June is almost here and Kentucky has yet to record a 90 degree day, although southern KY could get there on Friday (although I wouldn’t hold my breath). The median date for the first 90 degree day of the year is 6/2 for Bowling Green and 6/7 for Louisville. But more importantly, what does the rest of the summer look like?
While the summer of 2007 ended in record-breaking fashion with numerous records related to the drought and heatwave, no such extremes are expected for the summer of 2008. There are three important synoptic features that will drive the weather this summer; 1) the weakening La Nina, 2) the continued cold PDO, and 3) saturated soils in the Midwest.
1) While it appeared back in February that this very strong La Nina could persist for another year, recent model projections discussed in the latest ENSO diagnostic discussion (a weekly must-read for any meteorologist) now suggest that the La Nina should transition to neutral ENSO conditions by late summer. La Nina is characterized by a northward-shifted and stronger than normal jet stream that often is associated with warm and dry conditions over the Mid-South during summer (like 2007). This weakening state of La Nina and a transition to neutral ENSO will result in a “normal” jet stream with no spatial preference. By itself, this “normal” summer jet stream should result in “normal” temperature and precipitation patterns for KY/TN.
2) The cold PDO, rather than the strong La Nina, has been the big factor in the cold and snowy winter/spring that occurred over much of the central United States. The cold PDO results in cold ocean temperatures off the west coast of North America which creates a mean trough over the NW quadrant of the United States and forces a downstream ridge over the Southeast (sound familiar??). This jetstream pattern creates the familiar storm track we have seen all winter/spring with cold and snowy weather over the Midwest and cold and rainy weather over the Mid-South (with dry conditions in the Southeast). This pattern was allowed to persist into May by the fact that there has been a split flow pattern around since April. This split flow pattern has featured a zonal sub-tropical jet with a very meridional polar jet with a very sharp baroclinic zone. A meridional polar jet is prone to blocking (negative NAO) which has allowed the recent severe weather pattern over the central United States to be so persistent. As the cold pool over central Canada is eventually exhausted in June, the polar jet stream will become more zonal, which will allow the Southeast Ridge to exert a stronger influence. By itself, the cold PDO during summer should result in a summer that is warmer and drier than normal.
3) Excessive soil moisture anomalies can propagate either drought or pluvial (wet) conditions. This map of soil moisture anomalies shows the spatial extent of wet soil over much of the central United States. Evaporative cooling from both the excess soil moisture as well as the lush vegetation will help to keep a mean trough located over this region as long as the soil moisture anomalies persist. This mean trough should lead to additional precipitation which then maintains the wet soil. By itself, the wet soil in the central United States should result in a summer than is wetter and cooler than normal.
What does it all mean?
Don’t expect any 100 degree days this summer and probably not a lot of 95+ degree days either. I did a quick analysis comparing the number of 100+ and 95+ degree days when both the preceding winter and spring were either very dry or very wet (top 15 all-time for each). I found six years (wet) and seven years (dry) for the analysis. The results show that 100+ (95+) degree days are three (more than two) times as likely when soil moisture at the end of spring is very dry compared to when soil moisture is wet. I think the summer will progressively move from cool and wet (relative to normal) in June to seasonably hot and dry (relative to normal) in August. Hurricane season may influence precipitation totals during July and August, but there is no way to incorporate that into summer season predictions.