Sunday, October 04, 2015

Low-Level Moisture vs Warm Air Aloft Today

Very interesting situation today as cold, closed low at 500 mb over California continues to dig southward. Pattern over the states is generally blocked with closed lows over the southern corners, so California low will be slow moving and gradually weakening. This morning's TWC sounding (above from SPC) indicates increased low-level moisture, a strongly veering wind profile below 300 mb, BUT very warm and dry air aloft above 650 mb. So, a mixed bag. Continued moisture increases could perhaps overcome the warm inversion and allow strong thunderstorms to develop. 

The 12 UTC PW analysis from CIRA at Colorado State (above) shows higher PW just to south of the border - so the forecast issue for today is indeed whether low-level CAPE can increase and overcome the warmth aloft. Yet another day where sounding data from Guaymas could have been very useful. Perhaps a special sounding from TWC would help? - but the warm, dry air is within the southwesterly air stream - so there's little observational data and the models are on their own wrt the evolution of the warm air.

The 06 UTC runs of the WRF model at Atmo keep things fairly suppressed today over southern Arizona and the warm air aloft seems to prevail in those forecasts. The WRF-GFS forecast of precipitation through 5:00 am MST tomorrow morning is shown below. However, the 12 UTC run of the NAM model is much wetter than the WRF runs were, so a cross-check of the new WRF forecasts later this morning would be in order.

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