May precipitation to date is generally far above normal west of the Sierra Crest and north of the Transverse Range, with most basins having already received more than 150% of monthly average. East Sierra and desert basins are generally lower but more variable, ranging from 50% below to 150% above. It should be noted that May average precipitation amounts are generally lower statewide than in the November to April period. This makes the percentages of normal slightly less dramatic than they might otherwise seem; they should also be expected to decrease through the rest of the month.
Most of the precipitation responsible for these totals fell over the weekend of May 4. Its effect will be discussed in subsequent sections.
Snowmelt had begun statewide before the May 4 storm. As a result SWE percentages have decreased as compared to the previous Water Resources update. Currently statewide average SWE is 99%[a] of normal-to-date, or 59% or April 1 normal.
While melt had begun by the later part of April, cooler temperatures the weekend of April 26 and May 4 helped slow it some. Additionally the May 4 storm increased AJ runoff volume in every California basin north of the Transverse Ranges. The images below compare forecasts on April 26 (top) with May 10 (bottom).
In the short term much higher than average temperatures are expected. The Climate Prediction Center 3-4 week outlook continues to suggest near normal temperatures for most of California. Due to the near-term warming snowmelt is expected to accelerate and drive peak melt earlier than previously expected as shown in the May 3 vs. May 10 forecast monthly probability comparison at Friant Dam, below.
Trends have generally continued as expected since the last update with the exception of added snow volume due to the May 4 storm. Year-to-date SWE remains near average. The warming trend in the next week is expected to drive peak melt earlier in May and reduce later-season runoff volumes.
[a]Wow, we continue with the abnormally normal year!
