Weather Situation Changing Slowly

This morning Simon is still being called a tropical storm, although the deeper cloudiness has mainly left the low-level circulation behind. The visible image above from 1430 UTC shows Simon located almost due west of the Baja Spur, with thick cloudiness extending from Baja and northwestern Sonora into southern California, Arizona, and the Great Basin. An IR image from the same time shows that currently deep and very cold cloud tops are very limited in extent and over Baja. The NHC currently forecasts the remnants of Simon to decay near the Arizona border southeast of Yuma.

The main role of Simon in the developing weather event over the Southwest will be the low-level moisture that feeds well north of Simon's remnants. The WRF model forecasts from early this morning remain very similar to those from yesterday. The forecast above is from the early WRF-NAM and is for 850 mb winds and dewpoint temperature - valid time is 6:00 am MST on Thursday morning the 9th. Very high dewpoint temperatures, for this time of year, extend northward all the way to southern Nevada as the weak, 500 mb trough from the Pacific consolidates over the lower Colorado River Basin. Significant CAPE and thunderstorm activity activity develops ahead of the trough, initially far to the west of the Tucson area. Also of interest, is that the forecast above indicates that Simon's remnant circulation is stuck on the mountains north of the Baja Spur and that a new low develops ahead of the trough, north of Yuma.

The 500 mb trough is forecast to move across Arizona very slowly and significant CAPE doesn't develop over eastern Pima County until Thursday in the WRF forecasts. The slow movement of the trough in the westerlies will mean that the potential is significant for locally heavy rains, particularly where southerly, moist winds interact with the terrain and/or where training echoes develop. The forecast below is from the early WRF-NAM and is of total rainfall amounts through 11 pm MST on Thursday night. The WRF-GFS version seems way too wet, to me, as it was in the Odile situation - the GFS version forecasts rain amounts of over 10 inches out at the far western edge of Pima County. Regardless, it appears that the overall event will continue to evolve slowly through the rest of the week.