Thursday, July 29, 2010

Extreme Moisture - Still No Winds

It's a bit of a difficult situation today. The morning TWC sounding (top image) is of a tropical nature with very high moisture content. The sounding data indicate an unstable lifted parcel with theta-w of 26C, and note that the 850 mb Td is an amazing 19C! It would seem that a good, hearty sneeze should instantly kick off a thunderstorm. However, the morning sounding is about 5 mm too moist wrt the GPS precipitable water - I don't know where that extra moisture is in the sounding plot, but I suspect that it's very near the surface. If so, this makes the sounding not quite as unstable as it would seem to be. Further, the wind profile remains light and variable (see the current, very pathetic hodograph in upper right of the SPC sounding plot), with no significant steering flow as of yet. So, we should expect strong storms to kick off, early, over the mountains. But it is not obvious where the storms will want to propagate to - perhaps wherever the most unstable air is at lower elevations.
The middle plot is the RAP 500 mb plot and analysis for 12 UTC this morning. It shows that the closed low that has been malingering over Texas for the past few days has opened up into an inverted trough that is moving westward in the subtropical easterlies. The inverted trough stretches from northwest Mexico northeastward to the Oklahoma Panhandle currently. However, the height gradients remain weak, and the strongest winds at 500 mb seem to be over northern New Mexico. So, the big question for the day is that of how will the winds change between now and midnight?
The morning NAM forecast run today indicates that that model keeps the steering level winds quite light, while increasing the upper-tropospheric from the northeast. This not a great shear profile for southeastern Arizona, since anvils tend to spread out over the lower elevations. The NAM indicates more precipitation by midnight over south-central Arizona and also over the southeastern mountains (see bottom panel which is 12-hour QPF ending at midnight tonight). However, the NAM forecasts a better wind profile tomorrow and also Saturday, with a really big precipitation event forecast for all of southern Arizona for the 12-hours ending midnight Saturday night. Can it hold off that long? I doubt it, but much will depend upon how things evolve today!

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