Monday, June 18, 2012

Large, 500 mb Anticyclone Forecast Over Central and Southern US

The global models forecast a large anticyclone over the US by the weekend, after a scorchingly hot week in the Southwest. Above are the GFS operational member forecast valid at 156-hours (Sunday afternoon at 5 pm MST) and the GFS ensemble average at the same time - both for 500 mb. The models, including the ECMWF, are also indicating showers over much of New Mexico and Arizona over the weekend, particularly at higher elevations. The 500 mb anticyclone is very large and there is a secondary center southeast of Tucson (see top graphic). In situations like this the devil is indeed often in the details. The anticyclone has very weak flow near and south of the main center, with the light winds extending deep into the tropics. With such a pattern there is often no transient feature present that can trigger an impulsive push of low-level moisture north of the border.

The 156-hour operational GFS forecasts for 700 mb and 850 mb are shown below. Both forecasts indicate increased moisture, particularly into southern Arizona. The PW forecast on the 850 chart indicates values of an inch to an inch and a half over all of southern Arizona. The moisture distribution appears similar to what would be produced by a Gulf of California surge. So, the key question is whether such a surge will occur at the end of the week - definitely something to follow closely. There will be low pressure over the Great Basin and increased southerly winds, but it is not yet obvious that these would have a fetch from regions of high moisture content. A tropical storm moving north of 20 degrees would certainly be favorable - the GFS forecasts a hint at a disturbance approaching southern Baja by Friday. The NHC has identified a region southwest of Cabo Corrientes that has a high potential for storm development.

Finally, an update on Hurricane Carlotta - the storm briefly reached Category 2 strength, before landfall on the southwestern coast of Mexico. There were heavy rains and flooding and mudslides with some loss of life. The storm weakened quickly after landfall. The high potential area currently identified by NHC may involve remnants from Carlotta.

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