Thursday, August 12, 2010

Some Promising Signs

I see that the Tucson NWS forecast this morning calls for 10% POPs each of the next 14 forecast periods (i.e., 7 days). This implies that there is not enough information contained in any of the model forecasts to discriminate between essentially peristence and the evolution of the day-to-day weather. Since my outlooks have been called dismal of late, I'll try to read something positive between the model lines this morning.
First a look to the south - the southern GoC has been affected by dissipating MCSs the last two mornings, and this morning there is what I call a "north-racer" at the southern end of the Gulf (see top image). These occur several times each summer and race to the north, hugging the eastern coast of the GoC. Once in a while they persist all day long. The middle, satellite image shows that cloud debris has poked northward toward southeastern Arizona and now extends to north of 30 degrees. The soundings at El Paso and in northern Mexico this morning show northeast to east steering winds and have substantial CAPE - thus, the MCS activity should shift northward today and tonight. All of this favors an eventual push of the subtropical air into southeast Arizona. The 500 mb pattern remains pretty dismal overhead today and the 200 mb chart (bottom) certainly looks like fall, but there'll be a good, vertical, wind-shear vector in place when the moist air pushes in beneath the westerlies. Tucson's sounding exhibits a bit more CAPE today and there is nice layer of southerly flow around 700 mb. Thus, I'd expect an upturn in storm activity this afternoon and possibly again tomorrow.
The NAM forecasts this morning continue to indicate an inverted trough(s) at 500 and 700 mb moving across, or perhaps stringing out over would be more accurate, northern Mexico. These features, along with the increased MCS activity in Mexico, should act to push the subtropical, low-level air northward, leading to a real upturn in storms and their intensity by Saturday evening. Tomorrow's situation will depend upon the evening and nighttime storms and outflows that develop today, particularly over northern Mexico and the Borderlands. So there are some features of interest to keep an eye on as the weekend approaches!
Finally, a brief summary of yesterday. Activity was down some and almost no gauges in all of southeast Arizona caught any measurable rainfall. I found only two gauges of the 93 gauge ALERT network with rain (0.24" at Brawley Wash out to the west). Indeed the cells out to the west of town produced a number of lightning strikes, and I got the slightest bit, not even enough to be called a spit, of a sprinkle from one of the anvils here at house. Thunderstorms, but no rain, were reported at Douglas, Ft. Huachuca, and Nogales.

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