Weather Discussion for Thursday July 3rd 2008

Prepared Thursday July 3rd 8:30 am

Low-level moisture has continued to push into the lower elevations of all of southern Arizona during the night. Moisture increases and southeasterly winds are most spectacular over the lower Colorado River basin as per Yuma surface observations, the KYUX Doppler velocities (See Figure 1 and Figure 2) and the GOES IPW (Figure 3). Winds at 1500 UTC this morning are gusting 20 to 30 kts at Yuma and the dewpoint has risen into the lower 70s. The VAD shown in Figure 2 indicates that the surge of GoC air may bextend up to 4,000 ft AGL.

The Tucson 1200 UTC sounding this morning shows a slight increase of low-level moisture and is a bit dry relative to GPS IPW. There is a very deep residual boundary layer from yesterday and some CAPE indicated over low elevations, with CAPE over the higher mountains likely to be quite substantial. This afternoon's CAPE at lower elevations will be determined by how the low level moisture is advected by low-level winds, any further increase in moisture, and also by any warming that might occur between 500 and 400 mb - so estimating desert CAPE is toughest aspect of this afternoon's forecast.

Steering winds are likely to be northeast to east at 20 to 25 kts around 0000 UTC, while upper-level winds will blow the anvils off to the southeast - an excellent shear configuration for severe thunderstorms in southeastern Arizona. With diurnal afternoon low-level winds from the west-northwest, the shear profile below 500 mb will be supportive of strong, mesoscale outflows.

The threat for severe macroburst winds today is high, and depending on how the CAPE evolves there may also be some heavy rains in lower elevations and possibly hail. Hail is quite likely in higher elevations.

All-in-all looks like a good setup for a significant thunderstorm day here in southeast Arizona and also in much of the Southwest. As is often the case here in southeast Arizona, the outlook for tomorrow will be strongly impacted by how convection evolves late today and tonight - the NAM indicates even more activity tomorrow but this could be impacted by strong outflows later today and tonight. An interesting 4th regardless.