Monday, August 18, 2014

Large-Scale Setting A Complicated Mess

The large-scale setting is quite complex this morning, and I've enhanced the NAM 500 mb analysis a bit (above). There are three anticyclone circulation centers over southwestern North America and these are shown in blue. There is a cyclone centered along the coast at the California/Oregon borde,r with weak troughs (red dash/dot lines) separating the various features. I've shown the 24-hour forecast position of the Four Corners anticyclone and of the coastal cyclone - basically the cyclone is forecast southward down the coast, while the anticyclone shifts significantly eastward (black). The NAM also forecasts a distinct vorticity maximum to be over Utah by tomorrow morning, and I suspect that the model is amplifying the south end of the Pacific trough and swinging it around the cyclone center. As all this happens, 500 mb winds become southerly over our area - quite a change from current easterlies.

The morning skewT plot for TWC is shown below. There is substantial cooling below 800 mb and it will be hard for the BL to recover, especially given considerable, residual cloudiness (7 am MST visible image is second below). Although it is clear over Cochise County and the early WRF runs seem to clear the skies quickly this morning. I suspect that storms will again tend to hang to the mountains today in southeastern Arizona. Precipitable water is highest out along the Colorado River Basin, where activity has shifted this morning. The early WRF forecasts seem to develop a very shallow, but quite unstable, BL by mid-day and apparently this layer convects in the forecasts. The observed sounding, while continuing with a nice shear profile, has warmed above 500 mb, and this has reduced CAPE available to storms. The NAM forecasts a very big storm day for southeast Arizona tomorrow. I will await Mike's analysis of the new WRF forecasts.

Finally, the wild card later in the week will be TS Lowell, which is developing southwest of Baja. The NHC current forecast is shown at very bottom. The GFS ensembles have a large spread in their forecasts of where Lowell will be in four days or so.

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