FLOOD06099 - Laserfiche WebLink

Home Browse Search

4 Fort Collins Coloradoan August 1976 The weather was right for floods EDITOR'S NOTE: The unique weather cond1Uons July 31, 1978, In the BIg Thompeon and Cache la Poodre eanyon& were monltored cloeeIy by the meteoro1ogiata of GeophysIcal R It D CorporaUon. This &rUcle, by John Hem, gtves an lnal.ght into the weather eventa leadlng up to the tragic "_ episode. The materl&.l In ltallce Is from porUons of a dally radio weather fon:ast compUed by the meteorolog1Bts. .. "Unstable a1r In the high countzy will lead to Dash flooding and plenty of cloud-to-ground /Jghtning so be carelul" - Noon, 30 July 1976, John Hem.. The weather cond1Uons necessary to produce a heavy localized rainfall near a mountain range are reasonably well known. A plentiful supply of moisture In the at. mosphere, a strong means of forcing that moisture uph1ll and a relatively Intense, but stationary cloud system to process the moisture into rain are needed. These conditions are rarely met along the Front Range of the Rocky Mountains. On the night of July 31, all conditions were present. The meteorological beginnings of the floods can be traced back to ArIzona, Canada and the Gulf of Mexico. On the morning of Friday, July 30, copious amounts of moisture were being lifted by towering thunderstorms from near the ArIzona desert floor high into the atmosphere and transported north. e.astward into the Colorado high country. Ught southerly winds over Arizona and New Mexico drifted the moisture northward. Far to the north, a chill mass of polar air was pouring off the Canadian prairies onto the northern Great Plains. By noon the leading edge of this air had slipped Into Wyoming and Nebraska. Behind the front temperatures In 60s and low 70s brought relief to plains people who had roasted In the muggy heat that 8tlll covered the central and southern plains. Friday afternoon temperatures peaked in the 1008 across Kansas while tropical humlrtHy made the air hard to breathe. The curtain had begun to 11ft on the Colorado disaster as these three air masses converged toward their tragic confrontation. ". . _ hJ(h country travelers should W prepared for extensive mountain thunderstorm activity which ",,'ill bring a threat of JJghtning to climbers and flash floods to campers. . . .. - 5:2tJ p.m. 30 July, 1916, John Henz BY FRIDAY evening the cool polar alr covered most of Nebraska Md Wyoming. Thunderstorms were breaking out on the Nebraska plains along leading edge of the front. Mountain thunderstorms had form- ej;t on the Western Slope and In southern mountalns of Colorado. On the Oklahoma and Texas plains ~rous thunderstorms broke out as the tropical aJr surged northward. "heaD' downpours later in day could produce local flash flood1; . . .rainfall amounts of an inch locally wirh heavier amounts near large thunderstorms." 6:26 a.m. 91 July, 1976 Vincent Scheetz Saturday morning the shallow leading edge of cool Canadian air slid "back door" Into northeastern Colorado. Moist southeast winds helped blow upslope .c1ouds to form over Larimer County getting the weekend off to a cloudy start. Local humidity was perceptibly higher. Extensive middle and hIgh cloudiness covered the Colorado high country from Arizona thun- derstorms of the previous day. A weak upper level disturbance began to move northward out of northern Arlwna and New MexIco. This disturbance would later help trigger the flood.producing thunderstorm system. 8" I I I I n ~' 8 I /1 I I f I I I I I \ I \ \ I ~ \ \ ,-- , ---.... ... \ ... , ...., I ...-;..-::id. I I I, '/' Bm I I I ..._..... , P GL&'H' ,'1' ARl(O I I C'OI'IlFoRT , I \ I : ''is; '" , I I I I I I ,: /, ~ ~ - ~ I " ~ . Satellite photo. shows the storm A satellite picture was loken on Colo. rodo Day. the day following the devos. tOling flood of the Big Thompson Conyon. The photo shows the heavy cloud cover over the conyon O(ea [marked with on Xl. The storm over the western slates was the heaviest actjvity over the nolion on 'hot day. 