Flood-Producing Rains in Northern and Central California, December 16-26/1955
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336 MONTHLY WEATHER REVIEW DEQEMBEB1955 FLOOD-PRODUCING RAINS IN NORTHERN AND CENTRAL CALIFORNIA, DECEMBER 16-26/1955 ROBERT 0. COLE AND JOHN P. SCAmON National Weather Analysis Center, U. S. Weather Bureau, Washington. D. C. 1. INTRODUCTION of vertical velocities, temperature features, and the im- Northern and central California was the scene of un- portance of strong low-levelflow. Furthermore, rainfall usually heavy rains and destructive flooding during the amounts are compared with past records and related to period December 16-26,1955. Rainfall amounts ex- bothgradient and topography. Most of the synoptic ceeded many existing records and flood conditions were charts used inthis article were reproduced from the described as the worst in the history of northern Cali- operationalcharts of theNational Weather Analysis fornia. Preceded by periods of moderate to heavy rains Center. Also, it should be emphasized that much of the during the first half of the month, the record-breaking rain and flood data were based on preliminary reports and deluge fell on rain-soaked soil and drained into already subject to future revision. More complete data willbe rain-swollen streams. Rivers overflowed their banks and printed in Climatological Data, National Summary for went on to cause the costliest flood on record for the area. December 1955. News reports listed 66 deaths asa result of the floods, and 2. ANTECEDENT CONDITIONS preliminaryproperty damage estimates exceeded $150,000,000. Thousands of people were forced to evacu- For the first two weeks of December, the 500-mb. flow qte their homes, five small towns were reported “wiped pattern over northern California was predominantly west out,” and transportation was disrupted by high water, to northwest which permitted several short-wave systems slides, and bridge washouts. to pass onto the Pacific coast. Specifically, one sea level December climatological data immediately available Low passed onshore just, north of Crescent City on Decem- emphasized theintensity of thisstorm period with a ber 1, and six occluded fronts passed over northern and number of new rainfall records. Oakland and Blue central California during the 2-week period. Other Low Canyon reported December 1955 as the wettest month centers entered either the coast of Washington or British on record. Fresno, Mt. Shasta,and Medford reported Columbia. These minor storms produced moderate to thewettest December on record. Stationswith longer occasionally heavy precipitation along the Pacific coast histories found comparisons in thelast century:San areanorth of MontereyBay. Manystations received Francisco reported the wettest December since 1889 and more than half their normal December rainfall by the Sacramento since 1867. 14th (table 2). At least two stations, Fresno and Oak- The occurrence of moderate to heavy rains inthis part land, received more than their December normal by mid- of the country is generally regarded as a normal winter- month, andseveral other stationsreceived nearly as much. time feature and has been described many times in liter- These antecedent rains soaked the soil, filled the streams, ature. Several investigators, Vernon [I], Martinand and prepared the scene for destructive flooding. Hawkins [ZJ,and Hughes and Roe [3J, to name a few, have found that the rain mechanism for northern and 3. SYNOPTIC FEATURES DECEMBER 16-26 central California is largely due to topography, a strong Around the middle of December, the 500-mb. flow southwesterly flow, andan ample supply of moisture. pattern exhibited a change in regime. A flat persistent Since the rainfall in this case was unprecedented, it is ridge along the Pacific coast moved inland, and the flow surmised that theinteraction of these factors was excessive over northern and central California became southwest- .both in time and amount. Therefore, this article is pri- erly. At the same time, a blocking High cell inthe marily concerned with a discussion of some of the more Eodiak-Bering Sea area began increasing in intensity, pertinent synoptic and climatological aspects of the heavy and a pronounced trough was established in the eastern rains and ensuing floods. Included in the discussion are Pacific Ocean. (See article by Andrews [4] on the mean comments on the persistency of the synoptic situation, circulation pattern.) In general, thistrough extended distribution and intensity of Low centers passing over from the Alaska Panhandle to Hawaii. 