 June in Central Sierras Yosemite NP to Lake Tahoe. - Showers increase in the tropical southern Mexico coast region (Jalisco to Oaxaca states during July and head north thru Sonora into Arizona starting about 7-8 July with more rains Sierra Crest.
- Interestingly, we are seeing an increase in rainfall for western Mexico north into Arizona/southern Rockies in July.
- There appears to be a continued development of August and September rains into W Mexico/Baja, Arizona and eventually in California.
- Regarding the emerging summer season and transition to autumn, it is more logical to remain with the guidance at hand from NOAA/CPC/NWS. An increase in thunderstorm (TSTM) activity is reasonable as we transition into September, as we consider the overall pattern. It would be constructive to consider transition to a potentially aggressive period for wildland fires in California as we go into October.
- Certainly lightning and wind are important influencers of the autumn fire weather season. So, even if some wet periodsoccur, presence of lightning/TSTMS and gusty/fickle winds are certainly risk factors to consider, even in a "wet fall".
Special Guidance Summary (June 30-July 30 into mid autumn)
- June 30-July 30: The usual occurrences of breeze are suggested for the end of June as a risk factor for Fire Weather.
- July should be targeted as a month of TSTMS in the Central Sierras, but with local gusty winds in the Central Sierra foothills, near TSTMS.
- July precipitation will tend to be focused in the higher terrain areas of the Central Sierra during 11-15 July. Rain-band mergers pose greatest risk for fires when convective cells in the Sierra foothills contribute to development of gusty afternoon-evening winds in addition to rains and TSTMS in the Sierra west slope.
- August — above average occurrence of TSTMS and lightning from strong convective systems that cover areas in the Sierra crest and west slopes. These can be fire producers for mid to late July and August.
- September would be the month for squall lines and frontal systems that are better organized. 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. Interaction of these tropical cyclone remnants with frontal bands and can add challenges to forecasting in early fall.
- October will tend to be similar to September with small rain-cell size. Another chance for heavy rains with larger cell sizes may become more likely, if moisture becomes abundant, and cutoff upper lows become more frequent or intensify. 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 be very limited. These tend to result in very small cumulus cells, and limited moisture for precipitation. If moisture is present, then rain is more likely to occur, which is logical. As moisture content increases in a tropical airmasses affecting Southern California, the probability of clusters of thunderstorms also may increase.
SOUTHERN CALIFORNIA
- PRECIPITATION DATES: MOUNTAINS 11-15 July
- SOUTHERN CALIFORNIA SAN BDNO/LA CO MTNS/ AND TEHACHAPI MTNS 11 to 15 July. Dry and hot, alternating with breezy and a little cooler as troughs occur with deeper moist layer at times. Again, high pressure continues to focus into California while easterly flow with tropical moisture and TSTMs focus in western Mexico and into SOCAL/San Bernardino/Riverside Coís.
- SOUTHERN CALIFORNIA DESERTS: Lack of TSTMS, lack of show ers, plenty of heat. Precipitation dates will tend to focus into late July and early August when the moisture reaches north into SOCAL from Mexico.
- VENTURA CO MOUNTAIN PRECIPITATION: RAIN SHOWERS/TSTMS will occur if there is moisture reaching northwestward from Mexico and Arizona.
Outlook into August —October
- There may be an increased chance for rains in August and September if the monsoonal moisture reaches farther N into SOCAL and the Sierras as currently indicated by GFS and CFSv2 models, working together. Our best strategy at this point is to monitor tropical cyclone developments. As the summer transitions into fall, we should expect more interaction from tropical (monsoonal) moisture with flow around cutoff lows and their associated fronts. Those associations will occur. August-September are prime months for those. In addition, with the developing El Nino pattern expected to affect W Mexico, SOCAL, may push more moisture further north into California than shown here. If this occurs, then there will tend to be some more opportunities for rains to occur into SOCAL mountains and deserts, and Arizona.
- The current synopsis is rains early in late summer-early fall seasons (August-September. October and November currently appear drier during a 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, and Nov 2026. This would be followed by an up-tick in rainfall starting in early Dec, and possibly in late Nov.
In December the winter pattern is more likely to strengthen under the influence of El Nino , and shows a different response and pattern of rains than in August — October and most of November. In mid to late November and December, the more intense rains focus further north in California, following the drier scenario developing for October and November. This occur in response to the increased activity of El Nino and subsequent response of the El Nino ñ fueled winter cyclonic storms and fronts increasing strength.
Figures and Text: Alan Fox, Fox Weather, LLC
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