Assessing Central U.S. Drought Risk

Fig. 1: NOAA/WPC 7-day rainfall forecast trends wetter North-central and Southeast U.S. and mostly dry in-between.

U.S. 7-day discussion: The upper level flow is quick along the U.S./Canada border and able to focus a persistent thunderstorm zone capable of producing a lot of rain across the Dakotas and into Minnesota through next week (Fig. 1). At the same time slowly evolving low pressure across the coastal Southeast U.S. encourages daily thunderstorm episodes also capable of generating excessive rainfall. Heavy rains also emerge across Mexico as monsoon season is underway. Areas not exposed to the heavy rains described are mostly dry.

Central U.S. hazards to crops discussion: Anomalous important heat risk is always important in July for the Great Plains. Heading into July the shallow soil moisture trend across most of the Great Plains is drier and deep layer soil moisture is dry in the western Great Plains. At a glance, the west/southwest Great Plains are at risk of an evolving drought and possibly a flash drought if sustained hot weather occurs. It’s mostly about temperature in July (unless gully-washer thunderstorms are occurring).

Forecast models indicate the first obvious excessive heat risk for the central/west Great Plains next Wednesday. Temperatures will exceed 100F over the dry soil conditions of the western Great Plains. The Corn Belt is very warm and humid. Next Thursday expect 105F centered on west/central Kansas. This location has both shallow layer and deep layer soil moisture deficits and is a concern for fast oncoming drought. On Thursday the heat (into the 90’s) will extend to the Corn Belt. A similar scenario next Friday, possibly a degree or two less hot for the western Plains to the Corn Belt.

July 11-13 looks very hot in the west, central and south Great Plains with widespread 90’s and several areas (with driest soils) exceeding 100F on consecutive days. The Corn Belt is into the 90’s and humid. Dangerous 97F heat in the Corn Belt is not expected. By mid-July thundershowers threaten to squash the excessive heat.

So…what’s the result? The hot weather is a concern for the Corn Belt. However, leading into the event the lack of a large area of soil moisture deficit either near the surface or in the deep layers saves the Corn Belt from crop damage. The west-to-central Great Plains are in danger of developing rapidly evolving drought conditions especially western Kansas to the Oklahoma Panhandle.

A second event is needed to create important issues. The GFS correctly indicates such an event redeveloping in the western half of the Great Plains July 18^th. The far western Corn Belt will have some exposure. The reason the oncoming drought concern in the western Great Plains is important is because the mid-summer climate pattern will work hard to find that zone and reproduce serious heat. Confidence is 3.5 on a 1-5 scale that scenario occurs. Will the second heat wave extend into the Corn Belt? That part of the forecast is uncertain but if it does dry-to-drought concerns for parts of the Corn Belt are on the table.

Bottom line? Developing west/southwest Great Plains. This zone becomes a hot air mass source region. The Corn Belt survives the first hot weather episode next week. A second (or third) event would produce some damage. The risk of a second or third event is on the table due to the developing nearby hot air source region.

Fig. 2: June 2020 precipitation rate anomalies across Europe and Western Russia.

Fig. 3: The GFS ENS 15-day percent of normal rainfall forecast.

EU/RU drought concerns: Drought important to crop areas in the EU/RU arena will occur during summertime. The highest risk is Southwest Russia into the Black Sea region where shallow soil moisture deficits are increasing due to short-term hot weather forecasts and not much rainfall. Further north the Russian spring wheat zones were dry in June and forecast to stay (Fig. 2-3) that way into mid-July increasing the risk of drought in that region. Climate probability forecasts indicate the Black Sea region has the hottest risk of any crop area in the northern hemisphere for summer 2020. So…the 15-day dryness and anomalous heat forecast for that zone is the onset of what should be more persistent in August.

In May, Europe was very dry and the attendant dry soil moisture conditions which evolved were impressive. Part of the rapid dry soil moisture development in May was also related to deep layer soil moisture reduction especially in Eastern Europe. However, the Europe climate has been fickle…alternating between very dry (May) and a wetter shift (June). The Europe Drought Observatory is fairly aggressive identifying dry soil areas across mostly non-crop zones. The 15-day outlook offers dry weather in France to Germany and Poland but no anomalous heat. Remember…it’s about temperature as mid-summer approaches. The hot weather focus is on Russia.

Bottom line? Southern District will encounter drought in summer 2020. The drought will reach westward to Eastern Ukraine. Dryness could extend farther north into the spring wheat zone. As for Europe no drought risk now. HOWEVER, the warm mid-latitude oceans indicate the risk of late summer stronger than normal subtropical ridging that can cause rapid onset on dry/hot weather over mid-latitude land masses is characteristic of the current climate pattern which puts Europe at risk for late season dry issues.

EU/NOAA “weeklies”: This product is generated differently from operational forecasts especially week-3 and beyond. Because of the difference at that time stamp forecast confidence is reasonable for the next 2 weeks but lowers dramatically week 3 (and beyond). This is why Climate Impact Company provides a week 2-4 outlook…the best interpretive process available.

The EU “weeklies” are issued at 2000 GMT Monday and Thursday. They indicate a cool/wet trough over Europe while to the east a general Upper ridge pattern over Russia. The model is biased toward the June upper air pattern is likely incorrect for the 4-week period (July 20 to Aug. 17). Expect an unsteady upper ridge pattern to become dominant. In North America the week-3 forecast is in error right-off-the-bat showing an upper ridge over the northern Continental Divide where persistent upper troughs have dominated late spring/early summer which should continue into mid-summer. The model “knows” the tropics will become busy but is unsure how to position the ridge due to that seasonal climate change heading toward August. In the U.S. large areas of soil moisture deficit are at risk for evolving, continuing and intensifying heat which is why the result of the hot weather the next 10 days in the Great Plains is very important as to late July and August.