Friday, September 18, 2015

More On Developing Storm Event

Various models continue to forecast a significant rainfall event developing for southern Arizona - there are timing and detail differences, but the potential appears to be increasing with each run of the models.

Plot of CG flashes above (from Vaisala and Atmo) is for 12-hours ending 06 UTC last night. There were isolated storms over the southeast corner of Arizona yesterday, but they seem to have missed surface-observing sites. The eastern Pacific IR satellite image for 1330 UTC  (below) shows deep cloudiness and cold cloud-tops stretching from the south end of GoC west-southwestward out to 120 degrees W. The subtropical disturbance associated with the eastern portion of the cloud masses is moving north-northwestward and will be the main player in our developing weather event, while a weak 500 mb cyclone west of Baja will be a second-string player. The GFS forecast (second below) of 500 mb features is valid at 12 UTC on Sunday morning and indicates the subtropical short-wave to be over northern Sonora and headed our way.

As I said earlier, timing is an issue wrt the onset of the event here in eastern Pima County. The NWS forecast for the airport indicates a 10% chance of measurable rain tomorrow afternoon and night. However, the 06 UTC runs of the WRF model at Atmo are much more aggressive in the forecast for tomorrow.

The WRF-NAM forecast of composite radar echoes (above) is valid at 4:00 pm MST tomorrow (the 19th) and its forecast of accumulated rain (below) is valid for period ending at midnight tomorrow. The GFS version of the forecast is similar.

The WRF forecasts are sometimes too aggressive with the onset forecast, and I suspect that tomorrow will play out somewhere between the NWS and WRF forecasts. The longer-term WRF-NAM forecast for rainfall amounts on the 5.4 km grid (at bottom - for period ending at 11:00 am on 21 September) is quite amazing, with truly prodigious rain amounts forecast for Baja and across north-central Mexico.

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