Friday, February 06, 2015

Look Ahead To Late Next Week

The long range forecasts by the ECMWF and the NWS GFS have become very similar toward the end of next week. So, here's a quick look. Above is the ECMWF forecast for 500 mb valid at 5 pm MST Thursday, February 12th (or 00 UTC on Friday the 13th). Below is the same forecast from the operational GFS. Both models forecast a Rex block configuration, with a closed low sitting beneath a strong ridge and closed anticyclone.

Since Rex blocks are, as the name implies, fairly stagnant, they don't produce strong vertical motion due to vorticity advection (note that at 500 mb there is very little vorticity advection indicated by either model). But, if the closed low portion of the block picks up substantial moisture and some convective instability, then things can become interesting (again due to slow movement or evolution of the large-scale pattern).

At the bottom is the GFS forecast of precipitation for the 48-hours ending at the time of the two 500 mb forecasts. The precipitation forecast indicates an apparent influx of moisture from low latitudes and substantial precipitation for the Southwest. All of the 12 GFS ensemble members forecast some precipitation for southeast Arizona at the end of next week. However, we need to keep in mind that in the last two similar situations (re: agreement among the ensemble members), one event fizzled while the other became a serious, record breaker. So, out at seven days we're essentially looking at a coin-flip, but one that could become interesting.

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