500-mb. Low northern California, relationship of the jet stream, strengthcenters appeared in this trough, 600 to 700 miles west of Unauthenticated | Downloaded 09/26/21 04:48 PM UTC DECE~BEB1965 MONTHLY WEATHER REVIEW 337 theSeattle area,on December 15, 18, and 24. These Lows moved slowly eastward or northeastward and were absorbed within two or three days into the circulation of a large Low pressure system centered over northern Canada. Moving eastward with the Low of the 24th, the persistent trough passed over the Pacific coast on December 27. At the surface, the first Low of the series moved over the Oregon-Washington coastline on the 16th and filled. It was accompanied by a weak troughover northern California that caused some lightprecipitation onthe 15th and slightly larger amounts on the 16th. Rainfall amounts with thisfirst Low were insignificant when com- pared with amountsthat fell later. On December 17, the rainfall intensity became a little heavier. This rain occurred in advance of an occluding Low center located about 1,600 miles west of San Fran- cisco on December 16. Reaching the Pacific coast by the 18th, this Low and accompanying fronts (fig. la) was im- mediately followed by a wave of considerable vigor on December 19. The surface and 500-mb. featuresare shown in figure 2a. The strong pressure gradient associ- ated with this wave created high winds at the surface and upper levels. Forty to sixty knot winds were reported by stations along the northern coast of California and by ships 100 to 200 miles off shore. At Oakland, the 850- mb. winds reached a maximum speed of 65 knots on the 19th. The presence of these strong winds plus a modest influx of moist tropical air produced exceedingly heavy rains over northern California. Somewhat smaller amounts fell over central California as the accompanying frontalsystem pushed southeastward (fig. la)and frontalized near Fresno. Stations in an area from Crescent City to a point south of Point Arena and thence inland to Sacramento and Mt. Shasta reported more precipitation for the 4-day period December 17-20 than they did for the next 4 days, the period of heaviest rainfall for the remainder of California northwest of Bakersfield. A 24-hour total of 3.27 inches between the 18th and 19th at the Sacramento City Office set a new December record. The second surge of heavy rain began on December 21. Late on this day, anincrease of warm moist air and strong southwest winds over northern California were associated with a fairly intense occlusion crossing the Oregon-Wash- ington coast (fig. Ib). In addition to this frontal system, another occlusion was located about 900 miles west of California and a developing wave in the Johnson Island area. By December 22, the secondoccluded fronthad passed over northern California (fig. 2b); and the wave was racing toward the Pacific coast. Both the 500-mb. contours and the surface isobars indicated a long tropical fetch,a very important factor in the California rain mechanism. The 18,800-ft. 500 mb. contour extended FIGURE1.-The progression of frontal systems associated withthe from the latitude of the Hawaiian Islands eastward to three main rain periods: (a) December 16-19, (b) December 21- 25, and (c) December 24-27. Solid lines represent positions at Oakland and Sacramento' North Of the Hawaiian 0430 PST and dashed lines 1630 PST. Fronts with the first two Islands,the coduence Of warm and air created a very rain periods had a more southerly trajectoryand moved faster Strong jet stream from the south Pacific Ocean to the coast than those in the final period. Unauthenticated | Downloaded 09/26/21 04:48 PM UTC 338 MONTHLY WEATHER REVIEW DECEMBER1965 FIGURE2.-Sea level and 500-mb. synoptic patt,erns related to the two periods of heaviest rainfall: (a) 0630 GMrr surface chart for December 19 with 500-mb. contours for 0300 GMT in dashed lines, and (b) the 1830 GMT surface chart for December 22 with 1500 GMT 500-mb. contours. The December19 chartrepresents a typical rainfall map for northern California with a strong south- west flow along the Pacific coast. On the December 22 chart, note the pronounced tropical fetch and theunusually rapid move- ment of the Pacific wave as indicated by its 6-hourly positions (squares). FIGURE3.-Strength of blocking action in thc Pacific is indicated by the departures of500-mb. heights from normal for (a)1500 GMT, December 19, (b) 1500 GMT, December 22, and (c) 0300 GMT, December 26. Values are in 100's of feet. Unauthenticated | Downloaded 09/26/21 04:48 PM UTC DECEMBER1955 WEATHERMONTHLY REVIEW 339 of North America. The wave from Johnson Island Andrews [4] has written that a blocking tendency had accelerated as it moved into this strong flow, traveling existed in the North Pacific area since late October. It approximately 2,375 milesin 24 hours or nearly 100 m.p. h. was not, however, until December 15 that theblock began It was during this period that newspapers reported that to achieve major proportions. Strong 500-mb. departures 100 m. p. h. headwinds had almost halted all Pacific flights from normal heights were noted on December 19 (fig. 3a) between California and Hawaii. One airline reported that ranging from a positive 1,800 ft. in the Bering Sea to a the strong winds extended as low as 6,000 feet and that it negative 800 to 1,000 ft.