Saturday, July 18, 2015

Better Sounding This Morning

Thunderstorm activity shifted to the west yesterday, leaving southeast Arizona with a down day - forecasts were very much overdone yesterday. The plot above of detected CG flashes (from Vaisala and Atmo) is for 24-hours ending at 6:00 am MST this morning (Saturday, 18 July). During the past 18-hours only 4 of the 92 ALERT stations (less than 5 percent areal coverage) reported rainfall - these reports were in the far southwest portion of the network and were very slight amounts.

This morning's 12 UTC sounding from TWC on campus (above skewT from SPC) is much improved relative to yesterday morning's onion sounding (if interested in this terminology, see Zipser, 1977, Mon. Wea. Rev., 103, 1568-1589). The sounding exhibits considerable CAPE, that requires only moderate heating or orographic lift to get thunderstorms going. However, the winds aloft profile remains very poor, with light and variable winds below 500 mb and with southwesterly steering flow.  Best chance for storms this part of town will be from nearby development of storms on outflow boundaries. Sounding could certainly support locally heavy rains and also wet downbursts. So, a much more interesting day on tap today.

I took a quick look at the Atmo WRF forecast runs from 06 UTC and also the NWS HRRR from 12 UTC this morning. The models all forecast considerable storm activity across eastern Pima County by early afternoon - above is WRF-GFS forecast of composite radar echoes valid at 2:00 pm MST. Forecast below is for accumulated rainfall through 5:00 am tomorrow morning and is also from the WRF-GFS. Largest amounts are forecast for northwest Arizona, where the complex terrain is interacting with the strong northward push of mT air associated with TS Dolores. The model forecasts highest rain amounts in eastern Pima County to occur on the Catalinas and Santa Ritas, with a minimum sitting right over our location here - that amounts to a persistence forecast.

No comments:

Post a Comment