How this forecast works

The model, what it does and doesn't know, and where the numbers come from.

How the call is made

Each day the National Weather Service issues extended-range MOS guidance for every station on the map. That bulletin gives maximum and minimum temperature, dewpoint, cloud, wind and precipitation in twelve-hour blocks out to eight days.

Those blocks are expanded into an hourly temperature and humidity curve, and Wallin severity values are accumulated from the hours spent in each temperature band while the leaf is wet. Two versions are computed, one assuming the day stays dry and one assuming it rains, since the bulletin gives a probability rather than an outcome.

Those figures, with the hours in each temperature band and the chance of rain, go into a small model fitted separately for each lead day. Its output is the probability that the day turns out conducive, meaning that the observed weather scores at least one Wallin severity value. It is not a prediction of how many.

What it does not tell you

Conducive means the weather would support infection if inoculum were present. It is not a report that late blight is in your field, and it is not a spray recommendation on its own.

The model has no knowledge of your variety, your last spray, your irrigation, or whether blight has been found nearby. Those change the decision more than the weather does.

The two thresholds

18 accumulated severity values is the traditional Wallin trigger for starting fungicide applications, counted here from your 50% emergence date. Wisconsin uses it, and it is the figure Michigan growers will recognise from the weekly Wisconsin newsletter.

30 accumulated severity values is not a second start point. In the Michigan system it marked the move from the minimum to the maximum labelled application rate. Counted from emergence it falls in late July in a typical season, and across the three recorded Michigan outbreaks it was reached an average of 17 days before the report.

Wallin 1962; MacKenzie 1981; Baker, Kirk, Stein & Andresen, HortTechnology 15(3), 2005; Wharton, Kirk, Baker & Duynslager, Computers and Electronics in Agriculture 61, 2008.

Provenance

The street map is Leaflet over OpenStreetMap tiles. If the tiles cannot be reached the page falls back to state and county boundaries drawn into the page itself, from US Census cartographic geometry at 1:10m, so the map still works with no network at all.