Sunday, December 11, 2011

Very Significant Storm For The Southwest

The next storm system that will affect the Southwest has the potential to bring a significant rainfall episode to low elevations, as well a dangerous winter storm to higher elevations. The 2100 UTC water vapor image (above) indicates enhanced moisture west and southwest of Baja, while the next short-wave trough is digging southward off the northwest US coast. The moisture intrusion that is developing has had its origins both east of Hawaii and also with the large convective region that occurred the last few days around 130 W - see an earlier post. The interaction between the deepening short-wave and moisture from low latitudes will lead to an early development of precipitation across southern Arizona - likely sooner than indicated by some of the forecast models. The 36-hour NAM (from 1200 UTC run this morning - below) for 500 mb indicates a closed cyclone west of the LA Basin at 5 pm MST tomorrow. The forecast cyclone is being strengthened by vorticity advection into the bottom of the circulation from the long vorticity streamer that curves all the way back into southwestern Canada. Current model thickness forecasts indicate that precipitation in the lower elevations will remain rain, with heavy snowfall in the mountains.

The NAM forecast for total precipitation through 5 pm MST Wednesday afternoon is shown above - amounts are very substantial, especially for the eastern Arizona mountains. The GFS forecast for the same period (below) does not have such extreme amounts, but has forecast amounts of 0.75 to around 2 inches over a large portion of Arizona. The GFS ensemble forecast from 12 UTC this morning has all members forecasting significant precipitation for southeastern Arizona for the four consecutive 12-hour periods that begin at 5 am tomorrow! I can't recall a forecast quite like this since I've been watching the ensembles. It appears that this will be a very significant storm, and all who will be driving, or outside, at higher elevations (5 to 6 thousand feet and up) during the next couple of days should use caution as this storm unfolds.

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