Weather Discussion - 9:00 am Sunday June 29, 2008

There are many interesting things happening over the Southwest and so this discussion.

For the past week or so there have been daily thunderstorms over southeast, and sometimes central, Arizona. Moisture values and CAPE have generally remained marginal with IPW values of around 25 mm. Thursday afternoon (the 26th) IPW increased briefly above 30 mm and there was ample CAPE over lower elevations of southeast Arizona for severe storms and heavy rains to occur in the Tucson metro area. The street flooding here on the north side of town was quite impressive, even though at the house there was only 0.33" of rainfall. Temperatures in middle levels have remained quite cool through this period, with temps at 500 mb of -8 to -10 C, and thus there has been CAPE around even though moisture values were low.

On Thursday the high, subtropical values of IPW (>37.5 mm) had inched northward toward the end of the GoC and the borderlands. This morning the subtropical moisture has retreated southward and covers only about the southern third of the GoC.

This morning the Tucson sounding indicates that there will probably be CAPE present this afternoon at lower elevations in southeastern AZ. If we end up with 4 g/kg in the well-mixed boundary layer (BL) there will be slight CAPE, but it looks like the value could be 5 g/kg or even greater. Steering-level winds are easterly at about 20kt. There is a weak upper-level SW over New Mexico and the NAM forecasts part of this feature to move southwestward and interact with the northern end of a quite pronounced tropical wave. So the potential for storms appears enhanced today, especially given that 500 mb temperatures remain in the -8 to -10 C range across all of the southwest. There is also pronounced westward advection of higher moisture values across southern New Mexico. By afternoon there should be substantial moisture convergence over southeastern Arizona as the afternoon diurnal northwesterly winds butt into the easterly flow from New Mexico. Thus, there appears to be high potential for a significant storm day over southeastern AZ today, with severe macrobursts likely - given the high cloud bases and very hot subcloud BL.

In the longer term it finally appears that a wave of significance will move across the lower GoC. The models indicate substantial cool advection at 700 mb with the tropical wave that is now over Mexico and indicate significant storm activity likely in Sinaloa and southern Sonora - perhaps even moving out over the GoC. It would be nice to have some sounding observations over Mexico to help one evaluate the model forecasts, but all of northern Mexico has been without upper-air data for almost a month now. The evolution and movement of the wave, as predicted by the models, as well as the cool advection and rainfall would trigger a low-level surge of high moisture values into southern Arizona sometime between early morning on Tuesday and mid-day on Wednesday.

All in all, it looks like a number of very interesting days are on tap for us here in southeastern Arizona.

II. Problems With the TWC Soundings on 23 June 2008

Problems with the data from the RRS sondes are again starting to impact the analysis of the soundings taken at TWC and the forecasting of convection here in southeastern Arizona.

The FSL time series comparison of GPS IPW versus TWC IPW since the 19th of June indicates that 5 soundings were apparently too dry, while one sounding (1200 UTC 19 June) was very wet relative to GPS IPW. See Figure 1. Careful monitoring of the remotely sensed IPW continues to be the best way to make an initial assessment of how accurate a given TWC sounding might be.

The 00Z sounding last evening (see Figure 2) indicated a deep, well-mixed boundary layer extending up to 500 mb; however, the moisture structure was quite questionable, with very dry conditions off the surface and no recovery to a well-mixed value. This led to the "dry" sounding wrt IPW.

The 12Z sounding this morning (see Figure 3) indicates a very deep residual boundary layer, but the moisture structure is extremely erratic. The IPW values correspond well. Given the unrealistic physical nature of the Td data, I suspect that the agreement of the two IPW values is essentially a coincidence this morning. If the afternoon BL mixes toward 5 g/kg, then there is a small amount of CAPE present; however, if it mixes toward 4 g/kg then there is no CAPE present. Given the erratic Td data trace it is very hard to make a subjective forecast of the late afternoon sounding.

The Haboob of Saturday Evening June 21st 2008

This was probably the first significant event, for southeast Arizona, of the 2008 summer thunderstorm season. Widespread convection occurred over parts of southeast Arizona and particularly over southwest New Mexico and northern Mexico. See Figure 1.

High cloud bases and a very hot boundary layer led to an organized, mesoscale outflow that affected much of southeastern Arizona. Passage here in Tucson occurred about 4:30 pm local time. The outflow was well-depicted as a fine line on the KEMX reflectivity display see Figure 2.

Note that Douglas, Arizona, reported a severe convective gust of 51 kt at 3:32 pm local time after the haboob had moved passed. This severe storm event did not make it into SPC's list of wind events for the 21st.

We were returning to Tucson from Chicago and landed about 10:00 pm local time. The approach was very bumpy and winds were still gusty from the east to southeast. The air was heavily laden with dust, giving the 3/4 moon a very strange and dirty yellow-brown appearance.

REST OF MONTH LOOKS INTERESTING FOR SOUTHWEST

Both GFS and NAM model forecasts continue to be quite interesting for the Southwest.

Appears that there may be high-based, dry lightning storms over the mountains of far southeast AZ this weekend. Definitely not good, given the extreme dry conditions of last two and a half months. Only 0.09" of rain here at house since March 17th!

In the longer term, the GFS ensemble members keep a strong, middle-level anticyclone north Mexico/southern Plains, and then tend to shift it northward during last 10 days of month. Throw a tropical disturbance into this pattern and who knows what we might see down here.

I'm off to Wisconsin where it will be green and wet and probably muddy.

OVERVIEW of RRS SOUNDINGS 6 JUNE 2008

I have been involved in other things last few days. Have just taken a look at today's soundings and report the following:

0000 UTC soundings with superadiabatic layers aloft apparently due to wetbulbing:

UIL SLE BOI OTX FWD OAX SHV MSP TLH BMX APX MFL

ABR has an extreme super layer aloft but it is not due to wetbulbing. LBF flight missing.

12000 UTC soundings with superadiabatic layers aloft apparently due to wetbulbing:

UIL SLE ILX OTX DVN SGF

OUN sounding appears very strange for unknown reasons.