Page:CAB Accident Report, Northwest Airlines Flight 2.pdf/14

 APPENDIX

Report of weather conditions at Bismarck Airport on the evening of August 29, 1942, based on synoptic charts, five hours preceding and one hour succeeding the time of the accident, as taken from the weather maps of the U. S. Weather Bureau and prepared by the Board's Air Safety Specialist in Meteorology:

SYNOPTIC SITUATION

The weather map shows a combined mass of polar marine and polar continental air in Canada and covering the greater portion of North Dakota, western South Dakota, and the west and north portions of Wyoming, and tropical marine air to the southward of that area. These were separated at the surface by a warm front extended from just south of Duluth, Minnesota, thence northwest to a position between Fargo and Grand Forks, North Dakota, thence southwestward crossing southern boundary of South Dakota due south of Bismarck, and thence to southwest corner South Dakota to a wave crest in east-central Wyoming.

In North and South Dakota this front was moving northeastward about 10 m.p.h. but did not reach Bismarck at any time on August 29. In fact, there is no evidence of a frontal passage of any type at Bismarck at the time or near time of accident.

The tropical marine air which was convectively unstable was flowing northwestward and flowing over the polar air north of the line where the warm front was at the surface. Due to the lifting of this air mass, thunderstorms were forming in the tropical marine air.

In the case of the thunderstorm approaching Bismarck, convergence was occurring in the air flow to the north of the warm front which resulted in the deflecting of the surface wind at Bismarck temporarily to a north-northwesterly direction.

As the center of the thunderstorm passed to the north of Bismarck, the squall winds accompanying the thunderstorm combined with the northeasterly gradient flow caused a sudden increase in velocity with the wind shifting to northeasterly and becoming relatively strong and gusty. This occurred near the time of the accident.

Overcast had prevailed at Bismarck for several hours previous to the accident and until 7:05 p.m. when broken clouds were reported. Ceiling ranged from 3000 to 3600 feet except lowering to 2500 during heaviest part of storms. Visibility decreased from unlimited previous to 5:30 p.m., reaching 3/4 mile at 6:01 p.m., but was about two miles at the time of the accident. The surface winds which had been north-northwesterly from 4:30 p.m. to 6:01 p.m. shifted to north-northeastward at 6:10 p.m. increasing rapidly in velocity with a recorded but unfiled observation