Saturday, September 01, 2018

First Day Of Meteorological Fall

There was an interesting mix of clouds this morning before and just after sunrise - both lenticular type clouds and middle-level buildups were present (above and at bottom).

Very strong thunderstorms have been occurring since midnight across the Rim country and White Mountains - some of these may actually have been severe, based on the radar depictions.  Above shows Flash density since 1:00 am MST this morning (from and Vaisala) - quite impressive. The Flagstaff morning sounding (below from SPC) is quite cool at surface, but old BL above was very unstable. Winds were westerly but there was considerable veering in the hodograph, supporting idea that some of storms may have been severe (a classic Fall transition pattern).

The 500 mb chart and the TWC morning sounding (above and below from SPC) also show the transition conditions - 500 mb trough dominates, but residual mT air below 600 mb remains quite moist, with considerably more potential CAPE than was present yesterday morning. There is significant veering in the wind profile aloft, and isolated supercells are possible over much of the eastern quarter of Arizona this afternoon.

Each new run of the WRF models seems to come up with different details for the southeast Arizona forecast. I'll just show a graphic from the 06 UTC WRF-NAM. That model keeps storms today mostly east and north of Tucson. The GFS version is similar, however, it forecasts considerable nighttime activity over nearby higher elevations. Both versions forecast an active thunderstorm day tomorrow - below is the NAM version forecast of composite radar echoes valid at 7:00 pm tomorrow evening. Yet another watch and observe day as Fall gets underway.

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