3.1. Daily SST-Pentad Rainfall Analysis for 1999

Unlike the 1993-1997 monsoon seasons, the 1999 season was anomalously wet, especially in Arizona, resulting in a rise in state reservoir levels and the coolest summer temperatures in Phoenix in 32 years (Dr. Kelly Redmond, private communication). The main purpose of this analysis is to test at higher time resolution whether heavier rainfall, characteristic of monsoon conditions, follows after N. gulf SSTs attain or exceed 29°C. This occurred for each of the five seasons previously analyzed (e.g. Figs. 6-10, and Fig. 12).

Fig. 14. Northern gulf daily mean GOES SSTs contrasted with SSM/I rainfall amounts (dashed histograms) in the AZNM region. The timing of the severe flash flood event in Las Vegas is indicated.

As with Fig. 4, daily GOES-10 SSTs are plotted in Fig. 14 for the N. gulf, but also plotted are the 5 day cumulative rainfall amounts (dashed histograms) for the AZNM region, which are indicated by the right axis of this figure. The GOES SSTs < 30°C tend to be about 0.5°C cooler than the weekly MCSSTs. Hence, the threshold N. gulf GOES SST for supporting heavier rainfall should be about 28.5°C. This SST was exceeded on July 6th. To interpret GOES SSTs more precisely, a six-order polynomial fit was used to relate weekly MCSSTs to matching mean weekly GOES SSTs for the 1999 season. A GOES SST of 28.56 °C translates to a MCSST of 29.0°C, and this threshold SST is indicated in Fig. 14 by the dotted line. Recall that missing SSTs in Fig. 14 are due to cloud cover.

Also on July 6th, the first heavy monsoon rains arrived in Arizona, as indicated by daily SSM/I images (not shown). The first substantial rainfall histogram begins July 5th, but SSM/I and IR satellite images for the 5th indicate rainfall was light at best with some cloud cover over the eastern half of Arizona and western New Mexico. On the 6th, rainfall was much heavier throughout the southern half of Arizona, with relatively little rainfall over New Mexico. Over the next two days, as also indicated by GOES upper-level water vapor (WV) images, this moisture moved further west, resulting in the Las Vegas flood around 11 AM LST on July 8th. This flood caused $20,000,000 in property damage, and President Clinton declared Las Vegas a disaster area on July 19th (Haro et al. 1999). The arrival of rain in southern Arizona on the 6th is consistent with expected moisture transport times from the N. gulf, which are on the order of 9 hours for 10 m/s winds. The 850 mb NCEP streamline analysis indicates winds were south-southwesterly on the 6th, flowing into Arizona from the N. gulf, and an ETA model forecast for the 8th shows southerly 600 mb winds at 20 m/s (Haro et al. 1999).

This daily analysis of part of the 1999 season reinforces the previous analysis of the 1993-1997 seasons, indicating that a threshold N. gulf SST may exist which appears necessary for sustained heavy rains in Arizona. But while the lag between the N. gulf reaching 29°C (in terms of MCSSTs) and monsoon rainfall was less than a day for 1999, a considerable lag may exist in other seasons, as shown in Figs. 6-10 and Table 2.

_____________________________________________________________________________

 Table 2.  AZNM rainfall lag periods defined as time for rainfall to reach 
 or exceed 0.8 cm/pentad after N. gulf SSTs reach or exceed 29°C.
 cm/pentad after N. gulf SSTs reach or exceed 29 oC.

          Season      Lag Period (days)
           1993              5
           1994              7
           1995             19
           1996              4
           1997              2   
           1999              0
                                                             
           Mean             6.2
       std. Deviation       6.7
_____________________________________________________________________________