Friday, August 06, 2010

Very Complicated Situation Today

Brief overview of the situation today follows:
Sounding at Tucson this morning (top) indicates little to no CAPE; as did the sounding last evening before the thunderstorms at the airport. However, wrt GPS PW the sounding last evening was over 1 cm too dry and this morning's sounding is a bit more than 5 mm too dry. So, throw out all those indices computed by the sounding analysis software. There appear to be enhanced southeast winds at low-levels (both Tucson and El Paso) affected by both outflows and a decaying, backdoor front to the south. The Guaymas sounding this morning has moistened nicely and shows a good surge signal, enhanced by very strong outflow from the strong MCS that collapsed just to the south this early morning, in low-levels. Around the area, GPS PW values are: Yuma 19 mm; Tucson 32 mm; and Hemosillo 51 mm. So, highest moisture values extend northward from GoC into southeast Arizona. The vertical wind profile is not great for organized storms with winds from 600 mb up being from the south-southwest at 20 to 30 knots, indicating that anvils will try to spread out directly ahead of storms. this is a poor profile for rainfall in this part of town - unless, as I noted below, something develops very close to the south. Convective cloud bases indicated around 600 mb or so, with threat of gusty winds remaining because of the deep, hot boundary layer.
At 500 mb, the morning NAM analyzes (middle graphic) a nice inverted trough over the southern half of GoC, with a convectively enhanced vorticity maxima (see IR image of MCS activity over south GoC at 08Z this morning) southwest of Hermosillo. The Pacific trough remains along the California coast, and the center of the 500 mb anticyclone has slipped just south of the border in the Southwest. Further south, TD Seven E is spinning fairly vigorously south of Cabo Correntes - this system is forecast to become a TS and to move off to the west, passing southwest of the end of Baja. Many aspects of the current setting look like it's September rather than early August.
The NAM appears to forecast an afternoon and evening similar to yesterday and holds off the increased moisture and heavier rains until tomorrow. I'm inclined to think that better moisture may be in southeastern Arizona by evening and that there should be another increase in activity relative to last two days. However, the details of today are complicated by the cool outflows that have affected the area, and by clouds and even some midmorning storms. Thus, a very difficult day to forecast for specific points.


  1. Mike Leuthold9:29 AM

    About the 0Z sounding...Even the surface Td from the sounding was too dry and it's from a different sensor! Our Td was in the low 50's while they reported 45 and the distance from the two is only a couple hundred meters.

  2. John Diebolt9:08 PM

    I was looking forward to the first good night for lightining photography Friday evening.. Was out on Ina/Wade focusing on storms over the Catalinas and Oracle Junction. I got one bolt before the towers collapsed. Got home to look at Radar around 8:45PM with a complex moving west across Sonora and had these thoughts: Ordianarily might see a scenario this weekend where this complex over Sonora clouds us over and outflow generates some precip overnight/early morning leaving us very moist but lacking for solar insolation Saturday and relatively down. Sunday as southwesterlies begin to develop..action focuses just e/ne of Tucson and moves off to the NE with little development upstream to the SSW/SW and the result being many valley locales miss the action. The trough dynamics nearby this time around could make all the difference, overcoming our usual quandry in high-moisture situations. Plenty of opportunity to bust a high PoP forecast this weekend! From my limited time in Tucson (6 years) it seems it takes an exact mix of everything to generate a widespread precip event and if anything is the slightest bit fizzles. The vagaries of monsoon forecasting... bad sounding data..lack of Mexican data. Not the most enviable job but challenging!