Tuesday, April 5, 2011

Incredible North Pac Cyclone & Downstream Development

As discussed in the previous post, a major extratropical cyclone will form rapidly over the North Pacific today and tomorrow.  Although this cyclone moves into the high latitudes and makes landfall in western Alaska, it has important implications for the weather here in Utah through something called downstream development.

The GFS analysis for 1200 UTC 5 Apr (this morning) shows the incipient surface cyclone over the central Pacific with a broad anticyclone blanketing much of the eastern Pacific (bottom panel below).  A broad upper-level ridge is also found downstream of the surface low center (top panel).

Explosive cyclogenesis occurs over the next 36 hours as the cyclone deepens, get this, from 996 to 944 mb in 36 hours!  From 0000 UTC 6 Apr through 0000 UTC 7 Apr, the cyclone deepens 44 mb in 24 hours!  Incredible stuff.

If you compare the images above, you will see that the upper-level ridge amplifies dramatically during the cyclogenesis.  This is in response to strong warm advection and diabatic heating (from condensation and precipitation) ahead of the cyclone.  Downstream of the ridge, the flow veers to northerly.  In turn, the strong advection of planetary vorticity helps to carve and dig the downstream trough along the west coast of the United States.  It is this trough that will be bringing us a return to winter-like weather beginning on Thursday (the front tonight is merely a sacrificial lamb).

This case provides a good example of the importance of looking at what is happening not only regionally, but also on a planetary scale when forecasting.  Explosive cyclogenesis nearly always creates broad impacts on the large-scale circulation even in areas that are well removed from area being directly impacted.

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