Monday, August 21, 2006

Postmortem of yesterday and overview of morning observations today

Part 1 - Postmortem of yesterday: Storms were not widespread yesterday and a tropical-like squall did not materialize. So, yet another overly optimistic outlook - it's been a very tough summer for forecasting down here. Thunderstorms did move from the Catalinas into town last evening, and the airport got a direct hit by a storm with a moderate downburst and 0.33" of rain. I shot a photo of this storm at about 7pm when it was just west of the airport (i.e., looking south from our house) - CLICK HERE The simultaneous radar depiction of the storm CLICK HERE shows both the storm in the photo, and also the limited extent of local storm activity. As usual, we had about two hours of thunder here at the house, outflow, a sprinkle (trace of rain) and a great rainbow.

The most negative aspect of the day was the evolution of the thermodynamics during the afternoon (we knew we were dealing with a marginal wind profile). By evening, the Tucson sounding CLICK HERE showed drying had occurred during the day below 600 mb and that the afternoon BL was characterized by a moisture profile that was not well-mixed. This is a difficult negative to overcome, and the storm that made it to airport must have been propagating with a fairly strong downdraft (as the gust to 33 kt at TUS indicates).

Part 2 - Overview of conditions this morning: This morning's Tucson sounding CLICK HERE indicates moistening up to 500 mb, no residual BL is present (meaning substantial heating will be needed to build a deep BL over lower elevations), and there are fairly strong southeasterly winds present below about 650 mb (i.e., a downslope direction). Winds at upper-levels have veered again and are southwesterly around a weak trough over the West (so there is a good shear profile, except for the weak winds present from 500 to 350 mb). The key forecast issue is whether a well-mixed BL will be able to develop in the face of the downslope winds. (Mike Leuthold - let us know what the WRF model does if you can find time.)

On the larger-scale, standard level charts we find that:

At 700 mb: An anticyclone centered near Las Vegas is affecting most of Arizona, except the far southeast where the inverted trough over western Mexico appears to have resulted in southeast winds. The main anticyclone over southeast Oklahoma is separated from the western lobe (which actually has three weak centers) by a trough line extending from near Phoenix northeastward to east of Omaha. The 700 mb level is very moist at all sites upstream from Tucson (i.e., Albuquerque, El Paso, and Chihuahua MX).

At 500 mb: A western lobe of the anticyclone is centered in southern Utah, with a trough line separating it from the strong eastern anticyclone center over Oklahoma. This trough extends from near Tucson northeastward to central Nebraska. The southern part of this trough is an extension of the inverted trough over western Mexico - this trough appears to have moved westward away from the upper-level feature that has been nearly stationary for last couple of days over west Texas. Thus, the NAM may have it sort of right today, with the lower-level trough actually moving westward.

At 250 mb: The upper trough over west Texas/northeastern MX is very diffuse and hard to define - thus, this feature appears to be confined to the very upper troposphere, making it a likely non-player in the evolution of weather west of the Continental Divide. The TUTT seems very pronounced and shifted to a northerly location from central Mexico eastward across south Florida. Upper flow today is not nearly as diffluent as it was yesterday morning (a negative).

Why all this description? I think that today's 12 UTC observational data illustrate well how terribly difficult it can be to sort out all the potential interactions and evolutions of weak features which will come together to produce afternoon and evening thunderstorms. My gut says that today will be a much more active day than yesterday ended up being, but only time will tell which features dominate.

2 comments:

  1. Anonymous11:13 AM

    I don't really trust that sounding moisture profile...but pwat from satellite still looks reasonable and finally we have some flow in the low levels to push the storms off the mountains. Also low level flow is northwest so may not have problem with anvils...so let the fun begin.

    Pat Holbrook

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  2. Mike Leuthold3:54 PM

    Bob,
    Both NAM and GFS initializations had trouble this morning. The NAM inverted trough was initialized as a strong closed low over central Baja at 600 mb. WV imagery depicted no such feature that I could see but it certainly appears that there is something lower down due to all the SE winds in the morning sounding. WRF GFS fired White Mountains convection too quickly, but the rest looked pretty good and fired lots of convection over SE AZ as we are seeing as I type this. 12z WRF GFS has a big precip bullseye over southern Cochise county, SW NM and just south of the border late this afternoon and evening.

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