Hurricane Rosa And A Return Of Topical Moisture

Yesterday was again suppressed wrt thunderstorms across most of Arizona (exception - some storms along the Rim into the White Mountains) and northwestern Mexico. Graphic below shows CG flashes for 12-hours (from Atmo and Vaisala) and captures considerable thunderstorm activity within portions of Hurricane Rosa's circulation.

The PW is decreasing, while cooler mid-level air is moving south across Arizona - we end up having stable conditions much of rest of month because of the drying. The current WRF models forecast no activity our area until the moisture increases from the south on 30th and 1st of October.

Rosa is currently a Cat 1 hurricane and is forecast to intensify rapidly. Above is visible image showing Rosa at 7:00 am MST and below is NHC's forecast for Rosa from this morning through am on October 1st. The hurricane is rapidly strengthening and is forecast to become a major storm. But once it moves northward over cooler SSTs west of south end of Baja it begins weakening.

The exact path that the remnants of Rosa will follow is not really forecastable at this long-lead time. Forecast tracks from ensembles of several global models shown above indicate a very large range of possibilities at this time, and current NHC forecast is essentially the mean of these possibilities.

However, the hurricane's track and turn northward should result in its triggering a strong surge of GoC moisture into southwest Arizona and southern California. The forecast below (from 00 UTC WRF-GFS on 5.4 km grid) is of PW valid at 12 UTC on October 1st.

The return of subtropical moisture should lead to increasing shower and storm activity - we will have to wait for several days before we will get a feel for where and how much. Those who have been here for more than a few years know how very fickle events with remnant tropical storms can be.