Friday, October 02, 2009

Very Difficult Forecast Situation

First - low temperatures: this was the first morning this Fall with a low down in the 40s - 47F - here at the house (note that it was quite a bit cooler down at the Rillito pathway). The airport (TUS) only dropped down to 59F this morning.
Second - Olaf makes for a difficult forecast situation. This morning's NAM run continues to keep the significant precipitation along and south of the border. The circultion of Olaf is quite large and the moisture shield extends far to the north of the center. It seems clear that the circulation of Olaf at and above 500 mb will shear away quickly to the east after about 24 hours. However, the NAM indicates that the large circulation below 500 mb will continue moving to the north as a remnant low and that low-level southerly winds will affect southern Arizona beginning in 24 to 36 hours (perhaps sooner given the model's tendency to be slow on moisture advection) and continue out through 72 hours. Thus, there should be a large advection of low-level moisture across the border, ahead of the digging cutoff/short-wave at 500 mb. So, the interplay among all these features will produce a significant October precipitation event and the key question for here will be how far north the event can advance. It will be interesting to see what the atmo WRF runs indicate later this morning.

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