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April 1969 351 UDC 651.513:551.515.7:551.677.37(794)"1969.1" THEWEATHER AND CIRCULATION OF JANUARY 1969 Continued Strong High-Latitude Blocking and Flood-Producing Rains in California A. JAMES WAGNER Extended Forecast Division Weather Bureau, ESSA ,Suitland, Md. 1. MEAN CIRCULATION The strongest blockingNorthernthe in Hemisphere was over Greenland where 700-mb heights averaged 170m Blocking, which had become strongin the Western abovenormal for themonth (fig. 2). Slightlyweaker Hemisphereduring December (Green, 1969), increased blocking was locatedbetween Spitzbergen and Novaya further during January 1969. The Aleutianridge con- Zemlya. The magnitude of thisextensive high-latitude tinued to build and developed asmall High center (see blocking shows in the zonal profile of sea-level pressure themonthly mean 700-mb map (fig. 1)) withheights as for January 1969 comparedwith the January normal muchas 150 mabove normal (fig. 2). Anomalous flow(fig. 3). Sea-level pressureaveraged more than 8 mb from an unusually cold Canadian Arcticsource led to abovenormal north of 60" N. in the westernportion of frequent intensecyclogenesis just off the Paciiic Northwest the hemisphere. coast, reflected in a sharptrough with heights 110 m Cyclonic activity was stronger than normal in low and below normal. Flow from a separate center of cyclonic middle latitudesfrom the eastern Atlantic through the activity north of Hawaii contributed to strong confluence Mediterranean, and over eastern Asia where surface Lows inthe Californiaarea. This helpedproduce record- cross the cold landmass infrequently during midwinter. breaking heavy rains and disastrous floods in some areas The principal 700-mb monthly mean Low center which of the State. had been near the Taymyr Peninsula during December FIGURE1.-Mean 700-mb contours (decameters) for January 1969. Unauthenticated | Downloaded 10/04/21 03:52 AM UTC 352 REVIEWWEATHER MONTHLY Vol. 97, No. 4 1035 1030 1025 - m -i YI IO20 3 v) Y), (I 0 1015 1010 IO05 90 75 70 65 60 55 50 45 40 35 30 25 20 LATITUDE ( DEGREES ) FIGURE3.-Mean sea level pressure profile for the western half of theNorthern Hemispherebetween 20' N.and the Pole.Solid line shows observed for January 1969, and dashed line indicates the January normal. FIGURE2.-Departure from normal of mean 700-mb height (deca- meters) for January 1969. waterrelative to normal was also locatedabout 10' (see fig. 1 in Green, 1969) shifted to the Sea of Okhotsk farther north in 1962 (compare fig. 2 of Namias, 1963, (fig. 1) asheights fell rapidly over thenorthwestern with fig. 5A). Pacific and rose over the Arctic Basin. The watertemperatures cooled considerably relative The unusual circulation pattern during January 1969 to normal during January (fig. 5B) in thearea of cyclogen- around most of the Northern Hemisphere is demonstrated esis north of Hawaii, but remained a few degrees above by the positions of the various branches of the mean jet normal over most of the area from east of Hawaii to the streams (fig. 4). The westerlies were split intotwo branches California coast as tropical air was advected northeast- over both oceans; the northern branch was stronger inthe ward in the strong southwesterly flow at low latitudes Pacific, andthe southern branch was stronger inthe (figs. 1 and 2). The abnormallywarm water may have Atlantic. Over Asia and North America the jet stream increased the moisture content of these air m'asses above was south of its normal position. The strongest anomalies what it would have been otherwise. of wind speed were in thePacific where the blocking High, Water temperatures were also as much as 6'F above near the normal position of the jet south of the Aleutians normal in the western Pacific east of Japan (figs. 5A, B). (figs. 1, 4), reduced the mean 700-mb wind speed to 12 This additionalenhancement of the continent-ocean m/sec below normal, and the jet east of Hawaii produced temperaturecontrast may have further stimulated the a mean wind speed 12 m/sec above normal. intense cyclogenesis that occurred just to thenorth of The intensification of the deep and unusual Low north that area.However, as Namias (1969) pointsout, the of Hawaii, which was of crucial importance for the dis- wintertime,variability of the atmosphere .is smaller near astrous California rains, may be related to the preexistent the Asian coast than over thecentral Pacific. Hishy- distribution of sea-surface temperature in that partof the pothesis is that the climatological temperature contrast Pacific. During December, ocean temperatures north of between continent and ocean is so large that anomalies of Hawaii were more than 4°F above normal (fig. 5A). The water temperature near the coast change the magnitude normal seasonal southwardmovement of the westerlies of the thermal forcing field relatively little in that area. was aided this year by the blocking ridge south of the The January 1969, 700-mb height was only 70 m below Aleutians. The usually warm water and enhanced surface normal over Sakhalin (fig. 2) compared to 150 m above thermal gradient to the north of Hawaii may have