30-Day Weather Outlook for August 15 to September 15, 2024

Aug 16, 2024

BASIC PATTERN:

Large Scale Pattern

The focus of lingering upper troughs continues west of the Pacific Northwest, and troughing at 135-160W, and high pressure to the west but near to southern California (SOCAL). High pressure aloft develops at times across central California in this pattern. The focus of lingering upper troughs continues west of the Pacific Northwest, and troughing at 138-1550W, and high pressure aloft over southern and central California (SOCAL). High pressure aloft develops at times across central California in this pattern. However, midsummer troughing will tend to continue to keep temperatures more mild than usual in N California.
Troughing occasionally develops to the west of Central to S California. This induces flow of moisture into S California and the Sierras occasionally from Baja-W Mexico.
Predominantly dry SW flow aloft continues for N California. Oregon and Washington west and Washington northeast portions receive showers on the rain dates in late August and September.
Our IVTinit analysis shows the calculated estimate of one day (Day 0) Total Integrated Vapor Transport (TIVT24), based on the sea surface temperature anomaly pattern from the coast out the Dateline-180W. This is calculated by Fox Weather LLC ís CyclogenIVT AI application each day. It shows for us the potential areas for development of troughs and rains upstream from California and the West Coast. Physical units of TIVT are the same as for 24 hour TIVT from the Center for Western Weather and Water Extremes (CW3E).
For forestry and the remainder of Fire Season 2024 in NORCAL and Sierras with the bottom line: recurring hot periods and below average precipitation, but with increase in TSTM (thunderstorm) events in early to mid September.
Tropical cyclones are a regular feature across S Mexico and SW of Baja. Tropical cyclones in summer and early fall teleconnect with well above average temperatures for California, in general. Tropical cyclone remnants will head WNW and NW from the tip of Southern Baja California and mainly out to sea, steered by high pressure aloft over the eastern North Pacific near or west of California and in deserts region of the Southwest US.

Forecast for N California
NORCAL Precipitation: Aug 17, 23-24, Sep 2-5 6-9. Sep 15-16
NORCAL Hot-Dry Periods: Aug 19-26, Sep 5-6 and 11-14
NORCAL Mild and Breezy Periods: Aug 15-18, 27-Sep 3, 8-10 and 16
North Sierra Precipitation Aug Aug 17, 22-24, Sep 2-5, 7-8. and 15-16
Central Sierra Precipitation Aug 17, 22-24, Sep 2-5, 8-9 and 15-17
Coastal drizzle various times/dates for coast.

Forecast for Central California
Central Calif Precipitation: Coastal drizzle on various dates for coast.
Central Calif Hot Periods: Aug 19-26, Sep 4-7 and 10-15
Central Calif Mild Periods: Aug 16-18, 28-Sep 3, 8-9 and 16-17..

Forecast for S California
SOCAL RAINS: Aug 23-24, Sep 10-15
SOCAL HOT PERIODS: Aug 20, 23-24 and Sep 10-15
SOCAL Mild Periods: Aug Aug 17-18, 29-Sep 2 and 9-10

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The listings of dates normally included for hot and cold spells, and precipitation are approximate. They are based on GFS and our CFSDaily and CFSDailyAI products; and present expected trends in precipitation and temperature (FoxCFSDailyAI) to 5km. Our AI system gives some consideration to terrain and coastal influence. Although we consider the CFSv2 one of the better ways to represent basic weather in the sub-monthly time scale, performance of the CFSDaily. CFSv2 daily products during storm seasons has been especially inconsistent and difficult to apply to the useful criterion needed by cropland agriculture, and viticulture beyond the 15ñday GFS.

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Southern California Deserts Outlook
SOCAL deserts: above normal temperatures, and near normal rainfall for the period.

N and Central California and Sierras
Above average temperature overall, with recurrent very hot spells during most of August in Owens Valley to the SOCAL deserts area. Rainfall remains below average in remainder of August for NORCAL and Sierras.. Mild conditions occur in coastal valleys of Central and N California, for the most part, due to persistent troughing to deepen the marine layer and onshore flow..

September 2024 appears hot, but with a trend towards normal rainfall. Again, this suggests thunderstorms (TSTMS) in the N and Central Sierras. For Southern California away from the coastal influence area, we look for above average temperature overall. Rainfall will remain near average with the usual mountain/desert TSTMS. In Sept we currently appear to have above average temperature but tending towards near average precipitation in the Sierras, and in mountains of Central to Southern California, but below average rainfall in NORCAL..

Monsoonal Conditions
Tropical cyclones will continue to focus in the ocean area S to WSW of Mexico and SW of Baja. Despite the dry anomaly, monsoonal moisture and thunderstorm conditions may occur a few times during tropical cyclone season due to easterly waves in SOCAL. For the outlook portion: September - October the monsoon appears to have mostly near normal rain amounts in Sierras and SOCAL mountains/deserts for August-September, but turns to lower than average rain amounts in Sierras and SOCAL mountains/deserts for October.

Looking Farther Ahead
California temperatures appear to return to below normal in N California during October. Precipitation is below average for that month. It turns cooler after mid Oct, with rains starting to develop North Coast from north Mendocino Co northward through about end of Oct. Nov appears to be warmer than average, but with above average precipitation more likely to focus on Oregon coast northward.

(Terms and Definitions Used In This Weather Outlook)