Christopher C. Burt is the author of 'Extreme Weather; A Guide and Record Book'. He studied meteorology at the Univ. of Wisconsin-Madison.
By: Christopher C. Burt , 20:10 GMT le 01 avril 2012
March Rains/Snow Make Dent in California’s Dry Winter + Interesting New Real-time Wind Map
As of March 12th California was on track towards one of its driest water seasons on record. San Jose, for instance, had only measured 3.13” since the beginning of the water season on July 1, 2011 (25% of normal to date). Most of the state was averaging 25-50% of normal rainfall and Sierra snow pack levels were also below normal at similar rates. Since March 13th a series of wet storms have saved the state from the prospect of drought and more storms are forecast over the coming week.
The biggest rainmaker was the storm of March 13-16 (actually two storms back-to-back) that deposited over 20” of rainfall at Scott Creek in the Santa Cruz Mountains south of San Francisco. Ben Lomond picked up 17”. What was remarkable about this event was the extraordinary rain shadow effect that came into play. During the 24-hour period from the morning (5 a.m.) of March 13 to the morning of March 14 (5 a.m.), 14.22” of rain fell at Scott Creek whereas at San Jose Airport, just 25 miles away as the crow flies, a total of just .07” accumulated (and at the home of WU Senior Meteorologist Shaun Tanner near San Jose, NO measureable rain fell)! Over the course of the four-day storm San Jose finally managed to capture .68” of precipitation whereas Scott Creek had a 20.32” total. This was the most extraordinary example of a rain shadow I have ever heard of in the U.S. outside of Hawaii.
This isohyetal map of the San Francisco Bay Area illustrates the amazing rain shadow that the Santa Cruz Mountains cast over the San Jose metro area. San Jose Airport averages 14.90” per season (July 1-June 30) whereas Scott Creek averages 60.50”. The two sites are 22.5 miles apart. I’ve placed an ‘X’ over each site on the map and what their respective 4-day precipitation amounts were March 13-16. Map base shows average seasonal rainfall by 2” increments. From U.S. Army Corps of Engineers, San Francisco.
Here is a brief list of how the past two weeks changed the seasonal rainfall accumulations for some key California cities:
The percentages are what season to-date normals were on 3/13 versus 3/31. The biggest improvement has been over central and northern California. Note that Oakland received more rain in the past two weeks than it had accumulated for the entire season (July 1-March 13) prior to the onset of the recent rains.
The season, however, is still on track to be the driest rain year since 1990-1991 for at least the central portion of the state (assuming that normal rainfall occurs this coming May-June).
The Great April 1880 California Storm
Of course, there is the hope that the late March rainy trend will continue into April. In fact, one the wettest storms in central California history occurred on April 20-21, 1880. This was an extraordinary storm: Sacramento experienced its greatest 24-hour rainfall on record when 7.24” fell on April 20-21 and its two-day total was an astonishing 8.37” both still-standing records. Sacramento’s 14.02” total for the month is the 2nd greatest monthly total for any month of the year since their records began in 1850. San Francisco was on the southern edge of the heaviest precipitation band but still picked up and impressive 3.20” on April 20-21 and 6.43” for the week of April 14-21. Its monthly total of 10.06” remains the wettest April on record since measurements began in 1850. In Napa Valley, north of San Francisco, an amazing 14.70” fell in 24 hours at Mt. Helena and 14.70” was also measured at Helen Mine further north. These figures still stand as the greatest April 24-hour rainfalls in California history. In the Sierra Nevada snow accumulations were off the charts. In fact a world-record single storm snowfall amount of 194” (over 16 feet!) fell at the railway depot Norden on April 20-23 at an elevation of about 7500’. The snow was so heavy it collapsed snow sheds over the railway near Summit (Donner Pass area). Summit depot itself recorded 298” (almost 25 feet) of snow during the month of April, Emigrant Gap totaled 201”, and Truckee 124”. For the winter season of 1879-1880 Summit accumulated an astonishing 783” (over 65 feet) of snow.
Interesting Real-time Animated Wind Map for CONUS
On a completely different topic, Jan Null of Golden Gate Weather Services forwarded me this great map of real-time surface wind flows and speeds for the lower 48 states. I checked its accuracy against actual surface data and it seems to be pretty close to the mark. If you hold your cursor over a point on the map the actual measured winds speeds will pop-up for that location. It will be fun to look at this the next time a big extra-tropical or strong tropical storm makes a pass over the U.S.
Christopher C. Burt
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