Thursday, July 13, 2006

Gulf Surge Index on-line at Creighton Univ.

Art Douglas, Creighton University, computes a real-time Gulf Surge Index (GSI) daily. The index is computed from the surface data observed hourly at Yuma, Arizona. Values less than 10 index no surge detected, values greater than 10 indicate a surge event, and values of 60 or greater indicate a major surge event. Values for 2005 and 2006 to date are at:

For the past week: no surge events July 6 - 9. Surge events with values in range of 10 to 20 have occurred on July 10, 11, and 12..

The slow increase in moisture has lead to a situation over southern Arizona where high values of CAPE have developed at the same time that boundary layer temperatures have increased. The TUS sounding this morning is quite potent, but the winds aloft are weak. There is likely to be a dramatic increase in storms today, despite the NWS current POPs of zero for the local Tucson area.


  1. Oh of my favorite subjects...gulf surges. I have several points of confusion and frustration thru the years about surges. They are:

    1) Some people have used a hard cutoff to define surges...say southerly winds or moisture fluxes at YUM above a certain speed/level. Use of a hard cutoff implies a lack of significant atmospheric response if index is epsilon below cutoff. My the atmospheric response that abrupt?

    2) Moisture flux, per se, has little dynamical or thermodynamic meaning when it comes to increasing PW. Flux convergence of moisture does.

    3) What is the statistical relationship between Gulf surges and concomitant or subsequent sig. weather, i.e. CB's and rain? How much of the moistening and where over the low deserts of AZ can be directly attributed to "surges"?

    Help me understand please!


  2. There is no hard cutoff value for deciding if a surge event is occurring, but the GSI value of 10 picked up 95% of the subjective hand picked surge events observed 1950 to present. The GSI places a magnitude of strength to each event for intraseasonal and interannual comparitive purposes. In the future it would be logical to look at what magnitude a surge event needs to obtain for the event to become a better rain producer in southern AZ or have a better chance of extending to Las Vegas or Phoenix.

    Moisture convergence is most certainly more important from a dynamic standpoint, but the GSI was aimed at developing a climatology of surge events entering at a single point where synoptic reports are of very high quality (<98% were missing). The GSI uses hourly synoptic data to determine changes assoicated with the surge.

    Rainfall peaks in NW Mexico one day before a surge event (often the MCs trigger), while in southern AZ the rainfall peaks the day of and one day after the surge event. This is all in Nick's thesis.

    Nick also examined moisture transport at Yuma and this data is also calcualted on the GSI page. We found that he GSI data were easier to use when trying to identify weak gulf surges. It appears that once Yuma has a dew point >15C, that the moisture transport calculations are more sensitive to changes in wind speed and not so much to changes in dew points (say 18C vs 21C).

    The GSI is primarily meant to be used as a measure of gulf surge magnitude over the long historical record of synoptic reports from Yuma.