IEM Daily Feature
Tuesday, 12 September 2023

Change in 90 Day SPI

Posted: 12 Sep 2023 12:01 AM

The seven day total precipitation ending this morning around 7 AM is considered for the weekly update to the US Drought Monitor issued on Thursday. This period has seen a couple of rounds of rainfalls hitting various parts of the state. The focus of today's daily feature is assessing if any of it was enough to move the drought needle? While there is no one-to-one association of drought classification to any precipitation index, a commonly consulted value is trailing 90 day standardized precipitation index (SPI). The left hand map shows the most recent US Drought Monitor depiction along with long term climate sites drought classification based on 90 day SPI. The top right panel shows the weekly change in 90 day SPI over the past seven days with the arrow showing the direction of change over that period. The bottom right shows a count of stations meeting various before and after SPI values. The first thing that may catch your eye are the very negative red lines in the upper right panel. These can be explained by the fact that the trailing 90 day window dropped some heavy rainfall events from 3 months ago as the window slid forward. This is a huge caveat when looking at a given number of day SPI value as the big rainfall events can introduce noise as they come in and leave the time window. Another caveat is that replacing a week of June with a week of September lowers the climatological accumulation, which is effectively a positive addition to SPI without it even raining! So getting back to the original question, the last week of rain didn't do too much to erase any of the drought that continues to plague the state.

Good = 11
Bad = 2