Rex block is similar to what MrSpock explained above; the classical picture of it involves a low stuck in the middle of a ridge....and can very well happen when the ridge is too sharp and allows a low to bend back southwestward into it, giving the overall pattern this appearance of the "high over low" situation.
As with any block, it means the overall pattern isn't going to change much in the short-term until something comes along and busts up the block, whether by weakening the ridge or weakening the low.
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