Tuesday, August 19, 2014

Quick Look At Today

Abundant moisture is  inland over the Southwest, extending north across Utah and into Wyoming. Flash flood watches are posted over wide areas, including southeastern Arizona for today, and there are warnings north of Phoenix due to early morning storms up there. The NWS POPs for the airport are currently forecast at 60% for both today and tonight.

The closed cyclone at 500 mb continues to move southward along the California coast, and a weak, leading trough has moved northward across southern Arizona. The morning sounding plot for TWC is shown above. The trough passage has led to the southwest winds from 700 to 500 mb. Note the very weak middle tropospheric winds, which will let storms wander around today and probably propagate toward local regions of higher CAPE. The PW here is around 1.75", with values near 2.00" to the west and northwest.

Although there is a warm layer above 500 mb, low-level temperatures and moisture provide abundant CAPE, so that storms won't be hindered by conditions aloft. Low-level heating will be affected by the cloud cover, but storms should develop quickly whenever and wherever the sun breaks through.

Model forecasts are a bit of a mixed bag this morning. The latest NAM forecast of rainfall through 11 pm tonight (above) focuses on the higher terrain of southeast Arizona - problem with the forecast is that most of rain is forecast to fall before noon and this is unlikely. The early WRF-NAM from Atmo (below) forecast of rainfall for same period forecasts local areas of heavy rains over southeast Arizona, but with most activity to the north and west, where storms are already active. Interestingly, the early NAM forecasts the rain for southeast Arizona to occur this afternoon and evening - certainly more realistic than the new NAM forecast.

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