BASIC PATTERN:
Large Scale Pattern –
- We note that the sea surface temperature anomalies and IVTinit, there is support for a tendency for occasional troughing near the coast of California…Central and North. This includes San Luis Obispo Co from Pt Arguello north through the Bay Area to most of NORCAL coast and mountains, including Mendocino-Humboldt Co’s, Siskiyou’s, Trinity Co, NE California Plateau and Lassen Co, and the Sierras from Butte Co to Yosemite and the coastal and Sierra Redwood Parks (Sequoia etc).
- There continues support for troughs and upper lows to develop near Pt Arguello-Morro Bay. These maintain cool onshore flow with coastaldrizzle alternating with a shower on a few dates. Lows and troughs will tend to keep daytime temperature on the mild side.
- The sea surface temperature anomaly pattern (SSTA)suggests a continuation of troughing near the California coast, and the cool or briefly cold conditions continuing through May and even into the first week or two of June. There continue some residual cold fronts arriving from the N Pacific/off the Washington coast, with showers and still a bit of cool weather in NORCAL mountains and Sierras. Occasional cold periods could reappear a few times into N and Northcentral California areas to slow or lengthen a delayed spring bloom period.
- Precipitation Trend Precipitation Dates for N and Central California: Rain and showers May 14-16,20-26,29-31. Prospect for onset of summer in June…Both May and first part of June are shown by CFSv2 as recurring showery, late-spring-type periods. Cool weather normally accompanies the rains, while the warm-ups occur when skies clear. The near term issue for May in NORCAL and Sierras has been the late spring snow occurrence and snow pack.
- Precipitation trends in S and W Mexico during June and July-August…drier than normal during the early to mid summer. Sept turns wetter than normal in southern Mexico. Eventually, we should start to see warm sea surface temperature anomaly reappear SW Mexico coast and southern Mexico with above normal rainfall, as we transition into fall to early winter 2023 and strengthening of El Nino.
FORECASTS FOR CALIFORNIA
Forecast for Northern and Central California:
Norcal PRECIPITATION Trend: May 14-16, 20-26, 29-31.
Norcal MILD SPELLS: May 10-13, 26-27, 29-31.
Norcal COLD/COOL SPELLS: May 15-16, 21-25, 29-31.
Norcal FROSTS/FREEZES: May 17-18 upper 30s.
Central Calif PRECIPITATION: May 15-16 and 29.
Central Calif MILD SPELLS: May 10-14, 25-27.
Central Calif COLD/COOL SPELLS: May 15-16,24-25,28-29.
Central Calif DRY COLD WINDS: May 16-17 and 30.
Forecast for Southern California:
SOCAL RAINS: Dry, no rain.
SOCAL MILD TO WARM SPELLS: May 16-23, 26-28, 31.
SOCAL COOL OR COLD SPELLS: May 24-25 and 30.
In May the thermal forcing for strong and persistent coastal onshore flow is normally near its seasonal maximum, with the sea surface still cold, while inland temperatures by end of May try to experience some of the seasonal warmup.
Forecast for Central Sierra Nevada:
Central Sierra Nevada Precipitation - Dates of rain or snow: May 14-15, 17-27, 31, Jun 6-9.
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The listings of dates normally included for hot and cold spells, and precipitation are very 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 as one of the better ways to represent basic weather down in the sub-monthly time scale beyond the 15 –day GFS or monthly maps from CFSv2 or NMME.
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Southern California Deserts Outlook for May 12 - June 12, 2023
Highlights: Generally near normal rainfall in most areas. Cooler than normal in May-early June transition to warmer than normal in June.
Looking Ahead – Long Range Outlook - June 12 - July 12, 2023
NORCAL and Central California: In general, during this period, CFSv2 continues to show cool and showery conditions in the N and Central Sierras, Siskiyou’s/Cascades, and cold, showery conditions in the NE Plateau. After some cool and showery periods into early to mid June (per NMME), we finally emerge to a very brief stretch hot days for interior N and Central California. There continues risk of abnormal cool periods with showers and a thunderstorm central Sierras early to mid June, then turning warmer in last week of June to first part of July with seasonable showers.
Long Range Outlook SOCAL: For most of June…near normal precipitation for higher mountain areas, except a little above normal in some. Wetter than normal in lower mountain and foothill spots due to active moist marine layer.
Precipitation in SOCAL mountains and deserts: For May and most of June…near normal precipitation for higher mountain areas, except a little above normal in some. Wetter than normal in lower mountain and foothill spots due to active moist marine layer.
For SOCAL coast and valley areas: Watch for below normal temperatures early through mid June…cool days with drizzly coastal eddy conditions on the windward slopes, but about normal nighttime temperatures in SOCAL from coastal marine clouds in early through mid June. Turning warmer by end of June, with transition to warmer conditions. Would not expect summer monsoonal conditions until late July and August when the mountain/desert TSTMS of midsummer finally get going. We see indication of hot conditions for July, finally!
For the Sierras: After the cool May with showery conditions in the Central Sierras, we have a cool and showery first part of June too, with foothill-mountain showers. It appears that surface moisture from residual snow and moist soils and fuels may persist into mid June. From there, the expected cold sea surface temperature anomaly pattern appears to focus over Baja and SOCAL, favoring troughs with SE flow aloft into central and N California in most of June, per NMME (Ensemble) forecast map guidance. This teleconnects with hot conditions developing in Central and N California, as low pressure in and west of SOCAL produces downslope east winds in the Sierras and in the coastal mountains of N California. At some point, upper lows and southeast flow should finally launch us into an active mid-summer TSTM season in late July-August.
To follow the wet winter and spring, we should consider the possibility of a robust late May through June growth of fuels, i.e. dry brush understory in the forests. As the subsequent seasonal dry-off of these understory fuels occurs in late June-July, we could launch quickly into a robust fire season about late July or August.
(Terms and Definitions Used In This Weather Outlook)
Alan Fox & Zane Stephens...Fox Weather, LLC
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