Transition Day - 27 July 2013

Heavy storms moved across some parts of the Tucson area yesterday afternoon. The photo above (taken at about 2:20 pm MST) is looking south from the house at a storm that seemed to be coming this way. It produced heavy rains and a wet microburst that did roof damage and took down power poles near Fair and Park, a bit north of the airport (Trace at TUS and no winds, but DM reported gusts to 52 mph from the northwest and 0.58" of rain). Here at house we had gusts from the northwest of 45 to 50 mph out of a storm over the west end of the Catalinas. This outflow apparently led to new cell development to the east of the storm in the photo, so that we just had some spits and sprinkles that amounted to only 0.03" here. Graphic below shows lightning strikes for 24-hours ending at 6 am this morning. There was considerably more storm activity over central and eastern Pima County than forecast by the early WRF-GFS (for second straight day). So we have the question again of how well the model forecasts for today will be.

Across the ALERT network 48 sites had rainfall (52%) with 13 sites having 1/2" or more, 8 sites with over an inch and 1 site with just over 2 inches. Almost all the rainfall occurred before 5 pm yesterday afternoon.

The morning skew-T plot for the TUS sounding is fairly similar to yesterday's and remains very moist and with a chopped up, light and variable wind profile.The CIRA blended PW product (below for 6 am this morning) continues to indicate a large area of high PW over southern Arizona and the lower Colorado river Basin. There are signs of drier area intruding into the northern GoC and the forecast issue for today will be how quickly the drier air will invade southern Arizona. Skies are quite clear this morning, so the day is starting out differently than the past few days. The models forecast the 500 mb anticyclone to consolidate over northern Mexico  by tomorrow, bringing a dry-out from the south and southwest for much of the Southwest. The PW trends today will provide the information relative to the how quickly the transition to drier air will occur. The early WRF-GFS again forecasts a mostly down day across Pima County. But the forecast 6 pm TUS sounding for this afternoon again shows over 1500 j/kg CAPE and PW of 41 mm. Expect that storm activity will again exceed the forecast coverage, but key will be by how much. Steering flow may increase some by evening and be from west to east.

The extent of the drying forecast by the early WRF-GFS is shown by the forecasts of PW valid at 6 pm MST tomorrow (above) and at 6 pm on Monday the 29th (below).