Bowling Green continued its streak of five consecutive months of below normal precipitation Saturday, March 31st. Since November 1st, Bowling Green has had a cumulative rainfall deficit of 8.63 inches, making it the 5th driest such period since 1893. March was the driest (3rd all-time) and warmest relative to normal (9th all-time) of the five-month period. But does the dry start to the year necessarily mean that south-central KY will have a dry summer?
To help determine this, I looked at the four years that had a drier Nov-Mar period and calculated the rank of the following Apr-Aug period, which includes the key months for agriculture (the approximate growing season). There was no relationship between dry winters and dry summers, as the resulting distribution of the four summers that had data were random.
Year – Nov-Mar rank – Apr-Aug rank
1911 – 1st – 91st
1977 – 2nd – 44th
1918 – 3rd – 15th
1895 – 4th – 82nd
2007 – 5th – ???
Since the previous winter’s precipitation is not a reliable predictor of growing season precipitation, is there any guidance available? For the short term, both the 6-10 day and 8-14 day model forecasts show a cool but dry pattern across the eastern U.S. This cold outbreak will be ushered in by strong storms in the Tuesday-Wednesday time period. After that, a cool, dry northwesterly flow will limit precipitation until a southerly flow from the Gulf returns mid-month.
The key to a reversal in the developing drought conditions may be the upcoming hurricane season. While spared the wind and surge damage from Gulf hurricanes, Kentucky can receive a large percentage of its summer precipitation from the remnants of tropical cyclones. In 2005, rainfall from the remnants of Katrina dumped 5 inches of rain on Bowling Green, which helped to end a 40 day dry period from mid-July to late-August. While it is too early to accurately predict what type of hurricane season will occur in the Atlantic Basin, the state of ENSO will have a large role in the prediction. According to the weekly ENSO update, there is an even chance of neutral ENSO or La Nina conditions developing this summer in response to the weakening El Nino. While I have already addressed the relationship between hurricanes and La Nina here, most experts agree that the 2007 hurricane season will be more active than 2006 and should be above average although not nearly as active as the record-setting 2005. However, a shift to La Nina conditions may tilt the balance towards an above normal season.
Another issue with the potential for a La Nina this summer is the impact on the subtropical jet stream. La Nina summers are associated with a weakening of the sub-tropical jet and a strengthening of the polar jet. This could allow the west-Atlantic ridge to set up over the Southeastern U.S. for the summer, which would decrease stability and limit precipitation from air mass thunderstorms. Since soil moisture is so low from the recent dry March, a positive feedback mechanism could be established in late Spring where hot, dry surface air acts to reinforce the developing upper-level ridge as the summer heat builds in. Since the first two weeks of April are expected to be dry, the last two weeks of April could be a critical time to replenish soil moisture and avoid what could be a developing agricultural drought for Kentucky.