It take awhile for the county Assessor to catch up with the real-time values.
Also, if your home was purchased as an REO or short sale they may not allow for that in the decline of your value.
Most counties have a ssytem where you can petition for an adjustment once a year typically late in the third quarter.
Contact your local tax assesor to get the up to date procedure on how to do that. You may still have time!
I just signed on the answer this question for you but Karen and Ray answered the quesiton perfectly. If you have a recent appraisal demonstrating a market value lower than the county assesment you can petition to have your value reviewed. You can request the forms from the county website. The process is quite simple and usually just involves a meeting with a county appraiser to review your value documentaiton - your appraisal + other recent sales comps or other items that suport your idea of the reasonable value.