The potential for a significant precipitation event across most of Arizona continues to increase. I can put the developing event in perspective by noting the following. Here at the house there have been, to date, only two events that produced half an inch or more rainfall during all of 2013. On September 6th a direct hit by a small thunderstorm cell produced 0.66" in less than an hour. Back on February 20th a winter storm system provided an event with 0.63" of rain and snow here. That is it for the year so far.
The NAM forecast of the 500 mb level valid at 12 UTC next Saturday morning (November 23rd) is shown above. The pattern has evolved pretty much as the ECMWF forecast at long ranges - see post from last Saturday morning the 16th. The models at this time continue to forecast that this will be a fairly warm closed low with thickness values not dropping anywhere near "snow" values at low elevations in Arizona. Note the low over the Southwest is "blocked" into place by the amplifying trough over the Great Lakes, so its movement will be quite slow. The NAM model's forecast of total precipitation valid through midnight on Friday night is shown below, with a significant area of heavy rainfall centered over Arizona.
Forecasts from the early run of Atmo's WRF-NAM at midnight last night are shown above and below. The forecast above (both show total precipitation through midnight Friday) on the 5.4 km grid is similar to that from the NAM, with the main difference being the shift northward into Utah of the heaviest precipitation. The high resolution 1.8 km version (below) forecasts quite heavy amounts in central and southern Arizona, with amounts of 1 inch or more over a fairly large area. at htis time the 1.8 WRF is maximizing the event over southern Arizona - but this is of course still a couple of days away. The event will not be over at midnight Friday either - it's just as far out as the current WRF forecasts cover. So, the most interesting weather event in many months will be unfolding as the weekend approaches.
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