Wednesday, July 28, 2010

More of Same Today?

Not too much in way of changes since yesterday. The weak, west-east 500 mb (top image) trough in northwest Mexico has perhaps shifted a bit to the west and the same is true of the mid-to-upper tropospheric cyclone cutoff over north Texas.The 500 mb pattern remains very amorphous, with a huge subtropical high covering all of the southern 2/3rds of the country. The main center of the high has shifted westward to the northwest GoM. Height gradients remain very weak over the Southwest and Mexico.
The TWC morning sounding now indicates light southeasterly winds through much of the troposphere, but there is virtually no shear - see the hodograph above on the SPC middle graphic. Upper-levels have moistened following the active convection of yesterday. The CAPE remains moderate and the lifted parcel continues to have theta-w of about 25C. So, sunshine and the mountains will drive today's events - there is a large, mostly clear region form south of Tucson well into Mexico.
This morning's NAM forecast run indicates less precipitation today (bottom graphic is 12-hour rainfall ending at midnight tonight). This may be a bit conservative, if we get much sunshine. The NAM continues to forecast the Texas cyclone to open into an inverted trough, and to move steadily westward into the weekend, crossing most of the Southwest and northern Mexico. A pronounced region of upper-level difluence will likely be leading this feature, and the forecasts indicate that it will begin affecting southeast Arizona by this evening. Thus, an interesting weather week will continue, with lots of forecasting challenges.

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