Wednesday, July 23, 2014

Models Forecast Storms Today

Considerable increase in thunderstorm activity yesterday to our east and southeast. The storms produced outflows with easterly winds that moved across southeastern Arizona during the night. Both TUS and DM had east-southeast gusts after midnight - TUS 39 mph and DM 44 mph. The outflows kicked low-level moisture up some so that PW is now at 1.4 to 1.5 inches across most of southern Arizona except out to far west. Debris cloud and east winds kept early morning temperatures quite warm - airport low was a warm 87F.

The early WRF-NAM wasn't in yet when I looked this morning, so graphics are from the early WRF-GFS. The model forecasts storms (forecast of composite radar echoes above) developing across eastern Pima County by 6 pm today - July 23rd. Rainfall through midnight (below) is forecast mostly to occur west of the metro, but one cell goes right across the airport. 

The morning sounding for TUS (above from SPC) does not seem very promising for storms. Winds are light and variable through most of troposphere, except for the low-level easterlies (this usually favors storms out to the west - but last time we had this setup cells developed right over the metro area). Although PW is up, the afternoon BL will be quite deep with cloud bases up around 600 mb - which favors gusty outflows, as per last night. CAPE is slight with the morning sounding, partly due to the warm middle-level temperatures (-4C at TUS and ELP). Definitely a better sounding for higher elevations than for the low desert.

The sounding forecast for TUS valid at 2 pm from the WRF-GFS (below) shows important changes in the forecast conditions. Middle-level temperatures are down by a couple of degrees C (hard to see where this cooling comes from). The middle-level winds have increased to 15 to 20 mph and so the model predicts nice steering flow from mountains toward low deserts. The model also forecasts a significant outflow to move westward across Maricopa County this evening. I still think forecast remains tricky and will definitely take a look at the new, 12 UTC WRF forecasts.

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