The summer of 2007 is shaping up to be a hot and dry one for the mid-south. Here are the important factors…
1) A developing La Nina will keep the jet stream and storm systems to the north along the Canadian border
2) Quasi-stationary heat ridges supported by severe droughts over the Southwest and Southeast will weaken any storm that does move southward
3) Those same ridges could act as a saving grace by steering the remnants of an active Atlantic hurricane season into the mid-south
What does it all mean?
1) The latest ENSO diagnostic discussion from the CPC continues to suggest that a weak to moderate La Nina will develop over the next 1-3 months. Synoptically speaking, La Nina represents cooler than normal sea surface temperatures in the east-central Tropical Pacific which in turn leads to a weakening and a northward shift of the jet stream over the northern United States. If the La Nina forecast verifies, it means that summer storms and cold fronts will be located further north than a normal summer.
2) I think the biggest impact on our summer weather will be the developing drought in the Southeast. While the remnants of Barry soaked parts of the drought-stricken Southeast with 1-6 inches of rain, this region remains quite dry. Since summer drought can reinforce itself as subtropical highs migrate to regions with low surface moisture, the Bermuda high will likely shift further west than normal, which will lead to a couplet of strong ridges over the Southwest and Southeast with only a shallow trough between them over the Plains. This synoptic set-up will further weaken any summer storm and cold front from moving into the mid-south.
3) The wild-card this summer will be the Atlantic hurricane season. Hurricane experts predict an active season of 13-17 named storms including around 9-10 hurricanes. If the Bermuda high shifts westward as expected, hurricanes and tropical storms will be steered around the high and into the Gulf of Mexico. This means that storms are more likely to make landfall in a path similar to that of Dennis and Katrina from 2005 (NOTE: just the PATH is expected to be similar, not necessarily the strength of the storms). This means that the remnants of those storms will come directly north into TN/KY. This could be a summer where the majority of the rain we get comes from tropical remnants.
What to expect: Based on these three indicators, I think the summer of 2007 could be the warmest since 1999 and I think drought conditions will worsen across the Commonwealth. I think we will see the first 100+ degree readings since 1999 and it wouldn’t surprise me if we had several 100+ degree days. With the areas to our south so dry, humidity will be lower than normal, but that will lead to warmer temperatures than normal.
Could this forecast be wrong? Of course. With any seasonal forecast there is a lot of uncertainty. Here are the mitigating factors…
1) If the La Nina fails to materialize or is very weak, the jet stream will shift farther south than I am predicting which would lead to an increased likelihood for summer storms and cold fronts.
2) If the Southeast drought is terminated by above average June-July rainfall (such as a series of early season tropical storms or stalled fronts), the amplitude of the trough downstream of the Southwest ridge will increase and shift eastward. This would also allow summer storms and cold fronts to penetrate further south.
I will periodically update the status of my summer forecast with progress reports and make any adjustments due to any changes in the synoptic features.