Saturday, September 01, 2007

Weather Discussion 1 Sept. 2007

There have been many interesting changes from yesterday morning to this morning.

Atmosphere has moistened up some wrt IPW, but the soundings are very hard to interpret.

Last evening's TWC sounding (see Fig. 1) had a dry spike off the surface and then seemed to indicate a BL with a bit less than 8 g/kg and IPW of 25.8 mm (this was extremely dry wrt the GPS value). The evening sounding appeared to have no CAPE - but storms developed to the south of the airport around 0200Z and TUS carried a brief tstm (nice lightning visible from here with this cell).

This morning's sounding (see Fig. 2) has IPW of 37.5 mm, quite a large increase of almost 12 mm in 12 hours according to the soundings, but GPS data (see Fig. 3 from FSL) show a 12 hour increase of only about 2.5 mm. So, once again, the first forecasting dilemma for today is that of trying to figure out what the local thermodynamic conditions actually are.

The morning sounding has NO residual BL in moisture but a very deep residual BL in temperature. I'm going to guess that it's there (a residual BL in moisture) and that the mixing ratio we're heading toward is about 9 g/km. This would give a decent amount of CAPE this afternoon. But if I'm off by either +/- ~1 g/kg, the difference goes from a marginal day with mostly mountain storms to a very good day at low elevations with moderate CAPE available for storms. The deep BL and high cloud bases leads to either the potential for dry microbursts, or to severe hybrid downbursts at lower elevations - all depending upon what the afternoon BL actually looks like.

Am I frustrated trying to use bad sounding data to make a forecast? VERY!

Other interesting aspects of today's forecast:

Surface pressures are down 2 to 3 mb in south-central AZ compared to 24 hours ago.

An upper-level S/W trough is swinging southward across New Mexico, giving much of AZ flow aloft that is quite difluent.

Easterly low-level winds are still present but have diminished quite a bit. It will be important for the evolution of storms this afternoon if WNW diurnal winds can develop - the lower deserts to west and northwest are more moist than the local area.

Steering level winds are around 10 to 15 kt from the north, but anvil level winds are also northerly at about 20 kts. Thus, unless storms can propagate rapidly southward, anvil cloud will spread out ahead of the cells - not favorable for storms at lower elevations.

Thus, it's a very difficult call for lower elevations. Mountain and higher elevation areas should be very active. Will significant storms occur at lower elevations of southeast AZ? It's probably a coin toss, and we'll just have to see what kind of event unfolds this afternoon and evening.

As for TS/possible Hurricane Henriette, the NAM and the GFS have much different scenarios predicted for the storm. The NAM takes the storm fairly quickly into the area around the tip of Baja. This would not trigger a surge of low-level moisture into Arizona until the storm remnants actually moved to the north end of GoC or northern Sonora. The GFS takes the storm northward considerably further to the west, along a track that would trigger a significant surge. The GFS then brings the storm remnants right into southern AZ for a big rain event. However, it appears that only the operational GFS has this solution, and all the other GFS ensemble member forecasts from 0600 UTC hardly indicated that Henrietta exists. So, there are more coin tosses in the long-term outlook also.

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