In addition to being the Applications Integration Meteorologist for the NASA SPoRT program, I have retained my climate focal point duties at NWS Huntsville, AL. As a part of those duties, each week, I am honored to be able to provide feedback to the U.S. Drought Monitor (USDM). The USDM consists of a consortium of academic and government partners, including the University of Nebraska-Lincoln National Drought Mitigation Center (NDMC) and various agencies under NOAA and the USDA. Responsibility for ultimate authorship of the USDM each week falls to a rotating reponsibility of drought specialists within the various agencies and the UN-Lincoln NDMC. Each new week presents new challenges, particularly when assessing potential impacts due to low streamflow or low soil moisture for various parts of the country. Last week saw the introduction of D1 (moderate) drought conditions across portions of northern Alabama and southern middle Tennessee due to continued lack of rainfall and degradation of soil moisture and streamflows. This week, however, presented a new set of challenges, as parts (but certainly not all) of the area received some beneficial rainfall. This is depicted well in the in the 7-day Stage-IV precipitation map shown below.
As shown in Figure 1 above, rainfall was just around one quarter of an inch in portions of northeast Alabama, with amounts rising to around 2 to as much as 4 inches mainly in southwest portions of the Huntsville CWA, especially parts of Cullman, Franklin, and Colbert Counties. The image below shows precpitation anomalies for the same 7 day period.
As figure 2 above shows, precipitation was generally below normal for the week in portions of northeast Alabama, but was a little above normal in southern parts of our forecast area…with near normal precipitation in between. Since the USDM is updated on a weekly basis (Tuesday 12z – Tuesday 12z), the typical concerns involve how much precipitation fell during the weekly period and how this affected soil moisture and the hydrologic system. As meteorologists, we usually have a fairly good sense of rainfall amounts. However, we are often unsure how the soil and hydrologic systems respond to these inputs because there are so many factors to consider (runoff vs infiltration, evapotranspiration rates, etc). Additionally, these changes can be challenging to assess due to a general lack of in-situ observations. In northern Alabama, we are blessed with several soil moisture sensors as a part of the USDA Soil Climate Analysis Network (SCAN). However, this network is still relatively sparse for the purposes of drought monitoring, especially when decisions have to be made on sub-county scales. This is why the NASA LIS 1km domain over Alabama can be such a useful situational awareness tool for the purposes of drought monitoring. Indeed, such was the case just this week. While rainfall amounts ranged from slightly below normal to slightly above normal across the area, the question begging was…”how did the soil moisture respond to these inputs?” Did evaporation and evapotranspiration exceed the inputs to the system in western and southern areas? Or was it the converse? How about the lack of rainfall in the northeast. Were evaporation and evapotranspiration rates sufficiently low there so that the impact to soil moisture was minimal? These are questions that often need to be answered for weekly feedback to the USDM. After assessing rainfall amounts, a quick look at streamflows through the USGS WaterWatch site showed that they were still depressed after recent rains. So, now on to the 1km soil moisture assessment and the Alabama domain maps available through the NASA SPoRT web site (these are also available through the Southern Region LDM and AWIPS). The 0-10 cm soil moisture percentage quickly helped me to key in on areas where recent deficiencies were present, as shown below.
The What was quickly apparent was the relatively low soil moisture values in southern portions of DeKalb County and adjacent areas in eastern Marshall County, AL. Notice there, that values were similar to those in portions of central and southern Alabama (albeit on a smaller scale) where D1 or worse conditions existed in some places. These shallow (0-10 cm) soil moisture values will be the most responsive, of course, to short term rainfall anomalies. So, seeming shortages in soil moisture existed in portions of northern Alabama. So, the next question was…”how has this changed since last week?” Some rain had occurred, but was it enough to increase the soil moisture values. This is where the one-week change maps come into play, as shown in figure 4 below.
Notice in the above weekly difference plot that despite rainfall over the past week, soil moisture values decreased mainly across portions of northern and northeastern AL. Since soil moisture values already were lower in portions of southern DeKalb County than in other areas, then it seemed as though they were a likely candidate for expansion of the D1 (moderate) drought conditions that had been introduced in parts of the area last week. Essentially, the LIS had helped me to focus in on a particular area very quickly, reducing the investigative work that often goes into these types of assessments. As a result of the coupling of the Stage-IV data and the LIS soil data, I spoke with an Agricultural Extension Specialist at the Sand Mountain Research and Extension Center in Crossville, AL. He verified that crops in the area in question in southern DeKalb County were indeed under stress due to the low soil moisture, in addition to a couple of recent frosts. With all of this information at hand, the recommendation was made to the author of the drought monitor to extend D1 (moderate) drought conditions into this portion of the area. Because the integrated soil moisture difference plots indicated relatively minor improvement to areas that received rainfall this past week, and considering longer-scale (14-60 day) precipitation deficits and low soil moisture, drought designations were kept status-quo in other parts of the area for now.
By the way, I can’t post the latest draft of the USDM, but will post the official due out tomorrow morning. Due to the evidence provided by the NASA LIS and direct input from an extension specialist from the ACES, the D1 was extended into the areas recommended.
UPDATED – May 10th
Okay, as promised, here is the latest USDM for the state of Alabama…
Notice the area of moderate drought (D1) was extended southward through portions of DeKalb and Marshall Counties that were mentioned above. This may seem rather insigificant to those who aren’t familiar with the USDM and the types of decisions that go into this process. However, delineating these boundaries properly is important, especially for those (and the counties) that may be affected. Drought designations have implications particularly for agriculture and whether or not federal help is necessary to assist agricultural producers. The NASA LIS has been an invaluable tool in helping me to assess impacts due to lack of precipitation in the northern Alabama area.