Tropical storm Simon is now located southwest of the Baja spur with max winds estimated at 40 kt. Simon is weakening rapidly and becoming a remnant low, as it's middle-level circulation detaches and moves northward away from the low-level circulation. The visible image above, from 1430 UTC, shows that deep convection is no longer directly with the surface low, but is displaced from north through east of Simon. High-level cirrus from Simon now covers most of southern Arizona and some of southern California.
Low-level moisture from Simon has been feeding thunderstorms over western Mexico and Baja - plot of CG flashes detected through 0700 UTC is shown below. No thunderstorms yet this side of border, as PW remains very low - dewpoints have moved into the 60s out around Yuma, but morning sounding indicates moist layer is very shallow currently.
The sounding from Empalme this morning (second below) indicates 46 mm of PW and substantial CAPE and also CIN, so storms most likely on the mountains in Sonora. There is no organized wind field below 400 mb at Empalme this morning, so no strong advective fields are acting on the mT air as of this time over the GoC.
It appears that the middle-level vorticity associated with Simon will be picked up by the developing 500 mb trough west of southern California and move northward across western and northern Arizona, as the winds around the trough increase in speed. The NHC currently predicts the remnant depression from Simon to move into southern Arizona in a couple of days - track is quite similar to that forecasted for Odile last month. The WPC (once called the the Heavy Precipitation Center) predicts rainfall amounts through Wednesday as per graphic above - the WPC forecast indicates the heavy rains moving along the NHC predicted track for the remnants of Simon.
The rainfall forecasts from the early run of the WRF model at Atmo are quite different from the WPC forecast - graphics above and below show WRF rainfall predicted through 11 pm Wednesday night. Neither model forecasts much rainfall until cooler air arrives at middle-levels with the the California trough and thunderstorm activity develops. The GFS version (above) forecasts one swath of heavy rain stretching up the Colorado River Basin and extending into the western Rim Country, with a second swath across the northern GoC and into parts of Pima and Santa Cruz Counties.
The NAM version of the model (below) keeps the heaviest rainfall out in western Arizona. Both versions of the WRF forecast a weak surface low to move northward across western Arizona during the day on Wednesday. I took a careful look at the model forecasts on the 5.4 km grid and the low in Arizona seems to develop as the 500 mb trough approaches, with the Simon remnant low remaining to south and west (through 11 pm Wednesday). So right now it appears that anyone who wants more rain from this situation can find a model forecast that will make them happy - but which of the forecasts will be closest to what occurs in the real world?
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