Monday, July 16, 2018

Upper-Level Flow Forecast over the Western U.S.

This morning's 500-mb upper air pattern features an upper-level ridge, or anticyclone, centered off the southern California coast.  The anticyclone is zonally elongated, which means stretched in an east-west direction.  The flow at this level roughly parallels the 500-mb height contours, with lower heights on the left, which yields easterly flow to the south of the anticylcone over Mexico and the subtropical eastern Pacific Ocean, and westerly flow to the north over Canada and the Pacific Northwest. 

Such a pattern is not unusual in July, although in this case, the anticyclone is centered west of its climatological mean position over southern New Mexico and the easterlies to the south are a bit stronger than average. 

There are two important smaller-scale features at this level.  The first is a trough in the westerlies over the Pacific Northwest, indicated by a brown dashed line.  The second is a trough in the easterlies over northern Mexico, also indicated by a brown dashed line.  Meteorologists typically call such features short-wave troughs, although troughs in the easterlies are often called easterly waves

The ability to predict the movement and evolution of both the large-scale cyclones and anticyclones, as well as short-wave features, is critical for weather prediction.  During the monsoon, these features play an important role in moisture transport, convective initiation, and precipitation coverage and intensity. 

The loop below shows the GFS forecast for the next 5 days, ending at 1200 UTC (0600 MDT) Saturday 21 July.  Note how the short-wave features "pinwheel" around the anticyclone.  This is a defining characteristic of upper-level waves.  Short-wave features move faster than long-wave features.  The short-wave trough in the northerlies moves over the upper-midwest and amplifies, forming a stronger upper-level trough over the Great Lakes region.  Similarly, the easterly wave moves over the eastern Pacific and amplifies, closing off west of California.  Meanwhile, the anticyclone shifts slowly eastward, ending the loop centered over New Mexico and west Texas. 

The image below is for the end of the loop [i.e., 1200 UTC (0600 MDT) Saturday 21 July] to highlight those key features.  In addition, the position of the anticyclone and easterly wave opens up the potential for the transport of moisture later this week and weekend.  For example, such a pattern is one where the Sierra Nevada could see some thunderstorm activity. 

All of this is based on one model forecast produced by the Global Forecast System (GFS).  Typically the details of the forecast are sensitive to how all these features interact, and this is why meteorologists consult ensembles and multiple modeling systems to try and get a handle on the full range of possibilities. 

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