Saturday, October 02, 2010

Quick Look At Model Rainfall Forecasts

I've looked at several model forecasts this morning and will mainly contrast the rainfall forecasts in this post. The NWS NAM model pulls the 500 mb Pacific cutoff gradually northward, opening into the leading edge of the next Pacific trough. This puts southeastern Arizona into a moist southerly flow regime, especially tomorrow. The Univ. of Arizona Atmo version of the WRF model indicates that today will see a battle between dry air to the east and moist air to the west - with alternate pushes of wetter and drier air back and forth across the Tucson metro area. I also note that this morning's sounding at Guaymas has become very unstable. The San Diego sounding remains very unstable also, and heavy thunderstorms have continued over southern California - what a strange start to October over there!
The top two images show 24-hour predicted rainfall by midnight tonight from the 0600 UTC runs of the NAM (top) and the WRF (middle). Note that the two models both predict thundershowers today for southeastern Arizona. The NAM model forecasts almost all the rainfall off to the east of Tucson. The WRF model forecasts a band of showers producing rain from the south to the northwest of Tucson - so a substantial difference in the placement of the predicted showers for today. Note that the latest run of the NAM has shifted its rainfall back to the west, but still not as far west as the WRF.
The bottom graphic shows the 12-hour rainfall predicted by this morning's NAM forecast for the period ending at midnight tomorrow night - so the NAM forecasts a fairly substantial rain event tomorrow for much of Arizona.
Now, to sit back and watch how the actual events play out this weekend.

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