We contacted Howard Bluestein, School of Meteorology, University of Oklahoma, for the answer to this question. A jet streak is a localized maximum in wind speed. Let us assume that it is most pronounced aloft near the top of the troposphere, where wind speeds in midlatitudes tend to be strongest, and that the atmosphere is approximately in geostrophic balance. A jet streak is associated with a local maximum in the horizontal pressure-gradient force. The pressure field associated with a typical jet streak in the midlatitude westerlies is shown in Figure 1.
Lower pressure lies to the north.
The spacing of the isobars is closer together at the center of the jet streak, indicating a stronger pressure gradient there. The jet streak normally propagates eastward, as do most weather systems at midlatitudes during the winter season. If the jet streak has relatively high wind speeds, then air parcels move relative to it. An air parcel passing through the entrance region feels the effect of an increasing pressure-gradient force acting to its left, and it accelerates. Consequently, the air parcel is deflected to its left.