Wednesday, September 02, 2015

Interesting Day On Tap

Thunderstorm activity was very much diminished yesterday, as expected. Plot above shows CG flashes for 24-hours ending at 3:00 am MST this morning (from Vaisala and Atmo). Storms stayed around the periphery of the area, occurring primarily in regions where there had been no rain early Monday morning. Only 13 sites around the edges of the Pima County ALERT network reported rainfall. Couple of late afternoon sprinkles here - Trace.

This morning dawned gray, dreary, warm, and humid under heavy middle and high cloudiness, which has had a long fetch from low-latitudes The visible image below (for 7:30 am) shows the widespread cloud cover over much of Arizona and all of northwest Mexico. The degree to which the clouds keep temperatures down will be one aspect of the forecast puzzle for today - although the low temperature at TUS was a warm 83 F. 

The morning sounding from TWC (12 UTC above from SPC) indicates moderate CAPE this afternoon, but with some CIN above the top of the boundary layer (BL). Although upper winds are missing in this plot, other sources show that they are south-southwesterly at 30 to 35 kts, giving a somewhat uni-directional profile above the BL - except for the layer with weak westerly winds between 500 and 300 mb. Not a particularly favorable profile for parts of the metro area, since it favors anvil shading.

The larger-scale environment indicates weak upward motion, indicating a background environment favorably conditioned for storms. Temperatures are cool at 500 mb (chart below is 06 UTC WRF-NAM forecast of the 500 mb level valid at 5:00 am MST this morning). The forecast agrees well with the morning observations (note that Guaymas - much needed today - is again missing). The upper-level winds are very difluent over Arizona due to the Pacific trough. A weak short-wave over central Baja at 12 UTC is forecast to move northward across southeastern Arizona today. So there is a bit of a mixed bag today.

The 06 UTC WRF forecasts from Atmo both indicate a big storm day for the eastern half of Pima County (drier low-level air has inched eastward to around Organ Pipe National Monument). The models appear to heat things up too quickly, and the evolution of the cloudiness will be an important factor in the development and evolution of storms today.

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