stimu- normal south of the Aleutians and 90 m below normal lated abnormally strong cyclogenesis by providing greater north of Hawaii. than normal sensible and latent heat to the lower atmos- 2. MONTHLYWEATHER phere and impressing a low-level baroclinic fieldon air masses in the area. The unusualcirculation pattern over the western A physically similar situation that occurred somewhat portion of the hemisphereduring January 1969 was farthernorth was discussed in more detail by Namias related to strong contrast of temperature in the United (1963). In that case the center of negative height anomaly States. Confluence between Arctic air from Canada and was locatednear 40' N. (compare fig. SAof Namias, Alaska and subtropical air from the eastern Pacific led to 1963, with figs. 1 and 2) instead of 30' N. The warmest a mild GreatBasin and frigid Pacific -Northwestand Unauthenticated | Downloaded 10/04/21 03:52 AM UTC April 1969 A. James Wagner 353 FIGURE5.-Departure from normal of average sea-surface tempera- ture (OF) in the Pacific Oceanfor (A) December1968 and (B) FIGURE4.-Mean 700-mb geostrophicwind speed (meters per January 1969.Below normalareas areshaded, and areas more second) for January 1969. Solid arrows show axes of maximum than 2OF abcive normal have hatched lines (from Renner, 1968 wind speedduring January 1969, anddashed arrowsindicate and 1969, and from Japan Meteorological Agency, 1968 and 1969). normal January positions. northern Rockies (fig. 6). Temperatures were more than 8°F above normal at Winslow, Ariz., and Ely, Nev., the second and third warmest January of record, respectively. In contrast, the monthly mean temperature (-2.8OF) in Great Falls, Mont., was nearly 25OF below normal, and Billings, Mont., had its third coldest January in 35 yr of airport recordsand the coldestDecember-January in 75 yr. Temperatures averaged more than 4OF below normal over the Northern Plains, the Mississippi River Valley, and parts of the Southeast. Mildness in northern New England was due to easterlyanomalous maritime flow from the Greenland blocking High (fig. 2). The rest of the Nation eastof the Mississippi River had temperatures near or slightly below normal (fig. 6). FIGURE6.-Departure from normal of average surface temperature Juneau, Alaska, which was under the almost continu- (OF) for January 1969 (from Environmental Data Service, 1969). ous influence of Arctic air, reported its coldest January of record,more than18°F below normal,and tied the recordfor dryness since normal Pacific moisture failed Storms frequently formed to the lee of the Rockies and to reach the coast. Other stations along the Alaskan coast followed paths towards the Great Lakes, leading to fairly also reported extreme cold and dryness during January. extensive areas of heavyprecipitation in the northern The most newsworthy weather items during January Mississippi Valley and parts of the Midwest. Much of this 1969 were the heavy rains andfloods in southernCalifornia. precipitation was inthe form of freezing rain.Several The abnormallystrong southwest flow associated with cities from central Nebraska to northern Indiana reported an unusual subtropical branch of the jet stream brought glazing conditions on 7 to 11 days. heavy precipitation to California and much of the Nation Many stations from the west coast to the Great Lakes west of the Divide (figs. 2, 4, 7). Extensiveareas had reported the wettest or snowiest January of record, and more than twice the normal January precipitation, and dozens more had near records. Some of the highlights from . parts of southern California received more thanfour these are shown in table 1. timestheir January normal (fig. 7).No surface Highs Many localities in the Far West, particularly in and entered the West from the Pacific along the normal near the Sierras,reported the heaviest seasonal snow primary anticyclone track through Oregon. accumulations through January 31 in history, well over Unauthenticated | Downloaded 10/04/21 03:52 AM UTC 3 54 REVIEWWEATHER MONTHLY Vol. 97, No. 4 RCENTAGE OF NORMALPRECIPITATION TABLE1.-Selected record or near-record total precipitation and Jonuwv l96P snowfall during January 1969 Station Quantity R0lUUkS (inches) Los Angeles, Calif.: Civic Center ________ 14.94 Wettest January of record Airport ______ - - ____ __ 9.00 do. Fresno, Calif____________ 8.56 Wettest January in 91-yr record Blue Canyon, Cal if-...- 32.41 Wettest January of record Bishop, Cali____________ 8.93 Wettest month of record -23.0. Snowiest January of record Long Beach,Calif _______ 11.24 Wettest Janukry of recod (since 1942) Sacramento, Calif_______ a 90 Wettest January in 50 yr and 0th wettest of record Eanta Maria, Calif _______ 7.09 Wettest January in 62 yr and 6th wetbst of remrd Medford, Oreg___________ 13.7 Third snowiest month ofrecord and 12 days with measurable snowset new record Salem, Oreg.. ...___ ~ ____ 21.9 Third snowiest Januaryof record 7.-Percentage of normal precipitation forJanuary 1969 FIGURE Walla Walla, Wash_____ __ 4.17 Third wettest January since 1873 (from Environmental Data Service, 1969).

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