Thursday, July 05, 2007

A very complicated weather situation

Discussion prepared 1530 UTC 5 July 2007

Last evening we were able to see, from Tucson, widespread anvil and even some convective towers in the distance from north to east to southeast. There were some impressive storms over, and near, far east and southeast Arizona and these produced outflows that have affected all of southeastern Arizona during the night - increasing dewpoints and precipitable water, while keeping low temps very high (Tucson's "low" this morning was 87F). Douglas reported a severe thunderstorm wind gust as several outflows passed there. These storms were associated with a weak 500 mb shortwave through that has come over the top of the western US ridge and moved south-southwestward, to the west of the amazingly persistent cyclone that has been anchored over the southern Plains for several weeks. This cyclone, which again this morning seems to have taken on warm-core tropical characteristics, is forecast by the models as (and appears to be) breaking apart with pieces going several directions. The part that moves into northern Mexico does not really swing across southern Arizona with middle-level winds forecast to remain northeasterly though 48 to 72 hours. See the following 500 mb analysis and forecasts from the NAM:

The morning upper-air soundings at Tucson and Phoenix have extremely deep residual boundary layers form yesterday. Tucson's old BL extends to 450 mb this morning! So, even if low-level moisture continues to increase, there will only be small CAPE available at low-elevations for very high-based convection this afternoon. This is a perfect setup for severe downbursts and strong outflows from more intense mountain storms that will try to move into the deserts. The Phoenix sounding is similar.

So, a key question for tomorrow appears to be: Will low-level moisture continue to increase slowly due to evaporation and recycling OR will deeper GoC moisture push in below 850 mb, giving the precipitable water a real kick from south of the border? Yuma appears to be experiencing at least a moderate southesterly push of moisture this morning, similar to the past several days. The KYUX Doppler winds - see the VAD display available at -

indicates that fairly strong SSE winds extend up to about 3000 feet at this time.

My feeling at this time is that the piece of the Texas cyclone that pushes off southwestward across the southern GoC will be strong enough to trigger a significant low-level surge of Gulf Moisture northward into Arizona after midnight tonight. Thus, I anticipate that the summer thunderstorm season is off and running, and that tomorrow will bring increased CAPE and more extensive storm activity across lower elevations of south-central and southeast Arizona.

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