Sunday, August 10, 2014

Favorable Wind Profile But....

Significant storms continued over southeast corner of Arizona yesterday - above graphic shows CG flashes detected through 10 pm last evening (9 August). Plot of 24-hour rainfall from MesoWest at 5 am MST this morning shows that coverage was widespread  over Cochise County, with several amounts exceeding an inch. Alas, low elevations of the Sonoran Desert (including the metropolitan Tucson and Phoenix areas) remained very suppressed. Across the Pima County ALERT network 11 sites (along the eastern and southern edges of the network) recorded very light amounts.

A moderately strong outflow from storms to the south moved across metro Tucson around 8 pm last evening - above is time series from Atmo. This morning's sounding (below) indicates that the outflow had little impact on PW - values across southern Arizona remain at 1 1/4 to 1 1/2 inches, which is a very marginal value for low-elevation storms. The morning sounding has a nice shear profile through the potential convective cloud depth with mid-level easterly steering flow and westerly winds aloft at anvil levels. Wind profile has actually improved since yesterday with weaker wind speeds up in the hostil, dry and warm air above 500 mb. Shear profile results from a large title of the anticyclone aloft - near Four Corners at 500 mb and over northwestern Mexico at 250 mb. What continues to be lacking at low elevations is significant CAPE - I can forecast only a sliver of CAPE over metro Tucson this evening. An increase in low-level moisture would improve the situation (values are higher out over the desert to the west and toward Phoenix). Some cooling in the warm layer above 500 mb would also help. There seems to be some weak cool advection present in the 400 to 300 mb layer. So, all things considered, we may see the storm situation improve by late afternoon.

The early run of the WRF-NAM model at Atmo forecasts a small cluster of storms to move across the metro area around sunset today - below is the model's forecast of total rainfall through midnight tonight. Strongest storms are forecast along the Border Lands to the southwest of Tucson, but the model forecasts outflow boundaries (from the storms to the southwest and from mountain storms to the northeast - east) to converge over Tucson area and to force the low-elevation storms. So, we will have to watch how the situation evolves this afternoon.

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