'llle cool polar air held daytime highs In Larimer County in the 80s and steady southeast winds near the ground blew at 1~2:i m.p.h. much of the morning and atternoon. The leading edge of the cool air rested on the Palmer Lake Ridge near Colorado Springs to the SOIJth and along the foothills from Denver to lAramie, Wyo. Temperatures south and west of the front rose to 96 in Grand Junction and 98 In Lamar compared to 84ln Fort Collins. ]'lo18t air converged along the front In Kansas and Nebraska. Gusty easterly winds sped this unstable air westward toward the Larimer County foothills. Above mOWltatn top level, wind speeds were very light and from the southeast. Mountain winds help, to "steer" thunderstorm systems. Unfortunately the winds over Larimer County would hold evening thunderstorms on the mountalns. All the ingredients for a serious flash flood threat were present awaiting a "trigger mechanism" to unleash their fateful fury. "Weather radar indicates isolated cells around the local area are now towering to nearly 50,000 feet. Since they are moving very slowly, these large cells will tend to produce focally hea~y rains wherever they rmload their burden of water. Slow movement TTJBkes such large storms ootentJ811y dangerous due to hea vy rains being conf1ned o~.er such 8 5IJ1all area. The potential for flash flooding in do~1lStream areas even ilong small washes and dry creek beds can be considerable In cases like. today. . . For those of you ~. f f ~ 1 , , , I / I f I I 4' Estimated rainfall in Big Thompson piled from several federal surveys. I f 1 I''''r~ DRAKt :v R\\I~1? , Cl I l. J I . . , , SCAi.l OJ M'Ll~ , S , Drainage Basin com. olann1ng to be outside thJs even1ng, you'll want to be sure to stay out of washes, cre€ks and other low Jy1ng !U't!&tl 1I they"'am In the '1ciJrity 01 these large thunderstorms. . . after this broadcast Is over you should monltor your radio for any new developments that mlght occur." 5:25 p.m. 31 July, 1978 Vincent Scheetz UP UNTIL :s p.m. thunderstorm activity along the front had been confined to Kansas and southeastern Colorado. However, several large thunderstorms began explosive development south of Denver and Boulder about 5:30 p.m. as unstable air along the front finally was able to burst through a shallow frontal inversion. Uke water bursting through a hole in a dam, the unstable air mushroomed into huge thun. derheads. The weak Arizona;New Mexico upper air disturbance had helped to Inmate these storms. Plenty of moistuTi! was avaIlable to feed the storms. The final ingredient, strong forcing of the moist air into foothllls, was less than one hour away. To the southeast of Denver a large severe thunderstorm dropped hall and heavy rains from about 4 p.m. to 15 p.m. Strong, cool southeasterly winds from this thunderstorm's downdrafts spread a wall of dust westward across Adams, Arapahoe, Jefferson and Boulder counties. nus "gust front" produced local winds to 4{) mph on the plains and dropped temperatures 115.20 degrees In minutes. The ~t front reached " l3ouLD~R CO...,-y Larimer County between 5:30-6:30 p.m. 'I'h1s gust front was the f1naI 1 trigger needed. It hastened the . development of a local inversion. near the gTOWld. Such inversions frequently cause winds near the inversion to double or triple their speeds over those of winds near the ground. Numerous meteorologists observed southeasterly winds estimated lit M m.p.h. just above-the surface, pump the unstable plains aJr into the foothills !rom6p.m. Wllli 9p.m. The thunderstorms over Jefferson County at 15:30 p.m. moved north. westward into the foothIlls to feed on the strong inflow of moist un. stable air being forced Into the Larimer County foothills. By 7; 30 p.m. this thunderstorm system towered to over 60,000 feet or 12 miles in height. Around 7 p.m. a "tornadic windstorm" blew through the Grand Lake area overturning saUboats and blOwing down ~s. Small hall and brief heavy raIn- showers were reported in Estes Park. The thunderstorm system was reaching maturity as It "locked Into" the rocky terrain near Hague's Peak around 7:30 p.m. Radar rainfall estlmateslndicate the storm was producing 2-4 inches of rain an hOilr with a peak production of near l'j Inches an hour around 8 p.m. Torrential raIns feU Into the rocky drainage basin of the Big Thompson from8;30-10;30p.m. To observers on the plains the stonn appeared as a huge black - WE.A1HER. P. 6, 001.1 . . GREelEY ; , WE~D (('''NT,\, Estimated general rainfall pattern on night of July 31 flood of Big Thompson, Poudre and Rist canyons. . SA1\.\ROA'o( ) , ..~ ~'!'to ;0,..; .'. 1" _~..~..:<fo.J,. ~ Air moss convergence velop flood potential. along Canadian Front helped de- ... r r ,.. ~J' ')- (~"'. ~ ( / ~ ~ ~j/ " PLRINS -' THUNDERSTnRlTl r\ r i- RELATIVE 'HINO SPHO/ DIREGTIOr-j r <' Slrong westerlies above mountain top and weak easter- lies below cause thunderstorms 10 migrate over the plains. " r ~ r ,- ( R'LATlvt WINO SPEED! DIRecTIoN ( MOUNTAIN ( T~UNOERSTOR" r ( ( WEST El'l\T Weak westerlies above mountain top and strong easterlies below cause thunderstorms to "sock" onto the mountains. .