DISCUSSION: As 2018 draws to a close, so does the first quarter of the water year for Northern California. The water year begins on October 1 and ends on September 30 as set by the United States Geological Survey (USGS) in order to accurately take account of snow that would fall in autumn and winter and not melt until the following summer. So far, across much of Northern California, the amount of precipitation has been below average to date.
A very dry October across much of Northern California has been one of the biggest factors in the below-average rainfall total in the region. In fact, much of the rain that has fallen was early in the month due to a flow of monsoon moisture that reached the San Francisco Bay Area bringing only a few hundredths of precipitation to areas including San Jose and Oakland while not producing any rain in the areas that usually get more such as Napa. The very dry October came as a result of several persistent high-pressure ridges which kept much of the storms from the Gulf of Alaska away from Northern California and steered them towards Oregon and Washington.
November began the same way October ended with very dry conditions dominating for most of the month until the day before Thanksgiving, when the first storm from Alaska moved in. This was the first storm in a series which came through for a few days until the end of the month. The storms had brought enough rain to Northern California to help reduce nearly all the deficit from the dry October and even in some places finished above average for the season. However, the Crescent City area near the border of Oregon finished with another below average month which increased its deficits while the other more northern coastal areas like Ukiah and Eureka finished above average. In addition, the precipitation that came at the end of the month was instrumental in helping with the containment of the Camp Fire as well as clearing out the smoke from the Bay Area.
December started with the last of the storms from November before having storms come in at a period of once a week. However, these storms produced about the same as November, but it was below average as December is a wetter month for Northern California. Northern California typically gets most of its rain in December through February as the jet stream which helps with the movement of storms is positioned further south. However, this year, the storm track has not moved as far south due to high pressure ridges that lasted longer in between the storm systems. It is only the first quarter of the water year and the months with the typical heaviest rainfall are yet to come for Northern California.
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©2018 Meteorologist JP Kalb