MONTHLY WEATHER SUMMARY
By Bill Mork
California Department of Water Resources
April 2004

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April 2004 continued in the warm and dry pattern established in March, except the April monthly average temperature departures from normal were not nearly as large as March; largest departure was at the Redding Airport with an average temperature of 62.5 degrees, 4.8 degrees above normal. The only real break was a period of cool, showery weather when the mean long wave trough position was near the West Coast April 13 - 21.

From preliminary data April precipitation averaged about 1/3 of normal. Most locations from the I-80 corridor south to Los Angeles and Santa Barbara had less than 25 percent of normal precipitation. Some locations with no measurable precipitation in April include Culver City, Gilroy, Lompoc, Madera, and San Luis Obispo. An upper level low the first three days of April and a cold upper level trough of low pressure April 16 - 17 provided some moderate to locally heavy precipitation which was enough for above normal monthly totals at some Southeast Desert locations which are normally very dry. The best example of this was Needles with 1.35 inches in April, 614 percent of the average of 0.22 inches.

Heaviest Southland totals the first three days of April include 1.10 inches at Alpine, 1.07 inches at Poway, and 0.99 inches at Yorba Linda. Heaviest totals same area on April 16 - 17 include 1.05 inches at Idyllwild and 0.78 inches at Palomar Mountain. Other large storm totals for April 13 - 21 include 5.88 inches at Stouts Meadow (Shasta drainage), 4.96 inches at Four Trees (Feather drainage), and 4.96 inches at Brandy Creek. The wettest location in April was Gasquet on the Smith River in extreme northwest California with 9.28 inches (143 percent of average), all falling in the wet period of April 13 - 21.

The largest daily precipitation totals in April were 3.60 inches at Gasquet on April 19 and 3.80 inches at Brandy Creek (west of Redding) on Apr 20. The Northern Sierra 8-Station Precipitation Index picked up 1.67 inches in April, 43 percent of average.

The average monthly temperature for all stations in the State was about 2 degrees above normal. The first half of the month was close to normal while the second half of April was much above normal. Only 6 stations in the State had below normal temperatures in April with Oakland the coolest at 1.4 degrees below normal. The record heat of April 24 - 28 had a lot to do with the month going above normal in most of the State. A strong upper level ridge of high pressure and offshore flow were the usual suspects as 157 record high temperatures were set or tied in California in that period. All-time record high temperatures for the month of April included 102 at King City (tied), 100 at Salinas, 99 at Paso Robles (tied), 98 at Sacramento (city), 95 at Napa (tied), and 94 at Richmond. The hottest of the many record highs was 109 at Death Valley on April 28. The greatest number of record highs was 64 on April 27 as 16 cities in Southern California had high temperatures of 100 degrees or more; hottest that day was 103 at Pierce College (Canoga Park).

Hydrological Summary and Outlook:

Snowpack melting continued at much above normal April rates, and only about half the April 1 pack remained on May 1. Snowmelt runoff seems to be about one month early this year and can be expected to taper off relatively early. Runoff forecasts were lowered because of the dry April and still show a strong north to south gradient, with much below average expectations in the southern half of the State. Reservoir storage is near average, which will help meet most water needs this year, but supplies may be short in the southern part of the Central Valley and eastern Sierra. Runoff so far this water year has been about 90 percent of average compared to 100 percent at this time last year. Runoff during April was nearly 80 percent of average for the month.

Forecasts of April through July runoff are 65 percent of average overall, ranging from near normal in the Trinity and northern Shasta Lake tributaries to 55 percent in the southern Sierra. Water year forecasts are somewhat better at 80 percent of average statewide. Snowpack water content dropped at about double the normal rate in April and now stands at about 50 percent of average for May 1 overall or 40 percent of the average for April 1. The rapid rate of melting swelled monthly streamflow to near normal on a number of major snowfed rivers in spite of the lack of rain. Last year the pack was 105 percent of average at this time as a result of a wet and cool April 2003.

Estimated runoff of the 8 major rivers of the Sacramento and San Joaquin River regions was 2.7 million acre-feet during April. Reservoir storage gained about 0.4 million acre-feet during the month, to end at just over average for the date. This was less than the normal gain of about 1.4 million acre-feet expected in April. With the reduction in expected late season snowmelt, not many of the major foothill reservoirs are likely to fill. Last year at this time, reservoir storage stood at 105 percent of average.



Page last updated 5/20/04.