The situation this morning provides a nice illustration of how, at times, the latest forecast models can be inconsistent at very short time frames. The reasons for this situation today appear to be the varying, in time, and weakly defined initial conditions off the northwest coast of the U.S.
First I'll show forecasts from the early (midnight) runs of the high-resolution WRF model at Atmo. The forecast of composite radar echoes from the WRF-GFS valid at 3 pm MST tomorrow afternoon (Thursday, January 9th) is shown above, indicating light showers and rain just brushing the southeast corner of Arizona. The same forecast from the WRF-NAM is shown below and indicates a widespread area of light showers and rain across all of the eastern half of Pima, extending east and northeastward into other parts of southeast Arizona.
Products from the NWS model forecasts used to initialize the early WRF runs are shown here. The forecast is of accumulated rainfall through midnight Thursday night (note the darkest green indicates amounts of 1/4 to 1/2 inch). Again there are substantial differences over southeast Arizona, which would be expected given the difference in the two versions of the WRF forecasts.
Finally, to further confuse the issue, the new runs this morning of the NWS NAM model have done an about-face for southeastern Arizona. The new NAM forecast of accumulated rainfall through midnight tomorrow is shown below - quite different than the earlier version just above.
So, what exactly will happen over Pima County tomorrow? Definitely increased cloudiness, but the chances of rain or showers at low elevations are likely to not be very great, since the main impacts of the amplifying short-wave will be off to the east of the Continental Divide. But, this is a situation where we'll have to wait to see what tomorrow brings.
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