Friday, September 07, 2012

Very Complicated Situation This Morning

The early WRF-GFS forecast from Atmo indicates another big storm day today (Friday September 7th). The point forecasts of temperatures and rainfall amounts from the WRF are shown in the table above. The forecast of composite radar echoes for 4 pm MST is shown below. However, the heavy storm activity at sunrise was not accurately forecast by the early WRF run (there was convection in the forecast but it was too far west and not as extensive or strong as that observed). So, the situation today is somewhat similar to that last Tuesday. Last Tuesday the feedbacks from the morning convection completely fouled up the entire WRF-GFS forecast for that day. A weak but cool outflow from the west has already moved through the metro area (at around 5 to 6 am). So, caution advised regarding the model forecasts.

The morning sounding from Tucson (above) has almost 2 inches of PW and is very unstable above the surface layer - again, very similar to last Tuesday morning. Winds aloft remain light and variable and very heavy rains have already been falling nearby. So, it is not clear how the thermodynamics will evolve. The visible satellite image (below for 7:30 am MST) indicates heavy  cloud cover for most of southern Arizona, but with some clear areas in Cochise County and northern Sonora.

The situation is very complicated due to both early outflows and the presence of two Mesoscale Convective Vortices (MCVs) over southern Arizona. One is out to the west, north of Yuma, and was spun up by the convective activity late yesterday. The other is spinning northwest of Tucson and has developed in response to the early morning MCS. The new NAM model forecast seems to consolidate these two over Tucson by afternoon, and it also forecasts heavy rainfall in our area. Will check to see what the new WRF-GFS forecasts do.

The WRF forecast does indicate that the cold front coming down the Front Range will back-door into southeast Arizona during the afternoon tomorrow and that PW will be decreasing due to the easterly low-level winds. The drying continues into Sunday.

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