BASIC PATTERN:
Large Scale Pattern –
- A tropical cyclone comes through SOCAL during 20-23 August with heavy rains, likelihood of flash flood events, and gusty winds. Another rainy spell occurs 25-27 August for mostly central and N California. More possible rains for SOCAL occur 29-31st from another tropical cyclone.
- However, once the anomalous rains have ended, we appear to return no drier than normal conditions, and recurrent heat. The number of hot days over 100 degrees in Fallbrook, Escondido, and interior San Diego County will tend above normal from end of Aug through most of September. Cool spells are 25-27 August, and 24-29 September when some tropical-type rains are possible. The longwave pattern favors troughing to recur in California at times, with relief from the heat. Cool periods are possible 26-27th, and September 10-11, as well as September 24-29.
- Precipitation Trend August 20-September 20. Precipitation Dates for N and Central California: Thunderstorms and tropical cyclone conditions are indicated for August 20-23 in SOCAL, then showers and TSTMS will focus into central and N California during the extratropical stage of the tropical cyclone, indeed an unusual event for central California.
- The sea surface temperature anomaly maps have continued to show a robust El Nino pattern along the Peru-Ecuador coast and along the equator westward to 160W. The warm SSTA pattern normally associated with El Nino is expected to continue expanding west to the Dateline through September.
- Precipitation In California for August 20-September 20. The Sierras have above normal rainfall from Tahoe south through the central Sierras in August and September mostly due to either tropical cyclone, or to fronts that entrain tropical moisture and produce rainfall, per latest CFSv2 guidance.
- However, for precipitation in the longer range, September..near normal rain amounts. Oct.. again turns hot and very dry through California and our state's mountain areas.
- In the fall, we transition from a mostly dry and unusually warm October to a wetter trend from mid to late November for NORCAL and N through Central California. This appears to be followed by a typically wet, El Nino pattern for December 2023, with heaviest rains focusing in N and Central California. The predicted pattern by NMME supports Atmospheric River events for December through Central California to about Ventura Co.
FORECASTS FOR CALIFORNIA
Forecast for Northern and Central California:
Norcal PRECIPITATION TREND: Aug 20-23, 25-27, Sep 11-12, Sep 24-27.
Norcal WARM/HOT SPELLS: Aug 20, Sep 1-8, Sep 14-20.
Norcal COOL SPELLS: Aug 21-23 24-28, Sep 11-12, Sep 24-28
Central Calif PRECIPITATION: Aug 21-25, 26-27, Sep 11-12, Sep 24-27
Central Calif WARM/HOT SPELLS: Aug 20-21, Sep 1-8, Sep 14-23
Central Calif COOL SPELLS: Aug 22-25, 27-28, Sep 11-12, Sep 24-27
Forecast for Southern California:
Socal RAINS: Aug 20-22, Aug 28-31, Sep 10-12, Sep 24-26.
Socal WARM TO HOT SPELLS: Aug 20, 24-27, 30-31, Sep 8-12 and Sep 18-24
Socal COOL SPELLS: Aug 22-23, 29-31, Sep 10-12, Sep 24-26
Forecast for Central Sierra Nevada:
Central Sierra Nevada Precipitation for: Aug 13-Sep 13, 2023 - Main dates of TSTMS are... Aug 20-26, 28-31, Sep 8-13, Sep 24-26
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The listings of dates normally included for hot and cold spells, and precipitation are approximate. They are based on our CFSDaily and CFSDailyAI products; and present expected trends in precipitation and temperature (FoxCFSDailyAI) to 5km. Our system gives some consideration to terrain and coastal influence. We consider the CFSv2 one of the better ways to represent basic weather down in the sub-monthly time scale beyond the 15th day GFS or monthly maps from CFSv2 or NMME.
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Southern California Deserts Outlook for August 20 - September 20, 2023:
Highlights: Above normal rainfall from tropical cyclone influences on the August 20-23, August 28-31.
Looking Ahead – Long Range Outlook - Sep 20-Oct 20, 2023
S California Coast through mountains: Warmer than normal. Near normal rainfall, consisting of mountain thunderstorms (TSTMS). There will be cool spells as troughs begin to develop along California with some showers into the Sierras.
Central and S California Deserts and Plateaus and San Bernardino Co central and east portions: Seasonably hot with near normal rainfall consisting of mountain and desert TSTMS Sep 20-22. High pressure and hot weather will tend to alternate with troughing, some showery conditions in the Sierras in late Sep. Hot dry conditions in first part of Oct.
Figures: Not issued with this summary. There may be an update later in the week with figures in the PDF version.
(Terms and Definitions Used In This Weather Outlook)
Alan Fox & Zane Stephens...Fox Weather, LLC
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