Tale Of Two Models

Yesterday, there were showers and also some thunderstorms around southeast Arizona. Here in eastern Pima County the storms stuck mainly to the higher elevations. Across the ALERT network there was 30% coverage of stations with measured rainfall. However, this is a bit deceiving as the coverage at higher elevations was considerably higher and at low elevations across the metro area coverage was near zero. In the Catalinas there were 3 sites with more than half an inch and White Tail had just over an inch. As the official start of Fall tomorrow approaches, the two versions of the early WRF model run at Atmo have a split-personality type forecast for what might transpire today.

This morning PW remains fairly high over southeast Arizona with values around an inch and a half (above is CIRA blended PW analysis for 12 UTC).  Drier air circulating around the 500 mb low that is over central California has pushed across southwest Arizona, with the moisture gradient being strong across central Pima County. Note that TS Polo is passing west of the south tip of Baja and weakening rapidly - but high PW associated with Polo extends about 2/3rds of the way up the GoC.

The TWC morning sounding (skewT from SPC) is shown below. Dry air has intruded to some degree into the lowest levels (this is the case much more so up in Phoenix). However, there appears to be some CAPE still present in the 800 to 700 mb level - which would favor mountain storms again. Dry and warm air at and above 500 mb continues to be a problem. The wind profile is transitional and favors some organized, strong to perhaps severe thunderstorms over the eastern third of the state.

As for this afternoon, the early WRF-NAM forecasts storms over eastern Pima County (forecast of composite radar echoes above is valid at 4 pm this afternoon). The GFS version however keeps storms to the east of Pima County - probably a toss-up at this time.

As for tomorrow, the WRF version forecasts a resurgence of low-level moisture into south central Arizona due to the influences of Polo - WRF-NAM forecast of PW below is valid at 3 pm MST tomorrow afternoon. The GFS version is much drier, so again a toss-up. Forecasts for convection tomorrow are similar to today's in each version of the WRF.

Both the ECMWF and the GFS forecast a strong, westerly trough into the Southwest and West by day 7, bringing in the Fall season for us by next weekend.