Thursday, July 09, 2009

Fool Me Once, Fool Me Twice.......

Yesterday I said that I’d be surprised if any ALERT stations in the Pima County network caught rain. Well, I was certainly surprised and quite amazed that an apparently (to me – Pat Holbrook had warned on Tuesday that Wednesday might be a big day) hostile environment produced a round of severe storms and heavy rains.

I shot a photo of storms approaching the house (taken about 5:45 pm on the 8th) from the south – see above – and we had 0.39” here at house. Airport got double that; 60 of 93 ALERT gauges caught rain; The Mt. Hopkins RAWS site recorded 2.60” and the Alert gauge on far south Wilmot had 2.13”; a number of warnings and alerts were issued by NWS; and severe storms were reported near Green Valley (wind) and San Manuel (wind and ¾” hail). So, it was not exactly a quiet day.

While the precipitable water was trending down in the morning, it stabilized and held through the afternoon, while Phoenix actually increased some. So, the northwest winds coming off the low desert were not bringing in drier air, as I’d expected. The big storms that developed on and around the Rincons generated strong outflows that moved to the north (downhill). The resulting convergence zone over the metro area produced the storms that moved across much of the city.

Doing a quick postmortem, I see that I had not accounted for the rapid shift eastward of the 500 mb anticyclone - from near Yuma on Tuesday evening; to northeast of Tucson yesterday morning; to south central New Mexico yesterday evening; to northwest corner Texas Panhandle this morning. So, it was a rapidly changing large-scale environment, which favors storms as opposed to a stagnant anticyclone. Pat H. or Erik P. had mentioned this fact, but it eluded me yesterday. The shifting high acted to shunt terribly hot 500 mb air northward and the shifting flow overhead appeared to bring in some slightly cooler 500 mb air from the vast data voids of northern Mexico.

So, conditions here in southeast Arizona fought off the deep dry air that impacted much of the west and north parts of state yesterday. This morning the PW is definitely trending down, surface flow from the south to southeast appears to be bringing in drier air. The NAM keeps some convection in southeast Arizona today and then shuts it down tomorrow, as very dry air in the upper-troposphere swings around the bottom of the upper anticyclone. Me, I’ll just watch today and see if my Tuesday outlook verifies two or three days late.


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