How to Break the Jet Stream

Two very powerful cyclones, one from the tropics and one from the mid to high latitudes, are going to conspire over the next several days to break the jet stream. 

The tropical cyclone is Typhoon Choi-Wan, currently southeast of Japan with maximum sustained winds of 110 knots. 

Source: Joint Typhoon Warning Center

The mid to high latitude cyclone hasn't formed yet, but is expected to develop over the Sea of Okhotsk on Wednesday.  The ECMWF HRES forecast below from Tropical Tidbits is valid 1800 UTC Wed Oct 7 and includes sea level pressure contours (black lines) and 500-mb height (color fill).  The big circle identifies the mid to high latitude system, the little circle Choi-Wan, which at this point is transitioning into an extratropical system, and the upper level ridge (solid black line) downstream of these two systems.  

Source: Tropical Tidbits

By 1800 UTC Saturday 10 October, those two cyclones have merged into a massive 976 mb low over the Bering Sea.  That system ultimately "breaks" the jet stream, perturbing it in a way that it breaks down into a very wavy "high amplitude" pattern downstream over the North Pacific and North America.  I've identified the wavy pattern with a black line (upper level ridge) and black dashed line (upper-level trough).  

Source: Tropical Tidbits

The details of that wavy pattern are why extended forecasts for Utah this weekend and early next week are challenging.  In particular, the depth and track of that upper-level trough are very sensitive to everything happening farther upstream where those two cyclones are merging. 

Expect some shifting forecasts until the lead time decreases and the models begin to converge on a solution for Utah.