Saturday, July 19, 2014

Slightly Improved Synoptic Regime

Another very quiet, down day for most of southern Arizona yesterday. CGs above for 24-h ending at 6 am MST this morning and rainfall for 24-h ending at 7 am from MesoWest below. There were early morning storms out west as forecast by the WRF-NAM early am run yesterday. The PW across most of southern Arizona has recovered to around 30 to 34 mm - better than it was, but still very marginal for storms.

The 12 UTC TUS sounding (skewT plot above) is a chopped-up mess with very dry low-levels but considerable middle-level moisture. The air a bit above 700 mb has a small amount of CAPE, accounting for the high-based storms out west and abundant cloudiness at sunrise. Most pronounced feature of the sounding is the strong, southerly winds in the upper-troposphere, produced by a strong height gradient between a weak shortwave moving north along Baja and the strong, upper-level anticyclone over northern Mexico. See the NAM 500 mb analysis for 12 UTC below. As this feature moves northward, drier air will advect into parts of the Southwest, coming in from south to north.

The early WRF-NAM forecast of PW (below, 5.4 km graphic is valid at 9 pm MST this evening) illustrates the start of this new round of drying. As this dry area expands, a fairly substantial moisture gradient develops across Pima County for tomorrow and Monday with very dry air out to the west. The model forecast is mostly dry for southeastern Arizona through Monday, with only some storms and light rainfall right along the border region. The exception is that the model forecasts an unusual, long-lived thunderstorm to develop during late afternoon near Sonoita and to move north-northwestward across parts of metro Tucson, before dissipating in Pinal County around 10 pm. Will watch to see if anything like this actually occurs.

No comments:

Post a Comment