BASIC PATTERN:
Map Discussion
Mostly dry and warm conditions prevail thru 20 Sep. IVTinit and GFS->WRF->Extended MtnRT troughing and a few light showers or TSTMS in the far N mountains (Siskiyou Co) NE Plateau, E Shasta areas 21 Sep. This reflects the continued support for troughing that we saw in yesterday's model forecast guidance. A few other showers may appear in the Sierras on 3-4 and 6-7 Oct and around 15-20 Oct. A mostly fair and dry period is suggested 13-14 Oct.
Large Scale Pattern -
- The IVTinit pattern (sea surface temperature anomaly forcing) farors troughing out at 140W. Some of these troughs will try to extend into northern California, especially around Sep 20, and 23-28, affecting mostly central and N California.
- Tropical cyclones will continue in Sep off SW Mexico, and may occasionally bring moisture near California later in Sep andearly Oct.
- 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 through Oct, then equatorial sea surface temperatures should turn colder than normal for Australia,
- Precipitation trends in longer range: . IVTinit support for troughing and associated rains appears to develop about 20 . It appears that there may be support for rains into NORCAL thence into the Sierras 23-28, and also showers into the Sierras and Tahoe Sep 30-Oct 1. More rains appear to develop during 3-7 Oct, followed by a mainly dry and warm period Oct 13-14, and some more dry and warm weather in November, especially in the first week or two.
- In the fall, we transition from a mostly dry and warm autumn (October to November 2023) to a wetter trend from about 1 December in NORCAL through Central California. This appears to be followed by a typically wet pattern developing for late Dec 2023. This week we included 6 months of the CFSv2 model results. CFSv2 suggests a later start of El Nino-related rains, occurring in mostly mid to late December, through January and February 2024. This actually is the preferred pattern For El Ninos, following a mid November maximum in precipitation. Warmest sea surface temperature in the southern summer (El Nino season) occurs in Jan-Feb, which is comparable to our July and August in the Northern Hemisphere. Bottom line: we still expect a fairly robust El Nino rainy season for Dec ñ Feb, but the timing is a bit uncertain this far in advance. It looks like a typical El Nino, with heaviest rains focusing in Central and S California, and more typical winter rain amounts for N California.
FORECASTS FOR CALIFORNIA
Forecast for Northern California:
NORCAL Precipitation: Sep 21, 24-26, 29, Oct 3-4, 6-7 and 17-18
NORCAL WARM/HOT SPELLS: Sep 27-28, Oct 1-2 and 11-15.
NORCAL COOL SPELLS: Sep 15-25, 28-30, 4-8 and 16-19
Forecast for Central California:
Central Calif Precipitation: Oct 7-8
Central Calif WARM/HOT SPELLS: Sep 26-29, Oct 1-5, 10-15
Central Calif COOL SPELLS: Sep 15-24, 30, Oct 6-9, 17-19
Forecast for S California:
SOCAL RAINS: Oct 8-9
SOCAL WARM TO HOT SPELLS: Sep 26-Oct 5 and 12-16
SOCAL COOL SPELLS: Sep 19-21, 23-25, Oct 7-9, 18-19.
Central Sierra Nevada Precipitation for: Sep 15 - Oct 15, 2023:
Main dates of TSTMS or rains... Sep 18, 20-21, 24-27, 30-Oct 1, 7-8
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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 15ñday GFS or monthly maps from CFSv2 or NMME.
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Southern California Deserts Outlook:
Highlights: Near normal precipitation is expected middle and late Sep.
Looking Ahead — Longer Range Outlook Oct 16 - Nov 15, 2023
N California and Sierras:
Periods of warmer than normal with well below normal precipitation are suggested through October and likely the 1st half of November. It turnsg wetter late in November as El Nino starts to set up off the coast.
S California:
Much above normal temperatures through October and the start of November with much below normal precipitation. Some cooling later in November with an increase in rain probability as El Nino develops.
(Terms and Definitions Used In This Weather Outlook)
Figures: Alan Fox
Text: Zane Stephens, Fox Weather, LLC,
Copyright © 2023 Fox Weather, LLC