Above average temperatures likely through period due to warm ocean temperatures.
High amount of bug kill trees and diseased forests will result in high large fire potential in alpine areas until first major precipitation event in the fall.
A near normal to slightly below normal number of offshore wind events expected.
Above normal large fire potential ending from north to south in Oct-Nov
WEATHER DISCUSSION
September 1, 2015 - Drier and warmer weather returned in August as high pressure strengthened over the Southwest. The number of thunderstorm days in August was far lower than July with only 12 days recorded across the district. Most of the storms occurred over Cleveland National Forest eastward into the desert with few storms occurring over the Sierras or other high elevations. Wetting rains were sparse. Fortunately, the majority of the storms were wet and produced some lightning. But lightning was responsible for starting the largest fire in the district so far the season; the Rough Fire in the Sierra National Forest. Since May, there have been unusually high amounts of rainfall across the district. There have been frequent intrusions of subtropical moisture highlighted by extratropical storm “Delores” which washed out I-10 and brought record rainfall to San Diego. Between these rainy periods, in June and August, the weather was seasonally warm and dry.
The enhanced late spring and summer rainfall begs the question of whether this unusually rainy pattern will continue into fall. In terms of sea surface temperatures across the Pacific (SSTs), all signs point to the affirmative.
FUELS AND DROUGHT OUTLOOK:
After a month of moisture recovery in the live fuels, August saw fuel moisture readings fall quickly. The rainfall of July did allow for some growth of fine fuels and brush. But by the middle of August, all new grow on brush and shrubs was arrested and the new grass crop quickly withered. Live fuel moisture readings fell back to the critically low readings seen in June prior to “Delores”.
Dead fuel moisture quickly fell as well and readings dropped back to record, or near record, low readings in the 10 and 100 hour fuels during the first heatwave in August. These dead fuels are an increasingly ubiquitous sight in the high country.The 4 years of drought along with the bark beetle infestation will kill millions of trees in the Sierra Foothills. An additional 3- 4% of the trees in this area may perish this year which will result in an additional dead fuel loading for years to come.
It will take a widespread and sustained wetting rain event to lower large fire potential across the area due to the unhealthy state of the forest and old growth brush areas. But these rains are expected to slowly spread from north to south in the fall. The rainfall along with cooler weather and shorter daylight hours should allow large fire potential to fall to near normal by December. This winter looks to be at least average if not well above average in terms of precipitation and wintertime use of local resources is unlikely.
Although the prospect of above normal precipitation is welcomed by all, even an especially rainy winter will not end the drought. Much of the state has seen 25- 35% of average precipitation since 2011 and it will take years (perhaps many years) to erase the precipitation deficits accrued these past 4 years. Furthermore, due to the expected subtropical origin of these storms, the snow level may be quite high this winter and reservoir recharge may not be as great as one would hope for in a rainy year. But the rains should help subsoil moisture readings and save many plants from the mortality that one more dry year would bring.
El Niño
SST’s across the equator remain far above normal at the current time. In fact, the Niño region 3.4, which has the greatest influence on California’s weather, has warmed above the 1.5˚C threshold to be classified as a strong El Niño. The Niño regions on either side are also warming strongly. Only the Niño region 1+2 which is closest to the South American coast has seen the warming level off, but this area may warm as well if easterly waves continue to “pile” warm water across this side of the International Date Line. Most computer ensemble members indicate a strong El Niño will remain in place through the winter with SST’s possibly peaking November – January.
Many comparisons have been made between the current El Niño and the two most recent strong El Niño’s which occurred in 1982-1983 and in 1997. At first glance, the strength of the current El Niño seems quite similar to the one seen in 1997. But this year above normal SST’s extend well north of the equator into areas well off the coast of Mexico as well as over most of the areas off the Pacific NW coast. These areas, particularly the area of warm water off the Pacific NW coast into the Gulf of Alaska, were present last winter. This area in the Gulf of Alaska may have been at least partially responsible for the blocking pattern which kept California warm and dry last January through March. Should this area remain in place this winter, there may be fewer “cold core” storms impacting the West Coast which were the type which brought heavy rain to California back in ’97.
Even if there aren’t as many cold storms generated in winter, the shear amount of additional moisture available this year across the Pacific should result in above normal precipitation. Even storms of modest intensity will likely produce some impressive rainfall. The heaviest precipitation this winter may occur over Southern California eastward into the Southwest. Rainfall amounts may be closer to normal in Central California.
The first significant troughs of the season are now reaching the Pacific Northwest at the time of this writing. These troughs should become deep enough to reach the central part of the state in October, bringing significant rainfall to the central coast and Sierras. Later in the fall (likely within a few days of Thanksgiving), significant rains should reach Southern California.
Therefore, there will be some periods of offshore winds prior to the arrival of wetting rains across Southern California. While no studies have conclusively linked the presence of an El Niño and resulting offshore winds, there have been fewer strong offshore wind events when a positive ENSO and PNA have been present at the same time. Expect offshore wind events to occur about twice per month through December which is at a near to slightly below normal frequency.
Source: Predictive Services
