Wednesday, August 08, 2007

Weather Update for Next Couple of Days

The last week has been very interesting with very high IPW combined with very warm middle level temperatures. It has seemed like Central America. Several very heavy rain events occurred with little or no cloud to ground lightning. There were three fatalities here in the Tucson area due to flash flooding.

Yesterday was quite interesting as skies cleared and IPW decreased steadily. Winds aloft here in southeast AZ were very light. With the abundant sunshine, a well-mixed afternoon BL built up to about 750 mb. So, the drying was apparently a result of weak advection plus subsidence - particularly from 700 to 400 mb. The BL has retained fairly impressive moisture, and dewpoints remain high in the SE part of state.

The TWC sounding has suspect structures this morning - more on this in a separate blog post later this morning - but it appears to me that with heating and slow deepening of BL, coupled with at least some degree of cool advection in middle levels, deep convection will return very quickly. This afternoon we may see Cbs over the nearby mountains. Then things get more complicated.

There is a substantial tropical wave moving westward into the eastern Pacific. Very impressive convective complexes occurred over the lower GoC and further south last night. Thus, with falling pressures in lower Colorado River Basin and southern AZ (about 2 mb during last 24 hours), southerly winds, and moisture advection, should increase up the GoC.

The NAM has had great difficulty last two weeks over Mexico. Disturbances persist in analyses and forecasts with apparently a lot of latent-heating induced spinup leading to stronger middle level winds in the model than have likely been present in the real atmosphere. The lack of observations over most of Mexico lets the model feedback on itself and develop structures that may or not be real. For example, this morning's run takes the main tropical wave to the west-northwest, but it also breaks off a piece to the north up the GoC - winds around this feature may again be too strong, but it is certainly an interesting situation worthy of careful monitoring.

Meanwhile, I'm really enjoying the return of some sunshine to the desert!

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