The Summer of Limited CAPE - 2009

Another day yesterday with no rain across the Pima County ALERT network. There were some brief, strong, high-based storms to the east of Tucson late yesterday afternoon, but again the strongest storms were along and south of the border.

The discussions of the last two days remain relevant. The wrinkles for today:

Dewpoints are up at some sites and down others, indicating the low-level moisture we have is sloshing around on local circulations.

Precipitable water values across southern Arizona are around 1.25” with the subtropical air still to the south, except for the Lower Colorado Basin where the northward push of high dewpoint air continues.

The Phoenix and Tucson soundings this morning show some CAPE but still indicate a layer of dry northerly flow around 600 to 800 mb. Cloud bases would be high and lots of kick needed to get things going at low elevations.

Heavy cloud covers much of southern Arizona but here in the southeast it appears that there’ll be some decent heating today – see 15Z visible image above.

The NAM forecasts are verifying decently and the potential exists for a couple of days with storms and rainfall. The potential would be much better if the lower levels were moist and more unstable.

The NAM forecasts a strong vorticity lobe and inverted trough in the lower half of troposphere to move across southeastern Arizona this evening, while upper winds are very diffluent. The vertical wind profile is forecast to be excellent for strong, organized storms (southeast at 700 mb; easterly at 500 mb; and southwesterly at 200 mb with speeds of 25 to around 50 kts). What more could you want? Higher CAPE!

Regardless, it appears there will be adequate instability for mountain storms to organize and propagate into the deserts. There is a threat of severe downbursts and outflow winds today – especially if we can keep the sunshine into mid-afternoon. My forecast for the backyard – 50% chance of storms and at least some rain.

Tomorrow – things tomorrow will be heavily dependent upon how the situation evolves during the next 18 hours or so. The models do indicate the vertical wind profile becomes southerly and uni-directional, which is a bad profile for here since storms tend to anvil out overhead. Models predict greater chances for rain over the weekend, but we’ll have to wait and watch.