30-Day Weather Outlook for September 14, 2024, to October 14, 2024

Sep 17, 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.
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 15-16th, Sierras on 16-17, mountain areas of far N California on 28-29.
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 LLCís 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 SierrasÖ with the bottom line: recurring hot periods and below average precipitation, but with a few TSTM (thunderstorm) and dry lightning events in mid September.
There appears to be a dry period between the end of TSTM events in SOCAL mountains-deserts and southern Sierras, and the start of rains in NW California in late Oct - mid Nov.
Tropical cyclone activity across southern and western Mexico reaches its seasonal maximum in September. A seasonal shift in the flow aloftdirects 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 NW recurving to NE from Southern Baja California.
California appears to have near average rainfall in second half of Sept. The next period of above normal rainfall for N and Central California appears to be in Dec.
Please see the monthly outlook maps from the CFSv2 model, posted at the end of pdf version of this report.

Forecast for Northern California
NORCAL Precipitation: Sep 15, 18-19, Oct 14. mountains and North Coast.
NORCAL HOT PERIODS Hot temperatures possible in foothills and 24-27 (100 possible in foothills again).
North Sierra Precipitation: Sep 15-16,18-19, 28-29,Oct 14.
Central Sierra Precipitation: Sep 15-17, 28-29.

Forecast for Central California
CENTRAL CALIFORNIA Precipitation: mostly shallow marine layer. Rain possible 15-17.
CENTRAL CALIFORNIA HOT PERIODS: remaining warmer than average. Hot spell, highs 100-105 on hotter days coastal foothills 24-27 Sep.
Central Calif Mild Periods: Sep 18-19 and 28-30.

Forecast for Southern California
SOCAL RAINS: Sep. 15-16.
SOCAL HOT PERIODS: Hottest 24-27 Escondido-Riverside fthlls 105. Temperatures remain above average.
SOCAL MILD PERIODS: SEP 15-17.

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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 showers/thunderstorms in N and Central California mountains/Sierras. Above average temperature
overall, with recurrent very hot spells during Sep in Owens Valley to the SOCAL deserts area. Rainfall remains below average in 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, with some days of above average temperatures. However, recurrent troughing deepens the marine layer and onshore flow.

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.

Looking Farther Ahead
California temperatures appear to return 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 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)