Sunday, December 18, 2011

QPF Very Difficult Even At 24-Hours

As the current storm system begins to approach from the west, the key question remains that of how much rain or snow will fall across southeastern Arizona. This is referred to as QPF (Quantitative Precipitation Forecast - how much precipitation, when and where). This type of forecast remains very difficult even at short time frames because so many small-scale factors affect the final outcome - for example, the details of terrain, development of deep convection, etc. While it is very easy to visit the NWS web site and find that the probability of rainfall at the airport is 90% today and 100% tonight, it is much more difficult to find what the official QPF currently is for the airport. Digging deep into the page, and after much work, I find that the NWS forecast for the 1 km grid box that includes the airport is that 0.43 inches of rain is expected to fall through noon tomorrow.

The above 500 mb chart is the morning NAM forecast valid at midnight tonight. The model forecasts the strongest vorticity advection at 500 mb to remain south of the border. Temperatures observed in the closed low are below -25C this morning, indicating that mid-level temperatures in southeastern Arizona will fall abruptly during the coming 18 hours (500 mb T over Tucson at sounding time this morning was -17C). There are many pros and cons at play with this system. No distinct southerly feed of moisture has developed and the system has been carrying sparse moisture within it's circulation. Although low-level moisture is higher to the south and east of Tucson, the forecasts by the models never bring this moisture across the divide. So, the storm's vertical motion will develop what precipitation it can, as it works on somewhat dry air. The models do have a substantial northern offset for the 700 mb low, and it tracks right across the White Mountains, so that is somewhat favorable.

The morning NAM QPF through noon tomorrow (above) indicates 0.10 to 0.25" possible for Tucson and surrounding area. The midnight QPF forecast from the Atmo early run of its WRF-GFS, below, shifts the focus of the event into Tucson metro and both the Catalinas and the Rincons - very much different than the NAM.

Finally, the WRF forecasted sounding for Tucson at 3 pm this afternoon (below) indicates that deep, moist convection will be possible for a few hours - complicating the entire QPF situation. So, there are many complicated factors at play - now to observe how all of this interacts and works out.

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