Sunday, March 29, 2015

High Clouds On Increase Next Couple of Days

A weak 500 mb low west of Baja will impact our weather the next several days - mainly by spewing some high and middle-level clouds across southern Arizona. The WRF-NAM model (06 UTC run) currently forecasts some weak radar echoes over southeast Arizona tomorrow (above is forecast of composite radar echoes valid at 9:00 pm MST Monday). The forecast of PW valid at same time (below) indicates values reaching amounts just over half an inch. So, not surprising that the model forecasts little in the way of rain to actually reach the ground.

Second graphic below shows the 500 mb forecast (on the 5.4 km grid) valid at 9:00 pm tomorrow. At that time the closed low is forecast to still be west of Baja, while a weak short wave to its north is inching across Arizona. 

The GFS average 500 mb heights from the ensemble runs are shown above (valid March 31st at 5:00 pm MST) and below (valid April 4th at 5:00 pm). The model indicates that the Southwest remains in weak flow through the next several days. But after that the main branch of the westerlies shifts southward as the Pacific ridge and split flow regime shifts (at least temporarily) west to the central Pacific. So even if precipitation with systems in early April stays north of us, we'll probably see some spring winds starting to blow dust and pollen around. Gesundheit!

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