Tuesday, July 01, 2014

The Early WRF-NAM Forecasts From Atmo

I am going to focus on the early WRF-NAM forecasts for the afternoon of the 3rd of July - that's two days off, and I know that it is very difficult at times to forecast what is going to happen this afternoon during the summer! Above is the WRF forecast for PW valid  at 11 pm on the 3rd for the large, 5.4 km grid. Note that the model forecasts a distinct surge of GoC low-level moisture into much of southern Arizona. Such a surge will be needed if we are are to get significant rains and thunderstorms into lower elevations. The WRF (driven by the 00 UTC NWS NAM forecasts) is generating the surge due to the strong low that the model forecasts to be over the southern half of Baja by Wednesday night. Such a surge could also result from outflows pushing into Arizona from MCSs over northern Sonora the next two nights - so there are interesting possibilities for us to watch for. The forecast below is of composite radar echoes valid at 8 pm MST on the third. Strong storms are forecast over eastern and central Pima County during the evening.

The model forecast sounding for TUS shown above is valid at 5 pm on the third. It indicates limited, high-based CAPE at the airport with just over an inch of PW. Such a sounding is supportive of strong to severe, downburst winds, dust, and light showers. The 10-m wind forecast below is valid at 8 pm on the third - note the very strong, mesoscale outflow forecast to be crossing parts of Pima and Pinal Counties.

The WRF-NAM forecast of accumulated rainfall (above valid through 11 pm MST on the third) indicates the most substantial rains for Pima County to come off the Catalinas and to maximize out west of the Tucson metro area. Meanwhile, this morning's 12 UTC run of the NWS NAM is available. The new NAM forecast of total rainfall through 11 pm on the third is below. This forecast is very aggressive with rain and apparently forecasts an intense MCS over northern Sonora, which would send significant outflows northward into Arizona.

However, the NAM seems to be having trouble with the two Pacific tropical storms, which are situated along the model's southern boundary. The model seems to forecast a single, consolidated storm that moves northward along Baja triggering the important Gulf Surge. The NHC forecasts for these storms are dramatically different than what the NAM is forecasting. So caution is obviously needed until we have better indications of what's actually happening south of 30 N. But it should be an interesting Holiday weekend, regardless of how things eventually evolve.

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