Tuesday, September 04, 2007

Henriette Leads to Rapidly Changing Conditions over Southern Arizona

A Very Difficult Forecast Situation

The last two days have been extremely suppressed and very dry in southern Arizona, with dewpoints yesterday falling into the upper 30s. This was the third dry-down of the summer that came in on easterly to southeasterly winds.

This morning a number of things have changed. There has been a northward surge of low-level moisture into southwest and south-central Arizona during the late night and early morning hours. This was likely triggered by strong winds to the northeast of Hurricane Henriette. At 7 am the moisture had increased dramatically at Yuma, Sells, and Sasabe. The VAD Doppler radar data at Yuma indicate that this surge is about 3,000 ft deep. It may provide enough destabilization for at least mountain storms over the southern third of the state this afternoon.

The track of Henriette will then be the key determining factor for the weather over southeastern Arizona. The models now are in fair agreement that Henriette should move into the southern GoC. Tomorrow there will be a strong isallobaric wind blowing down the GoC and that may slosh today's low-level moisture back toward the south. Then, the main issue becomes how far north the deep moisture and precipitation fields associated with the weakening storm will come.

It appears that the current trough in the northwest will move rapidly over the top of the middle-level anticyclone leaving some remnants of Henriette blocked and decaying over northern Sonora and the GoC. This would allow the moisture associated with the storm to move directly into at least southeastern Arizona and give the potential for a significant rain event.

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