Thursday, September 01, 2016

Isolated Storms Again For Start Of Fall

Nice sunrise this morning to begin meteorological Fall.

There was much more storm activity over southeastern Arizona yesterday afternoon than was forecast by the 06 UTC runs of the WRF model. Appears that the 06 UTC forecasts seriously under-forecast the increases in PW that occurred during the day yesterday. Flash density for detected CGs above from and Vaisala is for 24-hours ending at 7:00 am MST this morning. Rainfall over the ALERT network (below) is for same period. Over the network 14 sites measured 0.04" or more of rainfall yesterday. There were sprinkles and a Trace here just after dark.

This morning's sounding (as per skewT plot above from SPC) is very moist at 1.7 inches of PW and quite unstable. However, the big question again today is how will the PW evolve with time? The 06 UTC WRF forecasts from Atmo this morning again forecast only very isolated storms over eastern Pima County and keep most activity out to the west.

Above is the 12 UTC CIRA blended PW analysis, which shows the more moist air out to our west and also indicates a small region to our south-southeast with PW less than inch. 

The 06 UTC WRF forecasts bring this dry region into play for eastern Pima County this afternoon. The WRF-NAM forecast of PW valid at 5:00 am (12 UTC) this morning above has forecast the dry region shown on the CIRA analysis. The forecast valid at 1:00 pm below expands the dry region and decreases PW here at Tucson by about 10 mm. The drying is the reason the models forecast limited storm activity in our area this afternoon. So, the key question is whether the 12 UTC forecasts will continue this drying trend.

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