Late last week, Dr. Justin Glisan, State Climatologist of Iowa for the Iowa Department of Agriculture and Land Stewardship, wrote some really interesting comments about the National Weather Service Climate Prediction Center monthly outlooks. We’ve copied some of the text directly from his tweets and asked him for additional thoughts on the topic of weather forecasts this time of year!
As far as monthly weather outlooks from the NWS CPC, two products are issued each month:
- The initial or 0.5-month lead outlook in the middle of the month
- The final outlook issued on the last day of the month.
These outlooks are not like a weather forecast you see on TV or hear on the radio. They are probabilistic products, meaning that they give probabilities of behavior. Instead of a high temperature of 71 degrees tomorrow as seen in a forecast, climate outlooks may show a 60% chance of temperatures being above average two weeks out, for example.
For the May 2020 outlooks, the initial 0.5-month lead was released on April 16 while the final set of outlooks was issued on April 30. As you can see in the first image, there was a notable shift from the initial outlooks with increased chances for colder than average temperatures (blue) and elevated chances of drier conditions (brown). Initially, most of Iowa had slightly elevated chances for wetter conditions. It is important to note that the final monthly outlooks have a strong reliance on the first two-week period of the month. You’ll note that the final May outlooks closely track the general signals in the current 8-14 day outlooks (shown below).
Over the last few months, we’ve seen considerable flips between the initial and final outlooks, as well as the outlooks compared to reality. Just looking at how April ended up in Iowa, conditions were cooler and drier than what the outlooks indicated.
Short-term outlooks cover two time frames, 6-10 and 8-14 days and hence, give guidance out 1.5 to 2 weeks. These products are issued daily in the afternoon. Recent short-term outlooks have been fairly accurate; they’ve been showing a trend towards cooler and drier condition over the last several iterations. Iowa has been drier than normal for the last 30 days, though this has been counter-balanced by adequate to surplus sub-soil moisture. With a nice stretch of pleasant weather, farmers have been making great progress in the fields. Looking forward into the beginning half of May, conditions are looking cooler and drier. If we examine the May-June-July outlooks, probabilities suggest that Iowa will experience wetter than average conditions with no clear-cut signal for temperature.
The “So What” point of the story is that the longer-term outlooks provide general guidance for the month and season with short-term outlooks highlighting the upcoming expected behavior, such as shifts in temperature and precipitation patterns. Coupled with weather forecasts, the short-term outlooks provide great insight into the development of favorable windows for planting and harvest.