Tuesday, July 22, 2014

Dry Spell Continues

Dry spell here at house has now reached one week. The sad state of affairs was reflected by afternoon dewpoints yesterday when Tucson dropped to 39F and Yuma fell to 28F. This morning there is substantial middle and high cloud from Mexico and New Mexico circulating around the western lobe of the strong, 500 mb anticyclone (above image from Kitt Peak at 7:45 am). Lightning CG flashes through 5 am MST this morning (below) shows that only isolated mountain storms occurred yesterday in northern Mexico.

The WRF model runs at Atmo continue to forecast an increase in PW coming westward across the Continental Divide tomorrow, with a brief increase in thunderstorm activity over southeastern Arizona. Above is early WRF-NAM forecast of PW valid at 5 pm tomorrow (23 July) - WRF forecasts PW at TUS to reach 43 mm during the storm activity. Sounding forecast for 2 pm MST is shown below with PW at 37 mm but CAPE at model-forecasted max of 1300 m2/s2. My eyeball says the model algorithm estimate of CAPE is very optimistic and that amounts actually present tomorrow could be as much as 50% less than estimated. The PW forecast also presents a bit of a chicken/egg dilemma, since PW reaches its highest values due to the shower activity forecast.

The WRF-NAM forecast of total rainfall through midnight tomorrow night is shown below. Note that rainfall tends to be highest over the nearby mountains. I continue to think that this is a tricky situation that we'll just have to watch closely. The early WRF-GFS forecast is considerably more dry for tomorrow in Pima County, but forecasts storm activity today over in Cochise County, which the NAM version does not.

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