9:30 AM | Long-term improvement in the California drought

Water vapor loop shows moisture flowing from the Pacific Ocean into southern California; image loop courtesy University of Wisconsin/CIMSS

Overview
The drought in California has lessened in severity this year and significant rainfall over the next couple of days should continue to improve overall conditions.  A major storm will bring another round of drought-denting rain to much of the western US over the next few days and this will add to recent rainfall that has helped to ease drought conditions across much of California.  In fact, flash flooding is a near term threat in California from this storm and there is a risk of mudslides in areas that experienced wildfires earlier this year.  Drenching rain from this upcoming storm will first arrive in southern Oregon and northern California late Friday and then it’ll push southward into southern sections of the state including LA and San Diego by early Saturday morning.  

Total precipitation for the next 5 days; map courtesy NOAA

Storm
Water vapor is currently pushing northward into California from the Pacific Ocean and this moisture will contribute to the expected copious amounts of rain and snow in the next couple of days. The rainfall over the next couple of days can climb to 3 or 4 inches and snow is likely to reach the 1-2 foot level in the Sierra Nevada and southern Cascades from Friday into Saturday.  As the storm pushes into the region on Friday, colder air will spread inland and southward raising the prospects for those significant mountain snows.  All of this precipitation will no doubt have an impact on what is a high travel time period with the Christmas holiday coming later this weekend. Early in the weekend, the biggest problems will be primarily across the far western states, but by Sunday, this storm will spread snow into the Rockies and northern Plains and create dangerous travel conditions in that part of the country.  

Palmer Drought Severity Index (PDSI) from one year ago; courtesy NOAA

Drought update
The Palmer drought index, sometimes called the Palmer drought severity index and often abbreviated PDSI, is a measurement of dryness based on recent precipitation and temperature. It was developed by meteorologist Wayne Palmer, who first published his method in the 1965 paper Meteorological Drought for the Office of Climatology of the U.S. Weather Bureau.  The PDSI uses readily available temperature and precipitation data to estimate relative dryness. It is a standardized index that spans -10 (dry) to +10 (wet) and it has been reasonably successful at quantifying long-term drought.

The most recent look at the Palmer Drought Severity Index (PDSI); courtesy NOAA

A year ago near the end of 2015, there were portions of California in “extreme drought” status according to the PDSI.  In fact, much of the state was classified as in “drought” condition ranging from moderate-to-severe-to-extreme and no part of the state was doing any better than “near normal”.  This year, however, as 2016 comes to a close; California is in much better shape as suggested by the current PDSI values.  In fact, Northern California is currently either classified as “very moist” or “extremely moist”.  Central California has experienced “unusual moist spell” conditions in recent days and only the southern half of the state remains in drought (severe) status.  

This upcoming storm system will only add to the long-term improvement that California has seen in recent times. 

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com