30-Day Weather Outlook for September 5 to October 5, 2024

Sep 03, 2024

BASIC PATTERN:

Large Scale Pattern

The focus of lingering upper troughs continues west of the Pacific Northwest, and troughing at 135-145W off NORCAL and high pressure aloft over southern and central California (SOCAL). High pressure aloft develops at times across central California in this pattern. There is some support for cool nights (into the 40s)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. A few thunderstorms develop, mostly in the southern and eastern Deserts, and in the central and southern Sierra about Sep 2-3,8, and 29-30.
Predominantly dry SW flow aloft continues for N California. Oregon and Washington west and northeast portions receive showers on the above rain dates in September.
Our IVTinitô 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's 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 early to mid-Fall Fire Season 2024 in NORCAL and Sierras with the bottom line: recurring hot periods and below average precipitation, but with a few TSTM (thunderstorm) and dry lightning events in early to mid September. There appears to be a month-long dry period between the end of TSTM events in SOCAL mountains-deserts and southern Sierras, and the start of rains in NW California in mid Nov.
Tropical cyclone activity across southern and western Mexico reaches its seasonal maximum in September to October. A seasonal shift in the flow aloft directs tropical cyclones NW-N up the coast of Baja and into the Gulf of California, so we will need to keep an eye on that. Tropical cyclones in early fall teleconnect with well above average temperatures for California, in general. Tropical cyclone remnants will head mostly WNW from the tip of Southern Baja California.
Please note that California appears to have near average rainfall in Sept, but well below normal rainfall for the start of winter rains in Nov-Dec. By contrast, ORWA has a tendency for above or well-above average rainfall in late Nov - Dec 2024.

Forecast for N California
NORCAL Precipitation: Sep 10-12 and 15-16, 18-19.. mountains and North Coast.
NORCAL HOT PERIODS: Hot temperatures possible in fthills 24-27. Hot on Sep 3-6, 13-15 and 24-27 (105-108 possible fthills)
North Sierra Precipitation: Sep 10-12, 15-16
Central Sierra Precipitation: Sep 7, 10-12, 15-17, 29-Oct 3

Forecast for Central California
CENTRAL CALIFORNIA Precipitation: drizzle at coast.
CENTRAL CALIFORNIA HOT PERIODS: remaining warmer than average. Hot spell, highs 100-108 on hotter days cstl fthlls 24-28 Sep. Central Calif Mild Periods: Sep 7-8, 11, 18-19 and 30-Oct 2

Forecast for S California
SOCAL RAINS: Sep. 7-8 and 15-16.
SOCAL HOT PERIODS: Sep 3-6 and hottest 24-27ÖEscondido-Riversidefoothills 105-108 Temperatures remaining above average.
SOCAL MILD PERIODS: Sep 8-10, 15-18.

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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 average temperatures, and below average rainfall for the period. However, it appears that there are some significant rains that occur.

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 alternate with hot in coastal valleys of Central and N California, for the most part. Unstable airmasses break the marine inversion at times in N California, some days of above average temperatures. However. recurrent troughing to deepen the marine layer and onshore flow. September 2024 appears hot, but with a trend towards normal rainfall in the southern and eastern Deserts of SOCAL. 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, but with well below averagerainfall in NW Mexico (Sonora region), and Sierra Madre Occidental mountains. Despite the dry anomaly in Sonora MX, monsoonal moisture and thunderstorms may occur a few times during tropical cyclone season due to easterly waves.

Outlook portion:
September - October the monsoon appears to have mostly near normal rain amounts in Sierras and SOCAL mountains/deserts for Sep, but well below average rain amounts in Sonora MX-Sierra Madre Mountains for Sep and Nov. There is a possibility of hot offshore flow followed by rains from a tropical cyclone event in last week of Sep - first few days of Oct.

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. Mid Nov turns above average for precipitation in NORCAL and north-central Sierras. Dec turns to above average precipitation, in NORCAL from Bay Area north, and seasonably wet in the ORWA region (Oregon, Washington), both coastal and inland areas.

(Terms and Definitions Used In This Weather Outlook)