Friday, July 04, 2014

Quick Look At Morning Data

The Tucson morning sounding (skew-T plot above) has a number of interesting features. The PW is up to essentially 40 mm due mostly to the very moist, but cool, air near the surface, left behind by yesterday's rain and outflows. The cool layer at the surface extends upward to about 750 mb, which is a fairly deep layer of modified air. Above the cool and moist layer there is a classic "onion" sounding with  a downdraft layer of adiabatic, but dry air, below a deep saturated layer.

Key questions for the day relate to the thermodynamics of the sounding: 1) how extensive is the dry layer above 750 mb? Since this was produced by storm and mesoscale processes, its horizontal extent is quite important - will it quickly advect away? 2) Since a new boundary layer must build in the cool surface layer, there will be substantial CIN until old and new BLs merge. 3) How rapidly will the residual mid and high clouds dissipate/move away? This appears to be a day when a special, early afternoon sounding would be very useful.

It appears that new convection will develop most easily on higher elevation slopes and the strength and movement of storm outflows will play a key role in what happens at low elevations. The fact that some areas in eastern Pima County received little to no rain, means that surface BL growth will be fastest in those areas.

The Empalme morning sounding is shown below for contrast. The sounding exhibits more than 60 mm of PW and very substantial CAPE.

Surface temperatures around the metro area at 9:45 am are shown above - the range is from 80 to near 90F. Note that at 9 am MST yesterday the airport temperature was 8 F warmer than it is this morning. Visible satellite imagery for 9:15 am (below) shows the chaotic cloud situation with much residual mid and high cloudiness - but also with substantial breaks across southern Arizona.

All of this adds up to a very complicated situation for this afternoon. Sometimes a day that starts like today ends up with little or nothing happening at low elevations; however, other times outflows can drive convective development at low elevations.

The 12 UTC WRF-NAM forecasts convective storms again today across metro Tucson. But it forecasts substantial heating and a rapid development of a deep, thermal boundary layer. I will remain an observer today, since I think that the smaller-scale details of what will transpire meteorologically is actually beyond the capability of the forecast models.

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