Comparing Droughts: 2007 vs. 2000-2002

Peter Corrigan
Service Hydrologist

When talking about the drought this year people often refer back to the last significant drought that only recently affected the area, in our case it was the 2000-2002 drought. Many people want to know if this one is as bad (or worse?) and of course how bad will it get? It’s a lot easier to answer the first part of the question than the second!

Our ability to monitor drought in a more comprehensive and objective way began in mid-1999 with the advent of a new drought monitoring effort, the U.S. Drought Monitor or USDM. The USDM is a joint effort of several federal agencies within the U.S. Departments of Agriculture and Commerce (including the NWS national and local offices), the University of Nebraska-Lincoln and numerous other participants at the federal, state and local level. It provides a weekly “snapshot” of the relative drought situation across the entire U.S. using 5 categories ranging from the least serious D0 (Abnormally Dry) to the most serious D4 (Exceptional Drought). The other categories are D1 (Moderate Drought), D2 (Severe Drought) and D3 (Extreme Drought). See the USDM web site for the specific criteria used to define each category of drought.

Using the USDM as a tool to analyze drought, we can start to compare the current drought with the 2000-2002 drought. Ironically, it was five years ago almost to the week (mid-October 2002) that the 2000-2002 drought began to come to an end. That drought really began with precipitation deficits incurred during the incredibly dry month of October 2000. That was the driest month ever recorded at both Lynchburg (0.01”) and Roanoke (0.02”). Lynchburg climate records date all the way back to 1893, while Roanoke only back to 1948. Many stations in the NWS cooperative network also recorded zero rainfall for the month! Defining a drought week as one in which there is Moderate Drought (D1) or worse depicted anywhere in the Blacksburg Hydrologic Service Area (HSA), the 2000-2002 drought “officially” began the week of October 10, 2000. The drought continued through the rest of 2000 and into about the first half of 2001. By late July of that year however, enough rain fell to eliminate drought and that condition lasted until late October 2001, when D1 reappeared in parts of the area. From then until December 10, 2002 there was continuous drought in parts of the HSA. This represents an unbroken span of 58 weeks with D1 or greater drought in some part of the area. 2001 went down as the driest year ever (since 1948) at Danville with 28.66”. It was also was the 2nd driest ever at Roanoke with 26.66” (the driest was 1963 with 25.67”) and at Blacksburg with 31.61” (record driest was also 1963 with 26.95”). At Lynchburg however, the 2001 precipitation of 32.49” was only the 16th driest in their long period of record.

The drought worsened considerably in early 2002, mainly due to a very dry February which had only about 25% of normal precipitation. Of course the areal coverage and intensity of the drought varied considerably during the entire 58-week period. It appears that the lowest point was reached beginning with the June 18, 2002 issuance of the USDM that saw D4, or exceptional drought shown on the map for the first time over parts of the North Carolina piedmont. The D4 category according to the USDM, equates to the 100-year drought or the 1% chance of annual occurrence. D4 conditions persisted and worsened in the Blacksburg HSA, primarily over portions of the Virginia and North Carolina piedmont during the summer and into mid-October 2002. Finally, significant improvements over the region began to take place during mid-October 2002 due to the remnants of Tropical Storm Kyle moving into the southeastern U.S. and several other rain-producing weather systems. In the 3-week period ending October 29, 2002 all of D3 and D4 were eliminated from the HSA. By December 10, 2002 all D1 or worse drought was gone and the drought was “officially” over.

Drought Conditions from October 2002

The 2007 drought by comparison has been (so far) much shorter in duration. The initial appearance of D1 drought on the USDM was on the April 10, 2007 as D1 edged into the extreme southwest counties of the Blacksburg HSA. The Blacksburg WFO began issuing Drought Statements in mid-May as D2 or Severe Drought first appeared on the USDM. The drought has continued and slowly worsened over the summer and fall, in large part due to an extremely hot and dry August-September period. At several climate stations, including Bluefield and Blacksburg it was the driest August-September on record. In addition, the sustained often record-breaking heat aggravated the existing precipitation deficits. Very dry and generally warm weather has continued through the first two weeks of October with D4 drought conditions now poised to overspread the southern counties in our HSA soon, perhaps in a matter of weeks. The October 16 USDM issuance will mark the 29th consecutive week with D1 or worse drought somewhere in our area. This is exactly half the duration of the worst period of the 2000-2002 drought. The intensity of the current drought has also not yet approached the levels of 2000-2002. During the worst of that drought there were approximately 17 consecutive weeks where D4 conditions existed somewhere within the HSA, running from June 18 through October 8, 2002. As noted above D4 is only now beginning to creep into the extreme southwest HSA, barely spilling over into western Ashe and Watauga counties as shown on the USDM image below from October 9, 2007.

 

Drought Conditions from October 2007

 

So how long will the 2007 (and perhaps 2008) drought last? Weather and climate forecasts at the time periods required to make such a prediction are not highly reliable at this point. The Winter Climate Outlook sections of this newsletter gives details on the extended forecast, but suffice to say the outlook is not highly optimistic.