AN EVALUATION OF WEATHER FORECASTS FOR UNION _l.--Sumary. of. weathew immediately prior to U-Day: At noon om the dey preceeding the shot, it was felt that there was a good chance for the ine arE¥E a€°10,000 fees (which had broken off the ENWeiSW trough at 201. April near 13N 175E and had drifted te 1ON 16CZ) to expand and give southerly winds over the Northern MARSHALLS. A forecast was issued to this effect. Om the basis of this forecast an H minus 18 hour forecast was called for. The indraft poorly oentinued to move over ENIWETORK at 20,000 and 2§,000 feet but was Wocated below 20,000 feet. It was reasoned that since the indraft waa expanding, it would move little and keep the winds over the target area generally southwesterly at levels 20,000 feet and upe At ENIWETOK, however, westerly to northwesterly winds would prevail at 30,000 feet. Reconnaissance flights had been finding a great deal of weather be— tween 1ON and the equator and westerly winds about 2N. A weak center appear~ ed at 1500 feet at 3N 164B around noose 2. The Weather Forecast: 5/8 cumulus, base 1800 feet, tops S000 feet, with scattered isolated tops to 16,000 feet; 4/8 altostratus, base 19,000 feet, tops 21,000 feet; 4/8 cirrus, base 39,000 feet, tops 41,000 facet; scattered light rain showers. a. Observed weather: 4/8 cumlus, base 1800 feet; 2/8 altocumlus, base 18,000 faet; 4/8 thin cirrostratus, base 40,000 fcet. Very light rain showers had been reported three hours prior ta sho® tinge be Comments on weather: Wilson flights (reconnaissance aircraft near shot site) reported 1/8 to 6/8 cumulus, tops 3500 feet to 12,000 fost, generally being around 8,000 feet; 1/8 to 3/8 altocumulus, base 20,000 feet; | and 4/8 to 8/8 cirrestratus at 45,000 feet, following the detonation. Be= tween four to seven hours after the shot, light scattered rain showere were reported, 3. The Wind Forecast: HEIGHT (Thsds Ft) H-1@ 90 100/50 80 75 Lo/,s 110/30 85 110/50 70 110/15 50 270/15 65 60 55 45 40 35 Inel 1: 090/10 300/15 290/15 270/25 270/35 250/30 H-9 090/60 090/55 090/50 090/45 090/30 Hate OBSERVED ‘WINDS H-Hour 090/60 090/55 090/50 090/45 c90/30 110/25 270/15 280/25 110/25 270/15 280/25 220/09 260/35 260/10 250/40 260/35 260/40 24045 250/40 250/40 240/de 270/25 N=-38 270/25 260/28