Map Discussion
As shown on the map, and the IVTinit map (Figure 2) there continues SSTA-based support for robust W flow across the N Pacific from 180 to 150W, with a weak downstream ridge of high pressure at 135W. We should see some atmospheric river activity into Oregon later in Oct. Except for 8-12 Oct, the pattern has shifted to a mostly dry one for California. The Pacific Northwest also reflects a less wet scenario because the IVTinit pattern focuses most of its energy near 140W in the Pacific as we had been seeing about a month ago. As MJO-related activity along the equator recurs at 130-155W, both IVTinit and MJO will add their respective contributions to induce an intermittently stronger trough into N California and Sierras at times in Nov. Valid time for this CW3E/Fox map is 10am 2 Oct.
BASIC PATTERN:
Large Scale Pattern -
- The IVTinit pattern (sea surface temperature anomaly forcing) favors troughing in the N and NE Pacific at 150-160W and some downstream troughing in N California area at 112-118W (Nevada, Sierras, and E California. These troughs will tend to extend into Northern and Central California, around Oct 15-22. Rains are indicated for NORCAL and Central California and Sierras, and a few light showers may reach into SOCAL eastern mountains.
- Tropical cyclones will continue in southern and SW Mexico in Oct, but are unlikely to affect 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 classic El Nino pattern continues to develop with colder than normal sea surface developing in Indonesia from Sumatra east to New Guinea and the Australia north coast, per the typical pattern. In the northern latitudes, the ingredients to drive the robust westerly storm track appear also to be developing.
- Precipitation trends in longer range: up to 31 Oct: Consensus of CFSv2, GFS, considering IVTinit: Oct 2, with a front bringing showers into Sierras and Tahoe Oct 1ó2. A warm or hot period occurs 3-4 Oct, cooler 5-6 Oct, Cooler again 8-12th with showers from two fronts that reach down through California, including SOCAL. Watch for showers of rain and snow in the Sierras and Plateau areas of NE California. It appears to be mostly fair, and dry thru California 14 to 31 Oct.
- It should be emphasized that the short term climate prediction models (CFSv2 and NMME) generally are not agreeing on above normal precipitation for Dec and Jan. See the posted maps included at the end of this report for Dec 2023 and Jan 2024. Closer scrutiny suggests that what CFSv2 forecasts for Dec 2023 (above normal rainfall for all of California), NMME shows for Jan 2024. For Nov and Dec, the NMME maintains a mild and dry pattern for most of California, while the CFSv2 appears to show better detail, i.e. wet in Dec followed by a dry break for most of Jan 2024.
- In Nov, we transition from mostly dry and warm periods in early Nov to a brief wet period in mid Nov (11-17). From the Thanksgiving holiday weekend on through mid Dec it appears to be intermittently wet typical for El Nino. As mentioned last week, we are selecting the period mid Dec through Jan 2024 and Feb 2024 to have mostly above normal rainfall throughout California, due to the expected El Nino pattern. Bottom line: we still expect a robust El Nino rain season for Dec - Feb, but the timing is a bit uncertain this far in advance. The pattern of rains appears typical for El Nino, with highest rain anomalies 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 29-Oct 1, 8-12. Mainly dry 14-31.
NORCAL WARM/HOT SPELLS: Warm Oct 3-4, 15-20, 24-25.
NORCAL COOL SPELLS: Sep 29-Oct 2, Oct 5-6, 8-12, 22, 27-29.
Forecast for Central California:
Central Calif Precipitation: Sep 30-Oct 2, 8-12, Dry after 13 Oct.
Central Calif WARM/HOT SPELLS: Warm Oct 3-4, 15-20, 24-25.
Central Calif COOL SPELLS: Sept 30-Oct 1, 5-6, 8-12,22, 27-29
Forecast for S California:
SOCAL RAINS: Sep 29-30,Oct 1, 9,11, Dry after 12 Oct.
SOCAL WARM TO HOT SPELLS: Warm Oct 3-4, 15-20, 24-25
SOCAL COOL SPELLS: Sept 30-Oct 1, 5-6, 8-12,22, 27-29
Central Sierra Nevada Precipitation for: 1-30 Sep 2023:
Main dates of snow, rains or showers... Sep 30-Oct 2, 8-12, Dry after 12 Oct.
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.
Southern California Deserts Outlook for Sep 22 - Oct 31
Highlights: Near normal precipitation is expected Oct.
For Nov 1-30, 2023
N and Central California and Sierras:
Periods of warmer than normal with near to below normal precipitation are still possible in Nov. It turns briefly wet about Nov 11-18 and again during the Thanksgiving holiday period. There will be a couple of halting attempts at an Atmospheric River, but for the most part, forecast guidance suggests a basically Normal rainfall pattern for NORCAL. That said, one AR can kick monthly rainfall for NORCAL above normal. A more consistent El Nino pattern sets up near and west of the coast in Dec.
S California:
Above normal temperatures recur for most of Nov. Cool periods coincide with the rains during Nov 14-21, and the Thanksgiving holiday period. Wet conditions develop in mainly late Nov, and in Dec 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