September has had a warm start. Producers with late-maturing crops have welcomed this pattern. What will the rest of the fall season bring? The 6-10 day and 8-14 day outlooks favor a continuation of the warmer-than-normal pattern. The outlook for October also favors a continuation of the trend (Figure 1, lower right). The precipitation outlook (Figure 1, upper right) is less certain, with equal chances of above- or below-normal precipitation for all but extreme southeast, where there is a slight chance of below-normal precipitation.
Figure 1. October outlooks versus normal (Source: WDL and CPC)
Looking towards the end of the year (October-December), the temperature outlook continues to favor warmer-than-normal temperatures across the state. However, this does not indicate how those temperatures might be distributed. Coupled with the warmer-than-normal outlook for October, it does reduce the probability of an early freeze. With recent above-normal moisture in the central/eastern portions of Kansas, it takes a substantial cold air mass to cool off temperatures. Increased moisture will aid in temperatures remaining above normal and help prohibit an early freeze. An extended growing season would be welcome, due to the delayed crop development. Unfortunately, by October 1, normal low temperatures in the northwest drop to 42 degrees F and climatologically, growing degrees decrease daily through the remainder of the season.
After equal chances of above/below normal moisture for October, expectations of wetter-than-normal conditions return for November/December according to the Climate Prediction Center (Figure 2, lower right). Currently, the western third of the state has drier soil moisture conditions at the surface and would benefit from a normal precipitation pattern. This would especially aid in establishing fall-seeded crops such as winter wheat and canola. Further east, with above-average moisture to date resulting in increased evaporation and atmospheric moisture, this would increase the likelihood of additional heavy thunderstorms and rapid rain rates across the region when rainfall does occur. Because of this, flooding risks will remain elevated for east/central Kansas through the fall, even when dry periods develop between rain events.
Figure 2. Late fall outlooks versus normal for the October to December period (Source: WDL and CPC)
The science behind the outlook
The weak El Niño event has officially ended. This means other factors, such as antecedent conditions, the Madden-Julian oscillation, and placement of ridges will have more influence on how conditions develop. Historically, fall is a period of weak steering winds and jet stream across the central U.S., resulting in more benign conditions. Frontal passages become more common and are usually drier/colder. The pattern becomes more reliant on tropical conditions elsewhere and the location of any ridges of high pressure to our west.
The Atlantic and Gulf tropical season winds down and has some, but limited, impact on Kansas weather. Further west though, the Pacific is a prominent driver. The typhoon season in the west Pacific is a source for strong storm systems in the central/eastern United States in the early winter/late fall months. With very warm eastern Pacific temperatures (Figure 3), it is likely that a strong high pressure will continue to reside across that region. Therefore, when typhoons weaken and turn northward along or east of Japan, they ride the ridge across the northern latitudes over the Pacific/western U.S. coast and dip south into central/eastern U.S. These often bring very dynamic and active weather. So far, the west Pacific has seen six typhoons in 2019 and is forecasting 16 total (Tropical Storm Risk Consortium). This would suggest a near-normal season, likely resulting in one or two ex-typhoon driven events in the U.S. However, at this time, the tropics in that region are rather quiet with cooler-than-normal sea surface temperatures in the west Pacific. Medium range models also suggest a pattern that isn’t supportive of recurve events. Therefore, it isn’t likely these will have an impact and thus, equal chances of wet/dry exist in October.
Figure 3. Sea surface temperature anomalies as of September 20, 2019. The green H is centered upon abnormally warm sea surface temperatures and on the northern periphery of these temperatures (green line) the jet stream is often steered around (green arrow). Source: tropicaltidbits.com
To date, statewide temperatures have been mostly cooler than normal for 2019. This has had substantial impacts on cattle, growing degree days, and numerous industries. However, thus far in September (Figure 4), this pattern has switched to warmer than normal with state-wide drying. With forecasted rainfall and a storm system, this weekend (September 21 and 22, 2019) will usher in cooler weather – but still above-normal temperatures.
Figure 4. Daily statewide temperature departures for Kansas. September has provided the first prolonged warmer-than-normal period of the year. (Source: Kansas Mesonet and ACIS)
Is this the start of a trend?
Well - persistence is key in fall forecasts over the previous few years. With the exception of last year’s heavy rains and resulting cooler-than-normal temperatures - it is quite likely the trend of the warm falls will continue for 2019. Forecast models for the period remain quite confident that above-normal temperatures will be observed across the country. More often than not, these warmer conditions are the result of above-average night minimum temperatures. If above normal rainfall is received as predicted in November/December, it will increase surface moisture and limit the amount we can cool off at night. With a wet/warm signal, this is the most likely scenario.
However, if we do dry more than anticipated, we will see above-normal afternoon temperatures with cooler-than-average overnight lows. This would also not help with drought conditions in southwest Kansas. Also, should afternoon warm/dry scenario emerge, anticipate increased winds and wildfire potential as a result. This is especially a concern going into next spring with a large standing fuel load from the past year’s moisture.
Mary Knapp, Weather Data Library/Mesonet
mknapp@ksu.edu
Chip Redmond, Weather Data Library/Mesonet
christopherredmond@ksu.edu