Monday, November 28, 2011

Going into Outlier Mode

The atmosphere is about to go into outlier mode over the western United States, and the foundation was laid upstream over the north Pacific last night.  As shown in the satellite image below, at 11 PM MST, a weak cyclone (note the comma cloud) was just south of the western Aleutian Islands.  

0600 UTC 28 Nov 2011 GFS Analysis and IR satellite image
The surface analysis from the Ocean Prediction Center showed a central pressure of 1000 mb, with two ominous labels near the low center: Rapidly Intensifying and Developing Hurricane Force.  

Just six hours later, at 1200 UTC, the cyclone central pressure was 989 mb, an 11 mb drop in 6 hours.  

Thus, this cyclone is well on it's way to meet the criteria for explosive cyclogenesis, which requires an average deepening rate of 1 mb per hour for 24 hours.  Meteorologists sometimes refer to such rapidly deepening cyclones as "bombs" (Sanders and Gyakum 1980). 

A cyclone like this has major implications for the large-scale circulation.  In particular, it contributes to something known as downstream development, which is characterized by the subsequent development of high amplitude weather features downstream (east) of the cyclone.  For example, the GFS forecasts the development of a strong ridge of high pressure over the eastern Pacific and western Canada, followed by a surface cyclone over the southwest United States, over the next few days.    

The ramifications for the weather of Utah and the southwest United States are substantial.  As shown below, the GFS puts a deep upper-level low over the southwest at 1100 MST 1 Dec 2011 (Thursday morning).  Strong easterly flow at crest level (i.e., 700-mb) extends across northern Utah, and strong northeasterly flow extends across most of Nevada.

These are strong winds from an unusual flow direction, and could lead to downslope (a.k.a. canyon winds) along the Wasatch Front and, if they extend further southwest, Santa Ana winds in California. Climatological leeward areas may see rare cool-season precipitation, including the deserts of southern Nevada, Panamint Range of California, etc.

A wise man once told me to beware when the atmosphere is in outlier mode, as will be the case this week.  It's still far enough out that we can't read too much into the latest model runs, but the next few days bear careful watching.  Some unusual things are likely to happen over the Southwest.  

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