I wanted to quickly bring attention to the forecast for severe weather over the next few days & next week. We are currently beginning to setup a hefty Rex block with a strong cut-off upper-level anticyclone sprawling over Canada and a trough positioned off the Front Range. This kind of pattern is sure to provide fun forecasting as well as the occasional bust. There's a cut-off low that's slowly trekking E / NE giving the potential of severe weather and that will continue in the next few days. My hunch is that all the severe weather target maps for the next few days will be transitioned westward to account for the unanticipated "stronger than anticipated" ridge...
For next week, I wanted to point out why the models have difficulty in high amplitude, stationary regimes (such as the Rex block). The following shows the mean 500mb height contours of today's 12Z GFS ensembles valid next Monday (or maybe Tuesday). The shading represents the spread (or standard deviation) from the mean state, which is just the average of all the ensembles. Note that the spreads among the ensembles are fairly low west of the Miss. River (< 40m, on par or slightly higher than your typical day to day noise).
Now let's fast forward just 24 hours later and look at the same plot of 500mb heights.
Wham! Clearly there's action off the west coast! The spread among the ensembles is now > 100m, just west of the west coast. But, if we look closely, we can see why ... this is the typical diffluence region of a Rex block, and the model must take the strong jet streak (and associated vorticity) either to the north "over top" of the ridge axis or to the south into the developing cut-off. Yesterday, the models were taking most of the dynamics northward. Today, they're split, which explains the bullseyes in spread. Some of the ensembles, like the operational GFS itself is taking quite a bit of action to the south, which in additional to the dryline's 4000 cape may make things fun next week.....
Dima
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