The 500 mb Anticyclone and the TWC Sounding this Morning

1 – This morning the 500 mb anticyclone exhibits three separate centers: one over southern Nevada, one over eastern New Mexico, and one over southern Arkansas. Yesterday morning there was only one distinct center and that was over northeastern Oklahoma. So, the transition that has been advertised by the models is underway. By 72 hours the NAM indicates that the two eastern lobes of the high have been pushed south and weakened/dissipated, while the Great Basin anticyclone dominates and shifts to northern Arizona. This would leave us with a fairly typical summer pattern to begin the month of July.

There is also a distinct inverted trough at 500 mb this morning that stretches from northwest Texas to the southwest Mexican coast. The models seem to indicate that this feature looses its coherence with time and becomes several fragmented vorticity maxima. In some ways it appears that the NAM is behaving as it did last summer - generating numerous small-scale waves and vorticity maxima over northwest Mexico and the Gulf of California. The user is then left having to decide if the model is actually capturing atmospheric reality for any given forecast cycle.

2 – This morning’s TWC sounding, see above, provides a nice illustration of how precipitable water values can be high (as per 1.4 inches plus this morning) but with CAPE values quite low or non-existent. I’ve shown an estimated sounding for 5 pm this afternoon (it may be a bit optimistic since I’ve used 8 g/kg as the mixed boundary layer’s moisture content). There is only a sliver of CAPE present. So, again today, better CAPE values will mainly be restricted to higher elevations. The afternoon’s estimate has a lifted parcel with theta-w of 22C – a marginal value. For really serious thunderstorm potential here in Tucson I’d like to see a lifted parcel with theta-w of 23 to 25C.

It is interesting that there is a shallow layer of easterly winds near 600 mb (i.e., around cloud base). These winds may try to push some mountain developments toward the west, but the dominant steering flow should be the south-southwest winds between 300 and 400 mb.

PS – Storms yesterday in Arizona tended to occur mostly in the southeastern corner of state – southeast of a line from Alpine through Sasabe. The highest amounts I found online were 0.96” at Tombstone and 0.31” at the Carr RAWS station. One station in the Pima County Alert network recorded precipitation (0.04”) in past 24 hours.

Bob